Hysteretic model development and seismic response of unbonded post‐tensioned precast CFT segmental bridge columns

The study investigated the cyclic behavior of unbonded, post‐tensioned, precast concrete‐filled tube segmental bridge columns by loading each specimen twice. Moreover, a stiffness‐degrading flag‐shaped (SDFS) hysteretic model was developed based on self‐centering and stiffness‐degrading behaviors. The proposed model overcomes the deficiency of cyclic behavior prediction using a FS model, which self‐centers with fixed elastic and inelastic stiffnesses. Experimental and analytical results showed that (1) deformation capabilities of the column under the first and second cyclic tests were similar; however, energy dissipation capacities significantly differed from each other, and (2) the SDFS model predicted the cyclic response of the column better than the FS model. Inelastic time‐history analyses were performed to demonstrate the dynamic response variability of a single‐degree‐of‐freedom (SDOF) system using both models. A parametric study, performed on SDOF systems subjected to eight historical earthquakes, showed that increased displacement ductility demand was significant for structures with a low period and low‐to‐medium yield strength ratio and reduced displacement ductility demand in these systems was effectively attained by increasing energy dissipation capacity. Copyright © 2008 John Wiley & Sons, Ltd.     

Using Bézier curves to model gradual stiffness transitions in nonlinear elements: Application to self‐centering systems

A number of techniques are available for modelling nonlinear elements, but most available hysteretic rules do not capture the gradual stiffness changes that are typical of physical systems. In particular, there has not previously been a hysteretic rule with rounded hysteretic corners that could be used to model self‐centering elements, where multiple stiffness changes occur within one loading cycle. This paper presents a new hysteretic rule that allows the gradual stiffness transitions that occur in real systems to be modelled. In this paper, the rule is formulated for flag‐shaped hystereses, but it is shown that the same model also produces hystereses that can be used to model systems that are not self‐centering. The same technique could also be applied to round the corners of different backbone hystereses. A previous study has shown how abrupt stiffness changes can cause very large acceleration spikes, particularly in self‐centering systems. This paper shows that acceleration spikes due to stiffness changes may be reduced by designing systems to change stiffness more gradually, and that this typically has little effect on other aspects of the seismic response. When modelling structural systems, especially if they are self‐centering, sharp‐cornered hysteretic models may be used for initial analysis, but round‐cornered hysteretic models should be considered when using nonlinear rotational springs or when accelerations are of particular importance. Copyright © 2011 John Wiley & Sons, Ltd.     

A study on using wide‐flange section web under out‐of‐plane flexure for passive energy dissipation

It is not common to purposely subject the web of wide‐flange or I‐sections to out‐of‐plane bending. However, yielding the web under this loading condition can be a stable source of energy dissipation as the transition at the corner from the web to the flanges is smooth and weld‐free; this prevents stress concentrations causing premature failure and eliminates uncertainties and imperfections associated with welding. Further, short segments of wide‐flange or I‐sections constitute a simple and inexpensive energy dissipating device as minimum manufacturing is required and leftovers not useful for other structural purposes can be re‐utilized. This paper proposes a new type of seismic damper in the form of braces based on yielding the web of short length segments of wide‐flange or I‐shaped steel sections under out‐of‐plane bending. The hysteretic behavior and ultimate energy dissipation capacity is investigated via component tests under cyclic loads. The experimental results indicate that the damping device has stable restoring force characteristics and a high energy dissipation capacity. Based on these results, a simple hysteretic model for predicting the load–displacement curve of the seismic damper is proposed, along with a procedure for predicting its ultimate energy dissipation capacity and anticipating its failure under arbitrarily applied cyclic loads. The procedure considers the influence of the loading path on the ultimate energy dissipation capacity. Finally, shaking table tests on half‐scale structures are conducted to further verify the feasibility and effectiveness of the new damper, and to assess the accuracy of the hysteretic model and the procedure for predicting its failure. Copyright © 2010 John Wiley & Sons, Ltd.     

