DAMAGE EVALUATION OF BEAM-ENDS IN CONCENTRICALLY BRACED STEEL FRAMES DURING LONG-PERIOD AND LONG-DURATION GROUND MOTIONS

Abstract

 1. Introduction
 The long period and long duration ground motions of the 2011 Tohoku Earthquake caused large deformations in tall (> 80 m) braced steel moment frames. Many engineers and researchers have claimed the damage observed at the beam-ends in these high-rise buildings. Thus, one of urgent tasks is to develop a simple method for screening vulnerability of the beam-ends in braced steel moment frames. Some researchers have proposed several evaluation methods to capture damages at the beam-ends. In this paper, the validity of those methods is investigated, considering fracture of braces, and a simple is proposed that does not require time consuming time history response analysis.
 2. Response of Braced Steel Moment Frames under Long Period and Long Duration Ground Motion
 First, an approximately 85 m height building model is created to examine the accuracy of the proposed method for evaluation of the beam-end damage considering the brace fracture. 10 long period and long duration artificial ground motions, scaled to 3 kinds of velocity spectrums: 160,120,80 cm/s, and 2 observed ground motions, scaled to the velocity spectrum of Building Code in Japan, are adopted as input ground motions. This analysis suggests that the damage distribution of the beam-ends is similar to the story drift distribution. Observation of previous analysis shows that damage of the beam-ends with a weld access hole is frequently larger than those with no hole.
 3. Accuracy of Proposed Damage Evaluation Method by Previous Researcher
 The accuracy of a damage evaluation method using Manson-Coffin rule previously proposed by a researcher (Manson-Coffin method) is investigated. Miner's rule is used for the evaluation of the beam-end damage in the method, which is largely consistent with static testing results. Another evaluation method using local strain at a beam-end proposed by the authors is fitted to the Manson-Coffin method to improve the accuracy.
 4. Reference Ductility Ratio for Damage Evaluation
 The number of cycles at each ductility ratio is required to evaluate the damage of the beam-ends using the Miner's rule. This method involves running a time history analysis program. Several ductility ratios are introduced as a reference to reduce calculation time for the damage evaluation of the beam-end. Damage values of the beam-end calculated by one of the reference ductility ratios are consistent with those by the time history response results within a margin of error of plus or minus 30%.
 5. Index for Screening Vulnerability of Beam-ends in Braced Steel Moment Frames
 The energy ratio is introduced, defined as the earthquake energy spectrum VE to the maximum velocity spectrum S_v. For R_E = 2.0 - 3.5 and a maximum velocity is close to 160 cm/s, the beam-end fracture is likely. R_E is promising an index to determine the damage of the beam-end.
 6. Conclusions
 This research investigated the damage evaluation of beam-ends in braced steel moment frames during a long period and long duration motion. The results are summarized as follows.
 1) The damage distribution of the beam-ends is likely to be similar to the story drift distribution. The damage of the beam-end with a weld access hole is frequently larger than that with no hole.
 2) Damage values of the beam-ends calculated by one of the reference ductility ratios are consistent with those by the time history response results with a margin of error of plus or minus 30%.
 3) When the maximum velocity is close to 160 cm/s and the R_E ranges from 2.0 to 3.5, the beam-ends are likely fracture.