Evaluation of building period formulas for seismic design

Building period formulas in seismic design code are evaluated with over 800 apparent building periods from 191 building stations and 67 earthquake events. The evaluation is carried out with the formulas in ASCE 7‐05 for steel and RC moment‐resisting frames, shear wall buildings, braced frames, and other structural types. Qualitative comparison of measured periods and periods calculated from the code formulas shows that the formula for steel moment‐resisting frames generally predicts well the lower bound of the measured periods for all building heights. But the differences between the periods from code formula and measured periods of low‐ to‐medium rise buildings are relatively high. In addition, the periods of essential buildings designed with the importance factor are about 40% shorter than the periods of non‐essential buildings. The code formula for RC moment‐resisting frames describes well the lower bound of measured periods. The formula for braced frames accurately predicts the lower bound periods of low‐to‐medium rise buildings. The formula for shear wall buildings overestimates periods for all building heights. For buildings that are classified as other structural types, the measured building periods can be much shorter than the periods calculated with the code formula. Based on these observations, it is suggested to use C_r factor of 0.015 for shear walls and other structural types. Copyright © 2010 John Wiley & Sons, Ltd.     

Seismic response of Beirut (Lebanon) buildings: instrumental results from ambient vibrations

Resonance period is a key parameter in the seismic design of a structure, thus dynamic parameters of buildings in Beirut (Lebanon) were investigated based on ambient vibration method for risk and vulnerability assessment. Lebanon is facing high seismic hazard due to its major faults, combined to a high seismic risk caused by dense urbanization in addition to the lack of a seismic design code implementation. For this study, ambient vibration recordings have been performed on 330 RC buildings, period parameters extracted and statistically analyzed to identify correlations with physical building parameters (height, horizontal dimensions, age) and site characteristics (rock sites or soft sites). The study shows that (1) the building height or number of floors (N) is the primary statistically robust parameter for the estimation of the fundamental period T; (2) the correlation between T and N is linear and site dependent: T ≈ N/23 for rock sites and N/18 for soft sites; (3) the measured damping is inversely proportional to the period: the taller the building the lower is the damping; (4) a significant overestimation of the period exists in current building codes. However part of the large discrepancy with building code recommendations may be due to the very low level of loading.     

基于地脉动和地铁振动的钢筋混凝土建筑结构响应分析

在北京城区的一栋钢筋混凝土建筑(Reinforced Concrete building,简称RC)中,进行历时两天的地脉动和地铁振动观测.介绍了利用地脉动和地铁振动信号研究RC建筑结构响应的观测方法、仪器设备、数据采集和数据处理方法.对观测数据进行两种分析:(1)对连续的地脉动背景噪声,采用H/V谱比法;(2)对经过...     

Comparison between seismic vulnerability models and experimental dynamic properties of existing buildings in France

Elastic fundamental frequency is a key-parameter of simplified seismic design and vulnerability assessment methods. Empirical relationships exist in codes to estimate this frequency but they miss experimental data to validate them accounting for national feature of building design and, above all, corresponding uncertainties. Even if resonance frequency extracted from ambient vibrations may be larger than the elastic frequency (at yield) generally used in earthquake engineering, ambient vibration recordings may provide a large set of data for statistical analysis of periods versus building characteristics relationships. We recorded ambient vibrations and estimated the fundamental frequency of about 60 buildings of various types (RC and masonry) in Grenoble City (France). These data complete the set existing yet, made of 26 RC-buildings of Grenoble (Farsi and Bard 2004) and 28 buildings in Nice (France) (Dunand 2005). Statistical analysis of these experimental data was performed for fundamental frequencies of RC shear wall structures and the results are compared with existing relationships. Only building height or number of stories has a statistical relevancy to estimate the resonance frequency but the variability associated to the proposed relationships is large. Moreover, we compared the elastic part of capacity curves of RC and masonry buildings used in the European Risk-UE method for vulnerability assessment with the experimental frequencies. The variability is also large and the curves may not be consistent with French existing buildings.     