Self‐centering structural systems with combination of hysteretic and viscous energy dissipations

This paper presents an innovative set of high‐seismic‐resistant structural systems termed Advanced Flag‐Shaped (AFS) systems, where self‐centering elements are used with combinations of various alternative energy dissipation elements (hysteretic, viscous or visco‐elasto‐plastic) in series and/or in parallel. AFS systems is developed using the rationale of combining velocity‐dependent with displacement‐dependent energy dissipation for self‐centering systems, particularly to counteract near‐fault earthquakes. Non‐linear time‐history analyses (NLTHA) on a set of four single‐degree‐of‐freedom (SDOF) systems under a suite of 20 far‐field and 20 near‐fault ground motions are used to compare the seismic performance of AFS systems with the conventional systems. It is shown that AFS systems with a combination in parallel of hysteretic and viscous energy dissipations achieved greater performance in terms of the three performance indices. Furthermore, the use of friction slip in series of viscous energy dissipation is shown to limit the peak response acceleration and induced base‐shear. An extensive parametric analysis is carried out to investigate the influence of two design parameters, λ₁ and λ₂ on the response of SDOF AFS systems with initial periods ranging from 0.2 to 3.0 s and with various strength levels when subjected to far‐field and near‐fault earthquakes. For the design of self‐centering systems with combined hysteretic and viscous energy dissipation (AFS) systems, λ₁ is recommended to be in the range of 0.8–1.6 while λ₂ to be between 0.25 and 0.75 to ensure sufficient self‐centering and energy dissipation capacities, respectively. Copyright © 2010 John Wiley & Sons, Ltd.     

Seismic design procedure and seismic response of post‐tensioned self‐centering steel frames

Post‐tensioned (PT) self‐centering moment‐resisting frames (MRFs) have recently been developed as an alternative to welded moment frames. The first generation of these systems incorporated yielding energy dissipation mechanisms, whereas more recently, PT self‐centering friction damped (SCFR) moment‐resistant connections have been proposed and experimentally validated. Although all of these systems exhibited good stiffness, strength and ductility properties and stable dissipation of energy under cyclic loading, questions concerning their ultimate response still remained and a complete design methodology to allow engineers to conceive structures using these systems was also needed. In this paper, the mechanics of SCFR frames are first described and a comprehensive design procedure that accounts for the frame behavior and the nonlinear dynamics of self‐centering frames is then elaborated. A strategy for the response of these systems at ultimate deformation stages is then proposed and detailing requirements on the beams in order to achieve this response are outlined. The proposed procedure aims to achieve designs where the interstory drifts for SCFR frames are similar to those of special steel welded moment‐resisting frames (WMRFs). Furthermore, this procedure is adapted from current seismic design practices and can be extended to any other PT self‐centering steel frame system. A six‐story building incorporating WMRFs was designed and a similar building incorporating SCFR frames were re‐designed by the proposed seismic design procedure. Time‐history analyses showed that the maximum interstory drifts and maximum floor accelerations of the SCFR frame were similar to those of the WMRF but that almost zero residual drifts were observed for the SCFR frame. The results obtained from the analyses confirmed the validity of the proposed seismic design procedure, since the peak drift values were similar to those prescribed by the seismic design codes and the SCFR frames achieved the intended performance level under both design and maximum considerable levels of seismic loading. Copyright © 2008 John Wiley & Sons, Ltd.     

