Flush End Research Articles

Experimental and numerical investigations of seismic performance of stepped beam-column joints with different connection forms

The seismic resilience of prefabricated steel structures significantly depends on the design and construction of beam-column joints. To address the challenges related to the installation of prefabricated wall panels and overall system assembly, this study introduces a novel stepped beam-column joint. The effectiveness of different connection configurations on the seismic behavior of the joints was investigated. A series of cyclic tests were conducted on four joint specimens with different configurations, and their failure mechanisms, hysteretic curves, skeleton curves, stiffness degradation, load-bearing capacity, and energy dissipation capacity, were revealed. The experimental results showed that the stepped joint configuration offers favourable seismic performance, with notable asymmetry observed in the response patterns under positive and negative loading directions. Reducing the height of the stepped segment or the thickness of the end plate adversely affected the seismic performance, leading to early bolt failure. Consequently, a detailed finite element model of the stepped joint was developed and the model demonstrated satisfactory agreement with the experimental results. Furthermore, a parametric analysis focusing on variations in stepped segment's height was conducted, providing insights into optimization of the load-bearing capacity of the stepped joint. The difference in the performance between the stepped joints and traditional flush end plate joints is also highlighted.

Seismic performance of very short links in eccentrically braced substructures

Research on the seismic performance of eccentrically braced hybrid frames indicates that utilizing very short links can enhance structural stiffness and load capacity. However, there is limited investigation into their effectiveness within eccentrically braced substructures featuring flush end plate bolted connections. This study involves an experimental and numerical investigation to understand the behavior of very short shear links with flush endplates bolted connections in the substructure. An experimental study of 8 specimens was carried out, and the variables studied included loading protocol, links length ratio, flange thickness, and end plate thickness. The experimental results showed that the co-design methodology for flush endplates and bolts of very short links exhibited acceptable performance. Very short shear links bolted with flush endplates demonstrated an average overstrength coefficient of about 1.8 and an ultimate plastic rotation angle above 0.12 rad. Numerical analysis explored the effects of length ratio, web height-to-thickness ratio, and flange width-to-thickness ratio on overstrength behaviors and yield capacity of very short links. The analysis indicated that the length ratio was crucial in determining overstrength coefficients and the yield capacity of very short links. The study established modified formulas that consider the impact of different variables to more accurately determine overstrength behaviors and yield capacity based on formulas in the Chinese code.

Performance of short links with flush and extended end plate connections using FEM analysis

AbstractEccentrically braced frames are commonly used as a seismic resistant system, due to the good balance of stiffness and ductility. Bolted links have been proposed in the past to allow replacing the damaged dissipative components following a strong earthquake. The goal of this research work is to investigate the influence of the connection type on the performance of bolted links using the Finite Element Analysis. Numerical models of links are first calibrated using past experimental tests. Modelling criteria adopted to replicate the experimental behaviour of the links highlighted in the tests are reported. The achieved results obtained regarding a flush end plate bolted link and an extended end plate bolted link with different lengths are then used as benchmark to assess the influence of the link length and type of connections adopted on the response of the links. The Finite Element Method is used to investigate alternative solutions to the tested ones. Finally, the investigated options are compared to determine the advantages and disadvantages of each solution.

Simplified Modelling of Failure in High Strength Bolts under Combined Tension and Bending

Bolted connections are widely adopted in steel structures and their behaviour affects to a large extent the global response of the system. High-strength bolts of type HV are commonly employed. Under pure tension, these bolt assemblies usually fail by thread stripping. However, it was observed experimentally that, under combined tension and bending, the failure mode changes to fracture of the shank. The former loading condition commonly occurs in the case of thick extended end plate connections and the latter in the case of flush end plates. In order to analyse the behaviour of the structure, the finite element method (FEM) is usually employed. While there is a wealth of information on FEM modelling of bolts for standard loading conditions (e.g., tension), the authors are unaware of a model able to replicate both tension-only and combined tension and bending conditions. In this paper, a simplified approach to be used in the framework of FEM is proposed to model the behaviour of high-strength HV bolts which can replicate the failure mechanism of bolts under tension only and combined tension and bending. The bolt assembly is modelled with continuum elements, supplemented by a non-linear spring connecting the nut to the bolt shank. The spring captures the stiffness, resistance, and ductility of the bolt-to-nut threaded connection, reproducing the experimentally observed failure mode in the case of pure tension conditions. A simplified damage model is applied to the continuum finite elements used to model the bolt, which replicates shank failure under combined tension and bending as a result of large local stresses and strains occurring under these conditions. The proposed model captures with good accuracy the actual behaviour of high-strength HV bolts under tension only as well as under combined tension and bending.

