Structural Design Loads for the Home Inspector
by Nick Gromicko, CMI® and Ben Gromicko
General
Loads are a primary consideration in any building design because they define the nature and magnitude of hazards and external forces that a building must resist to provide reasonable performance (i.e., safety and serviceability) throughout the structure’s useful life. The anticipated loads are influenced by a building’s intended use (occupancy and function), configuration (size and shape), and location (climate and site conditions). Ultimately, the type and magnitude of design loads affect critical decisions, such as material selection, construction details, and architectural configuration. Thus, to optimize the value (performance versus economy) of the finished product, it is essential to apply design loads realistically.
While the buildings we are considering in this article are primarily single-family detached and attached dwellings, the principles and concepts related to building loads also apply to other similar types of construction, such as low-rise apartment buildings. In general, the design loads recommended here are based on applicable provisions of the ASCE 7 standard, Minimum Design Loads for Buildings and Other Structures. The standard represents an acceptable practice for building loads in the United States and is recognized in U.S. building codes. For this reason, the reader is encouraged to become familiar with the provisions, commentary, and technical references contained in the ASCE 7 standard.
In general, the structural design of housing has not been treated as a unique engineering discipline or subjected to a special effort to develop better, more efficient design practices. Therefore, this article partly focuses on technical resources that are particularly relevant to the determination of design loads for residential structures. As with any design function, the designer must ultimately understand and approve the loads for a given project, as well as the overall design methodology, including all its inherent strengths and weaknesses. Since building codes tend to vary in their treatment of design loads, the designer should, as a matter of due diligence, identify variances from both local accepted practices, and the applicable building code relative to design loads as presented in this article, even though the variations may be considered technically sound.
The complete design of a home typically requires the evaluation of several different types of materials. Some material specifications use the allowable stress design or ASD approach, while others use load and resistance factor design or LRFD. Therefore, for a single project, it may be necessary to determine loads in accordance with both design formats. This article provides load combinations intended for each method. The determination of individual nominal loads is essentially unaffected. Special loads, such as flood loads, ice loads, and rain loads, are not addressed herein. The reader is referred to the ASCE 7 standard and applicable building code provisions regarding special loads.
Load Combinations
The load combinations in Table 3.1 are recommended for use with design specifications based on allowable stress design (ASD) and load and resistance factor design (LRFD). Load combinations provide the basic set of building load conditions that should be considered by the designer. They establish the proportioning of multiple transient loads that may assume point-in-time values when the load of interest attains its extreme design value. Load combinations are intended as a guide for the designer, who should exercise discretion in any particular application.
The load combinations in Table 3.1 are simplified and tailored to specific applications in residential construction and the design of typical components and systems in a home. These and similar load combinations are often used in practice as shortcuts to those load combinations that govern the design result. This article makes effective use of the shortcuts and provides examples later in the article. The shortcuts are intended only for the design of residential light-frame construction.
TABLE 3.1 Typical Load Combinations Used for the Design of Component or Systems
Notes:
1. The load combinations and factors are intended to apply to nominal design loads defined as follows:
- D = estimated mean dead weight of the construction;
- H = design lateral pressure for soil condition/type;
- L = design floor live load;
- Lr = maximum roof live load anticipated from construction/maintenance;
- W = design wind load;
- S = design roof snow load; and
- E = design earthquake load.
The design or nominal loads should be determined in accordance with this section.
2. Attic loads may be included in the floor live load, but a 10 psf attic load is typically used only to size ceiling joists adequately for access purposes. However, if the attic is intended for storage, the attic live load (or some portion) should also be considered for the design of other elements in the load path.
3. The transverse wind load for stud design is based on a localized component and cladding wind pressure; D + W provides an adequate and simple design check representative of worst-case combined axial and transverse loading. Axial forces from snow loads and roof live loads should usually not be considered simultaneously with an extreme wind load because they are mutually exclusive on residential sloped roofs. Furthermore, in most areas of the United States, design winds are produced by either hurricanes or thunderstorms; therefore, these wind events and snow are mutually exclusive because they occur at different times of the year.
4. For walls supporting heavy cladding loads (such as brick veneer), an analysis of earthquake lateral loads and combined axial loads should be considered. However, this load combination rarely governs the design of light-frame construction.
5. Wu is wind uplift load from negative (suction) pressures on the roof. Wind uplift loads must be resisted by continuous load path connections to the foundation or until offset by 0.6D.
6. The 0.6 reduction factor on D is intended to apply to the calculation of net overturning stresses and forces. For wind, the analysis of overturning should also consider roof uplift forces unless a separate load path is designed to transfer those forces.
Dead Loads
Dead loads consist of the permanent construction material loads comprising the roof, floor, wall, and foundation systems, including claddings, finishes, and fixed equipment. The values for dead loads in Table 3.2 are for commonly used materials and constructions in light-frame residential buildings. Table 3.3 provides values for common material densities and may be useful in calculating dead loads more accurately.
TABLE 3.2 Dead Loads for Common Residential Construction
Dead Loads
Dead loads consist of the permanent construction material loads comprising the roof, floor, wall, and foundation systems, including claddings, finishes, and fixed equipment. The values for dead loads in Table 3.2 are for commonly used materials and constructions in light-frame residential buildings. Table 3.3 provides values for common material densities and may be useful in calculating dead loads more accurately.
TABLE 3.2 Dead Loads for Common Residential Construction
Notes:
1. Reference unit conversions.
2. Value also used for roof rafter construction (i.e., cathedral ceiling).
3. For partially grouted masonry, interpolate between hollow and solid grout in accordance with the fraction of masonry cores that are grouted.
TABLE 3.3 Densities for Common Residential Construction Materials
Live Loads
Live loads are produced by the use and occupancy of a building. Loads include those from human occupants, furnishings, non-fixed equipment, storage, and construction and maintenance activities. Table 3.4 provides recommended design live loads for residential buildings. As required to adequately define the loading condition, loads are presented in terms of uniform area loads (psf), concentrated loads (lbs), and uniform live loads (plf). The uniform and concentrated live loads should not be applied simultaneously in a structural evaluation. Concentrated loads should be applied to a small area or surface consistent with the application and should be located or directed to give the maximum load effect possible in end-use conditions. For example, the stair concentrated load of 300 pounds should be applied to the center of the stair tread between supports. The concentrated wheel load of a vehicle on a garage slab or floor should be applied to all areas or members subject to a wheel or jack load, typically using a loaded area of about 20 square inches.
TABLE 3.4 Live Loads for Residential Construction
Notes:
1. Live load values should be verified relative to the locally applicable building code.
2. Roof live loads are intended to provide a minimum load for roof design in consideration of maintenance and construction activities. They should not be considered in combination with other transient loads (i.e., floor live load, wind load, etc.) when designing walls, floors, and foundations. A 15 psf roof live load is recommended for residential roof slopes greater than 4:12; refer to ASCE 7-98 for an alternate approach.
3. Loft sleeping and attic storage loads should be considered only in areas with a clear height greater than about 3 feet. The concept of a “clear height” limitation on live loads is logical, but it may not be universally recognized.
4. Some codes require 40 psf for all floor areas.
The floor live load on any given floor area may be reduced in accordance with Equation 3.4-1. The equation applies to floor and support members, such as beams or columns, that experience floor loads from a total tributary floor area greater than 200 square feet. This equation is different from that in ASCE 7-98, since it is based on data that applies to residential floor loads rather than commercial buildings.
Equation 3.4-1
The floor live load on any given floor area may be reduced in accordance with Equation 3.4-1. The equation applies to floor and support members, such as beams or columns, that experience floor loads from a total tributary floor area greater than 200 square feet. This equation is different from that in ASCE 7-98, since it is based on data that applies to residential floor loads rather than commercial buildings.
Equation 3.4-1
It should also be noted that the nominal design floor live load in Table 3.4 includes both a sustained and transient load component. The sustained component is that load typically present at any given time and includes the load associated with normal human occupancy and furnishings. For residential buildings, the mean sustained live load is about 6 psf but typically varies from 4 to 8 psf. The mean transient live load for dwellings is also about 6 psf but may be as high as 13 psf. Thus, a total design live load of 30 to 40 psf is fairly conservative.
Soil Lateral Loads
The lateral pressure exerted by earth backfill against a residential foundation wall (basement wall) can be calculated with reasonable accuracy on the basis of theory but only for conditions that rarely occur in practice (University of Alberta, 1992; Peck, Hanson and Thornburn, 1974). Theoretical analyses are usually based on homogeneous materials that demonstrate consistent compaction and behavioral properties. Such conditions are rarely experienced in the case of typical residential construction projects.
