Cyclic performance of steel and composite bracing members

This paper describes an experimental study on the response of hollow and filled steel members to monotonic and cyclic axial loading. Monotonic tests were first performed on short specimens to establish their compressive and tensile axial resistances and to investigate the effect of infill on local buckling and ductility. These were followed by cyclic tests on longer bracing members with three different cross-section sizes. The presence of concrete infill was observed to influence the mode of failure displayed by the specimens, as well as their compression and tension load responses. The ductility capacities of the individual specimens are compared, and the effects of slenderness, steel strength and infill are quantified. The experimental findings are compared with the recommendations of a number of international codes of practice and previous research studies on the seismic response of steel braces. It is found that the infill contributes to the compression resistance of the brace, even after multiple inelastic load reversals, and that it can improve ductility capacity by preventing or limiting local buckling.     

Displacement-restraint bracing for seismic retrofit of steel moment frames

This paper presents a seismic retrofit method using wire rope (cable) bracing for steel moment-resisting frames. The retrofitted frame using the proposed bracing system exhibits ductile behavior and maintains seismic energy dissipation capacity to the same extent as the original bare frame. The bracing member does not act for small and medium vibration amplitudes. For large vibration amplitudes, the bracing member acts and restrains unacceptably large story drift. This retrofit method prevents an increase in the column compression force resulting from the brace action. Cyclic loading test results of the portal frames reveal fundamental characteristics of the proposed bracing system. Seismic response analyses are also conducted for the three-story moment-resisting frames. The effectiveness of the retrofit method is discussed in light of these test and analysis results.     

Intermediate HSS bracing members during seismic excitations: modeling,design, and behavior

Concentric hollow structural section (HSS) bracing members are used frequently in steel framed structural systems to resist seismic excitations. Finite element modeling of the HSS braces that utilizes the true stress-strain curves produces hysteresis responses that are reasonable matches to the experimental response. True stress-strain curves are obtained from coupon tests or stub-column tests while utilizing an exponential function or strain hardening rule with a trial and error procedure to obtain the hysteresis behavior. In the current study, the true stress-strain curves are directly obtained from tests on stub-columns extracted from the full scale HSS bracing members away from the mid-length plastic hinge after cyclic testing. Two experimental tests (Shaback 2001 and Haddad 2004) were used to validate the model. Results indicate that the stress-strain curves for these braces are not unique. A refined damage accumulation model for ultra-low-cycle fatigue is implemented to predict fracture of the brace tests. The refined damage model is then used in the finite element modeling to predict fracture of braces in a chevron braced frame of an eight-storey building subjected to selected ground motions analyzed using OpenSees program. Results indicate that all braces could sustain the selected earthquake records without fracture.     

基于OpenSEES数值模拟的工字形截面支撑低周疲劳性能研究

为研究工字形截面支撑低周疲劳性能,进行了18根Q235和21根ST12材质焊接工字形钢支撑在等幅轴向循环位移作用下的OpenSEES有限元分析,在验证模拟结果和试验结果吻合较好的同时,提出利用支撑几何参数(长细比、宽厚比、高厚比、屈服点)修正的表征应变作为预测支撑低周疲劳寿命的控制参量,给出了对应修正公式和寿命预测公式。表征应变与支撑低周疲劳寿命满足对数线性关系,据此预测的支撑低周疲劳寿命离散性较小且基本位于试验值1.5倍分散带内。对于变幅加载条件下的支撑寿命预测,利用Miner线性损伤理论对39组支撑试验数据进行以裂纹萌生寿命为终止点的损伤值统计,给出了容许损伤建议值作为预测寿命对应损伤值。而后,引入5组单斜工字钢支撑框架低周疲劳试验结果进行验证,其预测寿命均接近试验裂纹萌生寿命,可以较为保守地评估该类支撑的低周疲劳寿命,且支撑截面损伤分布与试验破坏模式保持一致。以上均证明该数值模拟方法和低周疲劳寿命预测方法的准确性和高效性,可为框架支撑结构低周疲劳性能的研究提供借鉴。     

Inelastic postbuckling response of steel trusses under thermal loadings

The paper presents a finite element procedure for the inelastic postbuckling analysis of steel trusses under thermal loadings. First, the force-displacement relation in total form is derived. Then an element is formulated to simulate the inelastic nonlinear behavior of truss members under thermal loadings. The actions due to restraint against elongation and material softening are referred to as thermal loads and reduced member loads, respectively. To account for the material softening at elevated temperatures, the EC3 reduction factors are adopted. The trusses are assumed to be either under constant temperature but increasing loads, or under constant loads but rising temperature. The generalized displacement control (GDC) method is modified to solve for the postbuckling paths of the trusses. The results indicate that the critical load (or temperature) of a truss decreases drastically as the preheating (or preload) increases, and that a truss shows a breaking strength much smaller than the elastic critical load, if the effect of yielding is taken into account. From the design point, the overall capacity of an elastic truss can be well represented by the critical load-temperature curve.     