Empirical relationships between natural vibration period and height of buildings in Singapore

Singapore is a classic case of a modern metropolis with low hazard but high exposure to the seismicity in Sumatra. Because of land shortage, more than 80% of the population lives in high‐rise residential buildings. As part of the efforts to assess the seismic performance of buildings in Singapore subjected to long‐distance Sumatran earthquakes, relationships between the natural vibration period and height of high‐rise public residential buildings in Singapore are derived empirically by conducting ambient vibration tests on 116 buildings. The measured buildings have a height ranging from 4 to 30 stories. The aspect ratio of buildings in plan is found to be insignificant in affecting the natural vibration period of the first mode of the buildings. The period‐height relationships are derived using regression analysis considering the site properties of a building. It is concluded that the vibration periods estimated from the proposed period‐height relationship for buildings located at soft‐soil site are about 40% longer than the vibration periods estimated for buildings located at firm‐soil site. Measurements are also conducted to study the influence of buildings on the measured frequency of the surrounding soil. For this purpose, two buildings with 25 and 30 stories located at firm‐soil site and soft‐soil site, respectively, are selected. It is found that the distance of building influence on the measured frequency of the surrounding soil may reach up to one building height for a firm‐soil site and two building heights for a soft‐soil site. Additional data of natural vibration periods of 19 instrumented residential buildings, which have height ranging from 9 to 30 stories, were obtained from the building response recorded during the September 30, 2009 Sumatran earthquake event. The natural vibration periods of these buildings are compared with those estimated using the proposed period‐height relationships, and the absolute differences are found to be less than 12%. Copyright © 2013 John Wiley & Sons, Ltd.     

Influence of masonry infill panels on the vibration and stiffness characteristics of R/C frame buildings

Vibration measurements were performed on two adjacent, three-storey reinforced concrete frame buildings with hollow clay brick infill panels. The first building was a bare frame and the second one was a similar frame infilled with brick panels. The fundamental period for the infilled frame building was much smaller than that of the bare frame building. Using shear beam lumped mass models and the vibration data the actual lateral stiffness of both buildings was identified. The lateral stiffness of the infilled frame building was found to be seven times that of the bare frame building. Four numerical models of the infilled frame building were constructed. The frame and floors were represented using an experimentally validated model and the infill panels by one of three commonly used ‘equivalent diagonal truss’ models or by plane stress finite elements. Only the plane stress finite element model produced a reasonable agreement with the experimental results. Copyright © 1999 John Wiley & Sons, Ltd.     

Earthquake performance assessment and rehabilitation of two historical unreinforced masonry buildings

The paper describes the earthquake performance assessment of two historical buildings located in Istanbul exposed to a M_w = 7+ earthquake expected to hit the city and proposes solutions for their structural rehabilitation and/or strengthening. Both buildings are unreinforced clay brick masonry (URM) structures built in 1869 and 1885, respectively. The first building is a rectangular-shaped structure rising on four floors. The second one is L-shaped with one basement and three normal floors above ground. They survived the 1894, M_s = 7.0 Istanbul Earthquake, during which widespread damage to URM buildings took place in the city. Earthquake ground motion to be used in performance assessment and retrofit design is determined through probabilistic and deterministic seismic hazard assessment. Strength characteristics of the brick walls are assessed on the basis of Schmidt hammer test results and information reported in the literature. Dynamic properties of the buildings (fundamental vibration periods) are measured via ambient vibration tests. The buildings are modelled and analyzed as three-dimensional assembly of finite elements. Following the preliminary assessment based on the equivalent earthquake loads method, the dynamic analysis procedure of FEMA 356 (Pre-standard and commentary for the seismic rehabilitation of buildings, American Society of Civil Engineers, Reston, 2000) and ASCE/SEI 41-06 (Seismic rehabilitation of existing buildings, American Society of Civil Engineers, Reston, 2007) is followed to obtain dynamic structural response of the buildings and to evaluate their earthquake performance. In order to improve earthquake resistance of the buildings, reinforced cement jacketing of the main load carrying walls and application of fiber reinforced polymer bands to the secondary walls are proposed.     

Fundamental periods of vibration of RC buildings in Portugal from in-situ experimental and numerical techniques

Since the early nineteen seventies we have been measuring the in-situ dynamic characteristics of the different structures built in Portugal, essentially based on ambient vibration and using expedite techniques. A data-base containing not only the fundamental dynamic characteristics of those structures but also their most important geometric and constructive properties has been created with the aim of setting correlations between construction typologies and fundamental periods or frequencies, and damping characteristics, and calibrate numerical modelling of those structures. This paper presents the main results for circa 197 reinforced concrete (RC) buildings, obtaining the fundamental period as a linear function of height or number of storeys for different typologies and situations, and showing that numerical models, made for a number of illustrative cases, can reproduce with great accuracy the in-situ measurements. The main parameters having remarkable influence on the overall correlation laws are identified and a measure of uncertainty deduced. Comparisons with published formulae for other regions of the world show that we can group these laws by regions with similar expression within each group but with large variations from group to group. Discussion on how to deal with the elongation of the periods of vibration due to moderate and large amplitude motion, causing changes in the seismic behaviour and on appearance of damage, will also be briefly introduced, keeping in mind current code practices.     