The most common method of determining lateral soil loads on residential foundations follows Rankine’s (1857) theory of earth pressure and uses what is known as the Equivalent Fluid Density (EFD) method. As shown in Figure 3.1, pressure distribution is assumed to be triangular and to increase with depth.
In the EFD method, the soil unit weight w is multiplied by an empirical coefficient Ka to account for the fact that the soil is not actually fluid and that the pressure distribution is not necessarily triangular. The coefficient Ka is known as the active Rankine pressure coefficient. Thus, the equivalent fluid density (EFD) is determined as follows:
Equation 3.5-1
Figure 3.1 Triangular Pressure Distribution on a Foundation Basement Wall
It follows that for the triangular pressure distribution shown in Figure 3.1, the pressure at depth h, in feet, is:
Equation 3.5-2
The total active soil force (pounds per lineal foot of wall length) is:
Equation 3.5-3
The EFD method is subject to judgment as to the appropriate value of the coefficient Ka. The values of Ka in Table 3.5 are recommended for the determination of lateral pressures on residential foundations for various types of backfill materials placed with light compaction and good drainage. Given the long-time use of a 30 pcf equivalent fluid density in residential foundation wall prescriptive design tables (ICC), the values in Table 3.5 may be considered somewhat conservative for typical conditions. A relatively conservative safety factor of 3 to 4 is typically applied to the design of unreinforced or nominally reinforced masonry or concrete foundation walls. Therefore, at imminent failure of a foundation wall, the 30 psf design EFD would correspond to an active soil lateral pressure determined by using an equivalent fluid density of about 90 to 120 pcf or more.
TABLE 3.5 Values of Ka, Soil Unit Weight, and Equivalent Fluid Density by Soil Type 1, 2, 3
Notes:
1. Values are applicable to well-drained foundations with less than 10 feet of backfill placed with light compaction or natural settlement, as is common in residential construction. The values do not apply to foundation walls in flood-prone environments. In such cases, an equivalent fluid density value of 80 to 90 pcf would be more appropriate (HUD, 1977).
2. Values are based on the Standard Handbook for Civil Engineers, Third Edition, 1983, and on research on soil pressures reported in Thin Wall Foundation Testing, Department of Civil Engineering, University of Alberta, Canada, March 1992. It should be noted that the values for soil equivalent fluid density differ from those recommended in ASCE 7-98 but are nonetheless compatible with current residential building codes, design practice, and the stated references.
3. These values do not consider the significantly higher loads that can result from expansive clays and the lateral expansion of moist, frozen soil. Such conditions should be avoided by eliminating expansive clays adjacent to the foundation wall and providing for adequate surface and foundation drainage.
4. Organic silts and clays and expansive clays are unsuitable for backfill material.
5. Backfill in the form of clay soils (non-expansive) should be used with caution on foundation walls with unbalanced fill heights greater than 3 to 4 feet, and on cantilevered foundation walls with unbalanced fill heights greater than 2 to 3 feet.
Depending on the type and depth of backfill material and the manner of its placement, it is common practice in residential construction to allow the backfill soil to consolidate naturally by providing an additional 3 to 6 inches of fill material. The additional backfill ensures that surface water drainage away from the foundation remains adequate (i.e., the grade slopes away from the building). It also helps avoid heavy compaction that could cause undesirable loads on the foundation wall during and after construction. If soils are heavily compacted at the ground surface or compacted in lifts to standard Proctor densities greater than about 85% of optimum (ASTM, 1998), the standard 30 pcf EFD assumption may be inadequate. However, in cases where exterior slabs, patios, stairs, or other items are supported on the backfill, some amount of compaction is advisable unless the structures are supported on a separate foundation bearing on undisturbed ground.
Wind Loads
General
Wind produces non-static loads on a structure at highly variable magnitudes. The variation in pressures at different locations on a building is complex to the point that pressures may become too analytically intensive for precise consideration in design. Therefore, wind load specifications attempt to simplify the design problem by considering basic static pressure zones on a building representative of peak loads that are likely to be experienced. The peak pressures in one zone for a given wind direction may not, however, occur simultaneously with peak pressures in other zones. For some pressure zones, the peak pressure depends on a narrow range of wind direction. Therefore, the wind directionality effect must also be factored into determining risk-consistent wind loads on buildings. In fact, most modern wind load specifications take account of wind directionality and other effects in determining nominal design loads in some simplified form. This section further simplifies wind load design specifications to provide an easy yet effective approach for designing typical residential buildings.
Because they vary substantially over the surface of a building, wind loads are considered at two different scales. On a large scale, the loads produced on the overall building, or on major structural systems that sustain wind loads from more than one surface of the building, are considered the main wind force-resisting system (MWFRS). The MWFRS of a home includes the shear walls and diaphragms that create the lateral force-resisting system (LFRS), as well as the structural systems, such as trusses that experience loads from two surfaces (or pressure regimes) of the building. The wind loads applied to the MWFRS account for the large-area averaging effects of time-varying wind pressures on the surface or surfaces of the building.
On a smaller scale, pressures are somewhat greater on localized surface areas of the building, particularly near abrupt changes in building geometry (e.g., eaves, ridges and corners). These higher wind pressures occur on smaller areas, particularly affecting the loads borne by components and cladding (e.g., sheathing, windows, doors, purlins, studs). The components and cladding (C&C) transfer localized time-varying loads to the MWFRS, at which point the loads average out both spatially and temporally since, at a given time, some components may be at near-peak loads, while others are at substantially less than peak.
Determination of Wind Loads on Residential Buildings
The following method for the design of residential buildings is based on a simplification of the ASCE 7 wind provisions; therefore, the wind loads are not an exact duplicate. Lateral loads and roof uplift loads are determined by using a projected area approach. Other wind loads are determined for specific components or assemblies that comprise the exterior building envelope. Five steps are required to determine design wind loads on a residential building and its components.
Step 1: Determine site design wind speed and basic velocity pressure.
From the wind map in Figure 3.2 (refer to ASCE 7 for maps with greater detail), select a design wind speed for the site. The wind speed map in ASCE 7 includes the most accurate data and analysis available regarding design wind speeds in the United States. The new wind speeds may appear higher than those used in older design wind maps. The difference is due solely to the use of the “peak gust” to define wind speeds, rather than an averaged wind speed as represented by the “fastest mile of wind” used in older wind maps. Nominal design peak gust wind speeds are typically 85 to 90 mph in most of the United States; however, along the hurricane-prone Gulf and Atlantic Coasts, nominal design wind speeds range from 100 to 150 mph for the peak gust.
If relying on either an older fastest-mile wind speed map or older design provisions based on fastest-mile wind speeds, the designer should convert wind speed in accordance with Table 3.6 for use with this simplified method, which is based on peak gust wind speeds.
TABLE 3.6 Wind Speed Conversions
Once the nominal design wind speed in terms of peak gust is determined, the designer can select the basic velocity pressure in accordance with Table 3.7. The basic velocity pressure is a reference wind pressure to which pressure coefficients are applied to determine surface pressures on a building. Velocity pressures in Table 3.7 are based on typical conditions for residential construction, namely, suburban terrain exposure and relatively flat or rolling terrain without topographic wind speed-up effects.
FIGURE 3.2 Basic Design Wind Speed Map
Map from American Society of Civil Engineers, ASCE http://publicecodes.cyberregs.com/icod/ibc/index.htm
TABLE 3.7 Basic Wind Velocity (psf) for Suburban Terrain
Step 2: Make adjustments to the basic velocity pressure.
If appropriate, the basic velocity pressure from Step 1 should be adjusted in accordance with the factors below. The adjustments are cumulative.
Depending on the type and depth of backfill material and the manner of its placement, it is common practice in residential construction to allow the backfill soil to consolidate naturally by providing an additional 3 to 6 inches of fill material. The additional backfill ensures that surface water drainage away from the foundation remains adequate (i.e., the grade slopes away from the building). It also helps avoid heavy compaction that could cause undesirable loads on the foundation wall during and after construction. If soils are heavily compacted at the ground surface or compacted in lifts to standard Proctor densities greater than about 85% of optimum (ASTM, 1998), the standard 30 pcf EFD assumption may be inadequate. However, in cases where exterior slabs, patios, stairs, or other items are supported on the backfill, some amount of compaction is advisable unless the structures are supported on a separate foundation bearing on undisturbed ground.
