Reverse Channel Connection Research Articles

Experimental Study on Composite Beam with Various Connections under Midspan Impact Scenarios

The performance of an immediate frame span subjected to a falling-debris-impact scenario is of great importance to investigate the spread of initial damage during a progressive collapse event. If the frame span was not able to resist the impact load from possible falling debris, it would become new debris and impact the lower floor with previous debris. After that, progressive collapse might probably happen. However, the impact mechanism considering the contribution of composite slabs of an immediate span remained unclear currently. Therefore, experimental studies on a series of three half-scale composite beam subassemblies were conducted under impact scenarios. The impact loads were applied via a high-performance drop-weight test machine. Various beam–column connections, including welded unreinforced flange-bolted web, fin plate, and reverse channel connections, which represented rigid connection, hinge connection, and semirigid connection, respectively, were investigated. The structural resistances of the specimens were provided by flexural action, compression arch action, and catenary action. Among them, the contribution of compressive arch action was limited and can be ignored. Flexural action always provided the most structural resistance contribution in the whole impact process, and catenary action provided structural resistance under large deformation. The specimen structural resistance increased with increasing deformation before the failure of connections. Good flexural resistance and deformation capacity were the key factors for structures to resist impact load. The reversed channel specimen had the best impact resistance.

Parametric analysis and simplified approach for steel-framed subassemblies with reverse channel connection under falling-debris impact

Up to now, most studies corresponding to building structures against progressive collapse are based on the column-removal scenario. However, other cases, i.e., falling-debris impact scenario, which may cause progressive collapse of building structures were limitedly studied. This paper presents numerical simulations and a simplified approach of steel-framed subassemblies with Reverse Channel connection with Extended End Plate (RC-EEP) under the falling-debris impact scenario. The numerical results demonstrate the ductility, load-carrying capacity, and energy-absorption capacity of the steel-framed subassemblies with different Span-to-Depth Ratio under various impacted locations. All specimens showed the same global deformation profile and same load-resistance mechanism under the impact load on the same locations. Under mid-span-impact scenario, axial elongation of connection primarily affected the ductility. Both catenary action and flexural action affected the load-carrying capacity. Failure displacement, maximum load-carrying capacity and energy-absorption capacity all decreased with the increase of the Span-to-Depth ratio. Under beam-end-impact scenario, both the ductility and the load-carrying capacity were primarily controlled by the shear deformation and shear strength of the connection close to the impacted location. The increase of the Span-to-Depth ratio had little effect on failure displacement while slightly reduced the maximum load-carrying capacity and energy absorption capacity. Smaller Span-to-Depth Ratio was recommended in the impact-resisting design for impact load on both impact locations. A simplified approach was also developed to predict the behaviour of the steel frame subassemblies up to total failure. The validation study indicated that the proposed models can represent the key responses of steel-framed subassemblies with different Span-to-Depth Ratio subjected to impact load on various locations, including the flexural action at pure bending stage, the development of flexural action and catenary action at the tension-bending stage, and the failure stage. The accuracy of the proposed approach was validated against the experimental tests and numerical simulations, in the aspects of internal force-displacement relationships, load-carrying capacity-displacement relationships, and energy absorption-displacement relationships. The proposed approach provided an accurate and efficient way to predict the impact resistance of specimens with various Span-to-Depth Ratios under both of the mid-span impact and the beam-end impact scenarios.

An analytical method to calculate temperatures of components of reverse channel connection to concrete filled steel section under fire conditions

The paper describes the development of an analytical model for calculating the temperatures of different components of reverse channel connections to concrete filled steel tubular sections. The proposed analytical model is based on the Lumped Capacitance Method and includes the calculation of the quantity of heat transferred at the interface between the steel tube and the concrete core. For unprotected connections, the connection is divided into two regions. One part of the connection is almost unaffected by the presence of the concrete core, on the contrary, temperature of the second part of the connection is significantly affected by the heat transfer at the steel/concrete interface. For protected connections, the entire connection is assumed to have the same temperature. Equivalent two-dimensional section factors are proposed for these connection parts. The analytical model is developed based on the results of extensive parametric studies under standard and different parametric fire conditions and validated against fire test results. The proposed analytical method is applicable to both fire protected and unprotected connections, including both the heating and cooling periods of the fire attack, and incorporating the effects of in-fill concrete.