Single precast concrete rocking walls as earthquake force‐resisting elements

Precast concrete walls with unbonded post‐tensioning provide a simple self‐centering system. Yet, its application in seismic regions is not permitted as it is assumed to have no energy dissipation through a hysteretic mechanism. These walls, however, dissipate energy imparted to them because of the wall impacting the foundation during rocking and limited hysteretic action resulting from concrete nonlinearity. The energy dissipated due to rocking was ignored in previous experimental studies because they were conducted primarily using quasi‐static loading. Relying only on limited energy dissipation, a shake table study was conducted on four single rocking walls (SRWs) using multiple‐level earthquake input motions. All walls generally performed satisfactorily up to the design‐level earthquakes when their performance was assessed in terms of the maximum transient drift, maximum absolute acceleration, and residual drift. However, for the maximum considered earthquakes, the walls experienced peak lateral drifts greater than the permissible limits. Combining the experimental results with an analytical investigation, it is shown that SRWs can be designed as earthquake force‐resisting elements to produce satisfactory performance under design‐level and higher‐intensity earthquake motions. Copyright © 2016 John Wiley & Sons, Ltd.     

Experimental and numerical study of a self‐centering prestressed concrete moment resisting frame connection with bolted web friction devices

The self‐centering prestressed concrete (SCPC) moment resisting frame (MRF) with web friction devices (WFDs) is a new type of structure that integrates advantages of post‐tensioned precast concrete MRFs and self‐centering steel MRFs. In this paper, the configuration of the connection and design guidelines are presented. To further reduce damage to the connection under cyclic loading and facilitate implementation in practice, several significant improvements are proposed and experimentally validated in this study, including the following: (i) the welded connection is replaced by the bolted connection; (ii) aluminum plates are used for friction instead of brass plates to reduce the material costs without decreasing the energy dissipation capacity; and (iii) post‐tensioned tendons at the corners of the beam are replaced by a bundle of tendons at the beam centroid in order to facilitate the field assembly. The resulting improvements of seismic performances are experimentally demonstrated by 10 cyclic tests of two full‐scale SCPC beam–column connections. Numerical simulation of the proposed connection is conducted using the Open System for Earthquake Engineering Simulation (OpenSees) to replicate the experimental results. Seismic behaviors are taken into account, such as the gap opening/closing at the beam–column interface, the self‐centering capacity, and the friction energy dissipation. Good agreement is observed between the numerical simulation and the test results. The proposed SCPC connection with bolted WFDs is demonstrated to have good performance when subjected to cyclic loading. Copyright © 2013 John Wiley & Sons, Ltd.     

Tensile strength equation for HSS bracing members having slotted end connections

In the previous study, the authors investigated the effect of w/t ratios on the behaviour of bracing members under symmetric cyclic loading in compression and tension. In this study, 11 bracing members with slotted end sections made of cold‐formed square hollow structural sections (HSS) were tested. The w/t ratios ranged from 8 to 28. Unlike the test results of other former studies obtained under compression‐oriented cyclic loading, the results of this study showed that bracing members having a smaller w/t ratio (<14) had less deformation and less energy dissipation capacity, and a shorter fracture life compared with other specimens. Such inferior behaviour resulted from early fracture at the slotted end section. This study compares tensile strength obtained from the design equations in the AISC LRFD manual and Eurocode 3 using the actual strengths of the tested specimens. This study found that for preventing early fracture in HSS bracing members, design fracture strength should be larger than design yield strength. Design strength equations are proposed for bracing members in special concentrically braced frames (SCBF). The proposed design equations are verified by experimental tests conducted under symmetric cyclic loading in tension and compression using two HSS bracing members designed according to the proposed equation. Copyright © 2007 John Wiley & Sons, Ltd.     