Wind Loads
General
Wind produces non-static loads on a structure at highly variable magnitudes. The variation in pressures at different locations on a building is complex to the point that pressures may become too analytically intensive for precise consideration in design. Therefore, wind load specifications attempt to simplify the design problem by considering basic static pressure zones on a building representative of peak loads that are likely to be experienced. The peak pressures in one zone for a given wind direction may not, however, occur simultaneously with peak pressures in other zones. For some pressure zones, the peak pressure depends on a narrow range of wind direction. Therefore, the wind directionality effect must also be factored into determining risk-consistent wind loads on buildings. In fact, most modern wind load specifications take account of wind directionality and other effects in determining nominal design loads in some simplified form. This section further simplifies wind load design specifications to provide an easy yet effective approach for designing typical residential buildings.
Because they vary substantially over the surface of a building, wind loads are considered at two different scales. On a large scale, the loads produced on the overall building, or on major structural systems that sustain wind loads from more than one surface of the building, are considered the main wind force-resisting system (MWFRS). The MWFRS of a home includes the shear walls and diaphragms that create the lateral force-resisting system (LFRS), as well as the structural systems, such as trusses that experience loads from two surfaces (or pressure regimes) of the building. The wind loads applied to the MWFRS account for the large-area averaging effects of time-varying wind pressures on the surface or surfaces of the building.
On a smaller scale, pressures are somewhat greater on localized surface areas of the building, particularly near abrupt changes in building geometry (e.g., eaves, ridges and corners). These higher wind pressures occur on smaller areas, particularly affecting the loads borne by components and cladding (e.g., sheathing, windows, doors, purlins, studs). The components and cladding (C&C) transfer localized time-varying loads to the MWFRS, at which point the loads average out both spatially and temporally since, at a given time, some components may be at near-peak loads, while others are at substantially less than peak.
Determination of Wind Loads on Residential Buildings
The following method for the design of residential buildings is based on a simplification of the ASCE 7 wind provisions; therefore, the wind loads are not an exact duplicate. Lateral loads and roof uplift loads are determined by using a projected area approach. Other wind loads are determined for specific components or assemblies that comprise the exterior building envelope. Five steps are required to determine design wind loads on a residential building and its components.
Step 1: Determine site design wind speed and basic velocity pressure.
From the wind map in Figure 3.2 (refer to ASCE 7 for maps with greater detail), select a design wind speed for the site. The wind speed map in ASCE 7 includes the most accurate data and analysis available regarding design wind speeds in the United States. The new wind speeds may appear higher than those used in older design wind maps. The difference is due solely to the use of the “peak gust” to define wind speeds, rather than an averaged wind speed as represented by the “fastest mile of wind” used in older wind maps. Nominal design peak gust wind speeds are typically 85 to 90 mph in most of the United States; however, along the hurricane-prone Gulf and Atlantic Coasts, nominal design wind speeds range from 100 to 150 mph for the peak gust.
If relying on either an older fastest-mile wind speed map or older design provisions based on fastest-mile wind speeds, the designer should convert wind speed in accordance with Table 3.6 for use with this simplified method, which is based on peak gust wind speeds.
TABLE 3.6 Wind Speed Conversions
Once the nominal design wind speed in terms of peak gust is determined, the designer can select the basic velocity pressure in accordance with Table 3.7. The basic velocity pressure is a reference wind pressure to which pressure coefficients are applied to determine surface pressures on a building. Velocity pressures in Table 3.7 are based on typical conditions for residential construction, namely, suburban terrain exposure and relatively flat or rolling terrain without topographic wind speed-up effects.
FIGURE 3.2 Basic Design Wind Speed Map
Map from American Society of Civil Engineers, ASCE http://publicecodes.cyberregs.com/icod/ibc/index.htm
TABLE 3.7 Basic Wind Velocity (psf) for Suburban Terrain
Step 2: Make adjustments to the basic velocity pressure.
If appropriate, the basic velocity pressure from Step 1 should be adjusted in accordance with the factors below. The adjustments are cumulative.
Open exposure: The wind values in Table 3.7 are based on typical residential exposures to the wind. If a site is located in generally open, flat terrain with few obstructions to the wind in most directions, or is exposed to a large body of water (i.e., ocean or lake), the designer should multiply the values in Table 3.7 by a factor of 1.4. The factor may be adjusted for sites that are considered intermediate to open suburban exposures. It may also be used to adjust wind loads according to the exposure related to the specific directions of wind approach to the building. The wind exposure conditions used in this article are derived from ASCE 7 with some modification applicable to small residential buildings of three stories or less.
Protected exposure: If a site is generally surrounded by forest or densely wooded terrain with no open areas greater than a few hundred feet, smaller buildings, such as homes, experience significant wind load reductions from the typical suburban exposure condition assumed in Table 3.7. If such conditions
exist and the site’s design wind speed does not exceed about 120 mph peak gust, the designer may consider multiplying the values in Table 3.7 by 0.8. The factor may be used to adjust wind loads according to the exposure related to the specific directions of wind approach to the building. Wind load reductions associated with a protected exposure in a suburban or otherwise open exposure have been shown to approximate 20% (Ho, 1992). In densely treed terrain with the height of the building below that of the tree tops, the reduction factor applied to Table 3.7 values can approach 0.6. The effect is known as shielding; however, it is not currently permitted by ASCE 7-98. Two considerations require judgment: Are the sources of shielding likely to exist for the expected life of the structure? Are the sources of shielding able to withstand wind speeds in excess of a design event?
Wind directionality: As noted, the direction of the wind in a given event does not create peak loads (which provide the basis for design pressure coefficients) simultaneously on all building surfaces. In some cases, the pressure zones with the highest design pressures are extremely sensitive to wind direction. In accordance with ASCE 7-98, the velocity pressures in Table 3.7 are based on a directionality adjustment of 0.85 that applies to hurricane wind conditions where winds in a given event are multidirectional but with varying magnitude. However, in “straight” wind climates, a directionality factor of 0.75 has been shown to be appropriate (Ho, 1992). Therefore, if a site is in a non-hurricane-prone wind area (i.e., design wind speed of 110 mph gust or less), the designer may also consider multiplying the values in Table 3.7 by 0.9 (i.e., 0.9 x 0.85 ≅ 0.75) to adjust for directionality effects in non-hurricane-prone wind environments. ASCE 7-98 currently does not recognize this additional adjustment to account for wind directionality in “straight” wind environments.
Topographic effects: If topographic wind speed-up effects are likely because a structure is located near the crest of a protruding hill or cliff, the designer should consider using the topographic factor provided in ASCE 7-98. Wind loads can be easily doubled for buildings sited in particularly vulnerable locations relative to topographic features that cause localized wind speed-up for specific wind directions (ASCE, 1999).
Step 3: Determine lateral wind pressure coefficients.
Lateral pressure coefficients in Table 3.8 are composite pressure coefficients that combine the effect of positive pressures on the windward face of the building and negative (suction) pressures on the leeward faces of the building. When multiplied by the velocity pressure from Steps 1 and 2, the selected pressure coefficient provides a single wind pressure that is applied to the vertical projected area of the roof and wall, as indicated in Table 3.8. The resulting load is then used to design the home’s lateral force-resisting system. The lateral wind load must be determined for the two orthogonal directions on the building (parallel to the ridge and perpendicular to the ridge), using the vertical projected area of the building for each direction. Lateral loads are then assigned to various systems (e.g., shear walls, floor diaphragms, and roof diaphragms) by use of tributary areas or other methods.
TABLE 3.8 Lateral Pressure Coefficients for Application to Vertical Projected Areas
Step 4: Determine wind pressure coefficients for components and assemblies.
- Open terrain: Open areas with widely scattered obstructions, including shoreline exposures along coastal and non-coastal bodies of water.
- Suburban terrain: Suburban areas or other terrain with closely spaced obstructions that are the size of single-family dwellings or larger, and extend in the upwind direction a distance no less than 10 times the height of the building.
Protected exposure: If a site is generally surrounded by forest or densely wooded terrain with no open areas greater than a few hundred feet, smaller buildings, such as homes, experience significant wind load reductions from the typical suburban exposure condition assumed in Table 3.7. If such conditions
exist and the site’s design wind speed does not exceed about 120 mph peak gust, the designer may consider multiplying the values in Table 3.7 by 0.8. The factor may be used to adjust wind loads according to the exposure related to the specific directions of wind approach to the building. Wind load reductions associated with a protected exposure in a suburban or otherwise open exposure have been shown to approximate 20% (Ho, 1992). In densely treed terrain with the height of the building below that of the tree tops, the reduction factor applied to Table 3.7 values can approach 0.6. The effect is known as shielding; however, it is not currently permitted by ASCE 7-98. Two considerations require judgment: Are the sources of shielding likely to exist for the expected life of the structure? Are the sources of shielding able to withstand wind speeds in excess of a design event?
