Influence of connection design parameters on the seismic performance of braced frames

Special concentrically braced frames (SCBFs) are commonly used lateral-load resisting systems in seismic design. In SCBFs, the braces are connected to the beams and columns by gusset plate connections, and inelastic deformation is developed through tensile yielding and inelastic post-buckling deformation of the brace. Recent experimental research has indicated that the seismic performance of SCBFs can be improved by designing the SCBF gusset plate connections with direct consideration of the seismic deformation demands and by permitting yielding in the gusset plate at select performance levels.Experimental research provides important information needed to improve SCBF behavior, but the high cost of experiments limits this benefit. To extend and better understand the experimental work, a companion analytical study was conducted. In an earlier paper, the inelastic finite element model and analysis procedure were developed and verified through detailed comparison to experimental results. In this paper, the model and analytical procedure extend the experimental results. A parametric study was conducted to examine the influence of the gusset plate and framing elements on the seismic performance of SCBFs and to calibrate and develop improved design models. The impact of the frame details, including the beam-to-column connections, the brace angles, and their inelastic deformation demands, was also explored. The results suggest that proper detailing of the connections can result in a large improvement in the frame performance.     

Mechanical models for the analysis of bolted T-stub connections under cyclic loads

The mechanical models used to simulate the complete behavior of full-scale bolted T-stub connections under cyclic loads are mainly treated in this paper. These mechanical models are composed of individual T-stub components modeled as nonlinear spring elements in order to reliably reproduce their various response mechanisms interacting with one another in the connection. The hysteresis behaviors of the T-stub components including bolt/flange uplift, stem elongation, and relative slip deformation combined with bolt bearing are simulated by the multi-linear cyclic stiffness models characterized from their actual force–deformation response mechanisms each. The nonlinear component springs, which contain these idealized stiffness properties, are implemented into the user joint element produced based on the mechanical model so as to numerically generate the complete behavior of the full-scale connections with considerable accuracy. The analytical predictions performed on the joint element are evaluated against the experimental tests with respect to stiffness, strength, and deformation. Thus, the adequacy of the proposed modeling approach is verified through comparisons between analytical predictions and experimental test results. Finally, it can be shown that the mechanical model proposed in this study has the satisfactory potential to predict the response of the T-stub components as well as the behavior of the T-stub connections through analytical studies.     

反复荷载作用下方钢管混凝土柱与钢梁连接节点非线性有限元分析

本文首先选择合适的本构模型作为约束混凝土循环本构模型的骨架曲线,提出了考虑局部屈曲和开裂的钢材循环本构关系;进而建立两类方钢管混凝土柱与钢梁连接节点———缀板焊接连接节点和穿芯螺栓-端板连接节点的空间非线性分析模型,对其在低周反复荷载作用下的滞回性能进行了非线性分析计算。结果表明:由有限元模型所得的单调荷载-位移曲线与试验所得的低周反复荷载作用下的骨架曲线极为相似,但在峰值荷载后差异较大;由有限元模型所得的在低周反复荷载作用下滞回曲线也与在反复荷载作用下试验所得的相一致。有限元模型能准确地预测上述两类节点的弹塑性行为和整体抗震性能,可用于节点滞回性能的非线性参数分析研究。     

Analytical frame approach for the axial stiffness prediction of preloaded T-stubs

The behavior of beam-to-column bolted connections can be predicted by means of the component approach, included in the Eurocode 3. This method accounts for any bolted connection typology once the basic components are identified and modeled. Many of these components are represented using the so-called equivalent T-stub. In some cases, these joints may need to be preloaded in order to hold fatigue loads or even just to increase the connection stiffness.In this paper, an equivalent frame model, which implicitly includes the bolt, is presented to predict the axial stiffness of preloaded T-stubs. This proposal, designed for hand calculation, takes into account compatibility requirements and prying forces. Besides, an advanced 3D finite element (FE) model of a representative T-stub test is done to characterize the deformation response and the boundary characteristics related to preloading, in order to support the analytical proposal. Furthermore, an effective width for stiffness calculation is suggested, based on an extensive plate FE parametric study.Results are validated by comparison with experimental data from the literature.     

