Static tests on steel-concrete-steel sandwich beams

Bi-Steel is an innovative form of steel-concrete-steel sandwich construction invented by Bowerman [Bowerman H, Coyle N, Chapman JC. An innovative steel/concrete construction system. The Structural Engineer 2002;80(20):33-8], in which the two steel plates are inter-connected by a series of transverse bar connectors simultaneously friction welded at both ends. In use, the steel units are assembled, welded, and filled with concrete. The behaviour of the friction-welded bar-plate connections embedded in concrete subjected to bar tension and bar shear have been described in papers [Xie M, Chapman JC. Static and fatigue tensile strength of friction-welded bar-plate connections embedded in concrete. Journal of Constructional Steel Research 2005;61:651-73; Xie M, Foundoukos N, Chapman JC. Experimental and numerical investigation on the shear behaviour of friction-welded bar-plate connections embedded in concrete. Journal of Constructional Steel Research 2005;61:625-49]. This paper presents an experimental investigation on the static behaviour of Bi-Steel beams. Eighteen beams having a range of span, depth, plate thickness and bar spacing, have been tested under static loading, and four elementary modes of failure were observed: tension plate yielding, bar shear, bar tension and concrete shear. The tests confirm that for ductile failure, beams should be designed to fail by yielding of the tension plate. Comparisons are made with current design recommendations and some alternative methods are proposed.     

Static and fatigue tensile strength of friction-welded bar-plate connections embedded in concrete

Friction-welded bar-plate connections are a basic structural component of Bi-Steel steel-concrete-steel sandwich construction. In Bi-Steel members, the bar-plate connections, embedded in concrete, are subject to tension, shear and bending. This paper describes experimental and numerical studies on the static and fatigue strength of the friction-welded connections with the bar loaded in tension. Finite element analysis is carried out to examine the effects of plate thickness, the collar (“flash”) formed after friction welding, and possible initial defects or fatigue induced cracks. It is found that except for 6 mm plate specimens, the static tensile capacity of the embedded connections is governed by the tensile strength of the bar connectors. In the fatigue tests, single fracture and double fracture mechanisms were observed. Experimental stress vs. life (S-N) curves are derived from the fatigue test results, and they lie between the F-type and the S-type curves specified in BS5400 Part 10. Further papers will describe static and fatigue tests incorporating other force components, and on Bi-Steel beams.     

Fatigue tests on steel-concrete-steel sandwich components and beams

In Bi-Steel steel-concrete-steel sandwich members, the friction welded bar-plate connections are subject to plate tension, bar tension and bar shear with associated bar bending. The relative magnitude of these force components depends on the beam geometry and the type of loading. The force components interact, so component fatigue tests were performed under single and combined loads, each with a sufficient number of stress ranges to enable stress vs. fatigue life (S/N) curves and equations to be defined. Fatigue tests were also conducted on eighteen beams, and the experimental fatigue lives are compared with the values estimated by means of a proposed interaction equation based on the experimental S/N equations derived from the component tests.     

Fatigue shear performance of concrete beams reinforced with hybrid (glass-fiber-reinforced polymer+ steel) rebars and stirrups

Reinforced concrete beams consisting of both steel and glass-fiber-reinforced polymer rebars exhibit excellent strength, serviceability, and durability. However, the fatigue shear performance of such beams is unclear. Therefore, beams with hybrid longitudinal bars and hybrid stirrups were designed, and fatigue shear tests were performed. For specimens that failed by fatigue shear, all the glass-fiber-reinforced polymer stirrups and some steel stirrups fractured at the critical diagonal crack. For the specimen that failed by the static test after 8 million fatigue cycles, the static capacity after fatigue did not significantly decrease compared with the calculated value. The initial fatigue level has a greater influence on the crack development and fatigue life than the fatigue level in the later phase. The fatigue strength of the glass-fiber-reinforced polymer stirrups in the specimens was considerably lower than that of the axial tension tests on the glass-fiber-reinforced polymer bar in air and beam-hinge tests on the glass-fiber-reinforced polymer bar, and the failure modes were different. Glass-fiber-reinforced polymer stirrups were subjected to fatigue tension and shear, and failed owing to shear.     

Developments in sandwich construction

Bi-Steel is a form of steel/concrete/steel sandwich construction in which the plates are connected by bars, which are friction welded at both ends simultaneously. Manufactured units are taken to site, joined by welding, and filled with concrete. The bar/plate connections are subject to bar tension, bar shear, bar bending and plate tension. The concrete acts with the compression plate and resists shear. The paper outlines research undertaken to establish design methods of Bi-Steel beams under static and fatigue loading.     