Wind directionality: As noted, the direction of the wind in a given event does not create peak loads (which provide the basis for design pressure coefficients) simultaneously on all building surfaces. In some cases, the pressure zones with the highest design pressures are extremely sensitive to wind direction. In accordance with ASCE 7-98, the velocity pressures in Table 3.7 are based on a directionality adjustment of 0.85 that applies to hurricane wind conditions where winds in a given event are multidirectional but with varying magnitude. However, in “straight” wind climates, a directionality factor of 0.75 has been shown to be appropriate (Ho, 1992). Therefore, if a site is in a non-hurricane-prone wind area (i.e., design wind speed of 110 mph gust or less), the designer may also consider multiplying the values in Table 3.7 by 0.9 (i.e., 0.9 x 0.85 ≅ 0.75) to adjust for directionality effects in non-hurricane-prone wind environments. ASCE 7-98 currently does not recognize this additional adjustment to account for wind directionality in “straight” wind environments.
Topographic effects: If topographic wind speed-up effects are likely because a structure is located near the crest of a protruding hill or cliff, the designer should consider using the topographic factor provided in ASCE 7-98. Wind loads can be easily doubled for buildings sited in particularly vulnerable locations relative to topographic features that cause localized wind speed-up for specific wind directions (ASCE, 1999).
Step 3: Determine lateral wind pressure coefficients.
Lateral pressure coefficients in Table 3.8 are composite pressure coefficients that combine the effect of positive pressures on the windward face of the building and negative (suction) pressures on the leeward faces of the building. When multiplied by the velocity pressure from Steps 1 and 2, the selected pressure coefficient provides a single wind pressure that is applied to the vertical projected area of the roof and wall, as indicated in Table 3.8. The resulting load is then used to design the home’s lateral force-resisting system. The lateral wind load must be determined for the two orthogonal directions on the building (parallel to the ridge and perpendicular to the ridge), using the vertical projected area of the building for each direction. Lateral loads are then assigned to various systems (e.g., shear walls, floor diaphragms, and roof diaphragms) by use of tributary areas or other methods.
TABLE 3.8 Lateral Pressure Coefficients for Application to Vertical Projected Areas
Step 4: Determine wind pressure coefficients for components and assemblies.
The pressure coefficients in Table 3.9 are derived from ASCE 7-98 based on the assumption that the building is enclosed and not subject to higher internal pressures that may result from a windward opening in the building. The use of the values in Table 3.9 greatly simplifies the more detailed methodology described in ASCE 7-98; as a result, there is some rounding of numbers. With the exception of the roof uplift coefficient, all pressures calculated with the coefficients are intended to be applied to the perpendicular building surface area that is tributary to the element of concern. Thus, the wind load is applied perpendicular to the actual building surface, not to a projected area. The roof uplift pressure coefficient is used to determine a single wind pressure that may be applied to a horizontal projected area of the roof to determine roof tie-down connection forces.
For buildings in hurricane-prone regions subject to wind-borne debris, the GCp values in Table 3.9 are required to be increased in magnitude by ±0.35 to account for higher potential internal pressures due to the possibility of a windward wall opening (i.e., broken window). The adjustment is not required by ASCE 7-98 in “wind-borne debris regions” if glazing is protected against likely sources of debris impact as shown by an “approved” test method.
Step 5: Determine design wind pressures.
Once the basic velocity pressure is determined in Step 1 and adjusted in Step 2 for exposure and other site-specific considerations, the designer can calculate the design wind pressures by multiplying the adjusted basic velocity pressure by the pressure coefficients selected in Steps 3 and 4. The lateral pressures based on coefficients from Step 3 are applied to the tributary areas of the lateral force-resisting systems, such as shear walls and diaphragms. The pressures based on coefficients from Step 4 are applied to tributary areas of members, such as studs, rafters, trusses and sheathing, to determine stresses and connection forces.
TABLE 3-9 Wind Pressure Coefficients for Systems and Components (enclosed building)
For buildings in hurricane-prone regions subject to wind-borne debris, the GCp values in Table 3.9 are required to be increased in magnitude by ±0.35 to account for higher potential internal pressures due to the possibility of a windward wall opening (i.e., broken window). The adjustment is not required by ASCE 7-98 in “wind-borne debris regions” if glazing is protected against likely sources of debris impact as shown by an “approved” test method.
Step 5: Determine design wind pressures.
Once the basic velocity pressure is determined in Step 1 and adjusted in Step 2 for exposure and other site-specific considerations, the designer can calculate the design wind pressures by multiplying the adjusted basic velocity pressure by the pressure coefficients selected in Steps 3 and 4. The lateral pressures based on coefficients from Step 3 are applied to the tributary areas of the lateral force-resisting systems, such as shear walls and diaphragms. The pressures based on coefficients from Step 4 are applied to tributary areas of members, such as studs, rafters, trusses and sheathing, to determine stresses and connection forces.
TABLE 3-9 Wind Pressure Coefficients for Systems and Components (enclosed building)
Notes:
1. All coefficients include internal pressure in accordance with the assumption of an enclosed building. With the exception of the categories labeled trusses, roof beams, ridge and hip/valley rafters, and roof uplift, which are based on MWFRS loads, all coefficients are based on component-with-cladding wind loads.
2. Positive and negative signs represent pressures acting inwardly and outwardly, respectively, from the building surface. A negative pressure is a suction or vacuum. Both pressure conditions should be considered to determine the controlling design criteria.
3. The roof uplift pressure coefficient is used to determine uplift pressures that are applied to the horizontal projected area of the roof for the purpose of determining uplift tie-down forces. Additional uplift force on roof tie-downs due to roof overhangs should also be included. The uplift force must be transferred to the foundation or to a point where it is adequately resisted by the dead load of the building and the capacity of conventional framing connections.
4. The windward overhang pressure coefficient is applied to the underside of a windward roof overhang and acts upwardly on the bottom surface of the roof overhang. If the bottom surface of the roof overhang is the roof sheathing, or the soffit is not covered with a structural material on its underside, then the overhang pressure shall be considered additive to the roof sheathing pressure.
5. Air-permeable claddings allow for pressure relief such that the cladding experiences about two-thirds of the pressure differential experienced across the wall assembly (FPL, 1999). Products that experience reduced pressure include lap-type sidings, such as wood, vinyl, aluminum, and other similar sidings. Since these components are usually considered “nonessential,” it may be practical to multiply the calculated wind load on any nonstructural cladding by 0.75 to adjust for a serviceability wind load (Galambos and Ellingwood, 1986). Such an adjustment would also be applicable to deflection checks, if required, for other components listed in the table. However, a serviceability load criterion is not included or clearly defined in existing design codes.
Special Considerations in Hurricane-Prone Environments
Wind-Borne Debris
The wind loads determined in the previous section assume an enclosed building. If glazing in windows and doors is not protected from wind-borne debris or otherwise designed to resist potential impacts during a major hurricane, a building is more susceptible to structural damage owing to higher internal building pressures that may develop with a windward opening. The potential for water damage to building contents also increases. Openings formed in the building envelope during a major hurricane or tornado are often related to unprotected glazing, improperly fastened sheathing, or weak garage doors and their attachment to the building.
Recent years have focused much attention on wind-borne debris but with comparatively little scientific direction and poorly defined goals with respect to safety (i.e., acceptable risk), property protection, missile types, and reasonable impact criteria. Conventional practice in residential construction has called for simple plywood window coverings with attachments to resist the design wind loads. In some cases, homeowners elect to use impact-resistant glazing or shutters. Regardless of the chosen method and its cost, the responsibility for protection against wind-borne debris has traditionally rested with the homeowner. However, wind-borne debris protection has recently been mandated in some local building codes.
Just what defines impact resistance and the level of impact risk during a hurricane has been the subject of much debate. Surveys of damage following major hurricanes have identified several factors that affect the level of debris impact risk, including:
TABLE 3.10 Missile Types for Wind-Borne Debris Impact Tests
Special Considerations in Hurricane-Prone Environments
Wind-Borne Debris
The wind loads determined in the previous section assume an enclosed building. If glazing in windows and doors is not protected from wind-borne debris or otherwise designed to resist potential impacts during a major hurricane, a building is more susceptible to structural damage owing to higher internal building pressures that may develop with a windward opening. The potential for water damage to building contents also increases. Openings formed in the building envelope during a major hurricane or tornado are often related to unprotected glazing, improperly fastened sheathing, or weak garage doors and their attachment to the building.
Recent years have focused much attention on wind-borne debris but with comparatively little scientific direction and poorly defined goals with respect to safety (i.e., acceptable risk), property protection, missile types, and reasonable impact criteria. Conventional practice in residential construction has called for simple plywood window coverings with attachments to resist the design wind loads. In some cases, homeowners elect to use impact-resistant glazing or shutters. Regardless of the chosen method and its cost, the responsibility for protection against wind-borne debris has traditionally rested with the homeowner. However, wind-borne debris protection has recently been mandated in some local building codes.
