Beam Bottom Flange Research Articles

The Influence of Box-Strengthened Panel Zone on Steel Frame Seismic Performance

During the Northridge earthquake, extensive brittle failures on the weld zones of the beam bottom flanges in the rigidity connection of steel special moment frames (SMFs) were detected. One of the primary reasons is the high-tensile strain demand created at the beam bottom flange zones due to positive bending. The weak panel zone of the I-section column exhibits more shear deformation, which promotes and accelerates the brittle fracture of the beam bottom flange weld zones. A box-strengthened panel zone can minimize the shear deformation of the panel zone of the I-section column, which may also reduce the inter-story displacement of steel SMFs and enhance their seismic behavior. In order to investigate this fact, in this research we carried out a model test of a steel frame with a box-strengthened panel zone to examine SMFs’ seismic performance and inter-story displacement, as well as testing the contribution of panel zone shear deformation to inter-story drift. Numerical methods were then used to investigate the influence of the axial compression ratio and beam-to-column linear stiffness ratio on the effect of shear deformation on the box-strengthened panel zone. Design recommendations are given based on the research results.

Cyclic behavior of dual-steel beam-to-column welded flange-bolted web connections

Cyclic loading tests on dual-steel beam-to-column welded flange-bolted web connections were conducted to quantify their moment resistance, plastic deformation and energy dissipation capacities. The test program consisted of five one-sided connection specimens, where Q355 grade beams and Q690 grade columns were used, to study the effects of beam-to-column welded flange connection details and panel zone shear strength on the connection seismic performance. Two types of welded connection details were considered; one was the traditional complete-joint-penetration (CJP) welded connections by use of backing bars, and the other type was those with the bottom backing bar further reinforced by a fillet weld. Three column web thicknesses were designed to arrive at strong, intermediate and weak panel zones, respectively. It was found that the CJP weld with the backing bar reinforced only under the bottom beam flange produced satisfactory performance and accommodated plastic rotations larger than 0.03 rad, while a maximum plastic rotation of 0.04 rad could be developed in the high-strength steel panel zone before fracture occurred. These results evidenced that those dual-steel connections could still sustain high seismic deformation demands.

Thermal and Structural Response of Beam-end Shear Connections During a Large Compartment Fire Experiment

The role of steel connections is essential in structural fire design and analysis for steel-framed composite structures. The current structural design provisions provide strength reduction factors of load-carrying members and their end-connection elements at elevated temperatures, based on small-scale experiments under uniform heating conditions. The realistic thermal and structural evolution in member connections, especially as part of full-scale floor assemblies exposed to a large compartment fire, has not been well characterized. A large compartment fire experiment was recently conducted on a 9.1 m by 6.1 m composite floor assembly as part of a two-story steel-framed building. The test assembly had a total of ten shear-tab (fin-plate) connections subjected to combined fire and mechanical loading. This paper presents the measured thermal response of these connections to fire and comparison with the corresponding Eurocode 3 predictions with two methods (1) incorporating the beam bottom flange temperature at midspan and (2) the section factor method. The results show that the Eurocode 3 methods conservatively predict the maximum temperature during heating and the cooling rate but overestimate the high-temperature strength of connections when estimated using the section factor method, showing that the Eurocode 3 simplified approaches are not meant to provide the details of the failure mode for connections. This study suggests that estimating the strength of connections using strength reduction factors may not guarantee a safe structural fire design. In addition, this paper estimated the total axial force (from slab and beam) at the composite connection via using the strain gauge measurements close to the column bases which were not exposed to fire. It suggests realistic axial load and rotational demand on the shear connection due to restraints to thermal elongation or contraction of supported members should be considered in future design guidance as should designing and detailing the connections for ductility to withstand the inelastic deformation demands during the heating and the cooling phases.

Experimental investigation of header end-plate beam-to-column composite connections with single-corner gusset plates

Experiments on 6 full-scale header end-plate beam-to-column connections with concrete slabs and single-corner gusset plates of bracing members were conducted to study the contribution of concrete slabs and gusset plates to the hysteretic behavior of this type of connection. The main failure mode of the connections was the out-of-plane distortion instability of the steel beam caused by the bottom-up beam web crack near the connecting weld between the header end-plate and beam web. The experimental results showed that the connection had a certain energy dissipation capacity. Considering the concrete slab and the gusset plate, the connection behaved as a partially restrained connection and had flexural strength equivalent to 80% and 64% of the plastic moment of the beam under positive and negative moments, respectively. At a connection rotation of 0.02 rad, the connection still had a flexural capacity equivalent to 65% and 30% of the plastic moment of the beam under positive and negative moments, respectively, which provided a reserve capacity to resist seismic collapse of concentrically braced frame structures after brace failure. To make the gusset plate rotate out of the plane freely during brace buckling, some specimens were opened in the slab around the gusset plate. The existence of openings reduced the energy dissipation capacity and the flexural strength of the connection by 6% and 5%, respectively. The beam bottom flange comes into contact and extrudes with the column flange to regain stiffness and strength under negative loads in the later stage.