Resistance of headed studs subjected to fatigue loading: Part I: Experimental study

In steel-concrete composite structures the transfer of longitudinal shear forces at the interface between steel and concrete is mostly realized by headed shear studs. Especially in bridges due to traffic loads these shear connectors are subjected to high-cycle loading and the fatigue resistance governs the design. In this first part of two companion papers a series of experimental work with standard EC4 push-out specimens is presented. The main purpose of these tests was to determine the fatigue life and a possible reduction of the static strength of the headed shear studs subjected to unidirectional cyclic loading. A further aspect was to examine the effects of the loading sequence and damage accumulation on the fatigue life. The results of the experimental investigations show that due to a crack initiation at the stud foot at 10%-15% of the fatigue life, an early reduction of the static strength is caused. Furthermore tests to examine the effects of the loading sequence on the fatigue life revealed that the linear damage accumulation hypothesis according to Palmgren and Miner on which the present design codes are based do not describe the real behaviour.     

Resistance of headed studs subjected to fatigue loading Part II: Analytical study

Most design codes consider the static and fatigue resistance of composite steel-concrete structures with separate verifications for ultimate limit state and the limit state of fatigue. For headed shear studs both verifications are based on worldwide performed experimental investigations with push-out specimens. The determination of the fatigue life of headed shear studs in recent European codes is based on the S-N curve developed from the statistical analysis of a great number of cyclic push-out tests. The S-N curve considers the shear stress range as the only loading parameter. In addition, the maximum shear force is limited at serviceability limit states in order to avoid significant inelastic behaviour. However, recent researches showed that beside the shear stress range, also the peak load and the static strength of the shear studs influence the fatigue life. Based on the results of the 71 push-out tests presented in the earlier companion paper, international push-out tests to determine the fatigue life of headed shear studs in solid concrete slabs are sorted and reanalyzed and analytical expressions are developed to predict the fatigue life and the reduced static strength after high cycle preloading. Furthermore, the linear damage accumulation hypothesis according to Palmgren and Miner is modified and improved to consider load sequence effects and non-linear behaviour.     

Behavior of channel shear connectors, Part I: Experimental study

In composite beams, shear connectors are commonly used to transfer longitudinal shear forces across the steel-concrete interface. This paper presents an experimental study on the behavior of channel shear connectors embedded in a solid concrete material slab under monotonic and low-cycle fatigue loading. The latter would be applicable to composite structures subjected to seismic events. Of specific interest are the behavior and effects of different concrete materials. A series of push-out specimens made of plain concrete, reinforced concrete (RC), fiber reinforced concrete (FRC) and engineered cementitious composite (ECC) were tested for this evaluation. The results show that the reversed cyclic shear strength of most specimens is 10%-23% lower than their monotonic strength. Also, using the polypropylene fibers (FRC specimens) has a slight effect on the shear strength and load-displacement behavior of the specimens; however, using the polyvinyl alcohol fibers (ECC specimens) causes considerable increase in ultimate strength and ductility of channel shear connectors. Finally, the experimental load capacities are compared with that suggested by North American design codes.     

Study on behaviour of angle shear connector in steel-concrete composite structures

In this study an experimental study of the behaviour of angle shear connector is presented. Six push-out tests on angle shear connector with two varied length angles were investigated. The shear resistance and shear stiffness with angles embedded in a normal concrete strength were predicted. The experimental results were also compared with standard design code formula and recommended values of design codes. The available experimental procedure for the determination of the shear force-slip behaviour of shear connector under static loading was adopted.     

钢筋混凝土偏心受压和偏心受拉构件的抗剪强度

本文讨论了钢筋混凝土构件在轴向力、弯矩和剪力共同作用下的抗剪强度。对171根构件试验结果的分析表明,用我国《钢筋混凝土结构设计规范》(TJ 10-74)中受弯构件的抗剪强度计算公式来计算偏压和偏拉构件的抗剪强度是不合适的,对于偏压构件偏于保守,而对于偏拉构件则偏于不安全。在分析试验资料的基础上,本文提出了一个考虑轴向压力和拉力影响的抗剪强度实用计算方法。计算结果与试验结果符合较好。     

Analysis of composite beams in the hogging moment regions using a mixed finite element formulation

Cracking of the concrete slab in the hogging moment region decreases the global stiffness of composite steel-concrete structures and also reduces the effect of continuity, thus making the structural behaviour highly nonlinear even for low stress levels. In this paper, the behaviour of continuous composite beams with discrete shear connection is investigated using a nonlinear mixed finite element model. The model includes appropriate nonlinear constitutive relationships for the concrete, the steel and tension stiffening effect. Furthermore, the discrete nature of the shear connection is embedded in the model and the tension stiffening effects are introduced in the analysis by using a concrete constitutive model proposed in the CEB-FIB Model Code 1990 which incorporates embedded steel. Special attention is paid to the hogging moment regions, where cracking occurs. Comparisons between the numerical analyses and experimental results in the current literature are undertaken to validate the accuracy of the model. Furthermore, a parametric study is carried out to study the influence of span length and degree of shear connection on the strength and ductility of continuous composite beams.     