Inelastic response of composite steel and concrete base column connections

This paper assesses the experimental results of monotonic (pushover) tests carried out on partially encased composite steel-concrete columns connected to the foundation block through the traditional bolted steel end plate and an innovative socket type system. These tests show that the structural response of the traditional connection is significantly influenced by the behaviour of the anchorage bolts. The latter cause large fixed end rotations and possess limited energy dissipation. Conversely, innovative composite steel-concrete base column connections with socket systems exhibit adequate overstrength, inelastic deformations and energy absorption capacity. Furthermore, socket-type connections are characterized by spreading of inelasticity at the base of the composite columns without damage localization on concrete and interface components. It can thus be argued that the innovative connection type assessed in this study is a viable solution for applications in framed structures fulfilling capacity design requirements, e.g. structural systems in earthquake prone regions.     

钢框架斜隅支撑体系抗震性能研究现状及展望

简述了钢框架斜隅支撑体系的特点,分析了节点连接方式与隅撑布置方式对斜隅支撑钢框架抗震性能的影响,并提出了钢框架斜隅支撑体系抗震性能的研究方向,以供参考。     

建筑电气抗震支吊架设计

根据现行规范,本文论述了抗震支吊架的设置原则,列出了电气母线槽、电缆梯架、托盘桥架、槽盒以及线缆的重量,分析了其重量与截面积的对应关系,进而给出需设置抗震支吊架的母线、桥架和槽盒的规格,为建筑电气专业机电抗震的前期设计提供技术参考。     

焊接工字形截面钢支撑低周疲劳性能试验研究

通过21根铰支焊接工字形截面支撑在6倍试件的受拉屈服位移幅值下的对称循环轴向加载试验,研究了具有不同几何特性的该类支撑构件的低周疲劳和耗能性能。研究发现,支撑翼缘直到断裂的疲劳损伤发展过程可分为可见裂纹萌生,穿透裂纹形成和穿透裂纹扩展3个阶段,并根据试验数据提出了估算支撑构件疲劳寿命和循环耗能能力的经验公式。统计分析表明,裂纹萌生疲劳寿命与弹塑性局部屈曲密切相关,当支撑翼缘的宽厚比越小,支撑的长细比越大时,其裂纹萌生疲劳寿命就越长。试验还表明,低周疲劳性能较优的支撑构件亦具有较高的耗能能力。为了防止框架-中心支撑结构中的支撑构件在强震下过早发生断裂破坏,在实际设计中除要限制板件的宽厚比之外,还应适当放宽现行规范对支撑构件长细比的限制。     

Structural response of stainless steel oval hollow section compression members

Cold-formed stainless steel oval hollow sections (OHS) offer the combined aesthetic appeal of circular hollow sections and stainless steel, together with the structural efficiency associated with cross-sections of differing geometric properties about their two principal axes. To date, no structural design guidance exists for these cross-sections, principally due to their relatively recent introduction and a lack of fundamental structural test data. This paper examines the structural response of stainless steel OHS compression members and presents design recommendations. A series of laboratory tests was carried out to generate fundamental structural performance data. Tensile coupon tests were initially performed to establish the basic material stress–strain characteristics of the sections. These were followed by stub column tests to determine the average compressive response of the cross-sections and flexural buckling tests to obtain ultimate load carrying capacity data for use in the determination of a suitable buckling curve for stainless steel OHS. Measurements of the geometric properties of the test specimens including initial imperfections were carried out. The full load–displacement responses of the specimens were recorded and have been presented herein. A finite element (FE) modelling programme was performed in parallel with the experimental study. Once the FE models had been validated against the test results, parametric studies were carried out to further investigate the influence of individual key parameters, including the aspect ratio and local slenderness of the cross-sections as well as the member slenderness. Based on the obtained experimental and numerical results, a class 3 limit for stainless steel OHS in compression and a suitable buckling curve for OHS columns have been proposed.     

Inelastic storey-buckling factor of steel frames

This paper first briefly reviews the modified effective length method in the literature for design of steel columns considering the inter-column interaction with the limitations outlined. To overcome these limitations, a new storey-buckling approach for steel frames is presented, where each storey is treated as a whole at the sway buckling, and its resistance is evaluated using the presented storey-buckling factor, which can be obtained from the defined storey slenderness using the relationships illustrated by the column curves in current codes. In total, 30 frame examples, considering the effects of storey number, the span number and the configuration of axial forces in columns, are adopted to examine the proposed method. Comparisons between FE results show that the presented method has good performances in predicting the failure load of considered frames. In FE analysis, the geometric and material nonlinearities as well as two types of geometric imperfections and longitudinal residual stresses are considered. Predictions from the notional load approach are also compared with FE results for some cases, which shows that these predictions may underestimate or overestimate the failure load of certain frames.The interaction between the sway buckling of the whole storey and the non-sway buckling of the weakest column may reduce the failure load of steel frames. In this study, a reduction factor is presented to consider this mode interaction, and comparisons with FE results show that this method can reasonably assess the adverse effects of this mode interaction.     

Inelastic behaviors of steel shear tab connections

Inelastic behaviors of shear tab connections commonly used in modern steel buildings are investigated in this work. Full‐scale steel shear tab connections with and without concrete slab physically tested by other researchers are closely simulated by non‐linear finite element (FE) method. Different nonlinear FE features (inelastic materials, surface‐based contacts and large geometric options) are included, and different solution strategies (Newton method and Explicit Dynamic method) are employed to balance computational effort and solution accuracy. The simulations extend our understandings on shear tab connections at micro levels, including stress distribution in the connection zone, movement of the neutral axis along the beam sections and normal stress distributions along steel shear tabs and concrete slabs. It is found that the shear tabs contribute to flexural strength of the beam‐to‐column connection and the elastic–plastic theory explains the observed behavior well only when concrete slab is not present. The composite steel shear tab connections have unsymmetrical behavior under negative and positive bending moments. The compressive concrete slabs significantly increase the flexural stiffness and strength of composite shear tab connections. It is also verified that the shear studs near steel columns play a key role for the composite connections. Copyright © 2013 John Wiley & Sons, Ltd.     

Cyclic axial response and energy dissipation of cold-formed steel framing members

This paper summarizes results from an experimental program that investigated the cyclic axial behavior and energy dissipation of cold-formed steel C—sections structural framing members. Fully characterized cyclic axial load–deformation response of individual members is necessary to facilitate performance-based design of cold-formed steel building systems. Specimen cross-section dimensions and lengths were selected to isolate specific buckling modes (i.e., local, distortional or global buckling). The cyclic loading protocol was adapted from FEMA 461 with target displacements based on elastic buckling properties. Cyclic response showed large post-buckling deformations, pinching, strength and stiffness degradation. Damage accumulated within one half-wave after buckling. The total hysteretic energy dissipated within the damaged half-wave decreased with increasing cross-section slenderness. More energy dissipation comes at the cost of less cumulative axial deformation before tensile rupture.     

Thermal response of steel framing members in open car park fires

For open car park structures, adopting a performance-based structural fire design is often justified and allowed because the fire does not reach flashover. However, this design approach requires an accurate assessment of temperatures in structural members exposed to car fires. This paper describes a numerical study on the thermal exposure on steel framing members in open car park fires. Steel temperatures are computed by the coupling of computational fluid dynamics and finite element modeling, and by analytical models from the Eurocodes. In addition, the influence of galvanization on the steel temperature evolution is assessed. Results show that temperatures in unprotected beams and columns are influenced by the section geometry, car fire scenario, modeling approach, and use of galvanization. Galvanization slightly delays and reduces peak temperature. Regarding the different models, CFD-FEM (CFD: computational fluid dynamics, FEM: finite-element method) coupled models predict lower temperatures than the Hasemi model, because the latter conservatively assumes that the fire flame continuously touches the ceiling. Further, the Hasemi model cannot account for the effect of reduced emissivity from galvanization on the absorbed heat flux. Detailed temperature distributions obtained in the steel members can be used to complete efficient structural fire designs based on the member sections, structure layout, and use of galvanization.     

变截面门式刚架地震反应研究进展

总结了国内外变截面门式刚架抗震设计和地震反应研究的主要方法及研究现状,指出了我国开展变截面门式刚架进行地震反应研究的必要性和迫切性,针对我国现有规范(程)中变截面门式刚架抗震设计规定的不足,对变截面门式刚架的地震反应进行了研究分析,提出了变截面门式刚架地震反应分析和设计方法的研究方向.     

Use of steel bracing in reinforced concrete frames

In this paper the use of steel bracing in concrete-framed structures is investigated. The investigation is carried out through a series of tests conducted on a number of model frames. The object of the tests was to determine the degree of effectiveness of different diagonal bracing arrangements to increase the in-plane shear strength of the concrete frame and to observe the relative behaviour of tension and compression braces. The important question of the proper connections between the steel braces and the concrete frame is also considered. The test results indicate a considerable increase in the in-plane strength of the frame due to steel bracing. As an overall conclusion it is noted that, with proper connection between the brace and the frame, the steel bracing could be a viable alternative or supplement to shear walls in concrete framed buildings in seismic areas.     

消能减震支撑体系研究与在高烈度区的工程应用

对高烈度地震区某6层混凝土框架结构办公楼应用由钢支撑和液体粘滞阻尼器组成的消能减震阻尼器支撑体系进行了抗震分析。利用ETABS软件对其在设防烈度8度(0.3g)的地震作用下的地震反应进行了非线性时程分析。计算分析表明,该类消能减震阻尼器支撑有良好的消能减震效果,为类似工程提供了较好的参考。     

Plastic design of CB-frames with reduced section solution for bracing members

The seismic behaviour of concentrically braced frames (CBFs) designed according to the current European provisions is unsatisfactory due to the premature out-of-plane buckling of columns. For this reason, a new design methodology, based on a rigorous application of “capacity design” criteria has been recently proposed. In addition, aiming at a reduction of the plastic out of plane deformations of gusset plates due to brace buckling and at the prevention of sudden impact load affecting connections at the end of the straightening phase, Eurocode 8 requires the limitation of the brace slenderness. This limitation leads to the oversizing of diagonals and, consequently, of beams and columns. Therefore, to avoid this problem a new design strategy for bracing members is suggested: the Reduced Section Solution (RSS). It allows the calibration of the diagonal yielding resistance, leaving the brace slenderness practically unchanged.The results of dynamic inelastic analyses carried out with reference to braced frames designed according to the proposed procedure, both with and without RSS, are compared with those obtained with reference to the same structural schemes designed according to Eurocode 8. The obtained results show that the proposed design approaches are able to assure a significant improvement of the seismic performance.     

Performance evaluation and strengthening of concrete structures with composite bracing members

Steel bracings, in different geometrical forms, are commonly used in steel and concrete structures. The lateral stiffness of structures with concentric bracings depends on the buckling capacity of compressive bracings; in turn, this buckling phenomenon leads to a decrease in the energy dissipation capacity. Composite bracings, composed of steel cores encased in concrete, can increase their capacities. In this paper, the behavior of composite bracings is studied in two parts. At first, a number of braced frames are selected and their behaviors under cyclic loading are studied. Then, using the data obtained from the first part, two existing concrete structures, a three story and a nine story building, are selected and strengthened against seismic loadings by both the conventional concentric steel and the latter composite bracing systems. The behaviors of these structures are then studied by the push-over method and the results for the two types of bracings are compared.     

Effects of eccentric steel bracing systems on seismic fragility curves of mid-rise R/C buildings: A case study

In this study, the seismic reliability of a mid-rise reinforced concrete (R/C) building retrofitted using eccentric steel braces is investigated through fragility analysis. As a case study, a six storey mid-rise R/C building was selected. The design of selected sample building was made with reference to 1975 version of the Turkish Seismic Code. The effectiveness of using different types of eccentric steel braces in retrofitting the building was examined. The effect of distributing the steel bracing over the height of the R/C frame on the seismic performance of the retrofitted building was studied. For the strengthening of the original structure, D, K, and V type eccentric bracing systems were utilized and each of these bracing systems was applied with four different spatial distributions in the structure. For fragility analysis, the study employed a set of 200 generated earthquake acceleration records compatible with the elastic code design spectrum. Nonlinear time history analysis was used to analyze the structures subjected to this set of earthquake accelerations generated in terms of peak ground accelerations (PGA), whilst monitoring four performance limit states. The fragility curves were developed in terms of PGA for these limit states; namely: slight, moderate, major, and collapse with lognormal distribution assumption. The improvement of seismic reliability achieved through the use of D, K, and V type eccentric braces was evaluated by comparing the median values of the fragility curves of the existing building before and after retrofits. As a result of this study, the improvement in seismic performance of this type of mid-rise R/C building resulting from retrofits by different types of eccentric steel braces was obtained by formulation of the fragility reduction.