Finite element modeling of bolted connections in thin-walled stainless steel plates under static shear

The recently performed experimental study indicates that the current Japanese steel design standards (AIJ) cannot be used to predict accurately the ultimate behavior of bolted connections loaded in static shear, which are fabricated from thin-walled (cold-formed) SUS304 austenite stainless steel plates and thus, modified formula for calculating the ultimate strength to account for the mechanical properties of stainless steel and thin-walled steel plates were proposed. In this study, based on the existing test data for calibration and parametric study, finite element (FE) model with three-dimensional solid elements using ABAQUS program is established to investigate the structural behavior of bolted shear connections with thin-walled stainless steel plate. Non-linear material and non-geometric analysis is carried out in order to predict the load–displacement curves of bolted connections. Curling, i.e., out of plane deformation of the ends of connection plates which occurred in test specimens was also observed in FE model without geometric imperfection, the effect of curling on the ultimate strength was examined quantitatively and the failure criteria which is suitable to predict failure modes of bolted connections was proposed. In addition, results of the FE analysis are compared with previous experimental results, failure modes and ultimate strengths predicted by recommended procedures of FE showed a good correlation with those of experimental results and numerical approach was found to provide estimates with reasonable accuracy.     

Design and behavior of recentering beam-to-CFT column connections with super-elastic shape memory alloy fasteners

This study investigates the performance of new composite (steel-concrete) moment connections through numerical simulations. The innovative aspects of this research lay in the use of end-plate connections between steel beams and concrete-filled tube (CFT) columns that utilize a combination of low-carbon steel and shape memory alloy (SMA) components. In these new connections, the intent is to use the recentering effect provided by super-elastic SMA tension bars to reduce the level of building damage and residual drift after a major earthquake. The low-carbon steel components provide excellent energy dissipation. The analysis and design of these structures is complicated because the connections cannot be modeled as being simply pins or full fixity ones; they are partial restraint (PR). A refined finite element (FE) model with sophisticated three dimensional (3D) solid elements was developed to perform numerical experiments on the PR-CFT joints with a view to obtaining the global behavior of the connection. Based on the behavioral information obtained from these FE tests, simplified connection models were formulated using 2D joint elements with nonlinear spring components. The behavior of entire connections under cyclic loads was examined. The results were compared with the connection behavior obtained from the 3D FE simulations and corresponding connection tests. Good agreement was found between the simple and sophisticated models, confirming the robustness of the approach.     

Analysis of brick veneer walls on wood frame construction subjected to out-of-plane loads

Modern residential brick masonry veneer construction has been investigated by separate laboratory testing of corrugated sheet metal tie connections and full-scale solid brick veneer wall panels. Based on these experiments, detailed three-dimensional finite element (FE) models were developed, representing the full-scale brick veneer wall panel specimens, including nonlinear inelastic properties for the tie connections. Upon calibration, the FE wall panel models effectively captured the static and dynamic experimental brick veneer wall behaviors at different response levels (up to and including tie damage and even collapse). Some parametric studies were then carried out using the FE wall panel models to evaluate the effects of certain combinations of tie connections and layouts on the out-of-plane performance of brick veneer walls, subjected to static and dynamic loading.     

Experimental evaluation of the seismic performance of modular steel-braced frames

Several studies have shown that the lateral response of concentrically-braced frames is dominated by the inelastic behavior of the bracing members. However, the overall performance of the entire frame depends on the frame configuration including its connections. In this study, the hysteretic characteristics of modular steel-braced frames under reversed cyclic loading are evaluated. The design and construction of the test specimen accounted for the unique detailing requirements of these frames. A regular concentrically-braced frame with similar physical characteristics was also tested for comparison. Both test specimens consisted of a one-storey X-braced system with tubular brace cross-section. This paper describes the behavior characteristics and provides a detailed comparison of the two systems to assess the strength, stiffness, inelastic force and deformation, and energy dissipation characteristics of the modular system. An analytical model capable of capturing the effect of the system’s unique detailing requirements is proposed and validated using the test results.     