Assessment of Design Approaches for Stainless‐Steel Joints Through an Equivalent FE Modelling Technique

AbstractConsidering the significant strain‐hardening and ductility characteristics of stainless steel and the influential role of beam to column joints in the overall response of structures, a clear understanding of their moment rotation behaviour appears fundamental. In the present study, a review of the European design code for three typical semi‐continuous beam‐to‐column joints (extended end plate, flush end plate and top‐and‐seat angle connections) is first carried out. Then, using a hybrid modelling strategy with uncoupled elastic‐plastic springs for the bolts and shell elements for the members, more refined 3D finite element (FE) models are developed and validated against available test results. An equivalent stress definition is then proposed for calculating the joint moment resistance. Finally, the initial rotational stiffness, moment resistance, rotation capacity, and failure patterns are used to evaluate critically the accuracy of the predictions of the European design code. As a result, the need for more comprehensive design guidance in accordance with the observed findings is pointed out.

Novel coupled modular steel structure and seismic tests on high-performance interconnection

Modular building registers the advanced stage of construction industrialization and the corner-supported steel modular building is the most widely applied. However, its structure, which is integrated by corner interconnections only, incurs the problems of structural redundancy and interconnection-assembly complexity. To this gap, this paper proposes the novel coupled modular steel structure and suitable interconnection, which significantly affects the overall structural response and assembly efficiency. Pseudo static tests of three full-scale cruciform joint specimens are implemented to study the seismic performance of interconnection. The results show that the currently proposed coupled modular steel structure could replace the incompletely-combined group-columns and laminated beams with single structural components to avoid the structural redundancy and reduce the assembly difficulty of interconnections. The proposed interconnection exhibits excellent seismic performance. Hysteretic loops of coupled beam and module beam are “bow” shape and “shuttle” shape, respectively, indicating their good hysteretic performance. Ultimate storey drift angles of coupled beam and module beam could reach 0.08–0.09 rad and 0.06–0.08 rad, respectively, which satisfy the requirement for SMF in ANSI/AISC 341-16 and indicate their excellent rotation capacity. The ductility coefficients of the specimens are 2.27–3.64 except for MC2, indicating their good ductility capacity. Adopting the flush end plate for coupled beam and high strength bolts for vertical splice is recommended for the interconnections to obtain better hysteretic performance and bearing capacity. This research provides brand new design for modular construction and references for future research on global response of coupled modular steel structure.

Seismic tests and nonlinear model of beam-CFDST column joints with blind fasteners

In recent years, substantial research efforts have turned towards concrete filled double skin steel tubular (CFDST) column due to its superior mechanical performance. However, an effective and convenient connection method between a CFDST column and a beam still needs to be investigated in depth. This paper tried to adopt the blind bolting technology in a CFDST column to beam connection. A series of joint specimens were prefabricated and designed with different end plate types, column hollow ratios and beam types to carry out pseudo-dynamic tests. Excellent seismic performance was detected for all test specimens. It was observed that increasing the column hollow ratio could moderately increase the ultimate strength and the specimen with extended end plate had higher ultimate strength stiffness compared with that with flush end plate. To accurately capture the dynamic response for this type of joint, an analytical model was developed considering the shear performance of panel zone and moment-rotation behavior of connection. Shear strength of CFDST column panel zone was modified from that of CFST column panel zone based on the yield line method for including the contribution of confined compression strut. It indicated that two potential yield mechanism might appear depending on the column hollow ration (χ). The analytical model could well predict the test results. The research and analytical results provided an effective connection method to assemble a CFDST column to a beam and supply a feasible analytical model to predict the joint performance.

