Concrete Filled Tubular Research Articles

Hysteretic performance of a new blind bolted connection to concrete filled columns under cyclic loading: An experimental investigation

The structural performance and reliability of a new blind-bolting technique is investigated in this study. The new blind-bolt is termed Extended Hollobolt (EHB) and is a modification of the standard Hollobolt. The EHB enhances the tensile resistance and stiffness of the fastener by anchoring it in the concrete infill of a tubular column. This paper reports on an investigation into the cyclic behaviour of end-plate connections to concrete filled tubular (CFT) columns using the EHB. A series of six full-scale connections were tested under quasi-static cyclic loading. The key parameters investigated were amplitude of cyclic loading procedure, bolt grade, tube wall thickness, and concrete grade. The strength, stiffness, rotation capacity and energy dissipation capacity of the connections were evaluated at different load cycles. The EHB provided stable hysteretic behaviour with appropriate level of strength and stiffness, where strength is comparable to that of standard bolt-and-nut fasteners and where rigid behaviour can be achieved. The influence of tube wall thickness and concrete grade on the performance of the connection in terms of strength, stiffness, ductility and failure mode is investigated. It is shown that the required performance can be achieved by controlling the tube wall thickness and concrete strength. The results indicate that the connection can offer energy dissipation capacity and ductility appropriate for its potential use in seismic design.

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.

Simple calculation model for evaluating the fire resistance of unreinforced concrete filled tubular columns

This paper presents a method for calculating the fire resistance of unreinforced axially loaded concrete filled circular hollow section columns, based on the guidelines of Eurocode 4 Part 1.2 for the fire design of composite columns. The different design approaches which are currently available worldwide are reviewed in this paper, focusing in particular on the European code, which is discussed and analysed in depth. Several of the aspects which are nowadays missing or thought to be arguable by researchers in the Eurocode 4 method are studied and on the basis of parametric studies a new proposal is developed, so that designers can easily apply the code in the future. The parametric studies investigate the main factors affecting the fire resistance of unreinforced concrete filled tubular (CFT) columns, such as the outer diameter, the thickness of the steel tube wall, the relative slenderness of the column at room temperature and the fire exposure time, widely covering the range of values which can be found in practice. From the results of these parametric studies, equations for obtaining the equivalent temperatures of steel and concrete are developed, and appropriate flexural stiffness reduction coefficients are integrated in the simple calculation model of Eurocode 4. The method presented here is valid for centrally loaded circular CFT columns filled with unreinforced normal strength concrete, and makes possible the calculation of slender columns, extending the current limitations in buckling length of the European code.

The 3D-numerical simulation on failure process of concrete-filled tubular (CFT) stub columns under uniaxial compression

Based on the heterogeneous characterization of concrete at mesoscopic level, Realistic Failure Process Analysis (<TEX>$RFPA^{3D}$</TEX>) code is used to simulate the failure process of concrete-filled tubular (CFT) stub columns. The results obtained from the numerical simulations are firstly verified against the existing experimental results. An extensive parametric study is conducted to investigate the effects of different concrete strength on the behaviour and load-bearing capacity of the CFT stub columns. The strength of concrete considered in this study ranges from 30 to 110 MPa. Both the load-bearing capacity and load-displacement curves of CFT columns are evaluated. In particular, the crack propagation during the deformation and failure processes of the columns is predicted and the associated mechanisms related to the increased load-bearing capacity of the columns are clarified. The numerical results indicate that there are two mechanisms controlling the failure of the CFT columns. For the CFT columns with the lower concrete strength, they damage when the steel tube yields at first. By contrast, for the columns with high concrete strength it is the damage of concrete that controls the overall loading capacity of the CFT columns. The simulation results also demonstrate that <TEX>$RFPA^{3D}$</TEX> is not only a useful and effective tool to simulate the concrete-filled steel tubular columns, but also a valuable reference for the practice of engineering design.

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.

