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183 of 234 <br />Thornton Tomasetti <br />PROJECT: 888 N Main Santa Ana - alternative seismic retrofit PROJECT #: S18003.01 DATE: 8/30/2018 <br />BY· LJ SHEET: of <br />SUBJECT· Gusset detail design considering interaction with <br />existing traming <br />CHECKED BY:DRAWING #. <br />. SUPPLEMENTARY STUDY AND CALCULATIONS <br />Consider 'accidental stiffening' effect of damper gusset assemblies: <br />- 1- Maximum damper force (appx 500 kips) occurs at point of maximum velocity, minimum <br />lateral deformation of the existing concrete building frame. For this situation, damper force <br />can be resolved as vertical (shear) and horizontal (bearing) components into column faces. <br />- 2- Negligible damper force occurs at point of zero velocity, maximum lateral deformation. <br />For this situation, all forces result from gusset assemblies interacting with the deformed <br />concrete building frame. <br />To the extent that gusset steel connects to adjacent columns at different elevations, the <br />steel will attempt to 'hold back' points of greater lateral deflection by 'bearing on' points of <br />lesser deflection (closer to floor below or beam above), an 'accidental stiffening' effect. <br />This supplementary study looks at <br />(a) forces in gusset assembly plates and welds by maximum damper force <br />(b) forces in gusset assembly plates and welds by the 'accidental stiffening' effect <br />(c) forces in columns by the 'accidental stiffening' effect <br />(d) changes in effective column stiffness by the 'accidental stiffening' effect <br />(e) changes in overall building behavior by the 'accidental stiffening' effect <br />Study approach <br />(a) resolve damper force into components, take forces out at columns <br />(b) use small stand-alone models to incorporate gusset assemblies in moderate detail as <br />members located at approximate center of gravity (varies with steel shapes) with A and I <br />properties stepped to simulate variations with steel shapes <br />(c) laterally displace the stand-alone models with 3" drift between Level 1 and Level 3, <br />rounded up from 225 year MRI drift results (in line with 200% velocity damper force case) <br />and show resulting axial, shear and moment forces in gusset assemblies and columns <br />(d) use stand-alone models with both damper bays and no-damper bays to determine what <br />stiffness modifiers reasonably simulate column behaviors observed <br />(e) apply stiffness modifiers to full model to determine how modal periods are affected <br />Conclusions <br />(a) proposed gusset assembly plate sizes and shapes OK for both damper force case 1 <br />and maximum frame deformation case 2 <br />(b) gusset assembly welding governed by 'accidental stiffening' effect case 2 demands <br />(c) column demands are consistent with proposed design: damper bay columns see higher <br />shears so FRP wrap is provided; non-damper columns see lower shears, can omit FRP <br />(d) overall building behavior (lowest modal periods) not significantly affected so by <br />inspection if previously determined DCRs increase slightly results would still be OK