Seismic assessment and retrofit of two heritage-listed R/C elevated water storage tanks

A seismic assessment and advanced retrofit study on two heritage-listed reinforced concrete (R/C) elevated water storage tanks is presented in this paper. The two structures were built between the late 1920s and the early 1930s as water suppliers for a coal power plant in Santa Maria Novella Station in Florence, and are still in service. The first, taller tank has a R/C frame supporting structure and is currently used as water supplier for trains and platform services. The second, shorter tank, with a shaft-shell supporting structure, is used as water tower for the Station. The dynamic behaviour of the fluid is simulated by means of a classical convective and impulsive mass model, for which a discrete three-dimensional schematization is originally implemented in the finite element analysis. The time–history assessment enquiry highlights numerical collapse of the frame structure in the taller tank, and unsafe tensile stress states in a large portion of the shaft structure of the shorter one, under seismic action scaled at the maximum considered earthquake level. Based on these results, two retrofit hypotheses are proposed, and namely a dissipative bracing system incorporating pressurized fluid viscous spring-dampers, for the taller tank, and a base isolation system including double curved surface sliders, for the shorter one. The mechanical parameters, design criteria and technical implementation details of the two rehabilitation strategies are illustrated. The verification time–history analyses in protected conditions show that a substantial enhancement of the seismic response capacities of both structures is attained as compared to their original configurations, with little architectural intrusion, quick installation works and competitive costs.     

The damped cable system for seismic protection of frame structures — Part II: Design and application

The complementary sections of the studies carried out on the damped cable system, whose experimental and numerical characterization and assessment analyses are described in the companion paper, are presented herein. The first section includes a criterion for a preliminary evaluation of the section area of cables, the second branch stiffness of spring‐dampers and the mutual installation preload, and suggestions for a simplified nonlinear dynamic computation of the damping coefficient of dissipaters. A second section follows, aimed at evaluating the influence of cable layout on damped cable system performance. A numerical enquiry is developed on a four‐story and an eight‐story RC plane frame, to assess their seismic response for several shapes of cables, and determine what geometrical configurations are the best performing ones. In the third section, a demonstrative application of the protective system, represented by the seismic retrofit of a hospital building with RC structure, is offered. The characteristics of the system designed for this case study, including locations, dimensions, layouts, and technical installation details of cables and spring‐dampers, are illustrated, and the improvement of seismic performance as compared with the original conditions, is finally assessed. Copyright © 2011 John Wiley & Sons, Ltd.     

屋盖MR智能隔震系统对升船结构顶部厂房地震鞭梢效应的模糊控制

基于抑制升船结构顶部厂房地震鞭梢效应的目的，本文提出了升船结构顶部厂房屋盖MR智能隔震模糊控制的思想。文中，在建立屋盖智能隔震升船结构计算力学模型的基础上，建立了屋盖MR智能隔震系统对升船结构顶部厂房地震反应模糊控制的设计计算方法。文中并以中国某大坝巨型升船结构为背景，设计了屋盖MR智能隔震系统对升船结构顶部厂房地震反应模糊控制的控制系统。仿真分析和对MR阻尼器的参数研究表明，安装合适的屋盖MR智能隔震系统并采用模糊控制策略能有效地抑制具有不确定参数升船结构顶部厂房地震反应的鞭梢效应，且模糊控制器能保持较好的稳定性能。     

An alternative approach for the seismic rehabilitation of existing RC buildings using seismic isolation

Usually, buildings with seismic isolation are designed to comply with an operational building performance level after strong earthquakes. This approach, however, may limit the use of seismic isolation for the seismic rehabilitation of existing buildings with low lateral strength or substandard details, because it often requires invasive strengthening measures in the superstructure or the use of expensive custom‐made devices. In this paper, an alternative approach for the seismic rehabilitation of existing buildings with seismic isolation, based on the acceptance of limited plastic deformations in the superstructure under strong earthquakes, is proposed and then applied to a real case study, represented by a four‐storey RC frame building. Nonlinear response‐time histories analyses of an accurate model of the case‐study building are carried out to evaluate the seismic performances of the structure, comparing different rehabilitation strategies with and without seismic isolation. Initial costs of the intervention and possible (future) repair costs are then estimated. Based on the results of this study, values of the behavior factor (i.e. response modification factor) higher than those adopted in the current codes for base‐isolated buildings are tentatively proposed. Copyright © 2015 John Wiley & Sons, Ltd.     