Numerical modelling of restrained structural subassemblies of steel beam and CFT columns connected using reverse channels in fire

This paper employs the general finite element software ABAQUS to numerically model the behaviour of restrained structural subassemblies of steel beam to concrete filled tubular (CFT) columns and their joints in fire. The simulations were conducted using 3-D brick elements to enable detailed structural behaviour to be obtained. For validation, this paper compares the simulation and test results for the three fire tests using reverse channel connection recently conducted at the University of Manchester. This comparison demonstrates that the 3-D finite element model is able to successfully simulate the fire tests. Afterwards, the validated finite element model was used to conduct a preliminary numerical study to investigate the feasibility of changing some of the connection details to enhance survivability of the structure in fire. Specifically, this investigation concentrated on developing connection methods to enable catenary action in the connected beam to be more fully developed. An example is to develop a hybrid flush/extended endplate and flexible endplate connection in which the tension part of the connection uses a flush/extended endplate for increased tensile resistance but the compression part of the connection uses a flexible endplate for improved ductility. It has been found that, without additional cost, using a hybrid extended/flexible endplate connection to replace a flush endplate connection has the potential to enable the connected beam to survive significantly increased temperature.

Shear and bending behaviour of fin plate connection to concrete filled rectangular steel tubular column — Development of a simplified calculation method

This paper presents the results of numerical and theoretical studies into the behaviour of simple welded fin plate to concrete filled tubular (CFT) columns loaded by bending and shear force. Such connections represent a simple single-sided joint solution to steel CFTs and are commonly used in practice. The current design guidance encapsulated in CIDECT Guide 9 does not cover the ultimate strength of such connections. Hence numerical models have been developed and used to perform extensive parametric studies from which a simple manual calculation method has been developed for evaluating the strength of the column component of rectangular CFTs under shear and bending load imparted through a fin plate connection. The ranges of parameters encompassed by the model include column cross-section size, column thickness and fin plate length. The simple manual calculation procedure is based on defining a rigid plate deformation pattern for the column face and then applying the virtual work principle. The column face deformation corresponding to the maximum load has been found to be strongly related to the column width to thickness ratio and a method for evaluating this relationship has been proposed based on extensive numerical results. The load–deflection curve consists of two segments: the first representing the elastic behaviour from classic plate theory, the second obtained by plotting the deflection at the maximum load against the maximum load, both of which are obtained through the simple manual calculation procedure. The simple manual calculation method is compared favorably with a large range of finite element simulations. The proposed method may be used in the component based method for joints involving a fin plate component in shear and bending.

Advanced model for predicting the fire response of concrete filled tubular columns

In this work, a nonlinear finite element three-dimensional model is presented and validated in order to study the behaviour of axially loaded concrete filled tubular (CFT) columns with circular cross-section exposed to fire. A realistic sequentially coupled nonlinear thermal-stress analysis is conducted for a series of columns available in the literature. The model is validated by comparing the simulation results with the real fire resistance tests. By means of this model, and extensive sensitivity analysis is performed over a wide range of aspects concerning the finite element modelling of the problem under study, including new key factors not studied previously. Based on this sensitivity analysis several modelling recommendations are given in this paper, which will be useful for future research work. The validated numerical model is furthermore employed to study and discuss the Eurocode 4 Part 1–2 simple calculation model, which is deeply analysed in this paper.

Tying behaviour of fin-plate connection to concrete-filled rectangular steel tubular column — Development of a simplified calculation method

This paper presents the results of numerical and theoretical studies into the behaviour of simple welded fin-plate to concrete-filled tubular (CFT) columns loaded by tensile force. Such connections represent a simple single-sided joint solution to steel CFTs. The current design method for similar connections under purely tensile load, in CIDECT Guide 9, based on a deformation limit of 0.03 of the tube width is shown to be inadequate when evaluating the ultimate strength of such connections. Hence numerical models have been developed and used to perform extensive parametric studies from which a simple hand calculation method has been developed for evaluating the strength of the column component of rectangular CFTs under tensile load imparted through a fin-plate connection. The ranges of parameters encompassed by the model include column cross-section size, column thickness and fin-plate length. The simple hand calculation procedure is based upon defining a rigid plate deformation pattern for the column face and then applying the virtual work principle. The column face deformation corresponding to the maximum load has been found to be strongly related to the column width to thickness ratio and a method for evaluating this relationship has been proposed based on extensive numerical results. The load–deflection curve consists of two segments: the first representing the elastic behaviour from classic plate theory, the second obtained by plotting the deflection at failure against failure load — both of which are obtained through the simple hand calculation procedure. The simple hand calculation method is compared favorably with a large range of finite element simulations. The proposed method may be used in the component based method for joints involving a fin-plate component in tension.

