Strength-level seismic forces used to evaluate the structure for resistance to seismic loads shall be based on the R-values tabulated in the regular code for similar lateral-force-resisting systems including consideration of the structural detailing of the members where such R-values exist. Where such R-values do not exist, an appropriate R-value shall be rationally assigned considering the structural detailing of the members.
- The forces need not exceed 0.75 times the seismic forces prescribed by the regular code requirements.
- For Risk Category I, II or III structures, near-fault increases in ground motion (maximum considered earthquake ground motion of 0.2 second spectral response greater than 150 percent at 5 percent damping) need not be considered when the fundamental period of the building is 0.5 seconds in the direction under consideration.
- For Risk Category I or II structures, the seismic base shear need not exceed 0.30W.
- For Risk Category III or IV structures, the seismic base shear need not exceed 0.40W.
When a building is to be strengthened with the addition
of a new lateral force resisting system, the R value of the new system can be used when the new lateral force resisting system resists at least 75 percent of the building's base shear regardless of its relative rigidity.
Evaluation and seismic improvement of unreinforced masonry bearing wall buildings shall comply with the California Existing Building Code (CEBC), Appendix Chapter A1 2013 Edition, and as modified by the CHBC.
- Alternative standards may be used on a case-by-case basis when approved by the authority having jurisdiction. It shall be permitted to exceed the strength limitation of 100 psi in Section A108.2 of the CEBC when test data and building configuration supports higher values subject to the approval of the authority having jurisdiction.
- CEBC Section A102.2 shall not apply to Qualified Historical Buildings in Risk Category III buildings and other structures whose primary occupancies are public assembly with an occupancy load greater than 300.
All deviations from the detailing provisions of the lateral-force-resisting systems shall be evaluated for stability and the ability to maintain load-carrying capacity at the expected inelastic deformations.
The seismic resistance may be based upon the ultimate capacity of the structure to perform, giving due consideration to ductility and reserve strength of the lateral-force-resisting system and materials while maintaining a reasonable factor of safety. Broad judgment may be exercised regarding the strength and performance of materials not recognized by regular code requirements. (See Chapter 8-8
, Archaic Materials and Methods of Construction.)
All structural materials or members that do not comply with detailing and proportioning requirements of the regular code shall be evaluated for potential seismic performance and the consequence of non-compliance. All members that would be reasonably expected to fail and lead to collapse or life threatening injury when subjected to seismic demands shall be judged unacceptable, and appropriate structural strengthening shall be developed.
A complete and continuous load path, including connections, from every part or portion of the structure to the ground shall be provided for the required forces. It shall be verified that the structure is adequately tied together to perform as a unit when subjected to earthquake forces.
Parapets and exterior decoration shall be investigated for conformance with regular code requirements for anchorage and ability to resist prescribed seismic forces.
An exception to regular code requirements shall be permitted for those parapets and decorations which are judged not to be a hazard to life safety.
Nonstructural features of historical structure, such as exterior veneer, cornices and decorations, which might fall and create a life-safety hazard in an earthquake, shall be evaluated. Their ability to resist seismic forces shall be verified, or the feature shall be strengthened with improved anchorage when appropriate.
Partitions and ceilings of corridors and stairways serving an occupant load of 30 or more shall be investigated to determine their ability to remain in place when the building is subjected to earthquake forces.
Seismic forces used to evaluate and improve nonstructural components and their anchorage, where required, shall comply with ASCE 41 or need not exceed 0.75 times the seismic forces prescribed by the requirements of the regular code.