基于强震记录的长周期位移反应谱特征分析

Characteristics of long-period displacement spectra based on strong ground-motion records

高层建筑、大跨结构和大型储液罐等结构在长周期成分丰富的地震动作用下会产生大变形甚至破坏。基于位移的抗震设计相较于基于力的抗震设计更能反映结构和构件的性态,但其亟需可靠的长周期位移设计谱。为此,选取近年来国内外23次地震的数字强震记录1 787条,分析了不同震级和断层距下长周期位移反应谱(简称位移谱)特性,研究了场地条件对于位移谱的影响,最后对现有位移谱模型的适用性和可靠性进行评价。主要结论包括:震级和断层距主要影响位移谱的幅值;位移谱的形态与震级和断层距基本无关,表现为随着周期的增大位移谱先快速增长,随后缓慢增长,之后趋于稳定或稍微下降至地表峰值位移(PGD)。采用PGD归一化的位移谱形态可由3个拐点周期划分为4个阶段,即快速上升段、缓慢上升段、下降段和稳定段。震级对以PGD归一化的位移谱的影响明显,随着震级的增大,归一化位移谱幅值增加,卓越周期变长;小震级(矩震级M_(w)≤6.5)时,卓越周期为1~2 s;大震级(矩震级M_(w)>6.5)时,卓越周期为6~10 s,甚至更大。场地条件对位移谱的影响与断层距有关,近断层时,场地越硬位移谱幅值在中长周期段(如周期T>5.0 s)越低,但断层距大于100 km时,位移谱与场地条件基本无关;场地条件对归一化位移谱的影响基本可忽略。现有归一化位移谱模型尚存在一定局限:曹加良模型未考虑震级对位移谱模式的影响,且模型不能模拟位移谱在长周期段下降并至PGD的过程;Faccioli模型对于矩震级M_(w)≥6.5时位移谱特征认识不够,10 s周期时位移谱并未达到峰值。

High-rise buildings, long-span structures, and large liquid storage tanks will suffer substantial deformation or even damage under the long-period ground-motions. Displacement-based seismic design, comparing to force-based seismic design, describes the structural response and the damage more explicitly, but needs reliable displacement spectra in long-periods. Therefore, using selected sets of 1 787 high-quality digital strong-motion records from 23 recent large earthquakes, the characteristics of displacement spectra with various magnitudes, rupture distances, and site conditions were analyzed. The reliability and feasibility of available displacement spectrum models were assessed based on the findings herein. The major results show that the amplitudes of displacement spectra are mainly affected by the magnitude and rupture distance, to which the spectrum shapes are insensitive. With increasing period, displacement spectra start to increase rapidly, then grow steadily, and finally stay constant or slightly reduce to peak ground displacement(PGD). The normalized displacement spectra can be divided into four stages by three key periods, including rapidly increasing stage followed by steady growth stage, and then descending stage and finally staying steady stage around PGD. The magnitude plays a major role in the amplitudes of PGD-normalized displacement spectra as well as predominant periods. The amplitude and predominant period increase with increasing magnitude.When magnitude M_(w)≤6.5, the predominant periods remain 1-2 s, while M_(w)>6.5, the periods mainly range in 6-10 s or even greater. The effect of site conditions on normalized displacement spectra depends on rupture distance. When the rupture distance is short, the spectral displacements on stiff sites are less than that on soft sites in medium to long period range(e.g., period T>5 s), while the rupture distance exceeds 100 km, displacement spectra show no dependence on site conditions. The normalized spectra, however, keep no considerable trend with site conditions. The normalized displacement spectrum models available exhibits drawbacks comparing to observations, e.g., the model proposed by Cao takes little account of magnitude dependence, and can barely simulate the mode of displacement spectrum decreasing to PGD in long period range. The model proposed by Faccioli is not suitable to predict displacements of magnitude M_(w)≥6.5 due to the limited number of records from large earthquakes during model-building, of which the peak value of the displacement spectra is not reached at 10 s period.