Seismic Rehabilitation of Existing Buildings

In the design of new structures subject to earthquake loading the use of nonlinear response history analysis (NRHA) can be an extremely effective tool for optimization of the lateral system, however it can be a time consuming endeavor. In the process of optimization the necessity of running multiple analysis iterations, each subject to a suite of ground motions, is computationally intensive and can slow down the design process. However, by utilizing a simplified simulation technique, the extended consecutive modal pushover (ECMP) analysis method, the analysis and data reduction effort in design optimization can be reduced. The consecutive modal pushover (CMP) analysis method (Poursh et al. 2009) is a multi-mode nonlinear analysis technique involving sequential application of different modal based load profiles. An extension of the CMP procedure, termed the extended consecutive modal pushover (ECPM) method has been developed by the authors to improve the predictive capabilities of the CMP method. In an ECMP analysis the sequence and sign (direction) of modal load patterns are varied, encouraging formation of different inelastic mechanisms with the goal of replicating the various mechanisms that are observed to develop in NRHA when using a suite of ground motion records. Since the results of an ECMP analysis are similar to that achieved in a more computationally intensive NRHA, the ECMP procedure is an ideal simplified method to use during design optimization, and when model size makes NRHA computationally unfeasible. This paper will present the ECMP analysis method and will describe a procedure for using ECMP analysis as a design optimization tool coupled with NRHA. The results of ECMP and NRHA will be compared for two actual buildings to demonstrate the new ECMP analysis procedure and its use as a design optimization tool.

Use of the Extended Consecutive Modal Pushover Analysis Method to Optimize the Design Process

This paper summarizes the seismic analysis and rehabilitation design of a fifteen story steel moment resisting frame building in Oakland, California, less than five miles from the Hayward Fault The building was constructed with connection details that were found to be vulnerable to fracture in the 1994 Northridge earthquake Previous testing of the existing connections indicated that the welded connections were highly vulnerable to fracture, and consequently the building presented a risk to life safety in the event of a major earthquake Seismic strengthening to meet the State of California Guidelines for the Seismic Retrofit of State Buildings Risk Level III resulted in a requirement to reduce the story drifts to 15% in a 475-year return period event After considering several retrofitting schemes including connection strengthening, BRBF's and seismic isolation, a scheme that included strengthening sixty percent of the existing pre-Northridge moment frame connections and adding viscous dampers was selected Since the existing moment connections had performed so poorly in previous testing, and because the size of the members in this building exceeded those that had been previously tested, a series of four full-scale tests were performed to confirm the validity of the connection upgrade design Two phases of advanced analysis techniques were performed in the rehabilitation design; a study phase with multi-mode two-dimensional nonlinear pushover analyses along with single-degree-of-freedom nonlinear dynamic time-history analyses; and a working drawings phase with nonlinear response history analyses of building frames to refine the scheme and perform member checks The extra steps taken beyond typical engineering practices were intended to provide better assurance that the project's performance goals would be met during the design basis seismic event

Timothy Graf

Papers

Use of the Extended Consecutive Modal Pushover Analysis Method to Optimize the Design Process

Seismic Analysis and Rehabilitation of a 15 Story Pre-Northridge Steel Moment Resisting Frame Building