Numerical study of characteristics of underground blast induced surface ground motion and their effect on above-ground structures. Part II. Effects on structural responses

Studies of structural responses and damage to high-frequency blast motion are very limited. Current practice uses some empirical allowable ground vibration limits in assessing structural performance. These empirical limits overlook the physical parameters that govern structural response and damage, such as the ground motion characteristics and inherent structural properties. This paper studies the response of RC frame structures to numerically simulated underground blast-induced ground motions. The structural response and damage characteristics of frame structures to ground motions of different frequencies are investigated first. The effects of blast ground motion spatial variations and soil–structure interaction on structural responses are also studied. A suitable discrete model that gives accurate response prediction is determined. A damage index defined based on the accumulated plastic hinge rotation is used to predict structural damage level. Numerical results indicated that both the low structural vibration modes (global modes) and the first elemental vibration mode (local) might govern the dynamic structural responses depending on the ground motion frequency and structural response parameters under consideration. Both ground motion spatial variations and soil–structure interaction effects are prominent. Neglecting them might yield inaccurate structural response prediction. The overall structural response and damage are highly ground motion frequency dependent. Numerical results of structural damage are also compared with some test results obtained in a previous study and with code specifications. Discussions on the adequacy of the code allowable ground vibration limits on RC frame structures are also made.     

Damage index based on stiffness degradation of low‐rise RC walls

Widely used damage indices, such as ductility and drift ratios, do not account for the influences of the duration of strong shaking, the cumulative inelastic deformation or energy dissipation in structures. In addition, the formulation and application of most damage indices have until now been based primarily on flexural modes of failure. However, evidence from earthquakes suggests that shear failure or combined shear‐flexure behavior is responsible for a large proportion of failures. Empirical considerations have been made in this paper for evaluating structural damage of low‐rise RC walls under earthquake ground motions by means of a new energy‐based low‐cycle fatigue damage index. The proposed empirical damage index is based on the results of an experimental program that comprised six shake table tests of RC solid walls and walls with openings; results of six companion walls tested under QS‐cyclic loading were used for comparison purposes. Variables studied were the wall geometry, type of concrete, web shear steel ratio, type of web shear reinforcement, and testing method. The index correlates the stiffness degradation and the destructiveness of the earthquake in terms of the duration and intensity of the ground motions. The stiffness degradation model considers simultaneously the increment of damage associated to the low‐cycle fatigue, energy dissipation, and the cumulative cyclic parameters, such as displacement demand and hysteretic energy dissipated. Copyright © 2014 John Wiley & Sons, Ltd.     

主余震序列型地震动作用下RC框架结构损伤耗能研究

强震发生后通常伴随着余震,余震的发生会加重结构损伤甚至引发倒塌。为了量化研究主余震序列型地震动对结构损伤的影响,以一栋3层钢筋混凝土框架结构为研究对象,选取了10条实际的主余震记录作为地震动输入,采用ABAQUS非线性有限元软件对该框架结构进行非线性动力时程分析,并依据结构局部和整体损伤耗能指标来评价主余震序列型地震动对框架结构累积损伤的影响。研究结果表明:序列地震会加剧结构底层柱的局部损伤耗能,特别是对底层中柱的影响更加明显;序列地震作用下的结构整体损伤耗能平均值相对于单主震作用下增加约30%;当余震与主震的第一周期谱加速度指标的比值Sa （T₁）_余震/Sa （T₁）_主震较大时,序列地震对结构损伤需求的影响更为显著。     

RC structures cyclic behavior simulation with a model integrating plasticity,damage, and bond‐slip

The behavior of reinforced concrete structures under severe demands, as strong ground motions, is highly complex; this is mainly due to the complexity of concrete behavior and to the strong interaction between concrete and steel, with several coupled failure modes. On the other hand, given the increasing awareness and concern on the worldwide seismic risk, new developments have arisen in earthquake engineering; nonetheless, some developments are mainly based on simple analytical tools that are widely used, given their moderate computational cost. This research aims to provide a solid basis for validation and calibration of such developments by using computationally efficient continuum mechanics‐based tools. Within this context, this paper presents a model for 3D simulation of cyclic behavior of RC structures. The model integrates a bond‐slip model developed by one of the authors and the damage variable evolution methodology for concrete damage plastic model developed by some authors. In the integrated model, a new technique is derived for efficient 3D analysis of bond‐slip of 2 or more crossing reinforcing bars in beam‐column joints, slabs, footings, pile caps, and other similar members. The analysis is performed by implementing the bond‐slip model in a user element subroutine of Abaqus and the damage variable evolution methodology in the original concrete damage plastic model in the package. Two laboratory experiments consisting of a column and a frame subjected to cyclic displacements up to failure are simulated with the proposed formulation.     