Theoretical-period characteristics of frame buildings designed under variable levels of seismicity and allowable-drift

Period equations provided by seismic codes are generally derived by calibration with recorded period data of buildings located in high seismicity regions. These period-equations do not account for either the design level of seismicity or the permissible limit of lateral drift. Buildings designed under high level of seismicity are expected to display higher stiffness and shorter periods than buildings designed under low-to-medium seismicity levels. In addition, buildings designed with high level of permissible lateral-drift are expected to display lower stiffness and longer periods than buildings designed with low level of permissible lateral-drift. In this study, the theoretical fundamental-periods of an ensemble of regular reinforced concrete(RC) and steel moment-resisting frame(MRF) buildings having 3-, 6-, 9-and 12-stories and designed with various levels of seismicity and permissible lateral-drift are evaluated. The obtained outcome indicates the sensitivity of the MRF theoretical-periods to the design seismicity and the permissible lateral-drift. The results also indicate the need for modifying the current period equations to realistically account for the variations in the design seismicity and allowable lateral-drift of MRF buildings.     

地铁列车曲线运行引起学校建筑物振动响应分析

地铁以其快捷、准时、运量大等优点,已成为重要的轨道交通形式,但由此引起的环境振动问题日益突出。针对杭州市地铁3号线曲线地段的某中学建设工程,利用有限元软件ABAQUS,对车辆-普通整体道床轨道系统的竖向耦合模型进行振动响应分析,得到考虑轨道高低不平顺影响的轨道振动源强。应用有限元软件MIDAS GTS/NX,建立双孔平行曲线盾构隧道-土-桩-建筑物系统的三维有限元模型。以轨道支点力作为激励对地铁列车运行时的隧道-土-桩-建筑物系统的振动响应进行计算,研究地铁振动波在地层中的传播规律和建筑物的动力响应特性。根据相关环境振动控制标准对建筑物的振动舒适性进行评价。结果表明：轨道加速度和扣件动支点力的最大值分别约为40 m/s²和30 kN;地层和建筑物的振动以竖向为主,水平Y向振动略大于水平X向振动;地面加速度随着距隧道中心线距离的增加而逐渐衰减;各建筑物楼层的振动主频位于16~40 Hz;部分建筑物楼层的振动响应水平已超出了规范的限值要求,建议对地铁轨道或建筑物采取适当的减振措施。     

北京市防震减灾中心结构地震反应观测及振动特性识别

以北京市防震减灾中心结构为例,进行了结构的地震反应观测及振动特性识别研究。基于半功率带宽法对结构脉动测试数据进行了分析,计算出了结构自振周期和振型以及相应的阻尼比。各振型都具有较大幅值的楼层分别位于3、6和8层,据此设计并建成了结构地震反应观测台阵。分析台阵地震记录识别出了结构的自振特性,同时功率谱曲线显示,井下数据存在高频特性,初步判定其来自钢套管振动。开展结构的数值模拟分析,利用观测楼层上地震记录的卓越频率不断修正结构数值模型,直至结构反应与地震记录的频率值相符,且与脉动测试数据基本一致,相对位移对比分析发现,在该模型基础上相对位移反应与相对位移记录基本吻合。结构地震反应观测和数值模拟分析较好地实现了结构地震反应观测台阵的观测目的。     