Just what defines impact resistance and the level of impact risk during a hurricane has been the subject of much debate. Surveys of damage following major hurricanes have identified several factors that affect the level of debris impact risk, including:
- wind climate (design wind speed);
- exposure (e.g., suburban, wooded, height of surrounding buildings);
- development density (i.e., distance between buildings);
- construction characteristics (e.g., type of roofing, degree of wind resistance); and
- debris sources (e.g., roofing, fencing, gravel, etc.).
TABLE 3.10 Missile Types for Wind-Borne Debris Impact Tests
Notes:
1. Consult ASTM E1886 (ASTM, 1997) or SSTD 12-97 (SBCCI, 1997) for guidance on testing apparatus and methodology.
2. These missile types are not necessarily representative of the predominant types or sources of debris at any particular site. Steel balls are intended to represent small gravels that would be commonly used for roof ballast. The 2x4 missiles are intended to represent a direct end-on blow from construction debris without consideration of the probability of such an impact over the life of a particular structure.
In view of the above discussion, ASCE 7-98 identifies “wind-borne debris regions” as areas within hurricane-prone regions that are located (1) within 1 mile of the coastal mean high water line where the basic wind speed is equal to or greater than 110 mph or in Hawaii, or (2) where the basic wind speed is equal to or greater than 120 mph. As described in Section 3.6.2, ASCE 7-98 requires higher internal pressures to be considered for buildings in wind-borne debris regions unless glazed openings are protected by impact-resistant glazing or protective devices proven as such by an approved test method. Approved test methods include ASTM E1886 and SSTD 12-97 (ASTM, 1997; SBCCI, 1997).
The wind load method may be considered acceptable without wind-borne debris protection, provided that the building envelope (i.e., windows, doors, sheathing, and especially garage doors) is carefully designed for the required pressures. Most homes that experience wind-borne debris damage do not appear to exhibit more catastrophic failures, such as a roof blow-off, unless the roof was severely under-designed in the first place (i.e., inadequate tie-down) or subject to poor workmanship (i.e., missing fasteners at critical locations). Those cases are often the ones cited as evidence of internal pressure in anecdotal field studies. However, garage doors that fail due to wind pressure more frequently precipitate additional damage related to internal pressure. Therefore, in hurricane-prone regions, garage door reinforcement or pressure-rated garage doors should be specified and their attachment to structural framing carefully considered.
Building Durability
Roof overhangs increase uplift loads on roof tie-downs and the framing members that support the overhangs. They do, however, provide a reliable means of protection against moisture and the potential decay of wood building materials. The designer should therefore consider the trade-off between wind load and durability, particularly in the humid climate zones associated with hurricanes.
For buildings that are exposed to salt spray or mist from nearby bodies of salt water, the designer should also consider a higher-than-standard level of corrosion resistance for exposed fasteners and hardware. Truss plates near roof vents have also shown accelerated rates of corrosion in severe coastal exposures. The building owner, in turn, should consider a building maintenance plan that includes regular inspection, maintenance and repair.
Tips to Improve Performance
The following design and construction tips are simple options for reducing a building's vulnerability to hurricane damage:
In view of the above discussion, ASCE 7-98 identifies “wind-borne debris regions” as areas within hurricane-prone regions that are located (1) within 1 mile of the coastal mean high water line where the basic wind speed is equal to or greater than 110 mph or in Hawaii, or (2) where the basic wind speed is equal to or greater than 120 mph. As described in Section 3.6.2, ASCE 7-98 requires higher internal pressures to be considered for buildings in wind-borne debris regions unless glazed openings are protected by impact-resistant glazing or protective devices proven as such by an approved test method. Approved test methods include ASTM E1886 and SSTD 12-97 (ASTM, 1997; SBCCI, 1997).
The wind load method may be considered acceptable without wind-borne debris protection, provided that the building envelope (i.e., windows, doors, sheathing, and especially garage doors) is carefully designed for the required pressures. Most homes that experience wind-borne debris damage do not appear to exhibit more catastrophic failures, such as a roof blow-off, unless the roof was severely under-designed in the first place (i.e., inadequate tie-down) or subject to poor workmanship (i.e., missing fasteners at critical locations). Those cases are often the ones cited as evidence of internal pressure in anecdotal field studies. However, garage doors that fail due to wind pressure more frequently precipitate additional damage related to internal pressure. Therefore, in hurricane-prone regions, garage door reinforcement or pressure-rated garage doors should be specified and their attachment to structural framing carefully considered.
Building Durability
Roof overhangs increase uplift loads on roof tie-downs and the framing members that support the overhangs. They do, however, provide a reliable means of protection against moisture and the potential decay of wood building materials. The designer should therefore consider the trade-off between wind load and durability, particularly in the humid climate zones associated with hurricanes.
For buildings that are exposed to salt spray or mist from nearby bodies of salt water, the designer should also consider a higher-than-standard level of corrosion resistance for exposed fasteners and hardware. Truss plates near roof vents have also shown accelerated rates of corrosion in severe coastal exposures. The building owner, in turn, should consider a building maintenance plan that includes regular inspection, maintenance and repair.
Tips to Improve Performance
The following design and construction tips are simple options for reducing a building's vulnerability to hurricane damage:
- One-story buildings are much less vulnerable to wind damage than two- and three-story buildings.
- On average, hip roofs have demonstrated better performance than gable-end roofs.
- Moderate roof slopes (4:12 to 6:12) tend to optimize the trade-off between lateral loads and roof uplift loads (i.e., more aerodynamically efficient).
- Roof sheathing installation should be inspected for the proper type and spacing of fasteners, particularly at connections to gable-end framing.
- The installation of metal strapping or other tie-down hardware should be inspected, as required, to ensure the transfer of uplift loads.
- If composition roof shingles are used, high-wind fastening requirements should be followed (i.e., 6 nails per shingle in lieu of the standard 4 nails). A similar concern exists for tile roofing, metal roofing, and other roofing materials.
- Consider some practical means of glazed opening protection in the most severe hurricane-prone areas.
Snow Loads
For design purposes, snow is typically treated as a simple uniform gravity load on the horizontal projected area of a roof. The uniformly distributed design snow load on residential roofs can be easily determined by using the unadjusted ground snow load. This simple approach also represents standard practice in some regions of the United States; however, it does not account for a reduction in roof snow load that may be associated with steep roof slopes with slippery surfaces (refer to ASCE 7-98). To consider drift loads on sloped gable or hip roofs, the design roof snow load on the windward and leeward roof surfaces may be determined by multiplying the ground snow load by 0.8 and 1.2, respectively. In this case, the drifted side of the roof has 50% greater snow load than the non-drifted side of the roof. However, the average roof snow load is still equivalent to the ground snow load.
Design ground snow loads may be obtained from the map in Figure 3.3; however, snow loads are usually defined by the local building department. Typical ground snow loads range from 0 psf in the South to 50 psf in the northern United States. In mountainous areas, the ground snow load can surpass 100 psf such that local snow data should be carefully considered. In areas where the ground snow load is less than 15 psf, the minimum roof live load is usually the controlling gravity load in roof design. For a larger map with greater detail, refer to ASCE 7-98.
FIGURE 3.3 Ground Snow Loads (ASCE 7-98)
Map from American Society of Civil Engineers, ASCE http://publicecodes.cyberregs.com/icod/ibc/index.htm
Earthquake Loads
General
This section provides a simplified earthquake load analysis procedure appropriate for use in residential light-frame construction of not more than three stories above grade. The lateral forces associated with seismic ground motion are based on fundamental Newtonian mechanics (F = ma) expressed in terms of an equivalent static load. The method provided in this section is a simplification of the most current seismic design provisions. It is also similar to a simplified approach found in more recent building code development (ICC).
Most residential designers use a simplified approach similar to that in older seismic design codes. The approach outlined in the next section follows the older approach in terms of its simplicity while using the newer seismic risk maps and design format of NEHRP-97 as incorporated into recent building code development efforts (ICC); refer to Figure 3.4.
In general, wood-framed homes have performed well in major seismic events, probably because of, among many factors, their light-weight and resilient construction, the strength provided by nonstructural systems such as interior walls, and their load distribution capabilities. Only in the case of gross absence of good judgment or misapplication of design for earthquake forces have severe life-safety consequences become an issue in light-frame, low-rise structures experiencing extreme seismic events.