A generalised component method for bolted angle connections

This paper extends the existing Generalised Component Method (GCM) to model the full-range behaviours of connections with bolted angles, by introducing two new components, i.e., the slip component to model the slip of bolts (in shear), and the gap component to model the contact of beam bottom flange and column due to large joint rotation. The key aspects and principles of the extended GCM are established, including its framework, modelling techniques, and the constitutive models of the new components and other important components. The sequences of springs activation and the moment–rotation curves of individual loading stages were analysed. The extended GCM is compared against existing experimental results on three typical bolted angle connections. It is found the proposed GCM can predict the full-range moment–rotation behaviour of the connections reasonably well, showing significant improvement than the current component method in Eurocode 3 and the three-parameter-power model (3PPM).

Research of a new hot-rolled connector connection to composite walls

The composite members in steel residential buildings often possess small sections and have high restrictions on the indoor space extrusion of structural components, and then traditional diaphragm type and internal or exterior stiffener connections are hardly applied. A new hot-rolled connector connection was proposed, which used the flange-to-web arch transition of hot-rolled steel beams as the intermediate connector. The smooth section transition of the connector at the column face junction can reduce the stress concentrations and premature fracture potentials of connections between H-shaped beams and small-size composite walls or CFST columns in steel residential buildings. Four hot-rolled connector connections with different connector designs and beam sizes were tested under cyclic loads, and the seismic performances and working mechanisms were investigated. Refined finite element simulations were performed to indicate the load transfer path and load sharing mode in the connection. Results indicated that the proposed connection displayed plump hysteretic relations and satisfactory energy dissipation ability. The strengthened connectors succeeded in inducing the high plasticity away from the wall or column face. The bottom beam flange to connector butt weld was prone to fracture. The HTC connection with a long HTC connecting beam and the two bolt-row construction as the web connection was suggested.

Seismic demands of steel moment resisting frames with inelastic beam‐to‐column web panel zones

AbstractThis paper intends to provide quantitative seismic response characteristics of steel moment resisting frames (MRFs) with inelastic beam‐to‐column panel zone joints. Two modeling approaches are proposed for this purpose along with a methodology to consider the fracture potential at the bottom beam flange in steel MRFs due to panel zone kinking. Both approaches, which are validated with available experiments, preserve the behavioral insights of inelastic panel zones. Nonlinear static and dynamic analyses are then conducted to quantify the seismic demands of 32 prototype steel MRFs while their panel zones exhibit various levels of inelastic deformations. Engineering demand parameter hazard curves are developed to interpret the results for a risk‐targeted seismic performance. The results demonstrate that steel MRFs with panel zone shear distortions of 10 to 15 times the shear distortion at yield, , have a mean annual frequency of collapse ranging from to , which is up to two times lower than corresponding results with code‐compliant steel MRFs (i.e., It is shown that steel MRFs with inelastic deformations of 10 in their panel zones, (a) enjoy up to 50% reduction in residual story drift ratios at a design basis earthquake; (b) their beam‐to‐column connections do not experience fractures due to panel zone kinking; and (c) local buckling in steel beams is very limited even at low probability of occurrence earthquakes. The above hold true when the current detailing and fabrication practice is employed. The findings have implications on seismic design and the post‐seismic repairability of steel MRFs.

Determining the effective slab width of composite joint with the weak-axis connection for ultimate bearing capacity

Extensive brittle failures on the weld zone of beam bottom flange in composite joints of SMRFs were observed during the Northridge earthquake. One of the main reasons is the sizeable tensile strain demand generated at the beam bottom flange zone under sagging bending. The key to accurately understanding the mechanical properties of composite joints is the effective width of the slab. However, the previous research is all aimed at the I-section composited joints with column strong-axis connection or CFST column connection. Therefore, the research conclusions may not suit the composited joint with the weak-axis connection. Beam column weak axis connection is as universal as the strong axis connection. Therefore, this paper used ABAQUS software to investigate the effective width of the slab in composited joints with the weak-axis connection. Firstly, it defines the ultimate state of bearing capacity corresponding to the analysis in this paper. Secondly, it investigates the influence factor on the effective width of the slab under the ultimate sagging moment and hogging moment. Thirdly, it uses Gray Correlation Theory and MATLAB to fit the calculation formula of the effective width of the slab under the ultimate sagging moment and hogging moment. Fourthly, it compares the slab's effective flange width in the previous experiment and the formula presented in the paper. Finally, analyze the applicability of the fitting formula to the strong axis rigid composite joints and T-shaped rigid composite joints of the I-shaped column and H-beams.