信息动态

建立可靠的力学模型,使其能模拟循环荷载作用下足尺T型节点的全过程弯矩-转角曲线。由于螺栓之间相互作用且对连接的转动刚度有影响,螺栓连接的性能很复杂。将节点模型设计为由各个T型组件组成,并用线性弹簧模拟各组件。通过从荷载-位移反应机制得出的多线性刚度模型,分析组件的性能,包括受拉螺栓隆起、T型翼缘的弯曲、T型腹板的拉伸、相对滑移变形、允许变形等。将这些属性代入节点单元中的弹簧,能较准确地分析足尺节点性能。基于基本弹簧理论,集中阐述刚度模型,更好地分析了螺栓连接的性能、失效模型及T型组件韧性。     

循环荷载作用下预制钢-混凝土混合连接件的有限元分析

提出一种用于钢-混凝土混合连接的非线性有限元分析方法。考虑了部分抗震设计的四个足尺钢-混连接点的详细的试验结果已在另一篇论文中予以讨论。然而,由于梁柱节点和试验构件特性的内在复杂性,试验研究是不够全面的。因此,提出了一种基于有限元模型和DIANA程序的分析方法：有限元模型通过新加坡南洋理工大学钢-混凝土混合连接的试验结果证实有效。     

Mechanical modeling of bolted T-stub connections under cyclic loads Part I: Stiffness Modeling

This paper deals with mechanical models which make it possible to reliably simulate the complete moment-rotation curves in the full-scale T-stub connections subjected to cyclic loads. The behavior of these bolted connections becomes complex because the various response mechanisms of individual connection components interact with one another and have influence on the overall rotational stiffness of the connection. Accordingly, the mechanical joint models are made up of individual T-stub components modeled as nonlinear springs. The behaviors of component members including tension bolt uplift, bending of the T-stub flange, elongation of the T-stem, relative slip deformation, and bearing deformation are reproduced by the multi-nonlinear stiffness models obtained from their force-deformation response mechanisms. These stiffness properties should be assigned into the component springs implemented into the joint element so as to numerically generate the behavior of full-scale connections with considerable accuracy. Thus, this part (Part I) intends to focus on describing the stiffness models, which are based on the basic component spring theory, in an effort to provide insight into the behavior, failure modes, and ductility of T-stub components in the connection.     

Finite element analysis of precast hybrid-steel concrete connections under cyclic loading

In this paper, a nonlinear finite element (FE) analysis of hybrid-steel concrete connections is presented. The detailed experimental results of the four full-scale hybrid-steel concrete connections with limited seismic detailing have been discussed in a different paper. However, due to the inherent complexity of beam-column joints and the unique features of the tested specimens, the experimental study was not comprehensive enough. Therefore, in this paper, an analytical investigation based on the FE models and using the DIANA software is presented. The FE models were validated using the experimental results of the hybrid-steel concrete connections tested in Nanyang Technological University, Singapore. The critical parameters influencing the joint’s behaviour, such as the axial load on column, the connection plate thickness, and the continuation of beam bottom reinforcement, are varied, and their effects, especially implications on code specifications, are studied.     

Seismic behavior of CFRSTC composite frames considering slab effects

This paper reports on an experimental and numerical investigation conducted on the seismic behavior of concrete-filled rectangular steel tubular columns (CFRSTC) composite frames. The experimental study was conducted by subjecting two full-scale composite frames to simulated seismic loads. Both frames were composed of CFRSTC and steel beams. One specimen was placed on a reinforce concrete (RC) floor slab and the other was not. The purpose of the test was to investigate the elasto-plastic performance of the CFRSTC composite frame system and to examine the effects of composite action on the behavior of composite frames. The test results showed that the stiffness, strength and energy-dissipating capacity of the CFRSTC frame increased significantly with the presence of the floor slab. Compared with a bare steel beam, the composite beam experienced a decrease in the rotation capacity from 0.046 rad to 0.026 rad. The shear deformation of the panel zone grew because of the composite action, which delayed the fracture of the beam. Finite element (FE) models were established to simulate the tested frames. The results of the FE model fit well with that of the test model in terms of stiffness, strength, hysteretic behavior and component deformation.     