Parametric study of steel flush and extended end-plate joints under column loss scenario

In steel-framed structures, joints play an essential role in providing a frame response to loading under persistence scenarios and especially in robustness analysis, which is very often executed in column loss scenarios. A type of joint used in framed structures is a bolted end-plate joint with extended and flush end plates. Experimental tests of six configurations of extended and flush end-plate joints were conducted to understand the actual joint behaviour under loading based on bending moment and tension force from catenary action. An advanced 3D FEM model of the joints was created using Abaqus software. Validated to own results of the experimental tests models were used in a wide parametric study to investigate the effect of the arrangement and thickness of the end plate, number, placement, and grade of bolts. The results provide practical recommendations for improving the joint characteristics required in accidental scenarios, for example, for enhanced resistance and rotation capacity under bending moment and tension force. The change from brittle to ductile failure mode was analysed as the second major parameter of joint ultimate behaviour. The thicker end plate and higher bolt diameter as the main component of the connection with the unstiffened column web was recommended to increase capacity in an accidental scenario. Finally, few proposals were presented to create innovative joints that demonstrate better performance in the column loss case: additional channels, saddles, and rings at bottom beam flanges. A significant increment in the load capacity and rotation capacity of the modified joints was achieved. The failure mode of the modified joints was changed moving the damage outside the joint zone. Further, the brittle destruction of bolts and end-plate fracture were avoided in the innovative models of joints.

A new analytical formulation for the stiffness and resistance of the additional plate in bending in beam-to-beam steel joints

This paper focuses on the stiffness and resistance of the additional plate in bending. This is a component which appears in beam-to-beam bolted steel joints, when a secondary beam with a flush end plate is attached orthogonally to the primary beam by an additional plate welded between its flanges. In this work, six experimental tests have been carried out to calibrate a finite element model which has been used to develop a wide parametric study. Thus, a new formulation for stiffness and resistance based on an equivalent frame is proposed.

Structural behaviour of steel-timber versus steel-concrete composite joints with flush end plate

An innovative structural floor comprising of steel frames and cross-laminated timber slabs (i.e. steel-timber composite) has been introduced recently as a sustainable alternative to conventional composite floors with reinforced concrete slabs. To assess the structural performance of the steel-timber and compare it to steel-concrete composite and bare steel, push-down tests were performed on geometrically identical full-scale cruciform subassemblies with flush end plate connections. The width of slab and type of connections between the CLT slabs and the size of bolts connecting the flush end plate to the column were the variables in the testing program. The size of bolts and width of CLT slabs had minor (or no) influence on the hogging bending moment resistance. However, increasing the bolt size and CLT slab width increased the stiffness of the STC beam-to-column connections. Furthermore, the CLT-to-CLT connection type had significant influence on the stiffness and flexural capacity of the STC subassemblies under hogging bending moment. The results of the laboratory tests showed that the rotational stiffness and hogging flexural resistance of the semi-rigid STC joints with flush end plates are comparable to those of SCC joints. The ductility of the STC joints with a flush end plate is also comparable to identical SCC joints, providing that tensile failure in timber slabs can be avoided.

The Influence of Steel Connection on Fire Resistance of Composite Steel-Framed Buildings

Steel connections play a significant part in enhancing the robustness of structures in the event of a fire. Therefore, it is important to examine the impact of steel connections on the fire resistance of composite steel framed buildings. In this paper, both the behaviour of steel connections and their effect on composite steel frames are analysed using the non-linear finite element computer software VULCAN at various temperatures. The chosen frame is subjected to ISO834 fire. A comparison between end plate connections, pinned connection and rigid connection has been carried out. By applying different compartment fires, some cases are studied to show the behaviour of steel connection when the fire is applied at certain beams. In addition, different plate thickness has been analysed to examine the behaviour of the chosen steel connection under ISO834 fire. From the analytical results, it was found that the beam with extended end plate is stronger and has better performance in terms of axial forces than those beams with flush end-plate connection. Furthermore, that extended end plate connection has highest limiting temperatures compared to the flush end plate connection. In addition, it was observed that the performance of end-plate connections is very close to rigid connection and very far from pinned connections. Furthermore, plate thickness has less of an effect on the influence of steel connection in regards to fire resistance. The article will conclude that the behaviour of composite steel framed buildings is largely dependent on the steel connection due to their high impact under fire condition. It is recommended to consider the extended end-plate in the design proposals because of its higher fire resistant properties compared to the flush end-plate connection. Finally, this paper shows that steel connection has an important effect on the fire resistance of composite steel framed buildings.