Development of a General Component-Based Connection Element for Structural Fire Engineering Analysis

This paper reports on the development of a general-purpose Eurocode-compliant component-based connection finite element for steel-to-steel joints in fire. The development begins by utilising the temperature-dependent connection component characteristics previously developed at the University of Sheffield to create a component-based connection finite element to model flush endplate connections. Subsequently the element was extended to a new connection type with high ductility, the reverse channel. The component models have been developed for the reverse channel under tension and compression. The element has been incorporated into the nonlinear global structural analysis program Vulcan, in which it has been used along with a static-dynamic formulation. The use of the element is illustrated by modelling a fire test at the University of Manchester in which reverse channel connections were used.

Temperatures and thermal boundary conditions in reverse channel connections to concrete filled steel sections during standard and natural fire tests

The paper presents fire test and numerical simulation results of temperature distributions in reverse channel connections to concrete-filled tubular columns during standard and natural fire tests. The experiments included a furnace fire test with the composite frame subjected to increasing fire temperature according to the ISO 834 standard time–temperature curve and two natural fire tests in a full-scale structure including a cooling phase. These fire tests examined the effects of fire protection, different connection locations and different connection dimensions. Temperatures at different connection components were recorded. The recorded connection component temperatures were simulated by using the SAFIR fire engineering software to calibrate the thermal boundary conditions in different connection components. From the numerical simulation results, it has been concluded that radiation to the inner surfaces of the reverse channel and the adjacent part of the column tube is only from the gas volume bounded by these surfaces. A lower flame emissivity value than the standard value should be used. For simplicity, a value of εf=0.1 is proposed. Also a lower value of the convective heat transfer coefficient of 10W/m2K can be used in the connection area.

Initial stiffness evaluation of reverse channel connections in tension and compression

The so-called Reverse Channel connection has been conceived for the purpose of accommodating the thermal expansion of beams so that premature failure due to thermal buckling is avoided. The connection is made of a channel-shaped element, welded along the tips of its flanges onto the face of a hollow section column; an endplate welded on the beam is bolted onto the web of the channel. In a fire situation, the thermal expansion of a reverse-channel supported beam causes extensive bending deformation of the connection, therefore preventing the development of significant axial stress in the beam. Furthermore, this connection offers a high rotational capacity, if designed properly, which is beneficial in a fire situation where excessive deflections of beams can be expected.This paper aims to provide analytical stiffness assessment tools for reverse channel connections in compression and tension under uniform temperatures. The proposed analytical models are compared to results of Finite Element simulations, which in turn have been benchmarked with experiments. In addition, a comprehensive parametric study is conducted in order to identify all influencing factors on the initial stiffness response: reverse channel geometry and thickness, plate thickness, bolt position, and bolt diameter. Correction factors that account for 3D effects and bolt size are presented and discussed.The obtained expressions for the reverse channel stiffness are found to provide an accuracy that is acceptable for structural applications and can, therefore, be used as a design tool.

Finite element prediction of reverse channel connections to tubular columns behavior

This paper attempts to introduce a standardized moment-rotation function for reverse channel flush end-plate connection (RCC). This function is expressed in terms of the geometric parameters for the purposes of either predicting the connection behavior or incorporating the behavior into a frame analysis computer program. With this objective a parametric study was carried out, based on modeling of 140 RCC under monotonic loading by using the general finite element package ABAQUS. Out of the six currently available functions, the Kishi–Chen and Richard–Abbott models were proved appropriate for fitting experimental M–ϕ data for RCC. The two functions were successfully used in this parametric study to express the relationships for 140 specimens, where the analytical and predicted data are in very good agreement. However, the results of the fitted functions indicate that the dimensionless form of Richard–Abbott function provides more accuracy than Kishi–Chen function. Furthermore, the comparison of Eurocode 3 model with numerical M–ϕ curves illustrated a significant overestimation of the knee region behavior for most of the cases, particularly for RCC with low initial stiffness.

