On the concept of earthquake resistant hybrid steel frames

The concept of hybrid steel frame system is presented in which mixtures of fully-rigid and semi-rigid steel connections are used in the 9 and 20-story SAC frames. Several different patterns and locations of semi-rigid connection replacements within the frame are examined in order to identify the hybrid frames with most energy dissipation capabilities. Inelastic dynamic analyses are conducted on the proposed selected frames by subjecting them to the Los Angeles earthquake records characterized as those with 10% probabilities of exceedance in 50 years and 2% probabilities of exceedance in 50 years. The maximum story drift for the hybrid frames are determined and compared to the “life safety” and “collapse prevention” limits as recommended by the FEMA 356. The roof-displacement time history, story drift, and member forces for the hybrid frames are reported and compared with those of conventional SAC frames without semi-rigid connections.     

Experimental analysis of seismic resistant composite steel frames with dissipative devices

This article presents the results of the experimental research performed at Politecnico di Milano, within the project Fuseis. The research project aims at developing innovative types of seismic resistant composite steel frames with dissipative fuses. In case of strong seismic events, damage will concentrate only in these fuses, without observing any significant damage in the structural elements such as steel beams, columns and reinforced concrete slab of the structure. After the seismic event, the repair work will be limited only to replacing the fuses. Four full scale tests are implemented in order to evaluate the performance of a composite steel frame with fuse devices under seismic actions in terms of moment rotation behavior of the joints, global energy dissipation, storey drift and frame stability.     

焊接及螺栓连接钢框架的循环加载试验研究

为了探讨、比较焊接和螺栓连接钢框架的极限承载能力、滞回性能及动力特性,本文进行了6榀1∶2比例的单跨双层钢框架循环加载试验和动力特性测试,其中焊接、端板螺栓连接、角钢螺栓连接钢框架各两榀。试验过程中每层框架都铺设了混凝土楼板和配重,水平循环荷载按三角形分布施加。循环加载试验表明:当焊接连接框架的焊缝质量较高时,钢框架具有良好的承载能力和滞回性能,其最终破坏模式是构件形成塑性铰而发生强度破坏,属于典型的延性破坏,框架侧向层间塑性变形可以达到层高的1/25,塑性变形主要来自梁柱连接节点域的剪切变形和柱、梁的弯曲变形;端板螺栓连接框架的承载能力和滞回性能也较好,耗能能力略低于焊接框架,框架侧向层间塑性变形可以达到层高的1/30,但端板焊缝容易发生断裂;角钢螺栓连接框架的承载能力和耗能能力相对较低,塑性变形主要发生在连接角钢和柱脚部位,且翼缘连接角钢容易发生低周疲劳破坏。动力特性测试结果表明:随着节点转动刚度的减小,框架自振     

钢框架栓焊梁柱连接节点的抗震设计

节点设计是整个钢结构设计的重要环节,提高钢框架梁柱连接节点的抗震性能是整个钢结构抗震设计的重要内容.本文主要介绍了提高钢框架栓焊梁柱连接节点的抗震设计的主要措施,主要方法包括:通过加腋和加盖板对节点进行加强,提高节点承载力;对梁局部削弱,使塑性铰从节点区外移至梁上;改进节点区焊接孔构造形式,缓解局部应力集中;妥善处理弧板和垫板,减少焊缝缺陷.     

Performance of bolted angle connections in progressive collapse of steel frames

This paper is mainly focused on the behaviour of bolted top‐seat angle connections with double web angles, categorized as partially restrained connections, in progressive collapse of semi‐rigid steel frames due to sudden column loss. The main characteristics of this type of loading are declared and the effect of the imposed boundary conditions on the motivated elements is depicted. To study the performance of frame connections under these conditions, refined nonlinear finite element modelling technique is used. The models are created based on the previous experimental studies and their accuracy is examined through a comparison to the results of these tests. New models are created under conditions of the progressive collapse and the behaviour of the connections is studied. The study showed that due to the applied boundary conditions, the connections behave stiffer and show a higher moment capacity. Based on the results of the numerical models, equations are presented to estimate the moment‐rotation response of the studied connections under progressive collapse conditions based on the beam and connection mechanical and geometrical properties and the connection rotation. Copyright © 2009 John Wiley & Sons, Ltd.     

半刚接钢框架-十字加劲钢板剪力墙结构滞回性能研究

对一榀单跨两层半刚接框架-十字加劲钢板剪力墙结构进行水平反复荷载作用下的抗震试验研究,系统分析结构破坏模式和耗能机理,研究节点刚度与加劲墙体的相互影响效果,得到了承载力,延性,刚度和耗能能力等指标。试验结果表明:该种结构具有良好的延性和耗能性能,安全储备高;节点刚度退化小,内填钢板的设置缓解了节点区自身的延性要求,半钢框架和墙板协同工作良好;加劲肋的设置改善了钢板的实际受力,提高墙体的承载力及刚度,减轻了滞回曲线的捏缩现象,减小钢板噪音及震颤。结构破坏模式为加劲肋屈曲,内填钢板以小区格局部屈曲为主,伴随相关屈曲;框架柱脚及梁柱半刚性连接部位形成塑性铰;试件面内呈弯曲破坏模式,研究为该种结构体系的工程应用和理论分析提供依据。     

Analytical expressions for preliminary design of dissipative bracing systems in steel frames

In this paper a direct displacement-based design (DDBD) method for seismic design of steel frames equipped with dissipative braces is proposed. Attention is focused on concentric braced steel frames with pinned beam-to-column joints in which the bracing system (with viscoelastic or elastoplastic dissipative devices) is the main seismic resistant component. The proposed design method uses an equivalent continuous model where flexural deformability and shear deformability are related respectively to columns and diagonals of the bracing system. In this way, analytical expressions of the required flexural and shear stiffness distributions are obtained. These expressions are quite simple and can be conveniently used in preliminary design of dissipative diagonal braces and columns. Examples are shown for steel frames with dissipative braces based on elastomeric dampers (viscoelastic devices) and steel frames with buckling-restrained braces (elastoplastic devices). Results of time history analyses are illustrated and discussed in order to evaluate the effectiveness of the proposed DDBD procedure.     

Analysis of the behaviour of stainless steel bolted connections

This study is focused on two types of bolted connections that are common in steel structures. They concern cover plate connections and T-stubs, where the bolts are loaded in shear or in tension. The Eurocode 3 requirements for stainless steel connection design are essentially the same as for carbon steel. The study considers the case of austenitic stainless steel for which the conventional elastic limit is relatively low compared to the ultimate strength. In bearing, criteria on deformation limits have to be considered for cover plate connections. In T-stubs, strain hardening of stainless steel exhibits a continuous increase of the applied load and can influence the failure mode. A finite element model is developed and validated for the two types of connections. A more extensive parametric study should be carried out to develop a better understanding of the behaviour of stainless steel connections.     

Influence of initial imperfection on the behaviour of extended bolted end-plate connections for portal frames

Initial imperfection in bolted end-plate connections is mainly induced by residual deformation of welding. Three typical initial imperfections, namely V-shape, C-shape and W-shape are identified in terms of the geometrical imperfection shapes for extended bolted end-plate connections based on a field study. Seven specimens of extended bolted end-plate connection, among which six specimens with V-shape initial imperfection and one without imperfection in the end-plates were tested, and the influence of the initial imperfection of the end-plate on the behaviour of the connections was investigated. Test results show that initial rotation stiffness of extended bolted end-plate joints decreases with increase of initial imperfection in the end-plate, however, the strength of the end-plate joints is only affected slightly by the imperfection. Besides, the thicker the end-plate is, the more reduction in the initial rotation stiffness relatively. Further parametric study on 14 extended bolted end-plate connections with three typical shapes of imperfections were conducted based on the finite element analysis. Among the three types of initial imperfection, the V-shape has the least influence, and the C-shape has the biggest influence on the initial rotation stiffness of the extended bolted end-plate joints. For initial geometrical imperfection in a V-shape, its influence on behaviour of extended bolted end-plate connections is negligible as far as the imperfection is limited to the criterion, giving H/300. The influence of initial imperfection in a W-shape on rotation stiffness should be acceptable if the initial imperfection is limited to the criterion. In the case of a C-shape initial imperfection in the end-plate, its influence on the rotation stiffness would be substantial; even it satisfies the requirement of the imperfection limit. The results of the study should be applicable to the bolted end-plate connections, whether flushed or extended end-plate beam to column joints in structural frames.     

Disproportionate collapse performance of partially restrained steel frames with bolted T-stub connections

Current design and regulatory documents require the assessment of certain buildings with regard to their vulnerability to disproportionate collapse through notional load-bearing element removal followed by an alternative path analysis. While several recent studies have examined the robustness of steel building frames with fully moment-resistant connections and their ability to sustain local damage to load-bearing elements, only limited research is available regarding the robustness of steel frames with partially restrained (PR) connections. This paper investigates the performance of steel frames with partially restrained connections fabricated from bolted T-stubs following damage to load-bearing columns. A macro-model of a bolted T-stub connection for use in nonlinear analysis is presented, verified through experimental data, and incorporated in a nonlinear finite element model to evaluate the performance of PR frames. Two prequalified T-stub connections are considered, one of which is intended to develop the full moment capacity of the beam. The evaluation is conducted for various floor plans of typical office buildings. The analysis indicates that the frames with strong T-stub connections can resist collapse in damage scenarios involving notional removal of one first-story column, while the robustness of the frames with weak T-stub connections is questionable.     

Fire after earthquake analysis of steel moment resisting frames

Fire following earthquake can cause substantially loss of life and property, added to the destruction already caused by the earthquake, and represents an important threat in seismic regions. On the other hand, even when no fire develops immediately after an earthquake, the possibility of later fires affecting the structure must be adequately taken into account, since the earthquake induced damages make the structure more vulnerable to fire effects than the undamaged one. The paper presents the evaluation of the fire resistance time for some unprotected steel moment resisting frames, in the hypothesis that they are already damaged by the earthquake, using advanced methods for earthquake and subsequent fire analysis, and using both standard and natural fire scenarios. Moderate and severe seismic actions are used for designing the steel structures. The influence of the damage level induced by the earthquake on the fire resistance is emphasized.     

Elastic buckling and second-order behaviour of pitched-roof steel frames

The aims of this work are two-fold: (i) to present the results of a study concerning the elastic in-plane stability and second-order behaviour of unbraced single-bay pitched-roof steel frames and (ii) to propose, validate and illustrate the application of an efficient methodology to design this type of commonly used frame. After (i) characterizing the relevant frame buckling modes and P-Δ second-order effects, and (ii) addressing the exact and approximate calculation of the associated bifurcation loads and secondary bending moments, the paper deals with the incorporation of these concepts in the definition of an efficient design procedure. In particular, it is clearly shown that, due to the rafter slope, the geometrically nonlinear behaviours of orthogonal beam-and-column and pitched-roof frames are qualitatively different. Finally, the proposed concepts and methodologies are illustrated through the presentation and discussion of numerical results involving fixed and pinned-base frames.     

Hysteretic energy response of steel moment‐resisting frames with vertical mass irregularities

Dynamic analyses were carried out to study the seismic response of high‐rise steel moment‐resisting frames in 16‐storey buildings. The frames are intentionally designed using three different design procedures: strength‐controlled design, strong column–weak beam controlled design, and drift‐controlled design. The seismic performances of the so‐designed frames with vertical mass irregularities were discussed in view of drift ratio, plastic hinge rotation, hysteretic energy input and stress demand. A demand curve of hysteretic energy inputs was also presented with two earthquake levels in peak ground accelerations for a future design application. Copyright © 2004 John Wiley & Sons, Ltd.     

高强螺栓端板连接钢梁-方钢管混凝土框架结构抗震性能研究

通过一榀高强螺栓端板连接的2层1跨半4∶7比例组合梁-方钢管混凝土模型框架的拟动力试验、拟静力试验和静力推覆试验,运用子结构方法,模拟了一榀10层3跨的平面框架,研究了钢管混凝土框架结构的破坏形态、位移响应、滞回特性和耗能等抗震性能。拟动力试验中,多遇烈度地震时,框架刚度降低很小;基本设计烈度地震时,框架刚度降低约13%;罕遇烈度地震时,框架刚度降低20%,框架层间位移角小于2.0%。拟静力试验中,最大层间位移角超过1.6%,部分构件出现初始局部屈曲,框架整体未出现强度退化。在静力推覆试验中,即使层间位移角超过6.0%,框架承载力仍未发生下降。高强螺栓端板连接节点能够满足抗震设计要求,在规范限定的按弹塑性设计的层间位移角限值范围内,经过合理设计的螺栓端板连接可视为刚性连接。试验结果验证采用螺栓连接的钢梁-方钢管混凝土框架具有良好的抗震性能,可在抗震设防区结构中推广使用。     

The seismic behaviour of steel moment-resisting frames with stiffening braces

The present paper deals with the seismic behaviour of steel structures which are designed in the attempt of exploiting the dual characteristics of moment resisting frames (MRFs) and concentrically braced frames (CBFs) as lateral force resisting systems. Three prototype frames are studied within the context of Eurocode 8 (EC8) provisions; these are MRFs which are traditionally designed, against ultimate seismic actions (ULS), without checking serviceability limit state rules (SLS-interstory drift limits). To fulfill these requirements concentrical braces are inserted in the frames, which are not considered in the collapse resistance of the structural system. The non-linear behaviour of these systems under ultimate seismic environment is studied and compared to the one of the unbraced MRFs. The influence of second order effects is also investigated.     

