A403.3 Design base shear and design parameters.

A403.3 Design base shear and design parameters. ... Despite any other requirement of Section A403.3 or A403.4, the total expected strength of retrofit elements added to any target story need not exceed 1.3 times the expected strength of the story immediately above, as long as the retrofit elements are located symmetrically about the center of mass of the story above or so as to minimize torsion in the target story.

Commentary: The added sentence implements the SEAONC recommendation to cap the required strength, consistent with FEMA P-807. Exception 4: For retrofit systems involving different seismic force-resisting systems in the same direction within the same story, resisting elements are permitted to be designed using the least value of R for the different structural systems found in each independent line of resistance if the following conditions are met: (1) Risk Category I or II building, (2) four stories or less above grade plane, and (3) the seismic force-resisting systems are composed of WSP shear walls, steel moment-resisting frames, steel cantilever columns, and steel braced frames. Values for Cd and Ω 0 shall be

consistent with the R value used.

Add the following subsection:

A403.3.1 Story strength. Calculation of story strength and identification of irregularities in Section A403.3 shall be based on the expected strength of all wall lines, even if sheathed with nonconforming materials. The strength of a wall line may be reduced to account for inadequate load path or overturning resistance.

Commentary: The expected strength of the story above may be calculated using the FEMA P-807 criteria given in Bulletin Section B5.1. The requirement to assume an adequate load path prevents underestimating the upper story strength. It will apply for calculations of weak story or soft story ratio in ASCE 31/41, CEBC Appendix A - Chapter A4, and other code-based procedures; application of the 1.3 cap on retrofit strength for ASCE 41 and IEBC Appendix A - Chapter A4 retrofits; and calculation of spectral capacity with FEMA P-807.

A403.5. Revise the subsection heading and the provision as follows:

A403.5. Deformation Compatibility and P Δ effects. The requirements of the building code shall apply, except as modified herein. All structural framing elements and their connections not required by design to be part of the lateral force-resisting system shall be designed and/or detailed to be adequate to maintain support of expected gravity loads when subjected to the expected deformations caused by seismic forces. Increased demand due to PΔ effects and story side sway stability shall be considered in retrofit stories that rely on the strength and stiffness of cantilever columns for lateral resistance.

Commentary: This revision is consistent with a change approved for the latest edition of IEBC. A403.8. Revise the provision as follows: