Tenth U.S. National Conference on Earthquake Engineering Frontiers of Earthquake Engineering July 21-25, 2014 10NCEE Anchorage, Alaska USING OPENSEES FOR ANALYZING A 9- STORY STEEL BUILDING UNDER POST- EARTHQUAKE FIRES N. Elhami Khorasani1 and M. E. M. Garlock2 ABSTRACT Post-earthquake fires are major hazards with possible intense consequences. Civil engineering design is moving towards the concept of resilient cities, which requires buildings with structural systems to perform adequately after extreme hazards and ensure human safety. Current codes and standards in fire engineering are mainly based on design at the component level and deterministic approaches, where uncertainties in variables are not directly incorporated in the design process. Reliability-based approaches offer better means of evaluating resilience in the face of extreme hazards. But to measure resilience in an earthquake-fire multi-hazard scenario, uncertainties in the system must be captured, and nonlinear analyses must seamlessly transfer from seismic to fire analysis. This paper discusses the successes and challenges of this seamless analysis using the open source software OpenSees. A 9-story steel building with moment resisting frames is subject to fire following earthquake while being modeled in OpenSees. With the newly added fire module in OpenSees, the software has the capacity to perform structural analysis for both seismic and thermal loads. This way, system-level analysis is performed in one software and interaction of members with each other, under both seismic and fire events, is then considered together. The current constitutive material model for steel at elevated temperature does not properly capture behavior in fire following earthquake scenario. The constitutive model is therefore modified to capture effect of plastic strains, and strain reversals during heating or cooling. The resultant moment calculation is also modified to be consistent with other finite element programs and properly capture combined axial load and moment effects. The new model is used to analyze the 9-story building under two earthquake hazard levels and multiple fire locations. Results show that the new constitutive model works well. The sample analyses indicated that the subsequent fire can increase the drift of the column on the perimeter on the order of 0.5% in the lower floors and 1% in the upper floors. Total drifts in FFE did not exceed 2% (including the residual from earthquake) for the scenario studied. 1Graduate Student, Dept. of Civil and Environmental Engineering, Princeton University, Princeton, NJ, 08542 2Associate Professor, Dept. of Civil and Environmental Engineering, Princeton University, Princeton, NJ, 08542 Elhami Khorasani N, Garlock, MEM. Using OpenSees for analyzing a 9-story steel building under post-earthquake fires. Proceedings of the 10th National Conference in Earthquake Engineering, Earthquake Engineering Research Institute, Anchorage, AK, 2014. DOI: 10.4231/D3PK0727C Tenth U.S. National Conference on Earthquake Engineering Frontiers of Earthquake Engineering July 21-25, 2014 10NCEE Anchorage, Alaska Using OpenSees for analyzing a 9-story steel building under post-earthquake fires N. Elhami Khorasani1 and M. E. M. Garlock2 ABSTRACT Post-earthquake fires are major hazards with possible intense consequences. Civil engineering design is moving towards the concept of resilient cities, which requires buildings with structural systems to perform adequately after extreme hazards and ensure human safety. This paper provides a step towards developing a methodology to evaluate the resilience of steel buildings under post-earthquake fires. With the newly added fire module in OpenSees, the software has the capacity to perform structural analysis for both seismic and fire loads. This way, system-level analysis is performed in one software and interaction of members with each other, under both seismic and fire events, is considered together. This paper presents the challenges to seamlessly transfer from seismic to thermal analysis in OpenSees. The current constitutive material model for thermal analysis does not properly capture behavior of the frame for fire that follows earthquake. The constitutive model is therefore modified to capture effect of plastic strains, and strain reversals during heating or cooling. The new model is used to analyze a 9-story building under two earthquake hazard levels and multiple fire locations. Introduction Fire following an earthquake is a major hazard in densely populated urban areas in seismic regions. There are historical examples where fire after an earthquake has caused considerable damage to structures. San Francisco 1906 and 1989, Tokyo 1923, and Kobe 1995 are a few of such examples [1]. While the occurrence of an earthquake and its consequences cannot be prevented, proper designs can minimize the damage. In a strong earthquake, buildings designed for seismic regions, may experience plastic deformations but do not collapse. However, such earthquakes may cause major destruction and damage gas and water