The collapse behaviour of braced steel frames exposed to fire

Progressive collapse mechanisms of braced two-dimensional steel-framed structures, subjected to fire heating, are investigated using a robust static–dynamic procedure developed by the authors. 20 cases have been analysed to provide a comprehensive view of the mechanisms of progressive collapse for these frames, with different bracing systems under different fire conditions. The influences of stiffness and strength of the bracing systems are also analysed. The results indicate that the pull-in of columns is one of the main factors which generate progressive collapse. Horizontal “hat truss” bracing systems have limited capacity to avoid pull-in of columns supporting the heated floor, although they can directly redistribute the vertical load lost by buckling columns to adjacent columns. On the other hand, vertical bracing systems have the effect, not only of increasing the lateral restraint of the frame, which reduces the pull-in of the columns, but also of effectively preventing the collapse progressing from local to global. Stronger vertical bracing systems can redistribute load from a buckled column to its surrounding structural members. Frames with a combined hat and vertical bracing system can be designed to enhance the capability of the frame as much as possible to prevent progressive collapse when a heated column buckles.     

Review on the modelling of joint behaviour in steel frames

Steel portal frames were traditionally designed assuming that beam-to-column joints are ideally pinned or fully rigid. This simplifies the analysis and structural design processes, but at the expense of not obtaining a detailed understanding of the behaviour of the joints, which in reality, have finite stiffness and are therefore semi-rigid. The last century saw the evolution of analysis methods of semi-rigid joints, from the slope-deflection equation and moment distribution methods, to matrix stiffness methods and, at present, to iterative methods coupling the global and joint structural analyses. Studies agree that in frame analysis, joint rotational behaviour should be considered. This is usually done by using the moment-rotation curve. Models such as analytical, empirical, experimental, informational, mechanical and numerical can be used to determine joint mechanical behaviour. The most popular is the mechanical model, with several variances (e.g. Component Method). A summary is given of the advantages and disadvantages and principal characteristics of each model. Joint behaviour must be modelled when analysing semi-rigid frames, which is associated with a mathematical model of the moment-rotation curve. Depending on the type of structural analysis required, any moment-rotation curve representation can be used; these include linear, bilinear, multilinear and nonlinear representations. The most accurate representation uses continuous nonlinear functions, although the multilinear representation is commonly used for mechanical models. This article reviews three areas of steel joint research: (1) analysis methods of semi-rigid joints; (2) prediction methods for the mechanical behaviour of joints; (3) mathematical representations of the moment-rotation curve.     

Hysteretic behaviour of flush end plate joints to concrete-filled steel tubular columns

The results of an experimental research study involving the testing of four bolted moment-resisting connections under simulated seismic loading conditions are presented. Each test specimen modeled the interior joint of a moment-resisting frame consisting of H-shaped steel beams and circular or square concrete-filled steel tube (CFST) columns using high-strength blind bolts. In order to investigate the seismic behaviour of the blind bolted flush end plate joints to CFST columns, the hysteretic performance, failure modes, stiffness degradation and energy dissipation of the connection type are evaluated in detail. The test parameters varied included the column section type and the thickness of the end plate. The experimental results indicate that both the blind bolted connections with circular and square sections exhibited excellent hysteretic behaviour in terms of their moment–rotation response, strain distributions and energy dissipation. Under cyclic loading, all tested specimens displayed large rotation ductility capacities, and the failure modes were similar to those under monotonic loads. The effects of cyclic loading on the behaviour of the composite joint were obvious, especially on load bearing and stiffness of the connections. The joint type exhibited excellent seismic performance, so that it can be effectively utilized in moment-resisting composite frame structures.     

Seismic design procedure development for cold-formed steel-special bolted moment frames

Seismic design provisions of Cold-Formed Steel-Special Bolted Moment Frames in the proposed AISI Seismic Standard were developed on the basis that ductility capacity is provided through bolt slippage and bearing in bolted beam-to-column moment connections, and that beams and columns are to remain elastic at the design story drift to resist the maximum force that can be developed in the connections. Based on the instantaneous centre of rotation analysis procedure, both the slip and bearing components of the connection resistance, expressed in the form of column shear and story drift, are presented and design values tabulated.     

不同构造形式延性节点的钢框架损伤退化滞回性能有限元分析

针对空间钢框架在低周反复荷载作用下的滞回性能、破坏机理和损伤退化性能等进行了有限元分析。研究结果表明:加腋型节点钢框架可以有效降低梁柱翼缘连接焊缝处应力,与其他构造形式节点相比,其耗能能力较强;翼缘板加强型节点可使梁柱翼缘焊缝处应力降低约20%。不同构造形式节点钢框架在低周反复荷载作用下承载力化正负向退化不一致;刚度退化曲线差别很小,钢框架节点形式的构造差异对其刚度退化曲线影响不显著。