Modeling the panel zone in steel MR frames composed of built-up columns

This paper proposes an analytical model for the panel zone of connections of steel moment-resisting frames (SMRFs) composed of built-up columns with double sections and a vertical continuity plate. Panel zone in these connections consists of a middle panel, which is the vertical continuity plate, and two side panels, which are the webs of double sections of the column. Since the general behavior of these frames is governed by deformation of the middle panel, this panel is assumed as the primary one and the two side panels are assumed to be secondary. The column cover plates connecting the double sections and vertical continuity plate play an important role in providing compatibility of deformations. The new model proposed in this study is based on the previously developed models for the panel zones of connections in which the webs of beam and column are in the same plane, however, a number of refinements are introduced to capture the behavior of this type of connections. This quadri-linear model can be used for monotonic loading and accounts for both shear and bending deformations of panel zone. The results of this model are compared to those of finite element (FE) models verified by full scale experiments. The proposed model shows a good agreement, especially in elastic range, with the FE results. The results of FE analysis for important parameters affecting the panel zone behavior have also been compared to those obtained from the model in order to confirm the proposed model.     

Behavior of bolted angle connections subjected to combined shear force and moment

In this paper, the effect of web angle dimensions on moment-rotation behavior of bolted top and seat angle connections, with double web angles is studied. Several 3D parametric finite element (FE) models are presented in this study whose geometrical and mechanical properties are used as parameters. In these models, all of the connection components, such as beam, column, angles and bolts are modeled using solid elements. The effect of interactions between components, such as slippage of bolts and frictional forces, are modeled using a surface contact algorithm. To evaluate the behavior of connection more precisely, bolt pretensioning force is applied on bolt shanks as the first load case. The results of this numerical modeling are compared with the results of experimental works done by other researchers and good agreement was observed. To study the influence of shear force on behavior of these connections, several models were analyzed using different values of shear force. The effect of important parameters, especially the effect of web angle dimension, is studied then. An equation is proposed to determine the reduction factor for initial rotational stiffness of connection using connection initial rotational stiffness, yield moment, the expected shear force and web angle dimension. The proposed equation is compared with other existing formulations and it was observed that the proposed model is a better estimator of connection behavior.     

Refined 3D finite element modeling of partially-restrained connections including slip

This study presents an approach for refined parametric three-dimensional (3D) analysis of partially-restrained (PR) bolted steel beam-column connections. The models include the effects of slip by utilizing a general contact scheme. Non-linear 3D continuum elements are used for all parts of the connection and the contact conditions between all the components are explicitly recognized. A method for applying pretension in the bolts is introduced and verified. The effect of several geometrical and material parameters on the overall moment–rotation response of two connection configurations subject to static loading is studied. Models with parameters drawn from a previous experimental study of top and bottom seat angle connections are generated in order to compare the analyses with test results, with good prediction shown by the 3D refined models. The proposed 3D modeling approach is general and can be applied for accurate modeling of a wide range of other types of PR connections. A pronounced effect of slip and friction, between the connection components is shown with connections having thicker (stiffer) seat angles. This study demonstrates the effects of clamping through the bolts and contact between the components on the overall non-linear moment–rotation response. Equivalent moment–rotation responses of pull-test simulations are compared to FE model responses of full connections without web angles. The moment–rotation from the pull test is shown to be equivalent to that of the full FE model for small rotations. As the rotation increases a softer response is shown by the pull tests.     

Exposed column base plate connections in moment frames — Simulations and behavioral insights

The response of exposed column base connections under axial and flexural loadings is controlled by complex interactions of various components within them, i.e. the column, base plate, anchor rods, and footing. Current approaches for strength characterization of these connections show good overall agreement with test data from specimens that are smaller relative to the prototype scale. However, internal stress distributions that underpin these approaches have not been independently verified, presenting challenges for the generalization of these approaches to full scale connections. A finite element (FE) simulation study is presented to offer insights into connection response with a focus on internal stress distributions. The 3-d FE simulations incorporate important aspects of behavior including contact, gapping and nonlinear multiaxial constitutive response of the various components. The simulations are rigorously validated based on multiple data streams obtained from a series of six experiments on exposed base plate connections. The results of the simulations indicate that current strength characterization and design approaches (that assume a rectangular bearing stress block) may grossly misrepresent the stress distribution under the plate. Thicker base plates tend to concentrate the stresses at the compression toe of the base plate. This significantly amplifies the bending moment in the base plate on the compression side of the connection, indicating that current approaches that do not consider this effect may be non-conservative. A simplified approach to address this issue is discussed. Similar issues are not observed for axial force estimates in anchor rods. Limitations of the work are outlined.     