FIGURE 3.4 Seismic Map of Design Short-Period Spectral Response Acceleration (g) (2 percent chance of exceedance in 50 years or 2,475-year return period)
Map from American Society of Civil Engineers, ASCE http://publicecodes.cyberregs.com/icod/ibc/index.htm
Determination of Earthquake Loads on Houses
The total lateral force at the base of a building is called seismic base shear. The lateral force experienced at a particular story level is called the story shear. The story shear is greatest in the ground story and least in the top story. Seismic base shear and story shear (V) are determined in accordance with the following equation:
Equation 3.8-1
When determining story shear for a given story, the designer attributes to that story one-half of the dead load of the walls on the story under consideration and the dead load supported by the story. For housing, the interior partition wall dead load is reasonably accounted for by the use of a 6 psf load distributed uniformly over the floor area. When applicable, the snow load may be determined. The inclusion of any snow load, however, is based on the assumption that the snow is always frozen solid and adhered to the building such that it is part of the building mass during the entire seismic event.
The design spectral response acceleration for short-period ground motion SDS is typically used because light-frame buildings, such as houses, are believed to have a short period of vibration in response to seismic ground motion (i.e., high natural frequency). In fact, non-destructive tests of existing houses have confirmed the short period of vibration, although once ductile damage has begun to occur in a severe event, the natural period of the building likely increases.
Values of Ss are obtained from Figure 3.7. For a larger map with greater detail, refer to ASCE 7-98. The value of SDS should be determined in consideration of the mapped short-period spectral response acceleration Ss and the required soil site amplification factor Fa as follows:
Equation 3.8-2
The value of Ss ranges from practically zero in low-risk areas to 3g in the highest-risk regions of the United States. A typical value in high seismic areas is 1.5g. In general, wind loads control the design of the lateral force-resisting system of light-frame houses when Ss is less than about 1g. The 2/3 coefficient in Equation 3.8-2 is used to adjust to a design seismic ground motion value from that represented by the mapped Ss values (i.e., the mapped values are based on a “maximum considered earthquake” generally representative of a 2,475-year return period, with the design basis intended to represent a 475-year return period event).
Table 3.11 provides the values of Fa associated with a standard “firm” soil condition used for the design of residential buildings. Fa decreases with increasing ground motion because the soil begins to dampen the ground motion as shaking intensifies. Therefore, the soil can have a moderating effect on the seismic shear loads experienced by buildings in high seismic risk regions. Dampening also occurs between a building foundation and the soil and thus has a moderating effect. However, the soil-structure interaction effects on residential buildings have been the topic of little study; therefore, precise design procedures have yet to be developed. If a site is located on fill soils or “soft” ground, a different value of Fa should be considered. Nonetheless, as noted in the Anchorage Earthquake of 1964 and again 30 years later in the Northridge Earthquake, soft soils do not necessarily affect the performance of the above-ground house structure as much as they affect the site and foundations (e.g., settlement, fissuring, liquefaction, etc.).
TABLE 3.11 Site Soil Amplification Factor Relative to Acceleration (short period, firm soil)
The seismic response modifier R has a long history in seismic design, but with little in the way of scientific underpinnings. In fact, it can be traced back to expert opinion in the development of seismic design codes during the 1950s (ATC, 1995). In recognition that buildings can effectively dissipate energy from seismic ground motions through ductile damage, the R factor was conceived to adjust the shear forces from that which would be experienced if a building could exhibit perfectly elastic behavior without some form of ductile energy dissipation. The concept has served a major role in standardizing the seismic design of buildings even though it has evolved in the absence of a repeatable and generalized evaluation methodology with a known relationship to actual building performance.
Those structural building systems that are able to withstand greater ductile damage and deformation without substantial loss of strength are assigned a higher value for R. The R factor also incorporates differences in dampening that are believed to occur for various structural systems. Table 3.12 provides some values for R that are relevant to residential construction.
TABLE 3.12 Seismic Response Modifiers for Residential Construction
Notes:
1. The R factors may vary for a given structural system type depending on wall configuration, material selection, and connection detailing, but these considerations are necessarily matters of designer judgment.
2. The R for light-frame shear walls (steel-framed and wood-framed) with shear panels has been recently revised to 6 but is not yet published (ICC, 1999). Current practice typically uses an R of 5.5 to 6.5, depending on the edition of the local building code.
3. The wall is reinforced in accordance with concrete design requirements in ACI-318 or ACI-530. Nominally reinforced concrete or masonry that has conventional amounts of vertical reinforcement, such as one #5 rebar at openings and at 4 feet on center, may use the value for reinforced walls, provided the construction is no more than two stories above grade.
Seismic Shear Force Distribution
As described in the previous section, the vertical distribution of seismic forces to separate stories on a light-frame building is assumed to be in accordance with the mass supported by each story. However, design codes vary in the requirements related to vertical distribution of seismic shear. Unfortunately, there is apparently no clear body of evidence to confirm any particular method of vertical seismic force distribution for light-frame buildings. Therefore, in keeping with the simplified method, the approach used in this article reflects what is considered conventional practice. The horizontal distribution of seismic forces to various shear walls on a given story also varies in current practice for light-frame buildings. Until methods of vertical and horizontal seismic force distribution are better understood for application to light-frame buildings, the importance of designer judgment cannot be overemphasized.
Special Seismic Design Considerations
Perhaps the single most important principle in seismic design is to ensure that the structural components and systems are adequately tied together to perform as a structural unit. Underlying this principle are a host of analytic challenges and uncertainties in actually defining what “adequately tied together” means in a repeatable, accurate, and theoretically sound manner.
Recent seismic building code developments have introduced several new factors and provisions that attempt to address various problems or uncertainties in the design process. Unfortunately, these factors appear to introduce as many uncertainties as they address. Codes have tended to become more complicated to apply or decipher, perhaps detracting from some important basic principles in seismic design that, when understood, would provide guidance in the application of designer judgment. Many of the problems stem from the use of the seismic response modifier R, which is a concept first introduced to seismic design codes in the 1950s.
Also known as “reserve strength,” the concept of overstrength is a realization that a shear resisting system’s ultimate capacity is usually significantly higher than required by a design load as a result of intended safety margins. At the same time, the seismic ground motion (load) is reduced by the R factor to account for ductile response of the building system, among other things. Thus, the actual forces experienced on various components (i.e. connections) during a design level event can be substantially higher, even though the resisting system may be able to effectively dissipate that force. Therefore, overstrength factors have been included in newer seismic codes with recommendations to assist in designing components that may experience higher forces than determined otherwise for the building lateral force resisting system using methods similar to Equation 3.8-1. It should be noted that current overstrength factors should not be considered exact and that actual values of overstrength can vary substantially.
In essence, the overstrength concept is an attempt to address the principle of balanced design. It strives to ensure that critical components, such as connections, have sufficient capacity so that the overall lateral force-resisting system is able to act in its intended ductile manner (i.e., absorbing higher-than-design forces). Thus, a premature failure of a critical component (i.e., a restraining connection failure) is avoided. An exact approach requires near-perfect knowledge about various connections, details, safety margins, and system-component response characteristics that are generally not available. However, the concept is extremely important and, for the most part, experienced designers have exercised this principle through a blend of judgment and rational analysis.
The concept of overstrength is relative to the design of restraining connections for light-frame buildings by providing the designer with ultimate capacity values for light-frame shear wall systems. Thus, the designer is able to compare the unfactored shear wall capacity to that of hold-down restraints and other connections to ensure that the ultimate connection capacity is at least as much as that of the shear wall system. Some consideration of the ductility of the connection or component may also imply a response modification factor for a particular connection or framing detail. In summary, overstrength is an area where exact guidance does not exist and the designer must exercise reasonable care in accordance with or in addition to the applicable building code requirements.
The redundancy factor was postulated to address the reliability of lateral force-resisting systems by encouraging multiple lines of shear resistance in a building. It is now included in some of the latest seismic design provisions. Since it appears that redundancy factors have little technical basis and insufficient verification relative to light-frame structures, they are not explicitly addressed in this article. In fact, residential buildings are generally recognized for their inherent redundancies that are systematically overlooked when designating and defining a lateral force-resisting system for the purpose of executing a rational design. However, the principle is important to consider. For example, it would not be wise to rely on one or two shear-resisting components to support a building. In typical applications of light-frame construction, even a single shear wall line has several individual segments and numerous connections that resist shear forces. At a minimum, there are two such shear wall lines in either orientation of the building, not to mention interior walls and other nonstructural elements that contribute to the redundancy of typical light-frame homes. In summary, redundancy is an area where exact guidance does not exist and the designer must exercise reasonable care in accordance with or in addition to the applicable building code requirements.