基于不同破坏指数的建筑物震害预测不确定性分析

结构地震破坏指数是将结构地震破坏程度进行量化的指标,其应用领域十分广泛。本文针对建筑结构震害预测工作,选取了7种典型的破坏指数,分别以5层和17层钢筋混凝土结构为模型,计算了在给定地震动作用下,结构模型对应于每个破坏指数的结构震害等级。计算结果表明:由于选取的破坏指数以及相应的破坏等级划分不同,建筑物的震害预测结果会存在较大的差异,尤其在接近设防烈度的地震作用下,建筑物的震害预测结果存在很大的不确定性。     

Numerical study of characteristics of underground blast induced surface ground motion and their effect on above-ground structures. Part I. Ground motion characteristics

Current codes of practice in assessing the blast ground motion effect on structures are mainly based on the ground peak particle velocity (PPV) or PPV and the principal frequency (PF) of the ground motion. PPV and PF of ground motion from underground explosions are usually estimated by empirical formulae derived from field blast tests. Not many empirical formulae for PF, but many empirical formulae for PPV are available in the literature. They were obtained from recorded data either on ground surface or in the free field (inside the geological medium). Owing to the effect of surface reflection, blast motions on ground surface and in the free field are very different. But not many publications in the open literature discuss the differences of blast motions on ground surface and in the free field. Moreover, very few publications discuss the blast ground motion spatial variation characteristics. As ground motion directly affects structural responses, it is very important to study its characteristics in order to more reliably assess its effects on structures. In this paper, a validated numerical model is used to simulate stress wave at a granite site owing to explosion in an underground chamber. Using the simulated stress wave, the relations such as PPV and PF attenuation as well as spatial variation of motions on ground surface and in the free field are derived. Discussions on the differences of the characteristics of surface and free field motions are made. Results presented in this paper can be used in a more detailed assessment of ground motion effect on structures.     

Integrating visual damage simulation,virtual inspection,and collapse capacity to evaluate post‐earthquake structural safety of buildings

A methodology is introduced to assess the post‐earthquake structural safety of damaged buildings using a quantitative relationship between observable structural component damage and the change in collapse vulnerability. The proposed framework integrates component‐level damage simulation, virtual inspection, and structural collapse performance assessment. Engineering demand parameters from nonlinear response history analyses are used in conjunction with component‐level damage simulation to generate multiple realizations of damage to key structural elements. Triggering damage state ratios, which describe the fraction of components within a damage state that results in an unsafe placard assignment, are explicitly linked to the increased collapse vulnerability of the damaged building. A case study is presented in which the framework is applied to a 4‐story reinforced concrete frame building with masonry infills. The results show that when subjected to maximum considered earthquake level ground motions, the probability of experiencing enough structural damage to trigger an unsafe placard, leading to building closure, is more than 2 orders of magnitude higher than the risk of collapse.     

Seismic fragility assessment of RC frame structure designed according to modern Chinese code for seismic design of buildings

Following several damaging earthquakes in China,research has been devoted to find the causes of the collapse of reinforced concrete(RC) building sand studying the vulnerability of existing buildings.The Chinese Code for Seismic Design of Buildings(CCSDB) has evolved over time,however,there is still reported earthquake induced damage of newly designed RC buildings.Thus,to investigate modern Chinese seismic design code,three low-,mid-and high-rise RC frames were designed according to the 2010 CCSDB and the corresponding vulnerability curves were derived by computing a probabilistic seismic demand model(PSDM).The PSDM was computed by carrying out nonlinear time history analysis using thirty ground motions obtained from the Pacific Earthquake Engineering Research Center.Finally,the PSDM was used to generate fragility curves for immediate occupancy,significant damage,and collapse prevention damage levels.Results of the vulnerability assessment indicate that the seismic demands on the three different frames designed according to the 2010 CCSDB meet the seismic requirements and are almost in the same safety level.     

