Steel-timber Composite Connections Research Articles

Numerical investigation of steel-timber composite floors with flush end-plate connection

Steel-timber composite floors have received much attention in the past years due to their lightweight, high construction and dismantling speed, low pollution and energy consumption. These floors consist of timber slab that is connected to steel beam by shear connectors. This paper investigates the behavior of steel-timber composite floors, which have flush end-plate beam-to-column connections. To achieve this, three-dimensional finite element models were prepared from several laboratory specimens and then validated against each other. The numerical and laboratory results were compared and validated, which shows the appropriate accuracy of the assumptions used for modeling specimens. In the following, parametric studies were conducted to investigate the factors affecting stiffness, strength, and rotational capacity such as type, diameter, and spacing of shear connectors, thickness and width of timber slab, the thickness of flush end-plate, and beam-to-column bolt size. Lastly, several mathematical formulas were presented to estimate the moment-rotation response of semi-rigid steel-timber composite connections.

Experimental study on shear performance of steel-timber screw connectors with grout pockets

The steel-timber composite floor is an environmentally friendly structural form. The shear connectors between the timber and H-steel are the key to designing the steel-timber composite floors, and the strength and ductility of the connection are directly affected by the failure modes of the connectors. A new connection that anchors the screw tips in the grout pockets is presented in this paper. In this connection, the glulam around the screw tip is removed, and the grout is added after the screws have been tightened in the glulam to form a hybrid anchorage of the grout and glulam to the screw. Nine groups of double-shear tests were conducted to investigate the influences of with or without grout pockets, the size of grout pockets, and the row number of connectors on the performance of steel-timber composite connections. Firstly, compared with the common specimens connected by screws, the screw tips were restrained more firmly in specimens with grout pockets. The peak load Fmax, initial stiffness ks,0.4 and ductility D of the specimens with grout pockets were increased by a maximum of 25.2, 55.9 and 33.6%, respectively. Secondly, with the increase in the radius and depth of the grout pockets, the secondary stiffness ks,0.7 and the peak shear capacity Fmax of the specimens increased accordingly. Thirdly, after the screw tips were anchored in the grout pockets, the screws stably exhibited the failure mode of double plastic hinges (model III), achieving active control of the failure modes of the screws. There is a critical ratio of grout anchor length to the total length of the screw (L0/L) such that the tip plastic hinge is close to the grout pocket. In the end, the shear capacity equation for failure mode of double plastic hinges (model III) in Eurocode 5 can be used to calculate the peak capacity of this new connection, and the calculation results have a large safety margin.

Behaviour of embedded bolted shear connectors in steel-timber composite beams subjected to cyclic loading

This paper concerns the cyclic behaviour of embedded bolted shear connectors in steel-timber composite joints. The effect of the strength and size of the shear connector and the orientation of the cross-laminated timber panels with respect to the direction of the load on the cyclic behaviour of this type of composite connection were investigated through an extensive experimental study. In total, eleven steel-timber composite connections having a steel section and mechanical shear connectors embedded in grout were tested under a low-cycle, high-amplitude loading regime. The equivalent viscous damping, ductility, strength impairment, energy dissipation and the failure modes of all composite connections were assessed in detail to characterise the behaviour of a mechanical shear connector under cyclic loading conditions. The experimental results show that embedded bolted shear connectors have high energy-dissipating capacity and ductility if adequate edge distances of the bolts and shear pockets are provided. In addition, a finite element model of the composite connection considering the non-linearities of the geometry, and interfaces between the components and materials is developed and verified against the results of the experimental study. Based on the results obtained from the simulation, it can be demonstrated that the developed FE model is able to predict the behaviour of the composite connection at the local and global level.