Tying resistance of reverse channel connection to concrete filled square and rectangular tubular sections

This paper presents the derivations of an analytical method to evaluate the tying resistance of reverse channel connection to rectangular/square concrete filled tube (CFT). The analytical derivations are based on results of an extensive series of numerical parametric study of this type of connection, covering a range of design parameters, including tubular section size, tubular section width to thickness ratio, reverse channel flange size and gap between the reverse channel flanges.Under a tying force, the tubular section may fail in two modes: formation of yield line mechanism or fracture under tensile membrane action in the tubular wall. In most cases, the membrane resistance governs. However, if the tubular section is thick, the yield line resistance is higher than the membrane resistance and membrane action does not develop. The tying resistance of the connection is the higher of the two values.This paper derives analytical equations for calculating these two typing resistance values. The membrane resistance depends on the ultimate lateral deformation of the tubular wall and this paper also presents a method for its calculation. Comparisons between the analytical calculation results and the numerical simulation results indicate that the proposed analytical equations give reasonably accurate calculations and the analytical method may be used in practical design.

Behavior of reverse-channel and double-reverse-channel connections to tubular columns with HSS

This paper reports on the possible achievements on moment–rotation (M–ϕ) characteristics of reverse channel flush end-plate connections (RCC) by using HSS reverse channel under monotonic loading. It also introduces a double reverse channel connection (DRCC), where the reverse channel is split into two pieces, by leaving a gap in between, for better access to bolts. The investigations were based on parametric studies performed by using a general finite element package ABAQUS. In most cases, the use of HSS reverse channel led to noticeable increase in ultimate flexural resistance and the rotational capacity of RCC and DRCC. The key for the failure of RCC was the ratio of wall thickness of channel to the thickness of flush end-plate. When the behavior of DRCC with HSS (S690) reverse channel was compared to RCC, there was an increase in ultimate flexural resistance and rotational capacity by up to 38.7% and 53.1% respectively, without compromise to the initial stiffness. It was also noted that the clear distance between the split reverse channels had effect on the deformation of DRCC. This distance should be kept to a minimum, 20% of the beam depth, and at the same time it should be practically adequate for bolt accessibility.

Numerical Study of Steel Beams in Sub-frame Assembly Validation of Existing Hand Calculation Procedures

The design methods currently proposed by the codes prescribe the strength assessment of structures to be based on their strength limit state. These design methods can be applied to isolated steel members to determine their design strengthin fire. The real response of a structural member is, however, more complex due to the thermal expansion and the presence of restraints against this expansion by the surrounding structure. It is therefore imperative to study the response of a structural member at high temperature in a way which includes its interaction with its surroundings. This paper focus on the numerical investigation of steel beams in structural frames connected to concrete filled tubular (CFT) columns through reverse channel connections and comparison to hand calculation procedures. Finite element models (FEM) of the sub-frames were validated against fire tests conducted on sub-frames and then their results were compared to the proposed simplified hand calculation procedures (HCM).

Reduction of Connection Resistance During Veselí Fire Tests

The paper is focused to the temperature distribution in the reverse channel connections to concrete filled tubular column during two fire tests on an experimental building in Veselí nad Lužnicí. Temperatures of connections without fire protection as well as fire protected connections were investigated. The connected beams were without fire protection which allows to take into account a membrane action of a composite ceiling without fire resistance reduction of the connections.

Parametric study on moment–rotation characteristics of reverse channel connections to tubular columns

The significance of this parametric study initiates from the need for further understanding of the behavior of semi-rigid/partial-strength I-beam to tubular column connections. This research attempted to gain a qualitative understanding of the influence of the geometrical configurations of reverse channel connections (RCC) on the moment–rotation (M-ϕ) response. Hence, ABAQUS (v.6.12) software was used to develop three-dimensional (3-D) FE models for thirty specimens. The FE models developed were validated against the experimental results available from literature where 268 FE models were used for sensitivity analysis. The main emphasis of this research was on the stiffness, strength, sources of deformability, rotational capacity and failure mechanisms of the RCC. All tests were able to achieve a rotational capacity beyond the minimum 0.03rad, in most cases more than 0.06rad and in one case reached 0.16rad. For the same configuration of RCC, it was found that the channel wall thickness to the flush end-plate thickness ratio is the key to the rotational capacity of RCC. In addition, the deformability of the column face was mainly dependent on the ratio of channel depth to square hollow section (SHS) width and the minimum value of this ratio is recommended to be kept equal to 0.72.