Global damage indexes for the seismic performance assessement of RC structures

When performing the seismic risk assessment of new or existing buildings, the definition of compact indexes able to measure the damaging and safety level of structures is essential, also in view of the economic considerations on buildings rehabilitation. This paper proposes two series of indexes, named, respectively, Global Damage Indexes (GDIs), which are representative of the overall structure performance, and Section Damage Indexes (SDIs), which assess the conditions of reinforced concrete (RC) beam‐column sections. Such indexes are evaluated by means of an efficient numerical model able to perform nonlinear analyses of the RC frame, based on the continuum damage mechanics theory and fiber approach. An improvement of a two‐parameter damage model for concrete, developed by some of the authors, which guarantees a better correlation between the Local Damage Indexes (LDIs) and the material's mechanical characteristics, is also presented. For the reinforcement, a specific LDI, named ‘steel damage index’, which takes into account the plastic strain development and the bar buckling effect, is proposed. The numerical model has been employed to simulate several experimental tests, in order to verify the accuracy of the proposed approach in predicting the RC member's behavior. Nonlinear static and dynamic analyses of two RC frames are carried out. The robustness of the method, as well as the effectiveness of the GDIs in assessing the structural conditions, are demonstrated here. Finally, comparisons between the evolution of GDIs and the achievement of the performance levels as proposed in FEMA 356 are reported. Copyright © 2009 John Wiley & Sons, Ltd.     

Development of seismic fragility surfaces for reinforced concrete buildings by means of nonlinear time‐history analysis

Fragility curves are generally developed using a single parameter to relate the level of shaking to the expected structural damage. The main goal of this work is to use several parameters to characterize the earthquake ground motion. The fragility curves will, therefore, become surfaces when the ground motion is represented by two parameters. To this end, the roles of various strong‐motion parameters on the induced damage in the structure are compared through nonlinear time‐history numerical calculations. A robust structural model that can be used to perform numerous nonlinear dynamic calculations, with an acceptable cost, is adopted. The developed model is based on the use of structural elements with concentrated nonlinear damage mechanics and plasticity‐type behavior. The relations between numerous ground‐motion parameters, characterizing different aspects of the shaking, and the computed damage are analyzed and discussed. Natural and synthetic accelerograms were chosen/computed based on a consideration of the magnitude‐distance ranges of design earthquakes. A complete methodology for building fragility surfaces based on the damage calculation through nonlinear numerical analysis of multi‐degree‐of‐freedom systems is proposed. The fragility surfaces are built to represent the probability that a given damage level is reached (or exceeded) for any given level of ground motion characterized by the two chosen parameters. The results show that an increase from one to two ground‐motion parameters leads to a significant reduction in the scatter in the fragility analysis and allows the uncertainties related to the effect of the second ground‐motion parameter to be accounted for within risk assessments. Copyright © 2009 John Wiley & Sons, Ltd.     

脉冲型地震动潜在破坏势参数的相关性分析

为了在众多参数中挑选其中最有代表性的参数,来解释和反映脉冲型地震动对结构的潜在破坏能力,以338条脉冲型地震动记录作为研究对象,分析地震动参数与中低层结构响应的相关性。选取了14个常用地震动参数,对各地震动参数之间的相关性进行分析,从中选出7个代表性地震动参数;并将脉冲型地震动输入中低层结构模型中计算结构响应,分析代表性地震动参数与结构响应的相关性,与基于非脉冲型地震动的相关性计算结果进行对比。选用了3层和7层2个RC框架结构作为中低层结构代表,其基本周期为0.62s和0.89s。结果表明:对于脉冲型地震动,对于3层结构时与结构响应相关性最好的为EPV,对于7层结构时与结构响应相关性最好的为PGV,因此可以用PGV和EPV作为表征脉冲型地震动对中低层结构潜在破坏能力的参数;而对于非脉冲型地震动,与结构响应相关性最好的参数为PGV,可以用PGV作为表征脉冲型地震动对中低层结构的潜在破坏能力的参数。因此,通过地震动参数来解释和表征脉冲型地震动对结构的破坏能力是可行的。     

Simulation of structural response under high‐frequency ground excitation

The structural response to high‐frequency ground motions is complicated due to the involvement of local‐mode vibration. At present such a characteristic is not well recognized and this can cause confusion over the analytical and experimental modelling of the corresponding response and damage. The fact that most existing regulatory guides for limits on allowable construction vibrations are necessarily simplified for administrative reasons calls upon the derivation of more sophisticated approaches for special cases. This requires accumulation of pertinent experimental evidence. This paper attempts to provide some insights into the local‐mode dynamic response characteristics, with emphasis on appropriate modelling techniques and experimental measurements. A preliminary testing program is reported, in which efforts were made to reproduce high‐frequency response with a reduced scale reinforced concrete model with shaking table facilities. The results demonstrate the dependence of the response amplitudes with the excitation frequency. On a ppv‐basis, the current test results indicate that a substantial increase of the allowable ppv value from those specified by various standards may be considered for structural damage to reinforced concrete building structures. More analytical and experimental data are needed for further evaluation of the local‐mode effects and to quantify their impact on the structural damage process. Copyright © 2001 John Wiley & Sons, Ltd.     