Finite Element Modeling on Shear Performance of Grouted Stud Connectors for Steel–Timber Composite Beams

Steel–timber composite (STC) systems are considered as an environmentally friendly alternative to steel–concrete composite (SCC) structures due to its advantages including high strength-to-weight ratio, lower carbon footprint, and fully dry construction. Bolts and screws are the most commonly used connectors in STC system; however, they probably make great demands on the accuracy of construction because of the predrilling in both the timber slabs and steel girder fangles. To address this issue, the STC connections with grouted stud connectors (GSC) were proposed in this paper. In addition, stud connectors can also provide outstanding stiffness and load-bearing capacity. The mechanical characteristic of the GSC connections was exploratorily investigated by finite element (FE) modeling. The designed parameters for the FE models include stud diameter, stud strength, angle of outer layer of cross-laminated timber (CLT) panel, tapered groove configurations, and thickness of CLT panel. The numerical results indicated that the shear capacity and stiffness of the GSC connections were mainly influenced by stud diameter, stud strength, angle of outer layer of CLT panel, and the angle of the tapered grooves. Moreover, the FE simulated shear capacity of the GSC connections were compared with the results predicted by the available calculation formulas in design codes and literatures. Finally, the group effect of the GSC connections with multiple rows of studs was discussed based on the numerical results and parametric analyses. An effective row number of studs was proposed to characterize the group effect of the GSC connections.

Moment-rotation model for steel-timber composite connections with slab continuity steel rods

Detailed 3D nonlinear finite element (FE) analyses of steel-timber composite (STC) cruciform subassemblies including material, geometrical and contact nonlinearities are carried out. In the STC subassemblies, the steel beams are connected to the column flange by fin plates and/or double web-cleats and the continuity of the cross laminated timber (CLT) slabs across the column is preserved by mechanically anchored rods. The FE models are verified against experimental data and it is shown the adopted constitutive law of materials (i.e. steel and timber) and proposed modelling strategies accurately predict the failure mode, local (load-strain) and global (bending moment-rotation) response of the STC connections. Variables influencing the bending moment-rotation response of the STC connections are identified and considered in a parametric study and a mathematical model for the moment-rotation of the STC joints is developed by non-linear regression of the data obtained from the parametric study. The calibrated mathematical model is employed for blind prediction of the moment-rotation of the STC connections and the accuracy of the model compared to the detailed 3D nonlinear FE models is demonstrated by a few examples. Lastly, nonlinear FE analysis of a multi-storey STC frame with nominally pinned connections is carried out and it is shown that the continuity steel rods (anchored in the CLT slabs) reduce the mid-span deflection of the STC beams by approximately one-third.

Mechanical behaviour of steel timber composite shear connections

This paper presents an experimental investigation on a Steel Timber Composite (STC) connection system composed of steel H-section and engineered timber, glulam produced from Larix gmelinii. Static push-out tests were conducted on this new composite connection type to investigate its yield characteristics as well as relevant failure modes. Effects of the type, diameter and spacing of shear connectors, and thickness of glulam on the mechanical response of this STC connection were carefully analysed. For all considered specimens, “two-hinge” yield mode was observed but the failure of joints were characterized by the simultaneous occurrence of embedding strength failure of glulam flanges and bending failure of bolts used as shear connectors. The ultimate load carrying capacities as well as yielding of joints were directly proportional to the bolt diameter but inversely proportional to bolt spacing. As the thickness of glulam was increased, both the ultimate load and the yield load of joints initially increased but then decreased. Self-drilling screw connected joints performed better than the bolt connected joints both in terms of stiffness and ductility. Experimentally obtained key design parameters were compared against those predicted using GB/T 50005–2017, NDS-2018 and Eurocode 5. It is worth noting that the code predictions are not suitable for design application due to significant conservatism or over-prediction of connection response; this shows a clear gap in the existing knowledge on STC connections. Foschi formula was used to predict the nonlinear load-slip behaviour of considered specimens and analytical predictions showed reasonable agreement with test results.

