1. Byung Kwan Oh 1,2, Yousok Kim 2, Tong Jun Cho 2, Hyo Seon Park 1,2 1 Department of Architectural Engineering, Yonsei University, Seoul, Republic of Korea 2 Center for Structural Health Care Technology in Buildings, Yonsei University, Seoul, Republic of Korea CENTER FOR STRUCTURAL HEALTH CARE TECHNOLOGY IN BUILDINGS 건축구조헬스케어 연구단 7. CONCLUSION Seismic retrofit cost for pre-Northridge SMRF is minimized using optimal method proposed by this paper. Hybrid retrofit combining more than two retrofit methods can save the total retrofit and restoration cost. In 1994, Existing SMRF buildings applying seismic resistance design are significantly damaged by brittle fracture of connections due to Northridge earthquake. After Northridge earthquake, performance based seismic design/retrofit and diverse seismic retrofit methods are robustly researched and developed. Seismic retrofit methods for SMRF with brittle fracture include connection upgrade (welded haunch, welded cover plate and slotted web, etc.), bracing such as BRB and damper, etc. 1. SEISMIC RETROFIT METHODS AFTER NORTHRIDGE EARTHQUAKE BRB + SLOTTED WEB BRB enhances global strength and stiffness of building. Slotted web enhances ductility of local connections. Both global and local seismic performance of building can be improved by this combined retrofit. [Fig. 1] BRB + SLOTTED WEB 2. HYBRID SEISMIC RETROFIT 3. APPLICATION OF OPTIMAL METHOD 4. PERFORMANCE BASED OPTIMAL SEISMIC RETROFIT The proposed performance-based optimal seismic retrofit method is applied to the SAC Benchmark three-story pre-Northridge SMRF building. Rehabilitation objectives for building are basic safety objective (K+P) and enhanced objective (O). Optimal seismic retrofit is performed for above rehabilitation objectives. Rehabilitation objective K+P is retrofitted by BRB. Rehabilitation objective O is retrofitted by BRB and BRB+SLOTTED WEB Probability of Exceedance Target Building Performace Levels Operational Immediate Occupancy (IO) Life Safety (LS) Collapse Prevention (CP) Earthquake Hazard Level 50%/50 yearABCD 20%/50 yearEFGH 10%/50 yearIJKL 2%/50 yearMNOP Basic Safety Objective (BSO) K ＋ P Enhanced ObjectiveO alone or N alone or M alone [Table. 1] Rehabilitation objectives 5. OPTIMAL SEISMIC RETROFIT RESULTS I. Rehabilitation objective K+P Rehabilitation objective K+P cannot be satisfied without retrofit. Fracture sequence before retrofit. 2F -> 3F -> 1F at connections BRB retrofit is performed for building. Fracture sequence after retrofit. (BRB) X6 -> (Connection) 1F/2F -> 3F After BRB retrofit, rehabilitation objective is achieved as shown in Fig. 4. II. Rehabilitation objective O [Fig. 2] Before retrofit [Fig. 3] After BRB retrofit [Fig. 4] Inter-story drift ratio [Fig. 5] Pushover curve After retrofit Before retrofit [Fig. 6] BRB retrofit [Fig. 7] BRB + SLOTTED WEB retrofit Rehabilitation objective O cannot be satisfied without retrofit. Fracture sequence before retrofit. 2F -> 3F -> 1F at connections BRB retrofit and BRB + SLOTTED WEB retrofit is performed for building. Fracture sequence after retrofit. (BRB) X3 -> X4 -> X6 -> (Connection) 1F -> 2F -> 3F After BRB retrofit and BRB + Slotted web retrofit, rehabilitation objective is achieved as shown in Fig. 8. [Fig. 8] Inter-story drift ratio After BRB + Slotted web retrofit After BRB retrofit Before retrofit After BRB + Slotted web retrofit [Fig. 9] Pushover curve Strength and stiffness of building are improved by BRB retrofit. Strength and stiffness of building are significantly improved by BRB retrofit and BRB + Slotted web retrofit. 6. COMPARISON OF COST BETWEEN BRB RETROFIT AND HYBRID RETROFIT 1. Retrofit cost BRB – 2.5in² X 2EA, 4in² X 2EA => $34474 2. Restoration cost 1) BRB – 2.5in2 X 2EA, 4in2 X 2EA = $34474 2) connection – $1473X18EA = $26514 => $60988 3. Total cost $68948+ $34474 => $95462 1. Retrofit cost 1) BRB – 2.5in2 X 3EA, 4in2 X 1EA = $30495 2) Connection – $1473X2EA = $2946 => $33441 2. Restoration cost 1) BRB – 2.5in2 X 3EA, 4in2 X 1EA = $30495 2) connection – $1473X12EA = $17676 => $48171 3. Total cost $33441+$48171=> $81612 I. BRB RETROFIT II. HYBRID RETROFIT Retrofit and restoration cost of hybrid retrofit is more inexpensive than those of BRB retrofit. $5012 per 1bay is saved.