Earthquakes cannot be prevented but sound design and construction based on research and compliance with building code requirements can reduce their effects. Worldwide, it is estimated that several million earthquakes occur each year,¹ but most are too small to be felt. They can occur anywhere; however, the likelihood of earthquakes strong enough to threaten buildings is especially high in certain geographic areas. Areas of particularly high seismic hazard in the U.S., for example, are shown in Figure 1 on page 2.

In North America, where wood-frame construction is common, loss of life due to earthquakes has been relatively low compared to other regions of the world.² The relative good performance of wood buildings is often attributed to the following characteristics:

▶ Lightweight. Wood-frame buildings tend to be lightweight, reducing seismic forces, which are proportional to weight.

▶ Ductile connections. Multiple nailed connections in framing members, shear walls and diaphragms of wood-frame construction exhibit ductile behavior (the ability to yield and displace without sudden brittle fracture).

▶ Redundant load paths. Wood-frame buildings tend to be comprised of repetitive framing attached with numerous fasteners and connectors, which provide multiple and often redundant load paths for resistance to seismic forces. Further, when structural panels such as plywood or oriented strand board (OSB) are properly attached to lumber floor, roof and wall framing, they form diaphragms and shear walls that are exceptional at resisting these forces.

Photo: Lawrence Anderson, www.lawrenceanderson.net

The luxury Stella development in California includes four and five stories of wood-frame construction over a shared concrete pool-level podium. It was designed to meet requirements for Seismic Design Category D.

▶ Compliance with applicable codes and standards. Codes and standards governing the design and construction of wood-frame buildings have evolved based on experience from prior earthquakes and related research. Codes also prescribe minimum fastening requirements for the interconnection of repetitive wood framing members; this is unique to wood-frame construction and beneficial to a building’s seismic performance.

In addition to their other advantages—such as cost-effectiveness and sustainability—properly designed and constructed wood buildings complying with building code requirements help make communities more resilient to seismic hazards, because they are proven to perform well during seismic events. In California, for example, where wood-frame construction is common for public schools, an assessment of the damage to school buildings in the 1994 Northridge earthquake was summarized as follows: “Considering the sheer number of schools affected by the earthquake, it is reasonable to conclude that, for the most part, these facilities do very well. Most of the very widespread damage that caused school closure was either non-structural, or structural but repairable and not life threatening. This type of good performance is generally expected because much of the school construction is of low rise, wood-frame design, which is very resistant to damage regardless of the date of construction.”³

This continuing education course provides an overview of seismic-resistive design issues in wood-frame buildings with a focus on compliance with the 2015 International Building Code (IBC) and American Society of Civil Engineers/Structural Engineering Institute Minimum Design Loads for Buildings and Other Structures (ASCE 7-10). The information on code-conforming wood design contained in this course is based on the American Wood Council’s (AWC’s) 2015 National Design Specification® (NDS®) for Wood Construction, and 2015 Special Design Provisions for Wind and Seismic (SDPWS). The NDS and SDPWS are adopted by reference in the 2015 IBC.

Designing Wood Buildings to Withstand Seismic Forces

Seismic design forces are specified in the building code to allow for proportioning of strength and stiffness of the seismic force-resisting system. Structures with ductile detailing, redundancy and regularity are favored for seismic force resistance. These beneficial characteristics are specifically recognized in the seismic design requirements.

Source: U.S. Geological Survey

Photo: VanDorpe Chou Associates

Structures with ductile detailing, redundancy and regularity are favored for seismic force resistance. This structure includes repetitive wood framing and ductile nailed wood structural panel shear walls and diaphragms.

The IBC establishes the minimum lateral seismic design forces for which buildings must be designed primarily by reference to ASCE 7. While ASCE 7 allows use of a number of analysis procedures, the equivalent lateral force (ELF) procedure is most commonly used for seismic design of buildings in the U.S. This is particularly true for low-rise, short-period, wood-frame buildings. The ELF procedure relies on seismic force-resisting system design coefficients such as the response modification coefficient, R (often referred to as the R-factor), deflection amplification factor, Cd, and overstrength factor, Ωo. The R-factor is essential for determining design seismic base shear, V, which is used in the design of elements of the seismic force-resisting system. For short-period, wood-frame structures, seismic base shear, V, is calculated in accordance with Equation 1.

Design seismic base shear is proportional to effective seismic weight, W, the seismic hazard at the site represented by the spectral response acceleration parameter, SDS, response modification coefficient, R, and the importance factor, Ie. Since the R-factor is found in the denominator of the seismic base shear equation, as the R-factor increases for systems being considered, the seismic base shear forces decrease. For wood-frame buildings, values of the R-factor cover a wide range from R=1.5 to R=7.0 depending on the type of wood-frame seismic force-resisting system. (See Table 1.)

 

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