汶川地震近断层地震动作用下 结构地震响应特征分析

研究了具有不同自振特性的建筑结构在近断层速度脉冲型及非速度脉冲型地震动作用下的结构层间变形分布,揭示了近断层速度脉冲对工程结构地震响应的特殊影响. 从汶川M_S8.0地震近断层强震记录中选取两组典型速度脉冲型记录和非脉冲型记录, 根据确定的目标地震动强度水平,利用时域叠加小波函数法对选择的强震记录进行调整, 使之与目标地震动水平对应的加速度反应谱保持一致, 以此作为结构地震反应分析的地震动输入. 选取具有不同自振特征的3层、11层和20层典型钢筋混凝土框架结构, 建立有限元分析模型, 分别计算在速度脉冲型与非速度脉冲型记录作用下这些结构层间变形分布. 研究表明,速度脉冲型记录与非速度脉冲型记录作用下结构层间变形有明显差异, 且与结构自振特征有关.就低层结构的层间变形而言, 非速度脉冲型记录的影响较速度脉冲型记录的影响大. 随着结构自振周期的增加, 高阶振型的影响更加明显. 与非速度脉冲型记录相比,速度脉冲型记录的结构层间位移反应中值及离散程度较大. 速度脉冲型记录更容易激发高层结构的高阶振型, 产生较大的层间位移反应. 非速度脉冲型记录对中低层结构层间变形影响较大.因此, 在开展近断层结构地震影响评价时, 应考虑近断层速度脉冲的影响.      

Seismic vibration control of building structures with multiple tuned mass damper floors integrated

Floor isolation system (FIS) achieving very small floor accelerations has been used to ensure human comfortability or protect important equipments in buildings. Tuned mass damper (TMD) with large mass ratios has been demonstrated to be robust with respect to the changes in structural properties. This paper presents the concept of a TMD floor vibration control system, which takes advantages of both the FIS and TMD. Such a system is called ‘TMD floor system’ herein. The TMD floor system (TMDFS) in which building floors serve as TMDs can achieve large mass ratio without additional masses. Furthermore, multiple TMD floors installed in a building can control multimode vibrations. Then, an optimal design process, where the objective function is set as the maximum magnitude of the frequency response functions of inter‐storey drifts, is proposed to determine the TMD floor parameters. Additionally, the multimode approach is applied to determine the optimal locations of TMD floors if not all of the floors in a building can serve as TMDs. In addition to the numerical simulations, a scaled model shaking table experiment is also conducted. Both the numerical and experimental results show that the absolute accelerations of the TMD floors are smaller than those of the main structural storeys, which indicates the TMDFS maintains the merit of FIS while greatly reducing seismic responses of main structures. Copyright © 2013 John Wiley & Sons, Ltd.     

Application of subspace identification technique to long‐term seismic response monitoring of structures

This paper addresses the issue of structural system identification using earthquake‐induced structural response. The proposed methodology is based on the subspace identification algorithm to perform identification of structural dynamic characteristics using input–output seismic response data. Incorporated with subspace identification algorithm, a scheme to remove spurious modes is also used to identify real system poles. The efficiency of the proposed method is shown by the analysis of all measurement data from all measurement directly. The recorded seismic response data of three structures (one 7‐story RC building, one midisolation building, and one isolated bridge), under Taiwan Strong Motion Instrumentation Program, are analyzed during the past 15 years. The results present the variation of the identified fundamental modal frequencies and damping ratios from all the recorded seismic events that these three structures had encountered during their service life. Seismic assessment of the structures from the identified system dynamic characteristics during the period of their service is discussed. Copyright © 2014 John Wiley & Sons, Ltd.     

Shear building representations of seismically isolated buildings

Seismic isolation, with its capability of reducing floor accelerations and interstory drifts simultaneously, is recognized as an earthquake resistant design method that protects contents of a building along with the building itself. In research studies, superstructures of seismically isolated buildings are commonly modeled as idealized shear buildings. Shear building representation corresponds to an idealized structure where the beams are infinitely stiff in flexure and axially inextensible; columns are axially inextensible; and rigid floors are supported on these columns. Although it is more convenient to model and analyze a shear building, such an idealization may influence the seismic responses of seismically isolated buildings. This study presents a comparison of the seismic performances of seismically isolated buildings with superstructures modeled as shear buildings to those with full three dimensional superstructures. Both linear and nonlinear base isolation systems with different isolation periods and superstructures with different number of stories are considered.     

Accidental torsion in buildings due to stiffness uncertainty

Discrepancies between the computed and actual values of the structural element stiffness imply that a building with nominally symmetric plan is actually asymmetric to some unknown degree and will undergo torsional vibration when subjected to purely translational ground motion. Such accidental torsion leads to increase in structural element deformations which is shown to be essentially insensitive to the uncoupled lateral vibration period of the system but is affected strongly by the ratio of uncoupled lateral and torsional vibration periods. The structural deformations increase, in the mean, by at most 10 and 5 per cent for R/C and steel buildings, respectively, and by much smaller amounts for a wide range of system parameters. The increase in structural deformations due to stiffness uncertainty is shown to be much smaller than implied by the accidental torsional provisions in the Uniform Building Code and most other building codes.     

