强震下斜交桥地震反应与减隔震性能分析

针对斜交桥在破坏性地震中发生破坏和损伤的突出问题,采用铅芯橡胶支座(LRB)进行隔震和滞回耗能。基于OpenSees平台建立了不同斜度的传统非隔震和全桥采用LRB隔震的4跨斜交连续梁桥动力分析模型,沿2个水平方向输入远场地震动和具有向前方向性效应、滑冲效应以及无速度脉冲效应的近断层地震动,并进行非线性时程计算,研究桥墩和挡块的损伤状态、主梁旋转度、碰撞力与斜交桥斜度的关系以及LRB对斜交桥抗震性能的影响。结果表明:向前方向性效应和滑冲效应的脉冲型地震动作用下的斜交桥地震反应和损伤明显大于无速度脉冲近断层和远场地震动作用; 采用LRB隔震后,明显降低了固定墩的地震损伤,桥墩位移减震率可达到50%以上; LRB隔震桥主梁与挡块的间隙宜结合桥梁的地震风险和设计位移进行确定。     

近场地震下简支梁桥搭接长度需求分析

随着交通网络向偏远地区辐射,桥梁结构会不可避免的出现靠近断层的情况,受到近断层地震动的影响显著。为了研究近场地震作用下简支梁桥的搭接长度需求,基于ANSYS平台建立了简支梁桥的弹塑性分析模型。通过动力响应分析、弹塑性分析和抗剪能力分析,探讨了近场地震动的脉冲效应对简支梁桥搭接长度的影响。研究表明：近场地震动的脉冲效应放大了结构的地震响应,脉冲型地震和非脉冲地震作用时的最大墩梁相对位移分别为115.1 cm和41.5 cm;两类地震作用时的最大剪力分别为1 892.5 kN和1737.8 kN,最大剪力剪切强度比为0.82;当PGA≥0.4 g时,脉冲型地震作用下桥墩底部塑性铰区的最大转角超过极限转角,桥墩发生破坏,而非脉冲型地震PGA=1.0 g时的塑性铰转角小于极限转角;当PGA=1.0 g时,塑性铰破坏前墩梁相对位移为37 cm,占规范搭接长度的41.8%,近场地震作用下简支梁桥的搭接长度需求满足规范要求。     

近断层地震下在役简支梁桥横向抗震性能分析

为了研究材料劣化对近断层区简支梁桥横向抗震性能的影响,揭示支座+挡块组合隔震效果随桥梁服役时间的变化规律,以汶川地震中某实际桥例为背景,考虑钢筋锈蚀和混凝土碳化的时变特点,建立了桥例非线性时变分析模型,采用时程分析研究了桥梁不同构件的劣化损伤规律,并对挡块强度的时变影响规律进行了参数分析。结果表明:随着服役年限的增长,墩柱抗弯能力不断减小,损伤程度不断加剧;材料劣化提高了挡块的相对限位能力,使得支座位移下降,但由于墩柱损伤加剧,桥梁整体抗震性能下降;挡块强度越大,其限位能力越强,但会降低支座的"隔震"效果,增大墩柱塑性变形,且服役时间越长,挡块强度的增大对墩柱越不利。当挡块强度取20%～30%支反力时,不同服役年限下的支座和主梁位移都能得到有效的控制,而桥墩也处于可修复的损伤状态,对于桥例而言是合理强度取值,且与美国Caltrans规范吻合。     

点震源作用下三维沉积盆地地震动谱元模拟

基于高精度谱元法(SEM),以半空间中椭球形沉积盆地为例,研究了点震源作用下三维沉积盆地的地震响应。研究结果表明:沉积盆地对地震动幅值有显著的放大效应,对地震持时有明显的增长效应,地震动空间分布规律受控于震源频率和近断层倾角等因素。主要体现为以下几点:(1)震源函数主频率较高时,沉积盆地内部出现多个波峰波谷,局部聚焦增强,放大效应显著,如震源频率为2Hz时在沉积盆地中心位置的加速度峰值可达1Hz时的5.6倍;(2)随着断层倾角的增大,断层上盘效应逐渐减弱,地表加速度的峰值在逐渐减小,30°倾角断层沉积盆地中心的加速度达1.96m/s^2,是90°倾角断层加速度值的5.2倍;(3)震源频率和断层倾角对位移的影响没有对加速度的显著。     

Seismic protection of rocking structures with inerters

The seismic behaviour of a wide variety of structures can be characterized by the rocking response of rigid blocks. Nevertheless, suitable seismic control strategies are presently limited and consist mostly on preventing rocking motion all together, which may induce undesirable stress concentrations and lead to impractical interventions. In this paper, we investigate the potential advantages of using supplemental rotational inertia to mitigate the effects of earthquakes on rocking structures. The newly proposed strategy employs inerters, which are mechanical devices that develop resisting forces proportional to the relative acceleration between their terminals and can be combined with a clutch to ensure their rotational inertia is only employed to oppose the motion. We demonstrate that the inclusion of the inerter effectively reduces the frequency parameter of the block, resulting in lower rotation seismic demands and enhanced stability due to the well-known size effects of the rocking behaviour. The effects of the inerter and inerter-clutch devices on the response scaling and similarity are also studied. An examination of their overturning fragility functions reveals that inerter-equipped structures experience reduced probabilities of overturning in comparison with uncontrolled bodies, while the addition of a clutch further improves their seismic stability. The concept advanced in this paper is particularly attractive for the protection of rocking bodies as it opens the possibility of nonlocally modifying the dynamic response of rocking structures without altering their geometry.     