Evaluating a steel beam’s rigid connection to a concrete filled tubular column when submitted to dynamic load

Using prequalified connections during the structural design stage becomes increasingly necessary when developing structural engineering projects which include steel elements; this is so that the steel elements’ appropriate behavior can be ensured according to the structural system and seismic demand. Unfortunately, the international entities providing this type of information (i.e. FEMA) only have a limit series of prequalified connections and such series do not include rigid connections between steel beams and concrete filled tubular (CFT) columns having an extended end plate, which has become a very widespread building practice in Colombia. This paper describes the most important aspects of a study at the Universidad Nacional de Colombia concerning the behavior of a steel beam rigidly connected to a CFT-column, using six physical models having different width-thickness ratio (b/t) columns. ANSYS v.10 software was used for studying theoretical models (finite elements analysis) for comparative analysis of cyclic test theoretical and experimental results for each specimen presented for the qualification phase. The six tested specimens’ hysteretic curves are presented. Several conclusions are drawn concerning finite element validation for this type of connection and the influence of width-thickness ratio (b/t) variation and design recommendations for suitable behavior under dynamic loads when this type of connection was used.

Fire Resistance of Axially Loaded Slender Concrete Filled Steel Tubular Columns – Development of a Three-Dimensional Numerical Model and Comparison with Eurocode 4

In recent years, concrete filled tubular (CFT) columns have become popular among designers and structural engineers, due to a series of highly appreciated advantages: high load-bearing capacity, high seismic resistance, attractive appearance, reduced column footing, fast construction technology and high fire resistance without external protection. In a fire, the degradation of the material properties will cause CFT columns to become highly nonlinear and inelastic, which makes it quite difficult to predict their failure. In fact, it is still not possible for analytical methods to predict with enough accuracy the behaviour of columns of this kind when exposed to fire. Numerical models are therefore widely sought. Many numerical simulations have been carried out worldwide, without obtaining satisfactory results. This work proposes a three-dimensional numerical model for studying the actual fire behaviour of columns of this kind. This model was validated by comparing the simulation results with fire resistance tests carried out by other researchers, as well as with the predictions of the Eurocode 4 simplified calculation model.

Tests on impact behaviour of micro-concrete-filled steel tubes at elevated temperatures up to 400°C

Concrete-filled tubular (CFT) columns are being more and more utilized in construction of tall buildings and bridges. The CFT column system, which has been proved to have excellent load carrying capacity and ductility, by static and simulated seismic loading tests, also has good dynamic impact behaviour. The impact resistance of small-size micro-concrete-filled steel tubes under axial impact load at elevated temperatures up to 400°C was experimentally studied by using a spilt Hopkinson pressure bar. The stress and strain time history curves of the tested specimens were recorded to analyze the impact behaviour of CFT at elevated temperatures. The failure patterns and the influence of temperature on the impact resistance of CFT are discussed. The test results show that CFT has an excellent impact resistant capacity at elevated temperatures and the dynamic behaviour of core concrete under high temperatures was discovered. A simplified calculation method to determine the impact resistant capacity of CFT at elevated temperatures is presented, which is validated by the tested results.

