Wind- and Seismic Force-Resisting Systems

ASCE 7 recognizes several wood-frame seismic- and wind force-resisting systems. These include sheathed wood-frame diaphragms and shear walls and now, CLT diaphragms and shear walls.

Light wood-frame diaphragms and shear walls include 1) light wood-frame walls with wood structural panels rated for shear resistance; and 2) light wood-frame walls with structural panels made from other materials, including particleboard, structural fiberboard, gypsum wallboard, gypsum base for veneer plaster, water-resistant gypsum backing board, gypsum sheathing board, gypsum lath and plaster, and Portland cement plaster. SDPWS also allows wood-frame shear walls sheathed with wood structural panels to be designed to simultaneously resist shear and uplift from wind forces. The design requirements for wood-frame diaphragms and shear walls used in these systems are contained in Chapter 4 of the 2021 SDPWS.

The increasing popularity of mass timber structures and construction products such as CLT prompted an examination of how these materials, and the resulting taller buildings, may perform during natural disasters. However, until the release of the 2021 SDPWS, use of CLT as diaphragms or shear walls to resist wind and seismic forces had not been codified. The 2021 SDPWS contains new design provisions for CLT diaphragms and CLT shear walls. CLT diaphragm provisions are found in Section 4.5 and CLT shear wall provisions are found in Section 4.6 and Appendix B.

Figure 2: This USGS earthquake hazard map shows peak ground accelerations for a firm rock site having a 2 percent probability of being exceeded in 50 years.

Risk Category

Before considering provisions specific to wind or seismic hazards, identify the risk category of the building or structure as defined by the IBC. Risk categories are assigned to buildings to account for consequences and risks to human life (building occupants) in the event of a building failure. Risk categories are used to classify buildings and structures based on their importance and take into consideration occupancy, risk to human life, and societal need of the building or structure to function during and following an extreme event. For example, hospitals and other essential facilities are assigned the highest risk category (Risk Category IV) and therefore must meet more stringent design criteria. Detailed descriptions of buildings and structures associated with Risk Categories I, II, III, and IV are provided in 2021 IBC Table 1604.5.

Seismic Design and Wood

Earthquakes are an unpredictable fact of life in some regions in North America, particularly along the West Coast. The map in Figure 2, prepared by the United States Geological Survey (USGS), illustrates the seismic hazard for the entire country, with areas of greatest risk shown in red.

Although developers of building codes accept that some nonstructural and structural damage will occur, they seek to limit the likelihood of structural collapse and to ensure the superior performance of essential facilities such as hospitals and fire stations. These performance expectations are set in recognition of the fact that it is not economically feasible to prevent all damage in all buildings when designing for infrequent, large-magnitude earthquakes.

Earthquakes cannot be prevented, but proper design and construction—based on compliance with building code requirements—can reduce their negative impacts.

Referencing ASCE 7, building codes address the probability and severity of earthquakes by providing design requirements based on the site-specific seismic hazard and the building’s risk category.

ASCE 7 and model building codes such as the IBC provide maps that identify seismic hazards. Created in cooperation with the USGS and FEMA, these maps are updated regularly and assist engineers in designing buildings, bridges, highways, and utilities that will withstand seismic shaking.

Calculating Seismic Design Forces

The IBC establishes the minimum lateral seismic forces for which buildings must be designed, primarily by reference to ASCE 7-16. Base shear refers to the total horizontal force resisted through a structure’s lateral force-resisting system as the mass of the building accelerates due to ground shaking. The equivalent lateral force (ELF) procedure is the most used procedure for determining base shears. This is particularly true for low-rise, short-period, wood-frame buildings.

Base shear, or V, is proportional to the effective seismic weight of the structure and the seismic hazard at the site. The seismic hazard is represented by several variables: the spectral response acceleration parameter, SDS; response modification coefficient, R; and the importance factor, Ie. Effective seismic weight, or W, includes both dead load plus some live loads as prescribed in Section 12.7.2 of ASCE 7-16.

Table 1: Risk categories of the building or structure as defined by the IBC