Self-centering seismic retrofit scheme for reinforced concrete frame structures: SDOF system study

This paper presents the results of a parametric study of self-centering seismic retrofit schemes for reinforced concrete (RC) frame buildings. The self-centering retrofit system features flag-shaped hysteresis and minimal residual deformation. For comparison purpose,an alternate seismic retrofit scheme that uses a bilinear-hysteresis retrofit system such as buckling-restrained braces (BRB) is also considered in this paper. The parametric study was carried out in a single-degree-of-freedom (SDOF) system framework since a multi-story building structure may be idealized as an equivalent SDOF system and investigation of the performance of this equivalent SDOF system can provide insight into the seismic response of the multi-story building. A peak-oriented hysteresis model which can consider the strength and stiffness degradation is used to describe the hysteretic behavior of RC structures. The parametric study involves two key parameters -the strength ratio and elastic stiffness ratio between the seismic retrofit system and the original RC frame. An ensemble of 172 earthquake ground motion records scaled to the design basis earthquake in California with a probability of exceedance of 10% in 50 years was constructed for the simulation-based parametric study. The effectiveness of the two seismic retrofit schemes considered in this study is evaluated in terms of peak displacement ratio,peak acceleration ratio,energy dissipation demand ratio and residual displacement ratio between the SDOF systems with and without retrofit. It is found from this parametric study that RC structures retrofitted with the self-centering retrofit scheme (SCRS) can achieve a seismic performance level comparable to the bilinear-hysteresis retrofit scheme (BHRS) in terms of peak displacement and energy dissipation demand ratio while having negligible residual displacement after earthquake.     

Retrofit of reinforced concrete frames with buckling‐restrained braces

Damage to buildings observed in recent earthquakes suggests that many old reinforced concrete structures may be vulnerable to the effects of severe earthquakes. One suitable seismic retrofit solution is the installation of steel braces to increase the strength and ductility of a building. Steel bracings have some compelling advantages such as their comparatively low weight, their suitability for prefabrication, and the possibility of openings for utilities, access, and light. The braces are typically connected to steel frames that are fixed to the concrete structure using post‐installed concrete anchors along the perimeter. However, these framed steel braces are not without some disadvantages such as heavier steel usage and greater difficulties during the installation. Therefore, braces without steel frames appear to be an attractive alternative. In this study, braces were connected to gussets furnished with anchor brackets, which were fixed by means of a few post‐installed concrete anchors. The clear structural system and the increased utilization of the anchors allowed the anchorage to be designed precisely and economically. The use of buckling‐restrained braces (BRBs) provides additional benefits in comparison with conventional braces. BRBs improve the energy dissipation efficiency and allow the limitation of the brace force to be taken up by the highly stressed anchorage. Cyclic loading tests were conducted to investigate the seismic performance of BRBs connected with post‐installed anchors used to retrofit reinforced concrete frames. The tests showed that the proposed design method is feasible and increases strength as well as ductility to an adequate seismic performance level. Copyright © 2014 John Wiley & Sons, Ltd.     

COMPARATIVE STUDY ON THE SEISMIC RETROFIT OF A MID-RISE STEEL BUILDING: STEEL BRACING VS ENERGY DISSIPATION

The paper presents a comparative study of an existing retrofit for a mid-rise steel building using additional stiff steel braced-frames against an alternate retrofit using ADAS (Added Damping and Stiffness) passive energy dissipation devices. The subject building, located near Alameda Park in downtown Mexico City, is a ten-storey office building that was built in the 1950s. The structure was damaged during the 1985 Michoacán Earthquake because of resonant response with the site. The building was later retrofitted using additional braced frames according to the seismic provisions of Mexico's 1987 Federal District Code. The retrofit scheme was planned to take the structure away from resonant responses and to inhibit structural damage. A proposed upgrade using ADAS energy dissipation devices was studied to compare energy dissipation against traditional stiffening using steel braces as retrofit options for mid-rise buildings in Mexico City's lake-bed zone. Different sets of analyses were carried out to compare both alternatives: (a) three-dimensional elastic analyses; (b) limit analyses and; (c) nonlinear dynamic analyses for postulated site ground motions for a M_s=8.1 earthquake. Initial costs of the retrofit schemes were also studied. The comparative studies suggest that a retrofit using ADAS devices would have a better dynamic performance than the one using steel braces. However, the steel bracing retrofit provides more strength and its initial cost of retrofit is less than that of the ADAS retrofit. © 1997 John Wiley & Sons, Ltd.     

