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Designing for Earthquakes

Wood is a proven choice for seismic-resistive construction
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For inverted pendulum structures of timber frames (e.g., system G6. Timber Frames), seismic design coefficients (R=1.5, Ωo=1.5, and Cd=1.5) are applicable. Such systems include wood pile-supported structures where the wood member and its connections are designed in accordance with member and connection provisions of the NDS.

Seismic Design Category

The seismic design category for the structure at the site is critical for proper application of requirements for seismic design. The SDC is used as a trigger for permitted use of seismic systems and structural height limitations as seen in Table 3. It is also used to trigger applicability of special requirements associated with structural redundancy and structural irregularities in the building system. SDC is determined based on several factors:

▶  Soil properties at the site, or site class, which range through site class A, B, C, D, E and F. Site class A is associated with presence of hard rock. Site class F is associated with peats and/or highly organic clays, very high plasticity clays and very thick soft/medium stiff clays.

▶  Mapped values of seismic hazard

▶  Risk category of the structure

For short-period structures, such as most wood-frame structures, ASCE 7 allows determination of the seismic design category based on value of SDS and risk category alone (see Table 5) provided alternative criteria are met for structure period and diaphragm flexibility and for sites where mapped values of S1 are less than 0.75. Seismic design categories A, B, C, D, E and F reflect the range of possible categories under ASCE 7. They are similar to seismic zones found in previous codes; however, seismic design categories are more representative of the risk to a particular building because they incorporate the structure’s risk category, site conditions and mapped seismic hazard at the site.

SDC A represents a very low seismic hazard for which there are no seismic-specific limits on structural height, system type, structural redundancy or structural irregularities. Structures located in this category are not subject to design forces determined in accordance with the ELF. Beginning with SDC B, seismic forces in accordance with ELF are applicable and consideration must be given to special requirements for structural irregularities. Special requirements and limitations become increasingly significant beginning in SDC C. As can be seen in Table 3, as seismic design category increases, structural height limitations apply as well as limitations on the use of particular systems.


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Originally published in Engineering News-Record