Experimental Investigations on Laminated Rubber Bearings

Increasing application of base isolation as a seismic protection method has subsequently increased its analytical and experimental studies. Being the most critical part of the base isolated buildings, accurate evaluation of structural properties and precise modeling of isolation devices is of utmost importance in predicting the response of the buildings during the earthquakes. This technical note is concerned with experimental study on laminated rubber bearings. Free vibration and harmonic base excitation tests are performed on a three-storey building model to evaluate the properties of the structure, efficiency of the system, and effect of base excitations. It is found that these experimental methods can effectively be used for this purpose.     

Seismic Response of Adjacent Buildings Connected with Friction Dampers

The effectiveness of passive energy dissipation systems to improve seismic performance of connected buildings is now well established through extensive analytical and experimental investigations. However, the performance of buildings connected with friction dampers has not been looked into. In this paper, the investigation is carried out to study the structural responses of two adjacent buildings connected with friction dampers under various earthquake excitations. A formulation of the equations of motion for the two adjacent multi degree of freedom (MDOF) buildings connected with friction dampers is presented. The numerical study is carried out in two parts, namely (i) two adjacent MDOF buildings connected with friction dampers having same slip force in all the dampers and (ii) two adjacent MDOF buildings connected with friction dampers having different slip forces in the dampers. The effectiveness of the dampers in terms of the reduction of structural responses, namely, displacement, acceleration and shear forces of connected adjacent buildings is investigated. A parametric study is also conducted to investigate the optimum slip force of the dampers. In addition, the optimal placement of the dampers, rather than providing the dampers at all the floor levels is also studied to minimize the cost of the dampers. Results show that using friction dampers to connect the adjacent buildings of different fundamental frequencies can effectively reduce earthquake-induced responses of either building if slip force of the dampers is appropriately selected. Also, it is not necessary to connect the two adjacent buildings at all floors but lesser dampers at appropriate locations can significantly reduce the earthquake response of the combined system. Further, it is also observed that the reduction in the responses when the two MDOF buildings connected with 50% of the total dampers is almost as much as when they are connected at all the floors, thereby reducing the cost of the dampers significantly.     

The effect of near-fault directivity on building seismic collapse risk

Forward directivity may cause large velocity pulses in ground motion time histories that are damaging to buildings at sites close to faults, potentially increasing seismic collapse risk. This study quantifies the effects of forward directivity on collapse risk through incremental dynamic analysis of building simulation models that are capable of capturing the key aspects of strength and stiffness degradation associated with structural collapse. The paper also describes a method for incorporating the effects of near-fault directivity in probabilistic assessment of seismic collapse risk. The analysis is based on a suite of RC frame models that represent both past and present building code provisions, subjected to a database of near-fault, pulse-like ground motions with varying pulse periods. Results show that the predicted collapse capacity is strongly influenced by variations in pulse period and building ductility; pulse periods that are longer than the first-mode elastic building period tend to be the most damaging. A detailed assessment of seismic collapse risk shows that the predicted probability of collapse in 50 years for modern concrete buildings at a representative near-fault site is approximately 6%, which is significantly higher than the 1% probability in the far-field region targeted by current seismic design maps in the US. Copyright © 2012 John Wiley & Sons, Ltd.     

Description and analysis of building damage due to Pyrgos, Greece earthquake

On March 1993 an earthquake of magnitude M_s = 5·5 shook Pyrgos, a town in Western Peloponnissos, one of the most seismic prone areas in Greece. The damage induced to modern reinforced concrete buildings was rather light in contrast to the damage induced to historic and traditional buildings of adobe, stone or brick masonry which was severe. In order to study the causes of structural damage, detailed data are collected from a rather large statistical sample of 1023 masonry buildings and 22 reinforced concrete framed buildings with visible damage. For each building the number of storeys, the material of construction, as well as the type and the degree of damage are recorded. In addition, consideration is given to the site of the building within the town and the corresponding soil conditions. For reinforced concrete buildings, damage occured mostly in areas with relatively high estimated spectral accelerations and fundamental soil periods of vibration close to those of the buildings. Nevertheless, further analysis is required to explain the selective damage of a very small number of buildings. For masonry houses, the effect of soil conditions is more systematic. Moreover, the effects of the number of storeys as well as the age and material of construction appear to be dominant.