循环荷载特征对焊接工字形钢支撑低周疲劳性能的影响

通过14根铰支焊接工字形支撑在不同特征的循环轴向位移荷载下的低周疲劳试验，研究了循环轴向位移荷载的位移幅值、平均位移幅值及加载次序等因素对钢支撑低周疲劳及耗能性能的影响。研究发现，对称循环荷载中幅值越小，支撑翼缘局部屈曲发展越晚，其耗能及承载力退化也越平缓。文中提出了支撑在幅值6δ≤Δδ≤12δy的对称循环荷载下的疲劳寿命经验公式。试验表明，循环荷载的位移幅值是支撑疲劳损伤及耗能退化的最主要影响因素，过载峰效应及适当的平均压位移幅值改善了钢支撑低周疲劳及耗能性能。     

Rocking damage‐free steel column base with friction devices: design procedure and numerical evaluation

Earthquake‐resilient steel frames, such as self‐centering frames or frames with passive energy dissipation devices, have been extensively studied during the past decade, but little attention has been paid to their column bases. The paper presents a rocking damage‐free steel column base, which uses post‐tensioned high‐strength steel bars to control rocking behavior and friction devices to dissipate seismic energy. Contrary to conventional steel column bases, the rocking column base exhibits monotonic and cyclic moment–rotation behaviors that are easily described using simple analytical equations. Analytical equations are provided for different cases including structural limit states that involve yielding or loss of post‐tensioning in the post‐tensioned bars. A step‐by‐step design procedure is presented, which ensures damage‐free behavior, self‐centering capability, and adequate energy dissipation capacity for a predefined target rotation. A 3D nonlinear finite element (FE) model of the column base is developed in abaqus . The results of the FE simulations validate the accuracy of the moment–rotation analytical equations and demonstrate the efficiency of the design procedure. Moreover, a simplified model for the column base is developed in OpenSees . Comparisons among the OpenSees and abaqus models demonstrate the efficiency of the former and its adequacy to be used in nonlinear dynamic analysis. A prototype steel building is designed as a self‐centering moment‐resisting frame with conventional or rocking column bases. Nonlinear dynamic analyses show that the rocking column base fully protects the first story columns from yielding and eliminates the first story residual drift without any detrimental effect on peak interstory drifts. The study focuses on the 2D rocking motion and, thus, ignores 3D rocking effects such as biaxial bending deformations in the friction devices. The FE models, the analytical equations, and the design procedure will be updated and validated to cover 3D rocking motion effects after forthcoming experimental tests on the column base. Copyright © 2017 John Wiley & Sons, Ltd.     

Influence of connection typology on seismic response of MR‐Frames with and without ‘set‐backs’

The work presented is aimed at the investigation of the influence of beam‐to‐column connections on the seismic response of MR‐Frames, with and without ‘set‐backs’, designed according to the Theory of Plastic Mechanism Control. The investigated connection typologies are four partial strength connections whose structural details have been designed to obtain the same flexural resistance. The first three joints are designed by means of hierarchy criteria based on the component approach and are characterized by different location of the weakest joint component, leading to different values of joint rotational stiffness and plastic rotation supply and affecting the shape of the hysteresis loops governing the dissipative capacity. The last typology is a beam‐to‐column connection equipped with friction pads devoted to the dissipation of the earthquake input energy, thus preventing the connection damage. An appropriate modelling is needed to accurately represent both strength and deformation characteristics, especially with reference to partial‐strength connections where the dissipation of the earthquake input energy occurs. To this aim, beam‐to‐column joints are modelled by means of rotational inelastic springs located at the ends of the beams whose moment‐rotation curve is characterized by a cyclic behaviour which accounts for stiffness and strength degradation and pinching phenomena. The parameters characterizing the cyclic hysteretic behaviour have been calibrated on the base of experimental results aiming to the best fitting. Successively, the prediction of the structural response of MR‐Frames, both regular frames and frames with set‐backs, equipped with such connections has been carried out by means of both push‐over and Incremental Dynamic Analyses. Copyright © 2016 John Wiley & Sons, Ltd.     

腹板开孔形状影响耗能支撑滞回性能试验研究

腹板开孔耗能支撑有效地避免了传统中心支撑斜杆的失稳破坏,具有较强的耗能能力.为研究腹板开孔形状对支撑滞回性能的影响,对腹板开菱形孔、椭圆孔及长圆孔耗能支撑试件进行了低周往复加载试验,分析了试件在循环荷载作用下的破坏机理、滞回性能、承载能力、刚度退化及耗能能力.试验结果表明:耗能支撑试件滞回曲线饱满,耗能性能优越.耗能支...     