Three‐storey building retrofit: rocking isolation versus conventional design

Although modern seismic codes have undoubtedly led to safer structures, the seismic vulnerability of metropolitan areas is unavoidably governed by that of older buildings, which constitute the vast majority of the current building stock. Quite alarmingly, even relatively moderate intensity earthquakes have been proven capable of challenging their structural integrity, leading to severe damage or collapse. Therefore, there is an urgent need to assess the vulnerability of existing structures and to evaluate the efficiency of novel retrofit techniques. This paper studies experimentally the seismic performance of an existing three‐storey building, retrofitted through addition of shear walls. Emphasis is placed on the foundation of the shear walls, and two design alternatives are comparatively assessed: (a) conventional design according to current seismic codes and (b) ‘rocking isolation’ by reducing the size of the foundation. A series of reduced‐scale shaking table tests are conducted at the Laboratory of Soil Mechanics of the National Technical University of Athens. The physical model encompasses the structural system, along with the foundations, and the soil. The nonlinearity of structural members is simulated through specially designed and carefully calibrated artificial plastic hinges. The vulnerability of the original structure is confirmed, as it is found to collapse with a soft‐storey mechanism when subjected to moderate intensity shaking. The conventionally retrofitted structure is proven capable of sustaining larger intensity shaking, and the rocking‐isolated structure is shown to offer increased safety margins. Thanks to its inherent self‐centering mechanism, the rocking system is characterized by reduced permanent drifts. Copyright © 2014 John Wiley & Sons, Ltd.     

Seismic response of a case study soft story frame retrofitted using a GIB system

This paper presents results from a numerical investigation into the seismic retrofit of a soft story frame using a novel gapped‐inclined brace (GIB) system. The GIB system consists of a pinned brace and a gap element that is added to the first story columns of the frame. The inclusion of GIB elements in addition to increasing the lateral capacity of columns at the first story increases the post‐yield stiffness ratio of the system and reduces the P‐delta effects on the columns, while not increasing the first story lateral resistance or stiffness. This allows for the isolating benefits of the soft story to protect the upper floors of the structure from damage while avoiding excessive deformations and reducing the propensity for collapse. A six‐story RC frame with masonry infills on all floors except for the first floor is studied. The dynamic response of the retrofitted building using the GIB system is investigated numerically and is compared with the response of the original un‐retrofitted building and the same building in which masonry infills are added to the first story to mitigate the soft story response. Results from the nonlinear time‐history analyses indicate that the GIB system could provide a reliable seismic retrofit mechanism for soft story buildings, which greatly reduces the likelihood of collapse by increasing the displacement capacity of the soft storey and by reducing P‐delta effects, while minimizing the overall damage and losses in the building by taking advantages of the isolation that is provided by the soft story to the rest of the structure located above. Copyright © 2014 John Wiley & Sons, Ltd.     

Seismic fragility reduction of an unreinforced masonry school building through retrofit using ferrocement overlay

The Himalayan region is one of the major seismic areas in the world. However, similar to many other seismically active locations, there are substantial numbers of unreinforced masonry(URM) buildings; the majority of which have not been designed for seismic loads. Past seismic events have shown that such buildings are highly vulnerable to earthquakes. Retrofitting of these URM buildings is an important concern in earthquake mitigation programs. Most government school buildings in rural areas of northern India are constructed of unreinforced masonry. These school buildings are socially important structures and serve as a crucial resource for rehabilitation during any disaster. The effectiveness of ferrocement(FC) to create a URM-FC composite is described in this study by estimating the performance and fragility of a URM school building before and after a retrofit. Analytical models, based on the equivalent frame method, are developed and used for nonlinear static analysis to estimate the enhancement in capacity. The capacity enhancement due to retrofitting is presented in terms of the maximum PGA sustained and damage probabilities at the expected level of earthquake hazard.     

Seismic resilience of retrofitted RC buildings

Existing buildings can be at a greater seismic risk due to non-conformance to current design codes and may require structural retrofitting to improve building performance. The performance of buildings is measured in terms of immediate consequences due to direct damage, but the continuing impacts related to recovery are not considered in seismic retrofit assessment. This paper introduces a framework of retrofit selection based on the seismic resilience of deficient buildings retrofitted with the conventional mitigation approaches. The assembly-based methodology is considered for the seismic resilience assessment by compiling a nonlinear numerical model and a building performance model. The collapse fragility is developed from the capacity curve, and the resulting social, economic, and environmental consequences are determined. The seismic resilience of a building is assessed by developing a downtime assessment methodology incorporating sequence of repairs, impeding factors, and utility availability. Five functionality states are developed for the building functionality given investigated time interval, and a functionality curve for each retrofit is determined. It is concluded that seismic resilience can be used as a performance indicator to assess the continuing impacts of a hazard for the retrofit selection.     