Effects of near-fault and far-fault ground motions on nonlinear dynamic response and seismic damage of concrete gravity dams

As the forward directivity and fling effect characteristics of the near-fault ground motions, seismic response of structures in the near field of a rupturing fault can be significantly different from those observed in the far field. The unique characteristics of the near-fault ground motions can cause considerable damage during an earthquake. This paper presents results of a study aimed at evaluating the near-fault and far-fault ground motion effects on nonlinear dynamic response and seismic damage of concrete gravity dams including dam-reservoir-foundation interaction. For this purpose, 10 as-recorded earthquake records which display ground motions with an apparent velocity pulse are selected to represent the near-fault ground motion characteristics. The earthquake ground motions recorded at the same site from other events that the epicenter far away from the site are employed as the far-fault ground motions. The Koyna gravity dam, which is selected as a numerical application, is subjected to a set of as-recorded near-fault and far-fault strong ground motion records. The Concrete Damaged Plasticity (CDP) model including the strain hardening or softening behavior is employed in nonlinear analysis. Nonlinear dynamic response and seismic damage analyses of the selected concrete dam subjected to both near-fault and far-fault ground motions are performed. Both local and global damage indices are established as the response parameters. The results obtained from the analyses of the dam subjected to each fault effect are compared with each other. It is seen from the analysis results that the near-fault ground motions, which have significant influence on the dynamic response of dam–reservoir–foundation systems, have the potential to cause more severe damage to the dam body than far-fault ground motions.     

An experimental study of a damage‐controllable plastic‐hinge‐supported wall structure

The reinforced concrete (RC) shear wall serves as one of the most important components sustaining lateral seismic forces. Although they allow advanced seismic performance to be achieved, RC shear walls are rather difficult to repair once the physical plastic hinge at the bottom part has been formed. To overcome this, a damage‐controllable plastic hinge with a large energy dissipation capacity is developed herein, in which the sectional forces are decoupled and sustained separately by different components. The components sustaining the axial and the shear forces all remain elastic even under a rarely occurred earthquake, while the bending components yield and dissipate seismic energy during a design‐level earthquake. This design makes the behavior of the system more predictable and thus more easily customizable to different performance demands. Moreover, the energy dissipation components can be conveniently replaced to fully restore the occupancy function of a building. To examine the seismic behavior of the newly developed component, 3 one third‐scale specimens were tested quasi‐statically, including 1 RC wall complying with the current design codes of China and 2 installed with the damage‐controllable plastic hinges. Each wall was designed to have the same strength. The experimental results demonstrated that the plastic‐hinge‐supported walls had a better energy dissipation capacity and damage controllability than the RC specimen. Both achieved drift ratios greater than 3% under a steadily increasing lateral force.     

Softening effects of RC nonstructural flat walls on ductile concrete buildings

Nonstructural reinforced concrete flat walls architecturally designed as exterior/partition walls in concrete buildings were severely damaged by the 2011 earthquake off the Pacific coast of Tohoku. This damage was observed in the monolithic nonstructural flat walls of relatively old ductile concrete buildings. Although these flat walls might affect the overall seismic performance and behavior of a building, the nonstructural wall effects have not been clarified because of the complex interactions among the structural components. To understand these effects, this paper conducts an experimental and numerical investigation of the nonstructural wall effects, focusing on a typical residential building damaged by the 2011 earthquake. A single‐story, one‐bay moment‐resisting frame model of the building with a nonstructural flat wall was tested to clarify the fundamental behavior. The results reveal that the wall significantly contributed to the seismic performance of the overall frame until it failed in shear, subsequently losing structural effectiveness. Such experimental wall behavior could be simulated by the isoparametric element model. Moreover, the structural effects of the nonstructural flat walls on the global seismic performance and behavior of the investigated building were discussed through earthquake response analyses using ground motions recorded near the building site and pushover analyses. Consequently, the building damage could be simulated in an analytical case considering the nonstructural flat walls, showing larger inter‐story drifts in the lower stories due to softening of the walls. The analytical results also indicated that the softening of the nonstructural flat walls decreased the building ductility, as defined by ultimate inter‐story drifts. Copyright © 2017 John Wiley & Sons, Ltd.     