A Proposal for a Simple Method for Determining the Concrete Slab Width of Composite Beam-to-Column Joints

Extensive brittle failures in the weld zone of the beam bottom flange in composite joints of SMRFs were observed during the Northridge earthquake and the Kobe earthquake. One of the main reasons is the large tensile strain demand generated at the beam bottom flange zone under positive bending. The key to accurately understanding the mechanical properties of composite joints is the effective width of the slab. However, researchers have no consensus on the flange slab’s effective width for composite joints, even with the simplest solid concrete slab. Therefore, the best way to design the composite joint specimen is by following the current design specifications. However, each specification implements different ideas and approaches for specifying an effective slab width. This paper presents comparisons of the effective flange width provisions in China, the United States, Australia/New Zealand, and the European Union. The characteristics of each provision are briefly described and summarized. Numerical comparisons for positive and negative moment regions of continuous spans follow. Based on the commonalities and main differences among all these provisions, we propose a recommended equation for calculating the effective width of a solid concrete slab, reflecting the specification differences. The reliability of the recommended equation was verified with ABAQUS software analysis. The recommended equation can meet the fast and secure design requirements for composite joints in tests with concrete slabs.

The Effect of Degree of Composite Action on Reduced Web Section (RWS) Connections

AbstractDue to the concerns regarding the effects of the composite action on the seismic performance of beam‐to‐column connections leading to premature brittle fracture of the beam bottom flange, this paper introduces a comprehensive finite element (FE) analysis studying the effect of the degree of composite action to the performance of the bolted extended end‐plate (BEEP) RWS connections subjected to cyclic loads. Circular web openings with diameter equal to 0.8 of the beam's depth and two different parameters, namely the distance from the face of the column, S, and the degree of composite action, are investigated. Based on the results, it is confirmed that the degree of composite action of RWS connections is an important factor in seismic‐resistant design. It is concluded that the low degree of composite action with the use of a structural fuse, such as an RWS connection with localised beam web opening, can potentially avoid bottom flange fracture damage. It is found that composite RWS connections can also be employed to improve the seismic performance on steel structures.

Anti-collapse performance of composite frame with special-shaped MCFST columns

A spatial steel frame with concrete slab and special-shaped multi-partition concrete filled steel tubular columns was tested in a simulated scenario of progressive collapse. The load-deformation relationship of the frame with damaged middle column under vertical load is discussed. Parametric analysis is also conducted by using a validated FE model. The results show that the failure of the specimen resulted from the fracture of the bottom beam flange eventually. The configuration of side plate connection and multi-partition concrete filled steel tubular column both could provide a reliable load-redistribution path for additional loads due to single column loss. The designed frame exerts good robustness to resist the collapse resulting from middle column loss. The simulation shows that single-column removal would cause additional lateral force and moment on the adjacent columns, besides axial force. The length of side plate exhibits more influence on the performance of the frame than the thickness of side plate.

Investigation of side plate connections in an S-CFST column frame under a column-loss scenario

The progressive collapse resistance of four full-scale special-shaped columns composed of concrete-filled steel tubes (S-CFST column)-steel beam assemblies was studied in this paper. The failure modes, deflection profiles, vertical resistance development mechanisms and internal force development at the assumed plastic hinge section under the influence of infilled concrete, various span-to-height ratios of the steel beam and different connection details were compared. The test results showed that the fracture failure of all the specimens first occurred near the full penetration weld of the bottom beam flange or the bottom cover plate at the end of the side plate. Fracture failure of the steel beam web or shear plate occurred through the bolt holes. The beam chord rotation ranged from 0.055 to 0.088. The concrete infilled steel tube improved the ductility of the assembly and resulted in the development of the catenary mechanism, while reducing the span-to-height ratio of the steel beam resulted in the delay of the initiation of the mixed mechanism stage. Additionally, the development of the catenary mechanism was more sufficient for the specimen with the concrete infilled steel tube than the specimen without the concrete infilled steel tube under the same vertical displacement. The results showed that the ACPSP connection can avoid premature fracture of the full penetration weld and improve its progressive collapse resistance. By reducing the span-to-height ratio of the steel beam and using the ACPSP connection, the beam-column assemblies experienced not only the flexural and mixed mechanism stages but also the catenary mechanism stage. The GSPCP connection was proposed to improve the stress concentration in the ACPSP connection. Through further analysis by the verified finite element model, it is evident that the GSPCP connection can further improve the progressive collapse resistance of the S-CFST column-steel beam joint assembly.