核电站结构隔震研究进展

基底隔震已经大量应用于一般民用建筑中,被认为是在剧烈地震中保护建筑物最具发展前景的技术之一,但对于核电站这类特殊的建筑物应用较少,且研究比较薄弱。介绍了核电站水平及三维隔震装置的研发及性能测试,概述了核电站隔震系统抗震性能的研究进展和核电站隔震设计规范,指出了在核电站中应用隔震技术还有待解决的问题,对今后核电站隔震的研究提出了一些建议。     

Variable stiffness smart structure systems to mitigate seismic induced building damages

Upgrading noncode conforming buildings to mitigate seismic induced damages is important in moderate to high seismic hazard regions. The damage, can be mitigated by using conventional (e.g. FRP wrapping) and emerging (e.g. smart structures) retrofit techniques. A model for the structure to be retrofitted should include relevant performance indicators. This paper proposes a variable stiffness smart structure device known as the Smart Spring to be integrated on building structures to mitigate seismic induced damage. The variable stiffness capability is of importance to structures that exhibit vertical (e.g. soft storey) irregularities and to meet different performance levels under seismic excitation. To demonstrate the utility of the proposed retrofitting technique, a four‐storey steel building is modelled in MATLAB and appropriate performance indicators are chosen. Various return period seismic hazards are generated from past earthquake event records to predict the structure's performance. The performance improvement because of the retrofitting of building structures using the variable stiffness device is presented. Copyright © 2012 John Wiley & Sons, Ltd.     

基于隔震就位连接技术的高层钢框架结构建筑抗震技术研究

以抗震设防7度区的某高层钢框架结构建筑为研究对象,研究基于隔震就位连接技术的高层钢框架结构建筑抗震技术。对研究对象采用抗震就位连接技术,通过在隔震层中设置弹性滑板支座、厚层橡胶隔震支座化黏滞阻尼器等隔震装置提升研究对象自振周期,降低其上部结构受地震作用的影响程度,增强抗震性能。对研究对象进行模态分析与时程分析,结果显示,所研究隔震技术在显著提升研究对象自振周期,使其摆脱场地特征周期的同时,也能够延缓扭转振型的形成,显著降低地震作用对研究对象上部结构的影响,说明所研究技术的运用具有较好的抗震效果。     

Experimental investigation on a base isolation system incorporating steel–Teflon sliders and pressurized fluid viscous spring dampers

An experimental investigation on a base isolation system incorporating stainless steel–Teflon bearings as sliders, and pressurized fluid viscous spring dampers, is presented in this paper. In the system examined, dampers are connected to the base floor of an isolated building to provide the desired passive control of response in the superstructure, as well as to guarantee that it re‐centres completely after the termination of a seismic action. Two types of experiments were conducted: sinusoidal and random cyclic tests, and a pseudodynamic test in ‘substructured’ configuration. The cyclic tests were aimed at characterizing what follows: the hysteretic and strain‐rate‐dependent response of the considered highly non‐linear spring dampers; the normal pressure‐ and strain‐rate‐dependent frictional behaviour of steel–Teflon bearings, manufactured in compliance with the latest standards for this class of sliders; and the combined response of their assembly. The pseudodynamic test simulated the installation of the protection system at the base of a 2:3‐scale three‐storey steel frame structure, already tested in unprotected conditions by an earlier experimental campaign. Among other findings, the results of the performed tests, as well as of relevant mechanical interpretation and numerical simulation analyses, confirmed the linear additive combination of the dissipative actions of spring dampers and sliders in this mixed installation, and the high protective performance of the considered base isolation/supplemental damping system in a realistic earthquake simulation. Copyright © 2007 John Wiley & Sons, Ltd.     

Seismic strengthening of an under‐designed RC structure with FRP

The opportunities provided by the use of fiber‐reinforced polymer (FRP) for the seismic retrofit of existing reinforced concrete (RC) structures were assessed on a full‐scale three‐story framed structure. The structure, designed only for gravity loads, was subjected to a bi‐directional pseudo‐dynamic (PsD) test at peak ground acceleration (PGA) equal to 0.20g at the ELSA Laboratory of the Joint Research Centre. The seismic deficiencies exhibited by the structure after the test were confirmed by post‐test assessment of structural seismic capacity performed by nonlinear static pushover analysis implemented on the lumped plasticity model of the structure. In order to allow the structure to withstand 0.30g PGA seismic actions, a retrofit using glass fiber‐reinforced polymer (GFRP) laminates was designed. The retrofit design was targeted to achieve a more ductile and energy dissipating global performance of the structure by increasing the ductility of columns and preventing brittle failure modes. Design assumptions and criteria along with nonlinear static pushover analysis to assess the overall capacity of the FRP‐retrofitted structure are presented and discussed. After the retrofit execution, a new series of PsD tests at both 0.20g and 0.30g PGA level were carried out. Theoretical predictions are compared with the main experimental outcomes to assess the effectiveness of the proposed retrofit technique and validate the adopted design procedures. Copyright © 2007 John Wiley & Sons, Ltd.     

