۱- Introduction

۲- Overview of the FEMA P695 methodology

۳- Design and selection of archetypes

۴- Numerical modeling of archetypes and application of the methodology

۵- Evaluation of response factors

۶- Proposed modifications

۷- Evaluation of the proposed modifications

۸- Limitations of the study and future research needs

۹- Conclusions

References

Abstract

This paper reports the details of a numerical study undertaken to evaluate seismic response factors for steel buckling-restrained braced frames (BRBFs) using the FEMA P695 methodology. In the United States, BRBFs are designed according to Minimum Design Loads for Buildings and Other Structures (ASCE 7) and the Seismic Provisions for Structural Steel Buildings (AISC 341). Twenty-four archetypes were designed according to the U.S. specifications and their behavior was assessed by making use of non-simulated collapse models. The interstory drift, brace axial strain and cumulative brace axial strain demands under collapse level ground motions were determined. The results obtained indicate that the current seismic response factors are adequate in terms of interstory drift and cumulative axial strain demands. On the other hand, large differences between the design level and collapse level axial strains were reported, which can result in undesirable brace behavior. Modified approaches were developed to estimate the axial strains for collapse level ground motions. These include a modification to the deflection amplification factor and a modification to the AISC 341 requirements for expected brace deformations. The archetypes were redesigned using the proposed modifications and reevaluated using the FEMA P695 methodology. The results indicate that the proposed modifications result in axial strain demands that are in close agreement with the calculated demands.

Introduction

Steel buckling-restrained braced frames (BRBFs) are often used as lateral load resisting systems against forces produced by wind and earthquakes. BRBFs employ buckling-restrained braces (BRBs) which may be considered as hysteretic dampers. During a seismic event, BRBs yield in tension and compression and contribute to energy dissipation. As shown in Fig. 1, a typical BRB is composed of a core segment, de-bonding material and a buckling restraining mechanism [1,2]. The cross section of the core segment is usually reduced along the length to constrain yielding to a limited domain. The length of the yielding segment can be adjusted to meet the stiffness requirements. The axial load resistance and axial strain capacity of BRBs are the two most important parameters that should be determined at the design stage. BRB manufacturers use these key parameters to develop designs that meet these objectives. The AISC Seismic Provisions for Structural Steel Buildings (AISC 341) [3] provide guidance on the expected BRB deformation demands. According to this specification, BRBs shall be designed, tested and detailed to accommodate deformations corresponding to a story drift of at least 2% of the story height or two times the design story drift, whichever is larger. Qualifying cyclic tests for BRBs also employ the expected deformation demand as the main parameter for the loading protocol. In addition, individual brace test specimens are required to achieve a cumulative inelastic axial deformation of at least 200 times the yield deformation.