Deflection amplification has been applied in past and current seismic design codes to adjust the deflection or story drift determined by use of the design seismic shear load (as adjusted downward by the R factor) relative to that actually experienced without allowance for modified response (i.e., load not adjusted down by the R factor). For wood-framed shear wall construction, the deflection calculated at the nominal seismic shear load (Equation 3.8-1) is multiplied by a factor of 4. Thus, the estimate of deflection or drift of the shear wall (or entire story) based on the design seismic shear load would be increased four-fold. Again, the conditions that lead to this level of deflection amplification and the factors that may affect it in a particular design are not exact (and are not obvious to the designer). As a result, conservative drift amplification values are usually selected for code purposes. Regardless, deflection or drift calculations are rarely applied in a residential (low-rise) wood-framed building design for three reasons. First, a methodology is not generally available to predict the drift behavior of light-frame buildings reliably and accurately. Second, the current design values used for shear wall design are relatively conservative and are usually assumed to provide adequate stiffness (i.e., limit drift). Third, code-required drift limits have not been developed for specific application to light-frame residential construction. Measures to estimate drift, however, are in terms of nonlinear approximations of wood-frame shear wall load-drift behavior (up to ultimate capacity). In summary, deformation amplification is an area where exact guidance does not exist and predictive tools are unreliable. Therefore, the designer must exercise reasonable care in accordance with or in addition to the applicable building code requirements.
Another issue that has received greater attention in seismic design provisions is irregularities. Irregularities are related to special geometric or structural conditions that affect the seismic performance of a building and either require special design attention or should be altogether avoided. In essence, the presence of limits on structural irregularity speaks indirectly of the inability to predict the performance of a structure in a reliable, self-limiting fashion on the basis of analysis alone. Therefore, many of the irregularity limitations are based on judgment from problems experienced in past seismic events.
Irregularities are generally separated into plan and vertical structural irregularities. Plan structural irregularities include torsional imbalances that result in excessive rotation of the building, re-entrant corners creating “wings” of a building, floor or roof diaphragms with large openings or non-uniform stiffness, out-of-plane offsets in the lateral force resistance path, and nonparallel resisting systems. Vertical structural irregularities include stiffness irregularities (i.e., a “soft” story), capacity irregularities (i.e., a “weak” story), weight (mass) irregularity (i.e., a “heavy” story), and geometric discontinuities affecting the interaction of lateral resisting systems on adjacent stories.
The concept of irregularities is associated with ensuring an adequate load path and limiting undesirable (i.e., hard to control or predict) building responses in a seismic event. Again, experienced designers generally understand the effect of irregularities and effectively address or avoid them on a case-by-case basis. For typical single-family housing, all but the most serious irregularities (i.e., “soft story”) are generally of limited consequence, particularly given the apparently significant system behavior of light-frame homes (provided the structure is reasonably “tied together as a structural unit”). For larger structures, such as low- and high-rise commercial and residential construction, the issue of irregularity--and loads--becomes more significant. Because structural irregularities raise serious concerns and have been associated with building failures or performance problems in past seismic events, the designer must exercise reasonable care in accordance with or in addition to the applicable building code requirements.
A key issue related to building damage involves deformation compatibility of materials and detailing in a constructed system. This issue may be handled through specification of materials that have similar deformation capabilities or by system detailing that improves compatibility. For example, a relatively flexible hold-down device installed near a rigid sill anchor causes greater stress concentration on the more rigid element, as evidenced by the splitting of wood sill plates in the Northridge Earthquake. The solution can involve increasing the rigidity of the hold-down device (which can lessen the ductility of the system, increase stiffness, and effectively increase seismic load), or by redesigning the sill plate connection to accommodate the hold-down deformation and improve load distribution. As a non-structural example of deformation compatibility, gypsum board interior finishes crack in a major seismic event well before the structural capability of the wall’s structural sheathing is exhausted. Conversely, wood exterior siding and similar resilient finishes tend to deform compatibly with the wall and limit observable or unacceptable visual damage. A gypsum board interior finish may be made more resilient and compatible with structural deformations by using resilient metal channels or similar detailing; however, this enhancement has not yet been proven. Unfortunately, there is little definitive design guidance on deformation compatibility considerations in seismic design of wood-framed buildings and other structures.
As a final issue, it should be understood that the general objective of current and past seismic building code provisions has been to prevent collapse in extreme seismic events such that protection of life is reasonably provided, but not with complete assurance. It is often believed that damage can be controlled by use of a smaller R factor or, for a similar effect, a larger safety factor. Others have suggested using a higher design event. While either approach may indirectly reduce damage or improve performance, it does not necessarily improve the predictability of building performance and, therefore, may have uncertain benefits, if any, in many cases. However, some practical considerations as discussed above may lead to better-performing buildings, at least from the perspective of controlling damage.
Other Load Conditions
Other “forces of nature” may create loads on buildings. Some examples include:
In certain cases, forces from these phenomena can drastically exceed reasonable design loads for homes. For example, frost heave forces can easily exceed 10,000 pounds per square foot. Similarly, the force of expanding clay soil can be impressive. In addition, the self-straining stresses induced by temperature-related expansion or contraction of a member or system that is restrained against movement can be very large, although they are not typically a concern in wood-framed housing. Finally, the probability of a direct tornado strike on a given building is much lower than considered practical for engineering and general safety purposes. The unique wind loads produced by an extreme tornado may exceed typical design wind loads by almost an order of magnitude in effect. Conversely, most tornadoes have comparatively low wind speeds that can be resisted by attainable design improvements. However, the risk of such an event is still significantly lower than required by minimum accepted safety requirements.
It is common practice to avoid the loads noted above by using sound design detailing. For example, frost heave can be avoided by placing footings below a “safe” frost depth, building on non-frost-susceptible materials, or using other frost-protection methods. Expansive soil loads can be avoided by isolating building foundations from expansive soil, supporting foundations on a system of deep pilings, and designing foundations that provide for differential ground movements. Temperature effects can be eliminated by providing construction joints that allow for expansion and contraction. While such temperature effects on wood materials are practically negligible, some finishes, such as ceramic tile, can experience cracking when inadvertently restrained against small movements resulting from variations in temperature. Unfortunately, tornadoes cannot be avoided; therefore, it is not uncommon to consider the additional cost and protection of a tornado shelter in tornado-prone areas. A tornado shelter guide is available from the Federal Emergency Management Agency, Washington, D.C.
As noted at the beginning of this article, this article does not address loads from flooding, ice, rain, and other exceptional sources. The reader is referred to ASCE 7 and other resources for information regarding special load conditions.
Seismic Shear Force Distribution
As described in the previous section, the vertical distribution of seismic forces to separate stories on a light-frame building is assumed to be in accordance with the mass supported by each story. However, design codes vary in the requirements related to vertical distribution of seismic shear. Unfortunately, there is apparently no clear body of evidence to confirm any particular method of vertical seismic force distribution for light-frame buildings. Therefore, in keeping with the simplified method, the approach used in this article reflects what is considered conventional practice. The horizontal distribution of seismic forces to various shear walls on a given story also varies in current practice for light-frame buildings. Until methods of vertical and horizontal seismic force distribution are better understood for application to light-frame buildings, the importance of designer judgment cannot be overemphasized.
Special Seismic Design Considerations
Perhaps the single most important principle in seismic design is to ensure that the structural components and systems are adequately tied together to perform as a structural unit. Underlying this principle are a host of analytic challenges and uncertainties in actually defining what “adequately tied together” means in a repeatable, accurate, and theoretically sound manner.
Recent seismic building code developments have introduced several new factors and provisions that attempt to address various problems or uncertainties in the design process. Unfortunately, these factors appear to introduce as many uncertainties as they address. Codes have tended to become more complicated to apply or decipher, perhaps detracting from some important basic principles in seismic design that, when understood, would provide guidance in the application of designer judgment. Many of the problems stem from the use of the seismic response modifier R, which is a concept first introduced to seismic design codes in the 1950s.
Also known as “reserve strength,” the concept of overstrength is a realization that a shear resisting system’s ultimate capacity is usually significantly higher than required by a design load as a result of intended safety margins. At the same time, the seismic ground motion (load) is reduced by the R factor to account for ductile response of the building system, among other things. Thus, the actual forces experienced on various components (i.e. connections) during a design level event can be substantially higher, even though the resisting system may be able to effectively dissipate that force. Therefore, overstrength factors have been included in newer seismic codes with recommendations to assist in designing components that may experience higher forces than determined otherwise for the building lateral force resisting system using methods similar to Equation 3.8-1. It should be noted that current overstrength factors should not be considered exact and that actual values of overstrength can vary substantially.