Improved steel frame performance against progressive collapse with infill panels

Local failure of one or more components due to abnormal loading can induce progressive collapse of a building structure. Herein, improved structural robustness is targeted to improve steel frame performance against progressive collapse. A multi-scale analysis model is constructed; then, the anti-collapse performance of a plane steel frame structure with a flush end plate connection is comprehensively analyzed using nonlinear static analysis. To enhance the anti-collapse capability of the steel frame structure, reasonable addition of infill panels in indirect- and direct-influence areas is proposed. Infill panels properly arranged in direct influence area can give full play to the Vierendeel mechanism, and effectively improve combined ability of the remaining structure. And then 1/8–1/6 of the net span is recommended as the length of the infill steel panel for the steel frame structure with flush end plate connection.

Investigations on the fatigue behavior of end plate connections with prestressed bolts

In many areas of our infrastructure, as for instance the construction of crane runway beams, fatigue design of steel structures is a major issue. An important detail in practice, but insufficiently classified in the (Eurocode 3-1-9, 2005) detail catalogue, is the end plate connection with prestressed bolts. Detailed numerical simulations are often necessary for the fatigue check of the weld at the T-stub. The knowledge of the true bolt force required in the detail catalogue can usually also only be taken into account using finite element simulations. For these reasons, the detail ‘end plate connection with different weld configurations and prestressed bolts’ containing the two critical design cases mentioned above has been investigated. The analyzed details comprise butt welds as well as fillet welds and connections with overhanging and flush end plate connections.A literature review of fatigue in details of end plate connections is given. Moreover, the test program on three fatigue tests series is presented. After conducting experimental investigations on small-scale as well as large-scale specimens detailed numerical simulations have been realized. In order to assess the fatigue resistance of the welded connection, the effective notch stress concept has been used. Additionally, fatigue resistances of prestressed bolts have been investigated by recalculating the bolt force progress. After the verification of the numerical models, a parametric study has been conducted by varying geometric characteristics in order to give recommendations on the design of prestressed bolts and welds in end plate connections.

Experimental and numerical behaviour of blind bolted flush endplate composite connections

This research presents an experimental investigation and finite element model on the performance of composite beam-to-column flush end plate connections utilising blind bolts. Previous research recommended improvements to the design of these connections if they are required to withstand a low probability high consequence earthquake of a 1-in-2500 years return period. Therefore, two full-scale sub-assemblages of cruciform composite joints were tested under static and cyclic loading. The test results were used to establish the load-displacement characteristics of the connection. The experimental results demonstrated improved stiffness and strength when compared with a previous study. Two finite element models were developed and validated with the experimental data. Parametric studies were performed to investigate the influence of the thickness of equal angles, end plate thickness, shear connection ratio, reinforcement ratio and slab thickness on the connection behaviour.

Component tests and numerical simulations of composite floor systems under progressive collapse

This paper presents experimental and numerical studies on the behaviour of components and composite floor system under column removal scenarios. Shear connectors, double angle-cleat connections and flush end plate connections were extracted from a three-dimensional composite floor system and tested under different loading conditions. The load-displacement curves of these components were compared with the results predicted from existing models. A simplified numerical model was developed to simulate the behaviour of composite floor systems subject to progressive collapse, in which the properties of springs were determined from component tests. The model was then validated against experimental results in terms of load-displacement relationship and failure mode. Finally, parametric studies were conducted to investigate the effects of slab thickness, aspect ratio and reinforcement. Numerical results suggested that the aspect ratio of slabs has the most significant effect on the load capacity of composite floor systems under progressive collapse scenarios.

I-beam to square hollow column blind bolted moment connection: Experimental and numerical study

This paper presents the results of experimental investigations and numerical analyses carried out to investigate the performance of new blind bolted moment-resisting connections between square hollow section columns and open section beams, referred to herein as the extended and flush end plate. Eight full-scale specimens comprising four extended end plate and four flush end plate exterior connections were tested under monotonic loading. The moment-rotation curves, failure mode, initial stiffness and connection classification of the joints were obtained and discussed. The results showed that the end plate type has a significant influence on the flexural capacity as well as the rotational stiffness of the proposed blind bolted connection. Furthermore, the test also showed that for specific column sizes, an increase in the beam size and end plate thickness could reduce the connection strength. The experimental specimens were also simulated using finite element models and compared with the experimental results. Comparison amongst the experimental and three-dimensional finite element (FE) models shows that the modeling of finite element analyses for the prediction of the connection behavior is sufficiently accurate.