Trag‐ und Ermüdungsverhalten liegender Kopfbolzendübel unter Quer‐ und Längsschub

Structural and fatigue behaviour of horizontally lying shear studs subjected to vertical and longitudinal shear. In consideration of structural, manufacturing and economic aspects new interesting composite cross sections for buildings and bridges consisting of reinforced concrete and structural steel lead to a horizontally lying arrangement of headed shear studs influenced by edge proximity. Compared to common vertical shear studs the relatively small distance of horizontally lying shear studs from the edge of the slab leads to different bedding conditions of the connectors within the surrounding reinforced concrete material, which results in differing structural and fatigue behaviour of this type of shear connection. For future applications of horizontally lying shear studs on the basis of experimental and theoretical investigations design rules for the shear connection subjected to monotonic vertical shear, monotonic vertical and longitudinal shear and cyclic longitudinal shear are presented.     

槽钢剪力连接件的性能研究第一部分:试验研究

在组合梁的设计中,剪力连接件一般用于将纵向剪力转移到钢-混凝土的连接界面。对单调低周疲劳荷载作用下混凝土板中的槽钢剪力连接件的性能进行了试验研究。这种混凝土板将应用于地震作用下的组合结构,其中不同混凝土材料的性能和作用成为了研究重点。对一系列由素混凝土、钢筋混凝土(RC)、纤维钢筋混凝土(FRC)和水泥基复合材料(ECC)组成的构件进行推出测试。结果显示:大部分构件的循环抗剪强度要比它们在单调加载条件下的抗剪强度低10%~23%。同时,聚丙烯纤维(FRC构件)的采用对构件的抗剪强度和荷载-位移曲线也产生影响。然而,聚乙烯醇(ECC构件)的采用在剪力连接件的极限强度和延性上引起了显著的增加。最后,对比了试验构件的承载能力与北美设计规范建议的数值。     

Structural performance of composite joints using bent studs

A composite truss bridge is an efficient structural type where the merits of both the steel member and the prestressed concrete member are combined. The design of a connection for the truss bridge is of particular interest due to complex details resulting from concentrated forces at the composite joint. In this paper, experimental investigations were conducted to evaluate the performance of the joint with bent studs welded on a gusset plate. Push-out tests for static and fatigue loadings on a group stud connection were conducted to evaluate the direct shear strength and fatigue endurance of the joint. Because the joints are subject to various loading conditions, flexure-shear tests were also performed to verify the design provisions for the joints under a combination of tension and shear. In order to enhance the structural behavior of the connection, a bent shaped stud was proposed to increase the pull-out strength of the concrete slab. Test results showed that it is adequate to use Eurocode-4 for the evaluation of a shear connection which satisfies the requirement of minimum stud spacing specified in Eurocode-4. Since the shear load is concentrated at the joint structure of composite truss bridges, additional confining reinforcements are needed to enhance the horizontal shear strength of the shear connection. The design provision on the interaction of tension and shear for the joint structure gave conservative results. When the pyramid failure surface was assumed, internal studs had little effect on the pull-out strength of the joint. Constructability of the joint can be enhanced by decreasing the number of connectors and by strengthening the concrete slab through simplified design checks assuming four failure modes and their interactions.     

Test of shear transfer enhancement in symmetric cold-formed steel-concrete composite beams

An experimental programme was conducted to study the strength and behaviour of a bent-up tab shear transfer enhancement. Sixty eight push-out test specimens of cold-formed steel lipped channel sections (CFS) embedded in concrete planks which make use of the new proposed shear transfer enhancement called bent-up triangular tab shear transfer (BTTST) were tested in this programme. This paper summarises the results of the experimental study and develops an expression in order to predict the innovative shear transfer enhancement mechanism, BTTST in a new type of precast composite beam comprising CFS embedded in a precast concrete plank. The results show that specimens employed with shear transfer enhancements increase the shear capacities of the specimens as compared to those relying only on a natural bond between cold-formed steel and concrete. In this investigation, BTTST provided better performance in terms of strength. Furthermore the effects of different angles and sizes for BTTST, different thickness for CFS and different strengths for concrete are also discussed. It is concluded that the proposed shear transfer enhancement has sufficient strength and is also feasible.     