Experimental studies and numerical analysis of the shear behavior of fin plates to tubular columns at ambient and elevated temperatures

This paper reports the results of a recent experimental study into the behavior of welded finplate connections to both hollow and concrete filled tubular (CFT) columns under shear. Experiments have been performed at both ambient and elevated temperatures with the aid of an electric kiln. The observed failure modes include fracture of the fin plate and tearing out of the tube around the welds. By considering the results of previously published research, the current design method for similar connections under purely tensile load, in CIDECT Guide 9, based on a deformation limit of 3% of the tube width is shown to be inadequate when evaluating the ultimate strength of such connections. By comparing the results from the current test program which failed in the fin-plate with Eurocode guidance for failure of a fin-plate alone under shear and bending load it is shown that the column face influences the overall connection strength regardless of failure mode. Concrete in-fill is observed to significantly increase the strength of connections over empty specimens, and circular column specimens were observed to exhibit greater strength than similarly proportioned square columns. A finite element (F.E.) model, developed using ABAQUS, is presented and validated against the experimental results in order that extensive parametric tests may be subsequently performed. When validating the model against elevated temperature tests it was found that using reduction factors suggested in published research for the specific steel grades improved results over applying the generic Eurocode elevated temperature steel strength reduction factors.

Experimental study of structural fire behaviour of steel beam to concrete filled tubular column assemblies with different types of joints

This paper presents experimental results of structural fire behaviour of steel beam to concrete filled tubular (CFT) column assemblies using different types of joints. The joint types include fin plate, end plate, reverse channel and T-stub. The structural assembly was in the form of a “rugby goalpost”. In each test, loads were applied to the beam and then the structural assembly was exposed to the standard fire condition in a furnace while maintaining the applied loads. In total, 10 tests were carried out. In eight of the 10 tests, fire exposure continued until termination of the fire test, which was mainly caused by structural failure in the joints under tension when the beam was clearly in substantial catenary action. In the other two tests (one using fin plates and one using reverse channels), fire exposure stopped and forced cooling started when the beam was near a state of pure bending and just about to enter into catenary action. The results of the experiments indicate that even the relatively simple joints used in this study were able to allow the beams to develop substantial catenary action so that the final failure times and beam temperatures of the assemblies were much higher than those obtained by assuming the beams in pure bending. At termination of the tests, the beams reached very high deflections (about span/5), even then failure of the assemblies always occurred in the joints. Therefore, to enable the beams to reach their full potential in catenary action, the joints should be made to be much stronger. The results also indicate that reverse channel connection has the potential to be developed into a robust connection characterised by high stiffness, strength, rotational capacity and ductility. The beams in the two cooling tests developed high tension forces, however there was no structural failure in the assemblies. The principal aim of this paper is to present experimental results of joint behaviour in fire (which until now is lacking) to enable development of better understanding and rational design methods for robust construction of joints to resist extreme fire attack.

Behavior of Concrete Filled Tubes and Confined Concrete Filled Tubes under High Speed Impact

Concrete filled tubular (CFT) columns are being more and more utilized in construction of tall buildings and bridges. Xiao recently developed a new CFT column system, named as confined concrete filled tubes (CCFT), and validated its excellent load carrying capacity and ductility, by static and simulated seismic loading tests. This paper reports the impact behavior of CFT and CCFT stub columns. The impact responses of concrete filled steel tubes (CFT) and the confined concrete filled steel tubes (CCFT) were investigated using a high-speed gas gun. Experimental parameters included the confinement details and impact speed. Dynamic strains and the failure patterns are discussed. The test results demonstrate the effectiveness of lateral confinement to improve the dynamic impact behavior of CFT columns. Comparisons were also made between the test results and the analytical results using LS-DYNA. The trend of the stress wave propagation in the CFT specimen was reasonably well predicted.

Nonlinear seismic properties of the Second Saikai Bridge: A concrete filled tubular (CFT) arch bridge

The technology of concrete filled tube (CFT) arch bridges has developed rapidly in China since 1990. More than one hundred CFT arch bridges have been built or are now under construction. However, since the weight of a CFT arch rib is greater than that of a steel rib and the arch action is not effective in the out-of-plane direction, the seismic safety of CFT bridges should be evaluated. This paper examines nonlinear seismic responses of the Second Saikai Bridge, which is the first highway bridge in Japan to be a CFT arch bridge. This bridge has a main span length of 240 m. The natural vibration characteristics of this bridge are discussed using a three-dimensional FE model. A nonlinear seismic analysis is performed using the strong ground motions observed in the Hyogo-ken Nanbu Earthquake of Japan. The nonlinear seismic characteristics and seismic safety of this bridge are examined in detail by subjecting the bridge to earthquakes in the out-of-plane direction, longitudinal direction and combined out-of-plane and longitudinal directions. It is found that the analysis should consider the combined out-of-plane and longitudinal excitations.

