Earthquakes are a critical concern for design professionals working in some of the most populated regions of the U.S., particularly the West Coast. Because seismic events cannot be prevented, buildings in those regions must be designed to maintain structural integrity and keep occupants safe.
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. Photo: VanDorp Chou Associates
Recent seismic events—as well as research into material performance—have given designers and engineers critical insight into how buildings perform under such stress, which is reflected in building code and project applications.
Research and building code development have proven that timber components, assemblies and entire structures are capable of meeting or exceeding the most demanding earthquake and seismic design requirements. Wood products and systems give designers and engineers a readily available and robust selection of code-approved building materials that can help commercial and residential buildings and other infrastructure better withstand seismic events.
Wood-frame construction offers several characteristics to this end:
● Inherently flexible. Wood’s ability to withstand high loads for short periods of time and retain its elasticity and ultimate strength is an asset in seismic zones. ● Lightweight. Wood-frame buildings typically weigh less than those made of concrete and steel, reducing inertial seismic forces, which are proportional to weight and, therefore, are more extreme for heavier structures. ● Ductile connections. The ability to yield and displace without fracturing under an earthquake’s abrupt lateral stresses is an attribute of wood-frame construction, which features several nailed connections that allow it to respond to seismic events without critical failure. ● Code-compliant. Building codes prescribe minimum fastening requirements for connecting repeated wood framing members, which is unique to wood-frame construction and benefits its seismic performance. ● Redundant load paths. The numerous fasteners and connectors used in wood-frame construction offer multiple, often redundant, load paths for seismic forces, reducing the chance the structure will collapse if some connections fail. ● Strength and stiffness. The thickness of mass timber panels and the number and size of nails fastening the assemblies determine each component’s stiffness. Heavy bracing for shear walls can resist lateral distortion common in earthquakes.
The 2015 International Building Code (IBC) and the American Society of Civil Engineers/Structural Engineering Institute Minimum Design Loads for Buildings and Other Structures (ASCE 7-10) represent code and standards for seismic-resistive wood-frame buildings. These standards recognize how structures with ductile detailing, redundancy and regularity deliver high-performance seismic resistance.
Identifying the building risk category is critical to seismic-resistive design. The IBC and the ASCE 7 group common building types into four levels of risk to human life during a seismic event, from least risk to most risk:
● Risk Category I: Agricultural facilities and storage buildings ● Risk Category II: Houses, apartment buildings, offices and stores ● Risk Category III: Schools and assembly buildings with occupancy of greater than 300 ● Risk Category IV: Critical services, including power-generating stations and police and fire stations
Each risk category corresponds to a seismic scale rating (from 1.0 to 1.5) and allowable drift by story height (from 2.5% to 1.0%). The requirements for seismic base shear and drift control in building design are scaled by risk category. The stringent requirements applied to Risk Category IV structures due to their essential nature intends to limit structural and non-structural damage.
ASCE 7 groups wood-frame seismic-force-resisting systems (Table 12.2-1 in ASCE 7-10) accordingly: bearing walls, building frames and cantilevered columns. Three seismic-force-resisting coefficients are used to gauge the performance of these systems, helping designers and engineers select the right one for their application.
• R factor: Response modification coefficient (indicated by R) • Cd: Deflection amplification factor • Ω0: Overstrength factor
The National Design Specification (NDS) for Wood Construction is also helpful when designing wood-frame structures to withstand seismic events. The IBC-referenced design standard covers dimension lumber, glulam, structural composite lumber and CLT as well as fasteners, connections and fire design.
To learn more about wood as a resilient material during seismic events, see the resources below.
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