دانلود رایگان مقاله انگلیسی ارزیابی عوامل پاسخ لرزه ای برای BRBFs با استفاده از روش FEMA P695 - الزویر 2018

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.

Conclusions

A numerical study on seismic response factors for BRBFs has been presented. Twenty-four BRBF archetypes were designed according to the U.S. provisions using the current values of response factors recommended in ASCE 7. The FEMA P695 methodology was adopted for the evaluation process. Interstory drift ratio, brace axial strain and cumulative brace axial strain were used as performance metrics. The results showed that most of the frames performed satisfactorily when interstory drift and cumulative axial strains were considered. In general, the interstory drifts stayed below 5%, and the cumulative axial strains stayed below 200 times the yield strain, where these values can easily be accommodated by BRBFs. On the other hand, marked differences between the design axial strain and the axial strain demand for collapse level ground motions were reported. For most of the archetypes the axial strain demand was more than 2 times the axial strain capacity of the BRB. The differences are attributed to the underestimation of interstory drifts at the design stage and the differences between the demands produced by collapse level and DBE ground motions. A complementary study was conducted to investigate the deflection amplification for BRBFs. The results revealed that the amplification of deflections is non-uniform over the height of the building. The inelastic drifts were found to accumulate in the lower stories and the value of the deflection amplification factor (Cd) exceeded the current value of 5.0, which is recommended in the ASCE 7 standard. A Cd profile that varies over the height of the building was proposed which enables cost optimized solutions when compared with the use of a single valued deflection amplification factor.