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3 of 234 <br />Thornton Tomasetti Memorandum <br />TO <br />COMPANY <br />RE <br />CC <br />Michael Harrah <br />Caribou Industries <br />Alternative Methods <br />Executive Summary <br />Cam Simsir, Jackson Pitofsky, <br />Dean Schoenberg <br />FROM Leonard Joseph <br />DATE August 14,2018 <br />PROJECT NO S18003.01 <br />PROJECT 888 N Main Santa Ana tower <br />NAME <br />The City of Santa Ana Building Safety Division requested that an alternative seismic improvement <br />program proposed for the 888 North Main 10-story tower be developed and documented within <br />the framework of a recognized standard. This executive summary (in progress) is intended to <br />accompany Alternative Materials,Design and Methods of Construction form <br />APP-09 under CBC 2016, along with supporting documentation as it is developed. <br />ASCE 41-13 was suggested as a framework for developing the alternative seismic improvement <br />program. The owner and design team agrees with this approach with modifications appropriate <br />to the limited nature of proposed improvements. <br />1. Target Building Performance Levels 'c' and 'd' will apply to the tower concrete frame building <br />structure after alternative seismic improvements have been completed. These levels <br />correspond to 'Limited Objectives' of Life Safety and Collapse Prevention performance at the <br />50%/50 year Seismic Hazard Level as shown in ASCE 41-13 Table C2-2. This level is <br />proposed as practical and achievable while still providing a significant performance <br />improvement compared to building performance before retrofit. For building department <br />information we will present both before- and after-improvement performance measures. <br />2. Tier 1 and Tier 2 study approaches and flow charts in ASCE 41-13 will be addressed in brief <br />narrative form. With visibly robust and regular building geometry, and continuous concrete <br />floor slab diaphragms, most questions about load paths, transfers and connections are not <br />relevant. Building performance will be evaluated at a Tier 3 level using computer models <br />reflecting concrete section geometry, with appropriate modifiers for cracked sections, etc. <br />following established standards. Overall modeling will use simplified members with <br />properties and offsets calibrated to simulate behavior of more complex partial models <br />reflecting as-measured geometric conditions. <br />3. Tier 3 member capacities supporting after-improvement performance levels 'c' and 'd' reflect <br />a 'knowledge factor' K of 1.0 in Table 6-1 based on concrete strength tested at sufficient <br />tower locations to meet 'comprehensive testing program' requirements in ASCE 41-13 and <br />conservative rebar grades based on limited hardness tests. <br />707 Wilslme Bl\'d. Suite 4450 1 Los Angeles CA 90017-36 18 I T 213 330 7000 IF 213 330 7001 I www Th:IntznTO:lazer-1 Cen.