91.9309.1. Scope of Analysis. This division requires the alteration, repair, replacement or addition of structural elements and their connections to meet the strength and stiffness in conformance with this 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 wall lines to the foundation. Stories above the weak wall line shall be considered in the analysis but need not be modified.
91.9309.2. Design Base Shear and Design Parameters. (Amended by Ord. No. 184,081, Eff. 2/19/16.) The design force in a given direction shall be 75% of the design base shear specified in the seismic provision of ASCE 7. The value of response modification coefficient, R, need not be less than 3.5, provided the strengthening systems are not cantilevered column systems and the strengthened structure will not have vertical structural irregularities of either type 1a, 1b, 5a or 5b listed in ASCE 7, "Vertical Structural Irregularities" Table.
91.9309.4. Horizontal Structural Irregularities in Buildings with Three or More Stories. Structures with three or more stories having horizontal structural irregularities of either type 2, 3, 4, or 5 listed in ASCE 7, "Horizontal Structural Irregularities" Table, shall be altered to meet the additional requirements of those sections referenced in the table for the entire story with weak or open wall lines.
91.9309.5. Alternate Analysis, Base Shear and Design Parameters. Pursuant to Section 91.104.2.6, the Department may approve alternate design methodologies that improve the whole first story seismic performance that are at least equivalent to those prescribed by this division and that achieve the life safety objectives established by this division.
91.9309.6. Additional Anchorage Requirements for Buildings on Hillsides. Where any portion of a building within the scope of this division is constructed on or into a slope steeper than one unit vertical in three units horizontal (33-percent 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 Division 94 of the Los Angeles Building Code.
91.9309.7. 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 0.025 times the story height. 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.
91.9309.8. Pole Structures. 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.
91.9309.9. P-Delta Effect. The requirements of the Los Angeles 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.