Force–deflection relationship of reverse channel connection web component subjected to transverse load

This paper presents details of development of analytical formulations for evaluating the force–deformation relationship of reverse channel web under transverse tension. The force–deformation relationship is assumed to be a bilinear:linear response from start of loading until yield line formation and afterwards until ultimate resistance. The ultimate resistance is controlled by either tensile membrane action in the web until fracture, bolt fracture or punching shear resistance of the web. One row and multiple rows of bolts are considered. The ultimate deformation is derived according to assumed strain distribution in the web plate based on extensive numerical simulation results.The derivations are validated by comparisons against extensive numerical simulation results and against available test results.

An experimental study of the behaviour of reverse channel connection components to concrete filled steel tubes under tension at elevated temperature

This paper presents the results of a series of tests conducted on two components of reverse channel (RC) connection to concrete filled tubular (CFT) column to quantify the load–deflection behaviour of these components at both ambient and elevated temperatures. A total of 21 tests were performed, including 9 tests on the RC component and 12 tests on the face of the CFT section.The RC tests covered two channel widths (150 and 230mm), two channel web thicknesses (5.5 and 7.5mm), one and two bolt rows and three temperature levels (20, 550, and 750°C). The final failure modes were web fracture around bolt hole, bolt pull-out and bolt fracture. Before these final failure modes which exhibited evidences of membrane action, there were evidences of formation of yield lines around individual bolts and formation of yield lines along the length of the channel section. Due to different reductions in strength of bolt and steel plate at elevated temperatures, the failure mode may be different from that at ambient temperature.In the CFT tests, the loads on the tube surface were applied through two plates welded to the tube along the longitudinal direction. The test variables included two types of hollow section (square hollow section (SHS) and circular hollow section (CHS)), two distances between the welded plates (150 and 230mm) which corresponded to the two RC widths and three temperature levels (20, 550, and 750°C). Four failure modes were attained: weld failure, column face pull out, loading plate fracture and bolt failure.

Experimental behaviour of the reverse channel joint component at elevated and ambient temperatures

The reverse channel connection appears to have the best combination of desirable features under fire loading: moderate construction cost, ability to develop catenary action and extremely high ductility through deformation of the web channel (Ding and Wang, 2007). This paper presents the results of an experimental investigation of a reverse channel component conducted at the University of Coimbra as part of the European RFCS COMPFIRE Project, the main focus of which is to characterise the behaviour of steel joints that connect steel beams to concrete-filled tubular columns under natural fire loading. A series of tensile and compressive tests at ambient and elevated temperatures was conducted. The purposes of the experimental tests were to characterise the strength, stiffness and ductility of this joint component and to establish a relationship between force, displacement and temperature.

Experimental study of moment–rotation characteristics of reverse channel connections to tubular columns

This paper presents the results of an experimental study of the moment–rotation characteristics of reverse channel connections to tubular columns. Eight tests were carried out to investigate the effects of different parameters, including connection type (extended or flush endplate), reverse channel dimensions (thickness, width), connecting side of tube (narrow side or wide side), with or without concrete infill. This paper gives details of joint component deformation patterns, failure modes and complete connection moment–rotation curves. It has been found that using extended endplate can achieve connection performance required for semi-continuous design (sufficient rotation stiffness and moment resistance) and plastic design (sufficient rotation capacity); using concrete infill in the tube can greatly enhance both the connection rotation stiffness and moment capacity, with slight reduction in the connection rotation capacity; the key connection component is the reverse channel web and preferably hot-rolled tubes should be used to make the channels. Furthermore, the reverse channel web width should be minimised.

Experiments on reverse-channel connections at elevated temperatures

This paper reports on an experimental investigation of the behaviour of reverse-channel connections between steel beams and concrete-filled tubular (CFT) columns in fire conditions. The objectives of the tests were to assess the behaviour of beam-to-column connections subject to the combinations of significant tying and shear forces and large rotations, which can arise during a building fire, and to provide test data to characterise and validate simplified temperature-dependent component-based connection models. It has been found that the reverse-channel connections not only provide a practical solution for connecting steel beams to composite columns but they also possess high ductility and strength. Such high ductility allows greater beam deformations in fire while reducing the magnitudes of the induced axial forces, thereby enhancing the fire resistance of the connections without requiring them to be stronger.