Neue Untersuchungen zum Ermüdungsverhalten von Kopfbolzendübeln

New investigations on the fatigue behaviour of headed shear studs. In current national and international standards the determination of the ultimate load capacity and the fatigue life of headed shear studs subjected to high‐cycle loading take place with separate and independent verifications at the ultimate limit state, serviceability limit state and fatigue limit state, respectively. The effect of predamage due to fatigue loading is not considered. The paper deals with new research based on a comprehensive program of experimental investigations which considers the crack propagation through the stud foot and the local damage of concrete surrounding the stud as relevant consequences of high cycle loading. New design methods to predict the fatigue life and the residual strength after high cycle loading as well as the development of an improved damage accumulation hypothesis are the main subjects of this paper.     

Theoretical and experimental analysis of timber-to-timber joints connected with inclined screws

The aim of this paper is to propose a method for calculating the load-bearing capacity and stiffness of screws placed in an inclined position with respect to the shear plane and subjected to shear–compression load, to shear–tension load or to a combination due to their crossed X-position. At the same time, the authors also wish to highlight how the current method indicated by Eurocode 5 is partially unsuitable for describing the experimental results of strength and stiffness for these particular configurations. To this regard, push-out tests were carried out and the analysis of the resulting load–displacement curves has led to the definition of the main parameters describing the mechanical behaviour of the connections: strength, stiffness, effective number, ductility. The experimental values of strength and stiffness were then compared with the theoretical ones obtained using the Eurocode 5 calculation method and the calculation method proposed in this paper.     

Tension stiffening model for nonlinear analysis of GFRP-RC members

An analytical approach is presented to capture the tension stiffening effect of reinforced concrete member subjected to tensileforces. In this model, the tensile stiffening is described through an analytical relationship of stress–strain law in the cracking range which differs from the CEB-tension stiffening model that is based on the stress–strain curve of reinforcing bars using a bi-linear relationship. The bi-linear relation does not quite capture the post-cracking behaviour of reinforced concrete structures. In view of this, a parabolic branch is selected in the post-cracking phase possessing as asymptotic line to the stress–strain line of the bare bar that minimises the tension stiffening effect at the ultimate load level. This assumption is taken into account in view of material nonlinearities, the bond character and the tension stiffening effect. Analytical results are obtained and compared with experimental data for direct tensile load. In general, the computed results agree well with experimental results. Moreover, the effects of concrete strength, reinforcement ratio and bar diameter on tension stiffening are studied.     

Modelling beam-to-girder semi-rigid composite connection with angles including the effects of concrete tension stiffness

The present paper aims to propose a theoretical model for beam-to-girder steel–concrete composite connection. Typical web and seat angles connection was investigated in order to define its moment–rotation behaviour, including the initial stiffness, bending strength and rotational capacity. The proposed theoretical model describes the semi-rigid response of the connection and incorporates the tension stiffening effects of the concrete slab by means of a four parameter power function in which the concrete stiffness and cracking evolution are taken into account.The comparison between the theoretical model and the experimental results of four cruciform-type tests, including two types of connections, with and without web angles, resulted in accurate validation of the proposed moment vs. rotation model and allowed concluding on the importance of the presence of the web angles, which strongly contribute to the control of the concrete cracking process. In addition, the experimental results confirmed fundamental assumptions of the composite semi-rigid connections, as the effect of the concrete before and after cracking stabilization, the actual contribution of the bolts of the web connection and the shear lag effect in the slab.     

Numerical modelling of anchor bolts under pullout and relaxation tests

The anchoring of structure elements in a reinforced concrete foundation is often performed by means of cast-in-place anchors. Their main function is to transfer normal loads, tension and compression, and possible shear loads according to the force transferred to the base of the structure. This study is limited to the study of long and smooth anchor rods with an anchor washer on the end, embedded in concrete. The anchor rods are prestressed in order to minimize the effects of fatigue. In normal practice to design these anchors the long-term behaviour of concrete is not taken into account although concrete creep involves a decrease in the initial prestress level. This paper presents the experimental results of pullout and relaxation tests and a finite element modelling of the joint. The time-dependent behaviour of concrete is taken into account in a visco-elastic analysis using the creep laws given in Eurocode 2. The numerical results are coherent with the measurements obtained in experimental tests. The decrease in the prestress is correctly simulated and its evolution in time can be estimated.