長周期地震動に対する中規模ブレース付鋼構造骨組の梁端部損傷度評価

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  • DAMAGE EVALUATION OF BEAM-ENDS IN CONCENTRICALLY BRACED STEEL FRAMES DURING LONG-PERIOD AND LONG-DURATION GROUND MOTIONS
  • チョウ シュウキ ジシンドウ ニ タイスル チュウキボ ブレース ツキコウ コウゾウ ホネグミ ノ リョウタンブ ソンショウド ヒョウカ

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 1. Introduction<br> 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.<br> 2. Response of Braced Steel Moment Frames under Long Period and Long Duration Ground Motion<br> 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.<br> 3. Accuracy of Proposed Damage Evaluation Method by Previous Researcher<br> 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.<br> 4. Reference Ductility Ratio for Damage Evaluation<br> 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%.<br> 5. Index for Screening Vulnerability of Beam-ends in Braced Steel Moment Frames<br> The energy ratio is introduced, defined as the earthquake energy spectrum VE to the maximum velocity spectrum Sv. For RE = 2.0 - 3.5 and a maximum velocity is close to 160 cm/s, the beam-end fracture is likely. RE is promising an index to determine the damage of the beam-end.<br> 6. Conclusions<br> 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.<br> 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.<br> 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%.<br> 3) When the maximum velocity is close to 160 cm/s and the RE ranges from 2.0 to 3.5, the beam-ends are likely fracture.

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