Behaviour of restrained structural subassemblies of steel beam to CFT column in fire during cooling stage

During the well publicised Cardington structural fire research programme, parts of some connections suffered fracture during cooling. This led some to believe that fire spread and structural collapse may occur during the cooling stage of a fire and therefore this issue should be considered in structural fire engineering design. This paper focuses on steel framed structures using concrete filled tubular (CFT) columns and the objective of this paper is to find means of reducing the risk of structural failure during cooling. It reports the results of a study using the general finite element software ABAQUS to numerically model the behaviour of restrained structural subassemblies of steel beam to CFT columns and their joints both in fire, emphasising on the cooling stage. Validation of the finite element model was achieved by comparing the simulation and test results for the two fire tests investigating cooling behaviour, recently conducted at the University of Manchester on similar structures. In these two tests, the test assembly was heated to temperatures close to the limiting temperature of the steel beam and then cooled down while still maintaining the applied loads on the beam. One of the tests used reverse channel connection and the other test used fin plate connection. The finite element models give very good agreement with the experimental results and observations. Remarkable differences in tensile forces in the connected beams were observed during the tests depending on the beam temperature at which cooling started. This leads to the suggestion that in order to avoid connection fracture during cooling, it may be possible to reduce the limiting temperature of the connected beam by a small value (<50°C) from the limiting temperature calculated without considering any axial restraints in the beam.

Response and component characterisation of semi-rigid connections to tubular columns under axial loads

This paper examines the behaviour of open beam-to-tubular column bolted connections with angles subjected to direct tension and compression. Experimental and numerical studies as well as simplified mechanical assessments are presented. The response of seven tension tests and six compression tests on blind-bolted angle connections and combined channel/angle configurations are described and discussed in detail. Firstly, the experimental set-up, connection details and material properties are introduced followed by a detailed account of the results and observations from the tests. Based on the experimental results, the main behavioural patterns are identified and the salient response characteristics such as stiffness, capacity and failure mechanism are examined. It is shown that, for Hollo-bolted details, the distance between the blind-bolt and beam flange, the angle thickness and the column thickness have significant effects on the initial stiffness and tensile capacity of the connection. In addition, in the case of reverse channel connections, a direct relationship is deduced between the thickness of the channel component and the connection stiffness and capacity. It is also shown that the inelastic axial mechanisms exhibited by these types of connections are largely determined by the relative widths of the column/reverse channel and beam/angle components. Complementary finite element simulations are presented and utilised, together with the experimental findings, to highlight the main inelastic response characteristics for these forms of connections under direct axial action. Finally, the component-based method is extended to deal with angle connections between open beams and tubular columns employing Hollo-bolts or reverse channel components. To this end, expressions for the estimation of connection stiffness and capacity under axial actions are proposed and validated.

Methods of improving the survival temperature in fire of steel beam connected to CFT column using reverse channel connection

This paper presents the results of a numerical study, using ABAQUS, of the behaviour and methods of improving the survival temperatures in fire of steel beams to concrete filled tubular (CFT) columns using reverse channel connection. The beams are axially restrained by the connected columns and develop catenary action so their survival temperatures are primarily controlled by the joint tensile resistance and deformation capacity. Therefore, improving the beam survival temperature mainly relies on improving the joint performance. This study investigates five different joint types of reverse channel connection: extended endplate, flush endplate, flexible endplate, hybrid flush/flexible endplate and hybrid extended/flexible endplate. The connection details investigated include reverse channel web thickness, bolt diameter and grade, using fire-resistant (FR) steel for different joint components (reverse channel, end plate and bolts) and joint temperature control. The effects of changing the applied beam and column loads are also considered. It is concluded that by adopting some of the joint details to improve the joint tensile strength and deformation capacity, it is possible for the beams to develop substantial catenary action to survive very high temperatures. However, it is important that the additional catenary force in the beam is resisted by the connected columns.