近断层强地震动预测中的有限断层震源模型

提出了近断层强地震动预测中建立活断层上设定地震有限断层震源模型的方法和步骤.首先,根据地震地质和地震活动性调查以及地球物理勘探等资料,确定活断层的空间方位和滑动类型; 然后,根据地震定标律确定活断层的宏观震源参数; 第三,将高强体模型与k平方滑动模型相结合,产生断层破裂面上的混合滑动分布.在此基础上,预测了与1994年Northridge地震断层类型、矩震级(M_w6.7)基本一致的设定地震的有限断层震源模型.最后,将预测的有限断层震源模型与基于地震学的、使用动力学拐角频率的地震动随机合成方法相结合,预测了1994年Northridge地震近断层12个基岩台站的加速度时程,并和实际记录进行了对比.结果表明,用上述方法和步骤建立的有限断层震源模型是可行、实用的.      

Seismic performance of bridges with novel SMA cable-restrained high damping rubber bearings against near-fault ground motions

This study presents a novel type of shape memory alloy (SMA) cable-restrained high damping rubber (SMA-HDR) bearing, which is particularly suited to near-fault (NF) regions where the pulsing effect potentially exists in the ground motions. The working mechanism of the bearing is first described, followed by an experimental investigation on a full-scale SMA-HDR bearing specimen. The test results confirm the efficient restraining effect offered by the SMA cables, which contribute to 65% and 24.4% of the lateral load resistance and total energy dissipation, respectively, prior to the initial fracture of the SMA cables. The failure of the cables is initiated near the end grip where moderate stress concentration exists at this region. Following the experimental study, the numerical modeling strategy for the bearing is discussed, and a case study is then presented, demonstrating the application of the SMA-HDR bearings in the Datianba #2 highway bridge, a real project that first adopts the proposed bearings in the world. A simplified design process is introduced for the bridge with novel SMA-HDR bearings to mitigate the potential damage during strong earthquakes especially the NF ones. The system-level analysis on the prototype bridge shows that the novel SMA-HDR bearings equipped with ten 7×7×1.2 SMA cables in each bearing could reduce the average maximum bearing displacement (MBD) by nearly 30% compared with the conventional bridge with HDR bearings. The application of the novel SMA-HDR bearing can significantly alleviate the pounding effect, especially under the NF earthquakes. The presence of the SMA cables tends to increase the maximum force response of the piers, but this effect is minor and under control.     

Neural network analysis of overturning response under near-fault type excitation

Under strong seismic excitation, a rigid block will uplift from its support and undergo rocking oscillations which may lead to （complete） overturning. Numerical and analytical solutions to this highly nonlinear vibration problem are first highlighted in the paper and then utilized to demonstrate how sensitive the overturning behavior is not only to the intensity and frequency content of the base motion, but also to thc presence of strong pulses, to their detailed sequence, and even to their asymnletry. Five idealised pulses capable of representing ＂rupture-directivity＂ and ＂fling＂ affected ground motions near the fault, are utilized to this end ： the one-cycle sinus, the one-cycle cosinus, the Ricker wavelet, the truncated （T）-Ricker wavelet, and the rectangular pulse ＂Overturning-Acceleration Amplification＂ and ＂Rotation＂ spectra are introduced and presented. Artificial neural network modeling is then developed as an alternative numerical solution. The neural network analysis leads to closed-form expressions for predicting the overturning failure or survival of a rigid block, as a function of its geometric properties and the characteristics of the excitation time history. The capability of the developed neural network modeling is validated through comparisons with the numerical solution. The derived analytical expressions could also serve as a tool for assessing the destructiveness of near-fault ground motions, for structures sensitive to rocking with foundation uplift.     

Response spectrum of seismic design code for zones lack of near-fault strong earth- quake records

It was shown from the study on the recently near-fault earthquake ground motions that the near-fault effects were seldom considered in the existing Chinese seismic code. Referring to the UBC97 design concept for near-fault factors, based on the collected world-widely free-site records of near-fault earthquakes ground motions classified by earthquake magnitude and site condition, the attenuation relationship expressions of the acceleration spectrum demand at the key points within the long period and moderate period were established in term of the earthquake magnitude and the site condition. Furthermore, the near-fault factors’ expressions about the earthquake magnitude and the fault distance were deduced for the area lack of near-fault strong earthquake records. Based on the current Chinese Building Seismic Design Code, the near-fault effect factors and the modified design spectral curves, which were valuable for the seismic design, were proposed to analyze the seismic response of structures.     

Design spectra including effect of rupture directivity in near-fault region

In order to propose a seismic design spectrum that includes the effect of rupture directivity in the near-fault region, this study investigates the application of equivalent pulses to the parameter attenuation relationships developed for near-fault, forward-directivity motions. Near-fault ground motions are represented by equivalent pulses with different waveforms defined by a small number of parameters （peak acceleration, A, and velocity V; and pulse period, Tv）. Dimensionless ratios between these parameters （e.g., ATv/V, VTv/D） and response spectral shapes and amplitudes are examined for different pulses to gain insight on their dependence on basic pulse waveforms. Ratios of ATv/V, VTv/D, and the ratio of pulse period to the period for peak spectral velocity （Tv-p） are utilized to quantify the difference between rock and soil sites for near-fault forward-directivity ground motions. The ATv/Vratio of recorded near-fault motions is substantially larger for rock sites than that for soil sites, while Tvp/Tv ratios are smaller at rock sites than at soil sites. Furthermore, using simple pulses and available predictive relationships for the pulse parameters, a preliminary model for the design acceleration response spectra for the near-fault region that includes the dependence on magnitude, rupture distance, and local site conditions are developed.