In essence, the overstrength concept is an attempt to address the principle of balanced design. It strives to ensure that critical components, such as connections, have sufficient capacity so that the overall lateral force-resisting system is able to act in its intended ductile manner (i.e., absorbing higher-than-design forces). Thus, a premature failure of a critical component (i.e., a restraining connection failure) is avoided. An exact approach requires near-perfect knowledge about various connections, details, safety margins, and system-component response characteristics that are generally not available. However, the concept is extremely important and, for the most part, experienced designers have exercised this principle through a blend of judgment and rational analysis.
The concept of overstrength is relative to the design of restraining connections for light-frame buildings by providing the designer with ultimate capacity values for light-frame shear wall systems. Thus, the designer is able to compare the unfactored shear wall capacity to that of hold-down restraints and other connections to ensure that the ultimate connection capacity is at least as much as that of the shear wall system. Some consideration of the ductility of the connection or component may also imply a response modification factor for a particular connection or framing detail. In summary, overstrength is an area where exact guidance does not exist and the designer must exercise reasonable care in accordance with or in addition to the applicable building code requirements.
The redundancy factor was postulated to address the reliability of lateral force-resisting systems by encouraging multiple lines of shear resistance in a building. It is now included in some of the latest seismic design provisions. Since it appears that redundancy factors have little technical basis and insufficient verification relative to light-frame structures, they are not explicitly addressed in this article. In fact, residential buildings are generally recognized for their inherent redundancies that are systematically overlooked when designating and defining a lateral force-resisting system for the purpose of executing a rational design. However, the principle is important to consider. For example, it would not be wise to rely on one or two shear-resisting components to support a building. In typical applications of light-frame construction, even a single shear wall line has several individual segments and numerous connections that resist shear forces. At a minimum, there are two such shear wall lines in either orientation of the building, not to mention interior walls and other nonstructural elements that contribute to the redundancy of typical light-frame homes. In summary, redundancy is an area where exact guidance does not exist and the designer must exercise reasonable care in accordance with or in addition to the applicable building code requirements.
Deflection amplification has been applied in past and current seismic design codes to adjust the deflection or story drift determined by use of the design seismic shear load (as adjusted downward by the R factor) relative to that actually experienced without allowance for modified response (i.e., load not adjusted down by the R factor). For wood-framed shear wall construction, the deflection calculated at the nominal seismic shear load (Equation 3.8-1) is multiplied by a factor of 4. Thus, the estimate of deflection or drift of the shear wall (or entire story) based on the design seismic shear load would be increased four-fold. Again, the conditions that lead to this level of deflection amplification and the factors that may affect it in a particular design are not exact (and are not obvious to the designer). As a result, conservative drift amplification values are usually selected for code purposes. Regardless, deflection or drift calculations are rarely applied in a residential (low-rise) wood-framed building design for three reasons. First, a methodology is not generally available to predict the drift behavior of light-frame buildings reliably and accurately. Second, the current design values used for shear wall design are relatively conservative and are usually assumed to provide adequate stiffness (i.e., limit drift). Third, code-required drift limits have not been developed for specific application to light-frame residential construction. Measures to estimate drift, however, are in terms of nonlinear approximations of wood-frame shear wall load-drift behavior (up to ultimate capacity). In summary, deformation amplification is an area where exact guidance does not exist and predictive tools are unreliable. Therefore, the designer must exercise reasonable care in accordance with or in addition to the applicable building code requirements.
Another issue that has received greater attention in seismic design provisions is irregularities. Irregularities are related to special geometric or structural conditions that affect the seismic performance of a building and either require special design attention or should be altogether avoided. In essence, the presence of limits on structural irregularity speaks indirectly of the inability to predict the performance of a structure in a reliable, self-limiting fashion on the basis of analysis alone. Therefore, many of the irregularity limitations are based on judgment from problems experienced in past seismic events.
Irregularities are generally separated into plan and vertical structural irregularities. Plan structural irregularities include torsional imbalances that result in excessive rotation of the building, re-entrant corners creating “wings” of a building, floor or roof diaphragms with large openings or non-uniform stiffness, out-of-plane offsets in the lateral force resistance path, and nonparallel resisting systems. Vertical structural irregularities include stiffness irregularities (i.e., a “soft” story), capacity irregularities (i.e., a “weak” story), weight (mass) irregularity (i.e., a “heavy” story), and geometric discontinuities affecting the interaction of lateral resisting systems on adjacent stories.
The concept of irregularities is associated with ensuring an adequate load path and limiting undesirable (i.e., hard to control or predict) building responses in a seismic event. Again, experienced designers generally understand the effect of irregularities and effectively address or avoid them on a case-by-case basis. For typical single-family housing, all but the most serious irregularities (i.e., “soft story”) are generally of limited consequence, particularly given the apparently significant system behavior of light-frame homes (provided the structure is reasonably “tied together as a structural unit”). For larger structures, such as low- and high-rise commercial and residential construction, the issue of irregularity--and loads--becomes more significant. Because structural irregularities raise serious concerns and have been associated with building failures or performance problems in past seismic events, the designer must exercise reasonable care in accordance with or in addition to the applicable building code requirements.
A key issue related to building damage involves deformation compatibility of materials and detailing in a constructed system. This issue may be handled through specification of materials that have similar deformation capabilities or by system detailing that improves compatibility. For example, a relatively flexible hold-down device installed near a rigid sill anchor causes greater stress concentration on the more rigid element, as evidenced by the splitting of wood sill plates in the Northridge Earthquake. The solution can involve increasing the rigidity of the hold-down device (which can lessen the ductility of the system, increase stiffness, and effectively increase seismic load), or by redesigning the sill plate connection to accommodate the hold-down deformation and improve load distribution. As a non-structural example of deformation compatibility, gypsum board interior finishes crack in a major seismic event well before the structural capability of the wall’s structural sheathing is exhausted. Conversely, wood exterior siding and similar resilient finishes tend to deform compatibly with the wall and limit observable or unacceptable visual damage. A gypsum board interior finish may be made more resilient and compatible with structural deformations by using resilient metal channels or similar detailing; however, this enhancement has not yet been proven. Unfortunately, there is little definitive design guidance on deformation compatibility considerations in seismic design of wood-framed buildings and other structures.
As a final issue, it should be understood that the general objective of current and past seismic building code provisions has been to prevent collapse in extreme seismic events such that protection of life is reasonably provided, but not with complete assurance. It is often believed that damage can be controlled by use of a smaller R factor or, for a similar effect, a larger safety factor. Others have suggested using a higher design event. While either approach may indirectly reduce damage or improve performance, it does not necessarily improve the predictability of building performance and, therefore, may have uncertain benefits, if any, in many cases. However, some practical considerations as discussed above may lead to better-performing buildings, at least from the perspective of controlling damage.
Other Load Conditions
Other “forces of nature” may create loads on buildings. Some examples include:
- frost heave;
- expansive soils;
- temperature effects; and
- tornadoes.
In certain cases, forces from these phenomena can drastically exceed reasonable design loads for homes. For example, frost heave forces can easily exceed 10,000 pounds per square foot. Similarly, the force of expanding clay soil can be impressive. In addition, the self-straining stresses induced by temperature-related expansion or contraction of a member or system that is restrained against movement can be very large, although they are not typically a concern in wood-framed housing. Finally, the probability of a direct tornado strike on a given building is much lower than considered practical for engineering and general safety purposes. The unique wind loads produced by an extreme tornado may exceed typical design wind loads by almost an order of magnitude in effect. Conversely, most tornadoes have comparatively low wind speeds that can be resisted by attainable design improvements. However, the risk of such an event is still significantly lower than required by minimum accepted safety requirements.
It is common practice to avoid the loads noted above by using sound design detailing. For example, frost heave can be avoided by placing footings below a “safe” frost depth, building on non-frost-susceptible materials, or using other frost-protection methods. Expansive soil loads can be avoided by isolating building foundations from expansive soil, supporting foundations on a system of deep pilings, and designing foundations that provide for differential ground movements. Temperature effects can be eliminated by providing construction joints that allow for expansion and contraction. While such temperature effects on wood materials are practically negligible, some finishes, such as ceramic tile, can experience cracking when inadvertently restrained against small movements resulting from variations in temperature. Unfortunately, tornadoes cannot be avoided; therefore, it is not uncommon to consider the additional cost and protection of a tornado shelter in tornado-prone areas. A tornado shelter guide is available from the Federal Emergency Management Agency, Washington, D.C.
As noted at the beginning of this article, this article does not address loads from flooding, ice, rain, and other exceptional sources. The reader is referred to ASCE 7 and other resources for information regarding special load conditions.
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