Seismic Rehabilitation of Existing Buildings

Performance-based earthquake engineering requires numerical models that are able to reliably predict the expected component behavior of buildings under seismic excitations. The nonlinear modeling guidelines provided in ASCE-41-13 have historically been used in this context by practicing engineers. However, for steel wide-flange columns, the recommendations are primarily based on experimental data that was fairly limited until recently. This paper addresses many of the limitations present in ASCE-41-13 for the modeling of steel wide-flange columns. Recent experimental data from large- and full-scale tests on wide-flange steel columns, supplemented with detailed finite element studies, has been collected for a wide range of column cross-sections and applied compressive axial loads. Multiple regression analysis is utilized to develop multivariate predictive equations for the seismic response of steel wide-flange columns. It is found that the primary contributors that dominate the column response are the local web slenderness, member slenderness and the applied axial load ratio due to gravity loading. Guidelines for developing nonlinear component models for steel columns that can be directly used in both nonlinear static and dynamic analysis procedures are provided. The predicted responses are validated against test data that was not part of the original database. Furthermore, recommendations are proposed for modeling wide-flange columns subjected to varying axial loads as well as bidirectional loading; and a new limit for force-controlled elements is proposed for columns under high axial compressive loading and lateral drift demands.

Updates to the ASCE-41-13 Provisions for the Nonlinear Modeling of Steel Wide-Flange Columns for Performance-based Earthquake Engineering

A risk‐based life cycle cost strategy for optimal design and evaluation of control methods for nonlinear structures

In order to increase the accuracy of evaluating seismic response of structures, it is critical to conduct dynamic analyses based upon precise nonlinear models being as consistent as possible with the real conditions of corresponding structures. The concentrated plasticity model including one elastic element and two nonlinear spring elements at both ends has been considered within the research community for simulating beams and columns, counting the effect of strength and stiffness degradation. In this type of simulation, the axial force ratio generated in each structural component, which is a major factor in introducing nonlinear springs, has always been considered constant in the literature. The main objective of the present research is, therefore, to modify the fundamental weakness in this type of modeling approach; indeed, any variation of element’s axial effort, owing to redistribution of axial forces during an earthquake, is applied in the calculation of parameters of the concentrated plasticity model as a decisive step toward the development of nonlinear dynamic analysis. Moreover, an algorithm is presented for implementing this approach in the OpenSees software. Verification is established and the efficiency of the proposed method is illustrated through a reinforced concrete moment frame subjected to a specific record, as a case study building. Regarding the results, it is confirmed that the proposed algorithm is an appropriate tool for achieving quite a realistic nonlinear model and estimating reasonably accurate responses of structural systems with cyclic degrading behavior under earthquake loading.

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The Effect of Axial Force Variations on Nonlinear Modeling and Seismic Response of Reinforced Concrete Structures

粘性減衰モデルが弾塑性系の時刻歴応答解析に与える影響（その１）：減衰モデルの比較

Nonlinear dynamic (response history) analysis is being used increasingly in design practice for the performance-based seismic design of new buildings. In contrast to nonlinear static analysis, dynamic analysis requires more explicit modeling of cyclic response including strength and stiffness degradation as well as special consideration to selection and scaling of ground motions, definition of viscous damping, and other dynamic effects. To help bridge the gap between state-of-the-art in research and practice, the National Institute of Standards and Technology (NIST) has funded a multi-phase project through the Applied Technology Council (ATC) to develop improved modeling criteria and guidelines for nonlinear dynamic analysis. This paper highlights recently developed modeling guidelines and criteria for buildings, including both general modeling requirements as well as models and criteria that are specific to steel and concrete moment frame buildings. The general requirements address analysis and modeling requirements that are common to all material types and systems, including the relationship between modeling requirements and acceptance criteria, and the influence of modeling uncertainties. The steel and concrete moment frame guidelines incorporate the latest research information on modeling those systems. Illustrative examples are also summarized, which were used to demonstrate application of the guidelines.

Guidelines on Nonlinear Dynamic Analysis for Performance-Based Seismic Design of Steel and Concrete Moment Frames

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

Chia-Ming Uang

Papers

Guidelines on Nonlinear Dynamic Analysis for Performance-Based Seismic Design of Steel and Concrete Moment Frames

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