Self‐centering steel connections with steel bars and a discontinuous composite slab

Six cyclic tests were conducted on three full‐scale subassemblies to investigate the behavior of interior beam‐to‐column post‐tensioned (PT) connections. Strands were placed along each side of the steel beam web, passing through the steel column to provide precompression between the beams and a column. Top and bottom energy‐dissipating (ED) bars, passing through the column and welded to the beam, were used to increase the moment capacity and ED capacity of the connection. One of the subassemblies also had a composite concrete slab with discontinuity at the column centerline to eliminate restraint from the metal deck, reinforcement, and welded wire mesh. The objectives of this paper were to investigate the following: the durability of the connection by loading each specimen twice, the ED capacity of the ED bar, and the effects that the type of ED bar and type of composite slab have on the self‐centering behavior of the connection. The experimental results showed that: (1) the connection could sustain severe inelastic cyclic loading at least twice without strength degradation, (2) the ED capacity of the bar was much larger than that dissipated by a single AISC loading protocol, and (3) a specimen with a discontinuous composite slab, which opened freely at the centerline of the column, ensured the same self‐centering hysteretic behavior as the bare steel specimen. However, the decompression moment of the PT connection decreased significantly at each interstory drift, resulting in an early opening of a gap at the beam–column interface. Copyright © 2008 John Wiley & Sons, Ltd.     

Seismic behaviour of self‐centring braced frame buildings with reusable hysteretic damping brace

This paper presents the seismic behaviour of a concentrically braced frame system with self‐centring capability, in which a special type of bracing element termed reusable hysteretic damping brace (RHDB) is used. The RHDB is a passive energy dissipation device with its core energy‐dissipating component made of superelastic Nitinol wires. Compared with conventional bracing in steel structures, RHDB has a few prominent performance characteristics: damage free under frequent and design basis earthquakes in earthquake prone areas; minimal residual drifts due to the self‐centring capability of RHDB frame; and ability to survive several strong earthquakes without the need for repair or replacement. This paper also includes a brief discussion of the RHDB's mechanical configuration and analytical model for RHDB. The seismic performance study of RHDB frame was carried out through a non‐linear time history analysis of 3‐storey and 6‐storey RHDB frame buildings subjected to two suites of 20 earthquake ground motions. The analysis results were compared with buckling‐restrained brace (BRB) frames. This study shows that RHDB frame has a potential to outperform BRB frames by eliminating the residual drift problem. Copyright © 2007 John Wiley & Sons, Ltd.     

Shaking table test and numerical simulation of a 1/2‐scale self‐centering reinforced concrete frame

Self‐centering reinforced concrete frames are developed as an alternative of traditional seismic force‐resisting systems with better seismic performance and re‐centering capability. This paper presents an experimental and computational study on the seismic performance of self‐centering reinforced concrete frames. A 1/2‐scale model of a two‐story self‐centering reinforced concrete frame model was designed and tested on the shaking table in State Key Laboratory of Disaster Reduction in Civil Engineering at Tongji University to evaluate the seismic behavior of the structure. A structural analysis model, including detailed modeling of beam–column joints, column–base joints, and prestressed tendons, was constructed in the nonlinear dynamic modeling software OpenSEES. Agreements between test results and numerical solutions indicate that the designed reinforced concrete frame has satisfactory seismic performance and self‐centering capacity subjected to earthquakes; the self‐centering structures can undergo large rocking with minor residual displacement after the earthquake excitations; the proposed analysis procedure can be applied in simulating the seismic performance of self‐centering reinforced concrete frames. To achieve a more comprehensive evaluation on the performance of self‐centering structures, research on energy dissipation devices in the system is expected. Copyright © 2015 John Wiley & Sons, Ltd.     