lines, starting uncontrollable fires. When fire follows an earthquake, buildings with reduced strength or damaged fire protection materials may not have the capacity to resist the subsequent extreme event. Previous research, performed on structural performance of post-earthquake fires, has studied the problem (1) in different programming environments for seismic and thermal analyses and (2) within a deterministic approach [2 and 3]. The first shortcoming in such approaches is that switching between programs to complete seismic and thermal analyses requires certain 1Graduate Student, Dept. of Civil and Environmental Engineering, Princeton University, Princeton, NJ, 08542 2Associate Professor, Dept. of Civil and Environmental Engineering, Princeton University, Princeton, NJ, 08542 Elhami Khorasani N, Garlock, MEM. Using OpenSees for analyzing a 9-story steel building under post-earthquake fires. Proceedings of the 10th National Conference in Earthquake Engineering, Earthquake Engineering Research Institute, Anchorage, AK, 2014. idealizations, such as ignoring material degradation after earthquake, which would reduce the accuracy of the results. The second concern is that although performance-based guidelines for seismic design of structures have been well established, the available design procedures for fire are fairly new and largely based on perspective codes or performance-based guidelines with deterministic input variables. Yet the available data shows that considerable uncertainty exists in defining fire load density and properties of material at elevated temperatures. Therefore, a reliability approach to evaluating structures in fires is needed. The work presented in this paper is a step towards developing methodologies for probabilistic analysis of structures under post-earthquake fires. The program OpenSees is used to evaluate performance of a 9-story steel office building under fire and under fire following earthquake (FFE). The thermal module in OpenSees has been recently developed, and the paper illustrates the challenges to seamlessly transfer from seismic to thermal analysis in OpenSees. This paper includes the necessary modifications that the authors applied to the thermal module in OpenSees to make the analysis possible. Design Description The geometry and building description of the prototype Moment Resisting Frame (MRF), which will be used for the post-earthquake fire reliability analysis, is based on the SAC steel project. The SAC project included 3, 9, and 20-story prototype buildings located in Los Angeles, Seattle and Boston areas [4]. The buildings were designed as standard offices on both stiff and soft soil. The MRF in the present study is a 9-story frame that is located in downtown Los Angeles and has plan and elevations that are based on SAC buildings but considered only for stiff soil. The MRF is re-designed based on ASCE7-2010 specifications (Minimum Design Loads for Buildings and other Structures). 8 @13 8 ft 5@ 30ft. 18‘ 12’ 5@ 30 ft. foundation (pinned) Figure 1: Plan and elevation of the 9-story frame The floor plan and elevation of the 9-story structure is presented in Fig. 1. The building geometry consists of a square plan with 5 bays, each at 30 ft., in either direction. Girders are spaced at 30 ft while beams are spaced at 10 ft intervals. The 9-story building has a typical floor height of 13 ft with a basement height of 12 ft and ground floor height of 18 ft. The building consists of 4 MRFs on each side that are placed such that biaxial bending is avoided at corners. The MRFs in the two orthogonal directions are identical. The columns are pinned at the foundation, and laterally braced at the ground level. The design procedure is mainly based on the ASCE7 specifications, and the assumed gravity loads are consistent with the assumptions of the SAC models [4]. Seismic loads are according to the Equivalent Lateral Force (ELF) procedure in ASCE7 (2010). The design forces are calculated for the Site Class D (stiff soil). The design checks applied in the design of the MRF are based on the guidelines in the AISC Steel Construction Manual (2010), and AISC Seismic Provisions for Structural Steel Buildings (2010). The frame has a period of 2.0 seconds. Table 1 summarizes design of the 9-story MRF. Table 1: Design of the 9-story frame based on ASCE 7 -10 Interior Exterior Level Beam t t Column doubler, INT Column doubler, EXT 9-roof W24X76 W14X342 0.00 W14X257 0.00 8-9 W30X108 W14X342 0.47 W14X257 0.00 7-8 W33X169 W14X455 0.92 W14X370 0.00 6-7 W33X169 W14X455 0.92 W14X370 0.00 5-6 W36X194 W14X550 0.70 W14X500 0.00 4-5 W36X194 W14X550 0.70 W14X500 0.00 3-4 W36X194 W14X605 0.38 W14X550 0.00 2-3 W36X210 W14X605 0.63 W14X550 0.00 1-2 W36X210 W14X665 0.45 W14X605 0.00 Basement-1 W36X210 W14X665 0.45 W14X605 0.00 Modeling in OpenSees The program OpenSees is used to model and analyze the 9-story MRF under earthquake and fire. The OpenSees framework is an object-oriented software that was developed at the University of California, Berkeley, mainly for nonlinear analysis of structures under seismic loading.