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4 of 234 Thornton Tomasetti Memorandum Re: Alternative Methods Executive Summary Page 2 4. A field testing program of visual condition assessment, concrete cores, Ground Penetrating Radar (GPR) scans, surface hardness testing and limited x-rays has been completed. Although no existing structural drawings have been found, existing architectural drawings show member profiles consistent with field measurements. 5. Site visits for ASCE 41-13 visual condition assessment showed a concrete structural system in good condition, with no significant deterioration at the primary lateral load resisting elements. 6. Foundation elements are not visible and will not be tested or probed. This is considered acceptable because Target Building Performance Levels 'c' and 'd' at 50%/50 year Hazard Level imply low seismic demands for which foundation demands from seismic load combinations will be small, of comparable magnitude to those already experienced during its 50 year life to date. Subgrade conditions based on original (1960s) soil investigations at this site, and recent investigations nearby sites, are discussed in a geotechnical report to be submitted. 7. 'Seismic base' is taken at the Ground Floor for computer modeling, with lateral restraint there. Column continuity through the Ground Floor is modeled by one-story basement columns and existing concrete infill walls between them. Significant Ground Floor lateral restraint is provided by a continuous reinforced concrete slab tied to heavy perimeter basement walls, and by infill walls between tower perimeter columns in the basement itself. For Target Performance Levels 'c' and 'd' this modeling approach is appropriate. 8. For alternative improvement models, only viscous diagonal dampers are modeled as nonlinear elements. All other elements are modeled as elastic (with stiffness modifiers). 9. For determining after-improvement performance, Tier 3 Nonlinear Response History Analysis (NRHA) uses four input suites spectrally matched to the 73 year mean recurrence interval 5% damped spectrum by the geotechnical consultant. For comparison purposes only, Response Spectrum Analysis (RSA) uses the 73 year MRI 15% damped spectrum for after- improvement performance, for comparable base shears to the 73 year NRHA results. 10. Columns are anticipated to govern acceptance; the combined depth of typical perimeter beams and integral infill panels is several times larger than typical column widths so beams should not govern. This will be checked.