Evaluation of performance of non‐invasive upgrade strategy for beam–column sub‐assemblages of poorly designed structures under seismic type loading

Beam–column sub‐assemblages are the one of the most vulnerable structural elements to the seismic loading and may lead to devastating consequences. In order to improve the performance of the poorly/under‐designed building structures to the critical loading scenarios, introduction of steel bracing at the RC beam–column joint is found to be one of the modern and implementable techniques. In the present work, a diagonal metallic single haunch/bracing system is introduced at the beam–column joints to provide an alternate load path and to protect the joint zone from extensive damage because of brittle shear failure. In this paper, an investigation is reported on the evaluation of tae influence of different parameters, such as angle of inclination, location of bracing and axial stiffness of the single steel bracing on improving the performance through altering the force transfer mechanism. Numerical investigations on the performance of the beam–column sub‐assemblages have been carried out under cyclic loading using non‐linear finite element analysis. Experimentally validated numerical models (both GLD and upgraded specimen) have been further used for evaluating the performance of various upgrade schemes. Cyclic behaviour of reinforcement, concrete modelling based on fracture energy, bond‐slip relations between concrete and steel reinforcement have been incorporated. The study also includes the numerical investigation of crack and failure patterns, ultimate load carrying capacity, load displacement hysteresis, energy dissipation and ductility. The findings of the present study would be helpful to the engineers to develop suitable, feasible and efficient upgrade schemes for poorly designed structures under seismic loading. Copyright © 2016 John Wiley & Sons, Ltd.     

Hybrid simulation testing of a self‐centering rocking steel braced frame system

The self‐centering rocking steel frame is a seismic force resisting system in which a gap is allowed to form between a concentrically braced steel frame and the foundation. Downward vertical force applied to the rocking frame by post‐tensioning acts to close the uplifting gap and thus produces a restoring force. A key feature of the system is replaceable energy‐dissipating devices that act as structural fuses by producing high initial system stiffness and then yielding to dissipate energy from the input loading and protect the remaining portions of the structure from damage. In this research, a series of large‐scale hybrid simulation tests were performed to investigate the seismic performance of the self‐centering rocking steel frame and in particular, the ability of the controlled rocking system to self‐center the entire building. The hybrid simulation experiments were conducted in conjunction with computational modules, one that simulated the destabilizing P‐Δ effect and another module that simulated the hysteretic behavior of the rest of the building including simple composite steel/concrete shear beam‐to‐column connections and partition walls. These tests complement a series of quasi‐static cyclic and dynamic shake table tests that have been conducted on this system in prior work. The hybrid simulation tests validated the expected seismic performance as the system was subjected to ground motions in excess of the maximum considered earthquake, produced virtually no residual drift after every ground motion, did not produce inelasticity in the steel frame or post‐tensioning, and concentrated the inelasticity in fuse elements that were easily replaced. Copyright © 2014 John Wiley & Sons, Ltd.     

腹板摩擦式钢框架节点震后可更换性能的试验研究

主要针对梁腹板带有摩擦耗能螺栓的自复位钢框架节点结构进行抗震性能和可更换性能的试验研究,探讨该类节点在往复荷载作用下的滞回性能以及节点域的变形特征。在参数选型的基础上,对5组钢框架节点试件进行了低周反复荷载作用下的拟静力试验,其中:4组试件具有自复位能力,分析了各试件的承载力、刚度、耗能性能和滞回特性等性能。综合研究结果表明:所提出的拼接节点能够利用摩擦螺栓的滑移提高节点的耗能能力,有效减少梁和柱主体构件的损伤,同时预应力筋提供了结构的自复位能力。试验结果表明:在地震作用之后,通过更换腹板及摩擦螺栓可以使结构的承载能力和耗能性能与震前基本一致,从而实现结构功能的快速恢复。