Investigation of fatigue resistance of fillet-welded tube connection details for sign support structures

Stiffened and unstiffened fillet-welded tube-to-transverse plate connection details are widely used for mast-arm and base-plate connections for highway sign structures. However, due to repetitive wind loads, cyclic fatigue stresses are induced and they are the primary source of failure in welded connections at these locations. The resistance of fatigue critical details has been an on-going research topic because of limited experimental results and the variability in existing fatigue testing results. The main objective of this study is to evaluate fatigue resistance of fillet-welded tube connection details by utilizing the advanced fatigue tool in ANSYS Workbench platform. Finite Element (FE) models development and model validation using existing test data was presented. The resulting fatigue resistance from FE analysis was expressed in terms of fatigue life, fatigue damage, and fatigue safety factor to determine the fatigue performance of fillet-welded connections. Existing fatigue test data was grouped to perform a synthetic analysis and then analysis results were provided to determine input data and fatigue limit for the fatigue module. The local stress level at fatigue critical locations was evaluated using a static FE model for different number of stiffeners and boundary conditions. The results of this investigation provides fatigue resistance of fillet-welded connection details in the form of fatigue life, fatigue damage and safety factor for various connection parameters and structural conditions.     

Experimental behaviour of high strength steel end-plate connections

The use of high strength steel in construction has recently provided some challenges to structural engineers. One such challenge is to minimise the cross-section dimensions as the material provides higher strength, which brings great economic benefits. However, high strength steels exhibit high yield ratios and limited deformation capacity when compared to mild steel grades. This behaviour can be particularly important when structures are designed for abnormal loading conditions that produce inelastic deformations. In this situation, both members and connections have to develop sufficient ductility. The connections between members, in particular, are the regions where the material is exposed to higher deformation demands. To address these topics, an experimental investigation was undertaken of moment connections with end-plates made up of the high strength steel S690. The major contributions of this study are (i) the characterisation of the nonlinear behaviour, (ii) the validation of current Eurocode 3 specifications and (iii) the ductility analysis of high strength steel moment connections. The test results show that the tested connections satisfy the current design provisions for stiffness and resistance and achieve reasonable rotation demands.     

Flush Endplate Connections in Fire: Time-Dependent Behavior of Tension Bolts

Steel connections play a crucial role in maintaining the integrity and stability of steel building frames especially when exposed to fire temperatures. The behavior of flush endplate connections in fire is shown to be governed by tension bolt failure as bolts lose their strength and stiffness more rapidly at higher temperatures. As a result, the ability to predict the development of stresses in tension bolts of flush endplate connections at different stages of fire is of special importance. One of the factors influencing bolt stresses in fire is the thermal creep or time-dependent inelastic response of steel to elevated temperatures. Therefore, time- and temperature-dependent behavior of tension bolts of flush endplate connections in fire is the focus of this study. Stress-time histories in tension bolts are obtained by explicit consideration of thermal creep of steel in FE models of flush endplate connections at elevated temperatures. To better understand the effect of thermal creep on tension bolt behavior, the correlation between time-dependent rotational deformation of flush endplate connections and bolt stresses is also investigated. Further, the isochronous representation is utilized to study the rotational deformation and the tension bolt stresses under various applied moments ranging from 50% to 95% of the moment capacity and fire temperatures ranging from 450°C to 600°C with 25°C increment. Through such representation, it is indicated that the connection behavior is not only dependent on bolt strength degradation and applied moment, but also affected by the time duration of applied moments and temperatures. Also, with the inclusion of thermal creep of steel, the connection experiences higher rotation and excessive endplate deformation with stress relaxation leading to top tension bolt failure at earlier stages of fire. More specifically, for time exposure greater than or equal to 60 min, the failure temperature of the connection decreases from 600°C to around 550°C. Therefore, neglecting thermal creep of structural steel may result in an unsafe prediction of the overall response of flush endplate connections in fire.