Cyclic performance and fracture of wide flanged concentrically steel braced frames

ABSTRACT Concentrically braced steel frames (CBFs) are often used in buildings to resist lateral loads induced by wind and seismic excitations. Large-size wide-flange (WF) steel brace members could be selected and used in these frames. In the current study, a finite element model (FEM) with a strain-based fracture rule is presented to investigate the performance of a large-size WF brace member in single-storey CBF with different sizes of beams and columns when subjected to reversed axial displacements. The effects of restraining the far ends of the beam bottom-flange, increasing the thickness of tab-plates, increasing the number of bolts used to connect the tab-plates to the web of the beam, and the location of the work-point along the brace with respect the beam-column assembly, on the hysteresis response of the frame are investigated. Results suggest that it may be advantageous to reduce the number of bolts and to increase the bolt size to allow for an increase in the thickness of the tab-plates to resist the extreme rotational demand in high seismic regions, thereby preventing fracture of the weld between the tab-plates and the column flange.

Behaviour of MCFST column-steel beam connection with side plates in the scenario of column loss

Four multi-partition concrete-filled steel tubular (MCFST) joints with double side-plate (DSP) were tested in the simulated scenario of progressive collapse. The main parameter in the test is the joint connection type. Two kinds of joints with discrete and consecutive DSP are studied. The load-deformation relationship of the specimens is discussed. Parametric analysis is also conducted by using a validated FE model. The results show that the failure of four MCFST joints with DSP is all caused by the fracture of the bottom beam flange. The configuration of discrete and consecutive DSP both could provide a reliable load-redistribution path for the joint to resist additional loads due to single column loss. Increasing the width of discrete DSP and the length of consecutive DSP could improve the resistance and decrease the deformation ability of the joint. It is suggested that the extensive length of DSP from column wall (LC) should be larger than the width of the beam flange whilst the extensive height of DSP from beam flange (LF) should be designed to be close to the half width of the beam flange. Parametric analysis indicates that relatively thin DSP could make the connection more flexible to develop catenary action which would provide larger collapse resistance.

Bolted beam-to-column connection with buckling-restrained round steel bar dampers

In the design of steel building frames, comprehensive consideration is required to determine beam-to-column connections because their characteristics largely influence the performance of the whole frame. Recent studies by some researchers have investigated the application of dampers to bolted beam-to-column connections, where the dampers are often positioned between the bottom beam flange and the column. This study investigates the application of buckling-restrained round steel bar dampers to bolted beam-to-column connections. The configuration of the proposed connection and its design method are first described. Then, cyclic loading tests of six specimens are presented to clarify the structural performance of the proposed connection. The hysteretic behavior is largely influenced by the configuration of the damper, such as bar length, shank diameter, and damper position. Furthermore, finite element analyses of the damper and connection models are demonstrated for the detailed examination of their plastic behavior characteristics such as the spiral buckling deformation of the bar. The ratio of the beam shear force acting on the dampers is discussed based on the analysis results of the connection model.

Improved Steel Beam-Column Connections in Industrial Structures

Beam-to-column connection is a rigid connection used in steel moment frames which acts as the main resisting system in structural design. Haunches with double beam section height at the eave, ridge and crane bracket are usually used to resist the large bending moment at the critical locations of the steel frames. Damper devices can be used as the main source of producing forces used to reduce the bending moment due to different static and dynamic loads. This study focuses on improving the steel beam-column connection with the proposed control system in the eave connection which can also be used under the bracket which supports crane beam in industrial buildings. The purpose of this study is to describe the development of the beam-column steel connection incorporating dampers installed to beam bottom flange to reduce the effect of applied load on the frame without the need to create haunches to make the connection design more efficient.

