Designing for Earthquakes

Wood is a proven choice for seismic-resistive construction
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Structural Redundancy

The arrangement of structural elements within the building structure is recognized as significant to seismic performance. Buildings with a high degree of redundancy tend to perform better than those where structural resistance is concentrated in just a few elements. The goal of redundancy requirements in the code is to encourage redundant layouts of seismic force-resisting system elements. The redundancy factor, ρ, varies from 1.0 to 1.3 and has different criteria based on judgment applied to the various system configurations. The redundancy factor equals 1.0 in SDC B and C. For wood-frame shear walls in higher seismic design categories, use of a redundancy factor equal to 1.0 can often be accomplished for plans that have a regular layout and resistance provided at building perimeters, where aspect ratio (height-to-length ratio) of shear walls providing shear resistance is 1.0 or less, or, for cases where aspect ratio of shear walls is greater than 1.0, the minimum length of wood structural panel shear wall equals or exceeds the story height.

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 seismic forces.

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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 seismic forces.

Structural Irregularities

Structural irregularities are formed when the load path for strength or stiffness of a structure is interrupted thus concentrating demand on certain elements of the structure. Structural irregularities have been observed in prior earthquakes to cause a variety of problems that can range in seriousness from localized failure to an undesirable overall response causing total collapse. Irregularities are categorized in two groups: horizontal and vertical. Horizontal structural irregularities include: torsional irregularity, re-entrant corner irregularity, diaphragm discontinuity irregularity, out-of-plane offset irregularity and nonparallel system irregularity. Vertical structural irregularities include: soft story irregularity, weight irregularity, vertical geometric irregularity, in-plane discontinuity irregularity and discontinuity in lateral strength irregularity (e.g., weak story). Examples of common irregularities are shown in Figure 2.

 

[ Page 11 of 13 ]                
Originally published in Engineering News-Record

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