Vibration analysis of the Second Saikai Bridge—a concrete filled tubular (CFT) arch bridge

Concrete-filled steel tubular (CFT) arch bridges have been rapidly developing in China since 1990. Research has focused on the static behavior, thermal stress and erection technique, however, and there has been very little research of natural vibrations and dynamic responses of these bridges. Japan's first CFT arch bridge in highway bridges, the Second Saikai Bridge, is now under construction in Nagasaki Prefecture. Furthermore, this bridge has a pedestrian bridge that is suspended under the girder, which is rare. Therefore, the natural vibration properties are examined, and the influence of pedestrian bridge structure on the natural vibration of main bridge is discussed first. Response analysis under a moving vehicle and pedestrian is carried out, and the response characteristics and response level are clarified. The natural vibrations and responses are compared to those of CFT arch bridges that have been constructed in China. Results show the fine performance of both main bridge and pedestrian bridge of the Second Saikai Bridge.

Performance of steel–concrete composite structures in fire

AbstractThis paper presents a review of recent studies of the behaviour of steel–concrete composite structures in fire, including an assessment of research investigations and their implications on fire‐resistant design of composite structures. The paper focuses on the three main parts of composite structure: floor systems, columns and joints. The section on composite floor systems includes bending behaviour of conventional composite floor/beam system with composite slabs on top of a solid steel I‐beam; tensile membrane action in this system; slim‐floor construction; composite cellular beam; composite truss and composite beam with precast concrete slabs. The section on composite columns includes studies of concrete‐filled tubular (CFT) column; design of composite columns and performance of CFT columns within structures. The section on joints covers both steel and composite joints, including experiments on joints in fire; finite element simulation; the component method; behaviour of different joint components at elevated temperatures and implications of joint behaviour on steel and composite structural behaviour in fire. Within each section, wherever appropriate, the effects of whole structural behaviour are discussed. Furthermore, potential future research studies are identified to help understand the subject and develop improved performance‐based practical fire resistant design calculation methods.

T字形ダイアフラムで内部補強された角形CFT柱梁接合部の局部引張抵抗

Concrete filled tubular (CFT) frames have been widely applied in high-rise buildings since the early 1990s. However, there is a need to seek more cost-effective connection details, and this is especially true for square tubes. To meet this need, a relatively simple and cost-saving solution, in which a connection is stiffened with two interior T-shaped diaphragms, was experimentally investigated. This connection detail has been long used for steel frames; however, its application to the CFT frames has been limited because of the lack of information on the behavior and design guidelines. Based on the test results and observations obtained through this research, strength models and design formulas were derived using a limit analysis technique. Comparison with the test results showed that the proposed formulas provided accurate strength estimates.

Concrete-filled steel circular tubes subjected to constant amplitude cyclic pure bending

This paper describes an experimental investigation of the cyclic inelastic flexural behaviour of concrete-filled tubular (CFT) beams made of cold-formed circular hollow sections and filled with normal concrete. Cyclic bending tests were performed using a constant amplitude loading history on different CFT specimens with diameter-to-thickness ratios ( D/ t) ranging from 20 to 162. The CFT beams exhibited stable hysteresis behaviour up to the formation of plastic ripples and then showed considerable degradation in stiffness, strength, and ductility depending on the D/ t ratio. Seismic capacity parameters are presented including strength, stiffness, rotation capacity, hysteresis loops and modes of failure of the specimens. Peak moments obtained in the cyclic tests are compared with those obtained previously in monotonic tests and also with design moments predicted using the available design rules for composite beams. The deformation ductility demand was determined and used to derive new fully ductile section slenderness limits suitable for seismic design.