Failures and Prediction Methods of Lateral Buckling Strength of H-Shaped Beams Connected by Flush End Plates

This paper presents boundary conditions for lateral buckling of the H-shaped beams connected to columns by flush end plates which fall into the lateral buckling following the out of plane plastic deformations of the flush end plates. The study is conducted by the following procedure: First, the behavior of the connection is researched by the no buckling experiment under the cyclic loading. As a result, it is found that the relationship between the out of plane plastic deformations of the endplates and the bending moment at the beam end can be predicted. Next, the FEM analyses of the rigid-frame structures consist of the slender beams having flush end plates connection and the structures consist of the beams having rigid connection are conducted to research the stress conditions in the tension flanges and the compression flanges of the beams during falling into lateral buckling. As a result, it is found that the rigid connection is restraining warping of the beam more tightly than the flush end plates connection, then the maximum bending moment strength against lateral buckling of the beams connected by rigid connections is higher than those of the beams connected by flush endplate. We recommend that the boundary conditions for the lateral buckling of the beams connected by flush end plates should be regarded to be different from those of the beams connected by the rigid connections.

Dynamic increase factor for progressive collapse analysis of semi-rigid steel frames

An empirical and efficient method is presented for calculating the dynamic increase factor to amplify the applied loads on the affected bays of a steel frame structure with semi-rigid connections. The nonlinear static alternate path analysis is used to evaluate the dynamic responses. First, the polynomial models of the extended end plate and the top and seat connection are modified, and the proposed polynomial model of the flush end plate connection shows good agreement as compared with experimental results. Next, a beam model with nonlinear spring elements and plastic hinges is utilized to incorporate the combined effect of connection flexibility and material nonlinearity. A new step-by-step analysis procedure is established to obtain quickly the dynamic increase factor based on a combination of the pushdown analysis and nonlinear dynamic analysis. Finally, the modified dynamic increase factor equation, defined as a function of the maximum ratio value of energy demand to energy capacity of an affected beam, is derived by curve fitting data points generated by the different analysis cases with different column removal scenarios and five types of semi-rigid connections.

Experimental and numerical analyses of blind bolted moment-resisting composite joints to CFST columns with RC slab

In this paper, experimental and numerical analyses were conducted to investigate the structural properties and failure modes of the blind bolted flush or extended end plate joints between concrete-filled steel tubular (CFST) columns and composite beams. The experimental program and the main results of the tests were elaborated. Four full-scale subassemblages beam-tocolumn exterior joints to hot-rolled and cold-formed CFST columns were tested. The failure modes and moment-rotation relation curves of the joints were evaluated. The test results showed that all specimens exhibited favorable strength and stiffness, and larger deformation capacities. Meanwhile, finite element analysis (FEA) modeling of the specimens was built and analyzed. The results obtained from the FEA modeling were satisfactory agreement with the experimental tests results. Then, an extensive parametric study was conducted to investigate the influence of various parameters on the moment capacity and rotation stiffness of composite joints under positive or negative moment. The experimental studies and numerical analyses can improve the design of the typed joints to be used in moment-resisting composite frames.

Effect of boundary conditions on the behaviour of composite frames against progressive collapse

Boundary conditions, such as the horizontal and out-of-plane restraints, play an important role in structural resistance and deformation capacity of composite frames under column removal scenarios. Most studies only focused on interior frames with rigid horizontal and out-of-plane restraints, whereas limited data are available on the effects of weak restraints and out-of-plane rotations on the behaviour of composite frames against progressive collapse. In this study, a series of experimental tests were conducted on composite frames with different horizontal and out-of-plane restraints. Web cleat and flush end plate connections were used in the frames. Experimental results showed that when subject to in-plane loading, exterior composite frames could mobilise significant catenary action to mitigate progressive collapse and eventually column failure might occur under catenary action in bridging beams. For composite frames with out-of-plane rotation, the ultimate resistance was substantially reduced due to lateral torsional buckling of composite beams at large deformation stage. However, the flexural resistance of frames was less susceptible to boundary conditions compared to catenary action. Furthermore, bending moments at side columns were evaluated to gain insight into the behaviour of side columns under horizontal tension force at the catenary action stage. Moreover, component-based models were developed for composite frames and validated against test data. Design recommendations were provided in accordance with experimental and numerical results.