A. Scope of analysis. This Subchapter requires the alteration, repair, replacement or addition of structural elements and their connections to meet the strength and stiffness in conformance with the Building Code except as modified herein. The lateral-load-path analysis shall include the resisting elements and connections from the wood diaphragm immediately above any soft, weak or open-front wall lines to and including the foundation. Stories above the weak wall line shall be considered in the analysis but need not be modified. The engineer shall investigate existing conditions as applicable for the required analysis, including performing initial material testing and verification of existing conditions.
B. Design base shear and design parameters. The design force in a given direction shall not be less than seventy-five percent (75%) of that derived from the base shear as determined from the seismic provisions of ASCE 7 and design provisions as specified by the current Seismic Design Guidelines. The structure shall be analyzed and/or strengthened in order to mitigate the soft wall line and/or weak wall line deficiencies as defined in § 15.02.510.
C. Exception. Alternatively, the structure may be retrofitted per Appendix A4 of the California Existing Building Code, provided the entire story is analyzed and/or strengthened in order to mitigate the soft wall line and/or weak wall line deficiencies defined in § 15.02.510.
D. Lateral vertical systems. Strengthening systems with concrete walls or masonry walls, or steel braced frames shall not be permitted unless a full building analysis considering diaphragm stiffness and torsional behavior is performed.
E. Horizontal structural irregularities in buildings with three (3) or more stories. Structures with three (3) or more stories having horizontal structural irregularities of either type 2, 3, 4, or 5 listed in ASCE 7, "Horizontal Structural Irregularities", shall be altered to meet the additional requirements of those sections referenced in the table for the weak, soft or open-front wall lines being considered.
F. Alternate analysis, base shear and design parameters. The Building Official may approve alternate analysis and/or design methodologies that meet the same performance intent as those prescribed by this Subchapter and that achieve the objectives established by this Subchapter. A design criteria shall be submitted to the City for review and approval prior to submission of plans. Peer review shall be required when advanced systems, such as damped systems are utilized.
G. Additional anchorage requirements for buildings on hillsides. Where any portion of a building within the scope of this Subchapter is constructed on or into a slope steeper than one (1) unit vertical in three (3) units horizontal (33% slope), the lateral-force-resisting system, at and below the base level diaphragm, shall also be analyzed for the effects of concentrated lateral loads caused at the building base from the hillside conditions and comply with the provisions of the Building Code.
H. Story drift limitations. The calculated story drift for each retrofitted story shall not exceed the allowable deformation compatible with all vertical load-resisting elements and meet the requirements of the current Seismic Design Guidelines. The calculated story drift shall not be reduced by the effects of horizontal diaphragm stiffness, but shall be increased when these effects produce rotation. Drift calculations shall be in accordance with ASCE 7 requirements.
I. Cantilever column systems. The effects of rotation and soil stiffness shall be included in the calculated story drift where lateral loads are resisted by vertical elements whose required depth of embedment is determined by pole formulas. The coefficient of subgrade reaction used in deflection calculations shall be based on an approved geotechnical investigation conducted in accordance with approved geotechnical engineering reports.
J. P-delta effect. The requirements of the Building Code shall apply, except as modified herein. All structural framing elements and their connections not required by the design to be part of the lateral force resisting system shall be designed and detailed to be adequate to maintain support of design dead plus live loads when subject to the expected deformations caused by seismic forces. The stress analysis of cantilever columns shall use an effective length factor of 2.1 for the direction normal to the axis of the beam.
K. Ties, continuity and collectors. All parts of the structure included in the scope of analysis shall be interconnected and the connection shall be capable of resisting the seismic force created by the parts being connected as required per the Building Code.
(Ord. 2021-013, § 1(part))