Simplified design procedure for frame buildings with viscoelastic or elastomeric structural dampers

A simplified design procedure (SDP) for preliminary seismic design of frame buildings with structural dampers is presented. The SDP uses elastic‐static analysis and is applicable to structural dampers made from viscoelastic (VE) or high‐damping elastomeric materials. The behaviour of typical VE materials and high‐damping elastomeric materials is often non‐linear, and the SDP idealizes these materials as linear VE materials. With this idealization, structures with VE or high‐damping elastomeric dampers can be designed and analysed using methods based on linear VE theory. As an example, a retrofit design for a typical non‐ductile reinforced concrete (RC) frame building using high‐damping elastomeric dampers is developed using the SDP. To validate the SDP, results from non‐linear dynamic time history analyses (NDTHA) are presented. Results from NDTHA demonstrate that the SDP estimates the seismic response with sufficient accuracy for design. It is shown that a non‐ductile RC frame building can be retrofit with high‐damping elastomeric dampers to remain essentially elastic under the design basis earthquake (DBE). Copyright © 2005 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.     

Shake table testing of un‐reinforced brick masonry building test model isolated by U‐FREI

This paper presents a detailed study on feasibility of un‐bonded fiber reinforced elastomeric isolator (U‐FREI) as an alternative to steel reinforced elastomeric isolator (SREI) for seismic isolation of un‐reinforced masonry buildings. Un‐reinforced masonry buildings are inherently vulnerable under seismic excitation, and U‐FREIs are used for seismic isolation of such buildings in the present study. Shake table testing of a base isolated two storey un‐reinforced masonry building model subjected to four prescribed input excitations is carried out to ascertain its effectiveness in controlling seismic response. To compare the performance of U‐FREI, same building is placed directly on the shake table without isolator, and fixed base (FB) condition is simulated by restraining the base of the building with the shake table. Dynamic response characteristic of base isolated (BI) masonry building subjected to different intensities of input earthquakes is compared with the response of the same building without base isolation system. Acceleration response amplification and peak response values of test model with and without base isolation system are compared for different intensities of table acceleration. Distribution of shear forces and moment along the height of the structure and response time histories indicates significant reduction of dynamic responses of the structure with U‐FREI system. This study clearly demonstrates the improved seismic performance of un‐reinforced masonry building model supported on U‐FREIs under the action of considered ground motions. Copyright © 2015 John Wiley & Sons, Ltd.     

Effects of seismic isolation on the seismic response of a California high‐speed rail prototype bridge with soil‐structure and track‐structure interactions

With the launch of the high‐speed train project in California, the seismic risk is a crucial concern to the stakeholders. To investigate the seismic behavior of future California High‐Speed Rail (CHSR) bridge structures, a 3D nonlinear finite‐element model of a CHSR prototype bridge is developed. Soil‐structure and track‐structure interactions are accounted for in this comprehensive numerical model used to simulate the seismic response of the bridge and track system. This paper focuses on examining potential benefits and possible drawbacks of the a priori promising application of seismic isolation in CHSR bridges. Nonlinear time history analyses are performed for this prototype bridge subjected to two bidirectional horizontal historical earthquake ground motions each scaled to two different seismic hazard levels. The effect of seismic isolation on the seismic performance of the bridge is investigated through a detailed comparison of the seismic response of the bridge with and without seismic isolation. It is found that seismic isolation significantly reduces the deck acceleration and the force demand in the bridge substructure (i.e., piers and foundations), especially for high‐intensity earthquakes. However, seismic isolation increases the deck displacement (relative to the pile cap) and the stresses in the rails. These findings imply that seismic isolation can be promisingly applied to CHSR bridges with due consideration of balancing its beneficial and detrimental effects through using appropriate isolators design. The optimum seismic isolator properties can be sought by solving a performance‐based optimum seismic design problem using the nonlinear finite‐element model presented herein. Copyright © 2016 John Wiley & Sons, Ltd.