Full-range behaviour of double web angle connections

This paper presents recent experimental and analytical investigations on the full-range flexural behaviour of double web angle connections. Experiments were carried out on bolted double web angle connections with two different configurations featuring three and five bolt rows, respectively. For each connection configuration, five nominally identical specimens were tested to investigate the variability of connection behaviour, including joint stiffness, ultimate resistance and ductility, thereby making a total of ten tests. All tests were conducted well into the post-ultimate range until complete failure of the connection, examining the full-range connection response. The load transferring mechanism of the connection was dynamically changing, partly because the presence of bolt hole clearance caused certain bolts to be initially inactive, only to later become active in transferring load from the beam, and partly because when the beam bottom flange came into contact with the column flange under large connection rotation, the centre of rotation changed causing the compressive load transferred at the bottom bolt rows to unload and reverse. To capture the above complicated changes of load transferring mechanisms and thus capture the full-range behaviour of a bolted web angle connection, an advanced mechanical model was proposed. The model is based on the Component Method, and adopts a decision tree to track the evolution of the load-transferring mechanism of the connection, including the bearing state of each bolt, either under tensile or compressive load, the potential presence of contact between the beam bottom flange and the column flange, and the plastic deformation of the critical connection components. The proposed model was applied to the tested connections with excellent agreement observed between the model predictions and the experimental results. Not only producing full-range force-displacement curves that are very close to the experimental curves, the proposed model also provides useful information about the internal state of the connection, which was not obtainable from the tests.

Shear tab connection with composite beam subjected to transient-state fire temperatures

Purpose This study aims to develop a rational model to predict the thermal axial forces developed in shear tab connections with composite beams when subjected to transient-state fire temperatures. Design/methodology/approach Finite element (FE) models are first developed in ABAQUS and validated against experimental data available in the literature. Second, a parametric study is conducted to identify the major parameters that affect the behavior of shear tab connections with composite beams in the fire. This includes beam length, shear tab thickness, shear tab location, concrete slab thickness, setback distance and partial composite action. A design-oriented model is developed to predict the thermal induced axial forces during the heating and cooling phases of a fire event. The model consists of multi-linear springs that can predict the stiffness and strength of each component of the connection with the composite beam. Findings The FE results show that significant thermal axial forces are generated in the composite beam in the fire. This is prominent when the beam bottom flange comes in contact with the column. Fracture at the toe of the welds governs the behavior during the cooling phase in most FE simulations. Also, the rational model is validated against the FE results and is capable of predicting the thermal axial forces developed in shear tab connections with composite beams under different geometrical properties. Originality/value The proposed model can predict the thermal axial force demand and can be used in performance-based approaches in future structural fire engineering applications.

Elasto-plastic behavior of weak-panel beam-column joints with RC slabs under bidirectional loading

This paper presents an experimental and numerical study to examine the elasto-plastic behavior of the weak-panel type beam-column joints with RC slabs subjected to bidirectional and unidirectional loading. The effect of the RC slab on the behavior of the beam-column joints was investigated. The steel beam-column joint specimens were composed of wide-flange beams and square tube columns, and the panel zones were designed to yield before the columns and beams. The specimens failed at 0.04 rad story drift under bidirectional loading, while the specimens sustained a story drift of 0.08 rad under unidirectional loading. The existence of the RC slab led to various failure modes in this test, including the local buckling of the upper column, the weld fracture at the lower panel section, and the weld fracture at the beam bottom flange. It was worth noting that the panel-to-diaphragm weld cracks always initiated in the lower panel section and developed into severe fractures. Both the top and bottom surfaces of the slab suffered concrete crushing in the area around the column. Concrete cracks on the top surface were basically along the direction of the steel beam, while the cracks along the direction of the slab diagonal were also observed for the specimens under bidirectional loading. The RC slab increased the plastic strength of the specimens by 15%–20%. An equation was proposed to quantify the slab effect on the plastic strength of the specimens. Finite element analysis was conducted to validate the proposed equation.

A Repairable Connection for Earthquake‐Resisting Moment Frames

ABSTRACTMost steel moment frames are designed so that the beams act as structural fuses during earthquake loading. This approach, while effective for life safety, has poor resilience because it is difficult to replace beams in a building. A repairable steel moment frame connection has been developed that may be attractive to engineers and constructors. The connection has a fuse plate bolted to the beam bottom flange which experiences shear yielding during severe earthquakes, protecting both the column and the beam. After an earthquake, the fuse plates can be removed and replaced to unlock residual frame deformations and prepare the building for another event.Full‐scale experiments were conducted to validate the performance of this repairable connection. Two tests were performed with a W690×125 beam and a W530×196 column. The tests demonstrated rotation capacities of at least 0.05 rad, which exceeded AISC requirements for special moment frames. The fuse plate was effective in preventing beam damage in each test, and the second test demonstrated that the connection could be repaired by replacing the fuse plate.