Seismic performance assessment of a hybrid coupled wall system with replaceable steel coupling beams versus traditional RC coupling beams

This study assesses the seismic performance of a hybrid coupled wall (HCW) system with replaceable steel coupling beams (RSCBs) at four intensities of ground motion shaking. The performance of the HCW system is benchmarked against the traditional reinforced concrete coupled wall (RCW). Nonlinear numerical models are developed in OpenSees for a representative wall elevation in a prototype 11‐story building designed per modern Chinese codes. Performance is assessed via nonlinear dynamic analysis. The results indicate that both systems can adequately meet code defined objectives in terms of global and component behavior. Behavior of the two systems is consistent under service level earthquakes, whereas under more extreme events, the HCW system illustrates enhanced performance over the RCW system resulting in peak interstory drifts up to 31% lower in the HCW than the RCW. Larger drifts in the RCW are because of reduced coupling action induced by stiffness degradation of RC coupling beams, whereas the stable hysteretic responses and overstrength of RSCBs benefit post‐yield behavior of the HCW. Under extreme events, the maximum beam rotations of the RSCBs are up to 42% smaller than those of the RC coupling beams. Moderate to severe damage is expected in the RC coupling beams, whereas the RSCBs sustain damage to the slab above the beam and possible web buckling of shear links. The assessment illustrates the benefits of the HCW with RSCBs over the RCW system, because of easy replacement of the shear links as opposed to costly and time‐consuming repairs of RC coupling beams. Copyright © 2016 John Wiley & Sons, Ltd.     

Identification of the structural model and analysis of the global seismic behaviour of a RC damaged building

The study of the structural behaviour of damaged RC buildings during ground motion is a fundamental topic in the modern earthquake engineering. Many studies have been carried out in order to better understand the real evolution of the damage in RC buildings during a seismic event. In this work, a damaged RC building has been intensively investigated in terms of materials property and stiffness evolution in order to interpreting the structural and nonstructural surveyed damage. The peculiarity of this building is its damage sequence during the 2002 Molise earthquake. In fact, the town of Bonefro suffered moderate damage (MCS intensity VII), with the exception of the investigated reinforced concrete building. The October 31, 2002 event (M=5.4) caused some structural damage to this building. The second event (M=5.3), on November 1, 2002, increased substantially the damage level (grade 4 according to the 1998 European Macroseismic Scale). It occurred just while, due to fortuitous circumstances, a 5 min. seismic velocimetric recording was being taken. The working group has performed some frequency analyses based on the recording. Several non linear models have been defined to understand the damage evolution of the building and the local and global damage patterns through for static analyses. Finally, linear and non linear models have been developed with the main goal of identifying the characteristics of a reliable undamaged structural model.     

Near-fault ground motions with prominent acceleration pulses： pulse characteristics and ductility demand

Major earthquakes of last 15 years (e.g., Northridge 1994, Kobe 1995 and Chi-Chi 1999) have shown that many near-fault ground motions possess prominent acceleration pulses. Some of the prominent ground acceleration pulses are related to large ground velocity pulses, others are caused by mechanisms that are totally different from those causing the velocity pulses or fling steps. Various efforts to model acceleration pulses have been reported in the literature. In this paper, research results from a recent study of acceleration pulse prominent ground motions and an analysis of structural damage induced by acceleration pulses are summarized. The main results of the study include: (1) temporal characteristics of acceleration pulses; (2) ductility demand spectrum of simple acceleration pulses with respect to equivalent classes of dynamic systems and pulse characteristic parameters; and (3) estimation of fundamental period change under the excitation of strong acceleration pulses. By using the acceleration pulse induced linear acceleration spectrum and the ductility demand spectrum, a simple procedure has been developed to estimate the ductility demand and the fundamental period change of a reinforced concrete (RC) structure under the impact of a strong acceleration pulse.     

Seismic behaviour of hybrid systems made of PR composite frames coupled with dissipative bracings

The paper investigates the dynamic behaviour of hybrid systems made of partially restrained (PR) steel–concrete composite frames coupled with viscoelastic dissipative bracings. A numerical model that accounts for both the resisting mechanisms of the joint and the viscoelastic contribution of the dissipative bracing is introduced and briefly discussed. The model is first validated against experimental outcomes obtained on a one‐storey two‐bay composite frame with partial strength semi‐rigid joints subjected to free vibrations. A number of time‐history analyses under different earthquake ground motions and peak ground accelerations are then carried out on the same type of frame. The purpose is to investigate the influence of the type of beam‐to‐column connection and property of the viscoelastic bracing on the performance of the hybrid system. The inherent stiffness of the bare PR frame and the plastic hysteresis of the beam‐to‐column joints, which always lead to only limited damage in the joint, are found to provide a significant contribution to the overall structural performance even under destructive earthquakes. This remark leads to the conclusion that the viscoelastic bracing can be effectively used within the hybrid system. Copyright © 2008 John Wiley & Sons, Ltd.