Finite element modelling of steel-timber composite beam-to-column joints with nominally pinned connections

Non-linear 3D continuum-based finite element (FE) simulations are employed to investigate the structural behaviour of steel-timber composite (STC) beam-to-column connections with shear-tabs and/or double web-angles. In the FE models developed, the material non-linearities of the steel and timber are considered using constitutive laws cast in the framework of classical plasticity and continuum-damage mechanics, respectively. Furthermore, the non-linearities associated with geometry (small strain-large displacement) and non-linear contact (interactions between different components) are considered in the models. A sensitivity analysis is performed to assess the spurious mesh sensitivity associated with softening of the timber, and the 3D FE models of the STC subassemblies are validated against experimental data gathered from previous studies. It is shown that the FE models proposed can predict the structural response and the complex failure modes of STC connections with reasonable accuracy. Finally, a parametric study is carried out and the influence of variables such as the depth of shear tab/web angles or the number and size of the bolts, thickness of the timber slab the and degree of composite efficiency on the failure mode, the peak load carrying capacity and moment-rotation response of the nominally pinned STC beam-to-column connections are evaluated and discussed.

Cyclic behaviour of bolt and screw shear connectors in steel-timber composite (STC) beams

Feasibility and acceptable performance of the steel-timber composite (STC) system as a sustainable alternative to conventional steel-concrete composite have been demonstrated through recent static push-out and bending tests. But, structural behaviour and energy dissipation capacity of the STC connections subjected to cyclic loads have not been investigated yet. This study investigates the cyclic behaviour of STC connections with mechanical shear connectors. In total, twelve STC joints were fabricated by connecting the cross-laminated timber (CLT) panels to the flanges of a steel profile and the joints were subjected to low-cycle high-amplitude loading regime. Effects of the shear connector type (i.e. screw, high strength bolt), shear connector size and the orientation of CLT panels (outer lamellas parallel and/or perpendicular) with respect to the direction of the load were considered in the experimental program. The ductility, strength impairment and equivalent viscous damping which characterise the performance of a mechanical shear connector under cyclic loading conditions in steel-timber composite connections were assessed. The results of the cyclic tests demonstrated the high ductility and energy dissipating capacity of the steel-timber composite connections. A simple hysteretic model was proposed for steel-to-CLT composite connections with bolt and screw shear connectors and the model was calibrated against the results of laboratory experiments.

Structural behaviour of steel-timber composite (STC) beam-to-column connections with double angle web cleats subjected to hogging bending moment

This paper deals with an experimental study of steel-timber composite (STC) beam-to-column connections with double web angles, with the aim of developing an efficient composite flooring system that exhibits superior structural performance yet is simple and cost-effective from a construction perspective. The experimental program involved the testing of six cruciform subassemblies representing internal beam-to-column connections subjected to negative (hogging) bending moments. In four specimens, STC beams were connected to the flanges of a universal column by double web angles. Additionally, a specimen with bare steel beams and another specimen with steel-concrete composite (SCC) beams were fabricated and tested to provide benchmarks for comparing the structural behaviour of the STC with bare steel and SCC beam-to-column connections. It is shown that the STC connections exhibit semi-rigid behaviour with negative bending moment and rotation capacities larger than that of SCC connections. Lastly, an analytical model which is an extension of the existing component-based method is proposed for estimating the negative bending moment capacity of the STC connections with double web angles.

Semi-rigid partial-strength steel-timber composite (STC) connections with mechanically anchored steel rods

In steel composite constructions, replacing the concrete slabs with prefabricated timber slabs significantly reduces the embodied energy and carbon footprint and lowers the self-weight of the structure and the size of the columns and footings. Considering the advantages of the light-weight steel-timber composite (STC) floors, this paper presents the results of laboratory experiments conducted on six (STC) beam-to-column subassemblies with double angle web cleats and fin plate (nominally-pinned) connections. Preserving continuity of the slabs across the column increases hogging bending moment resistance and stiffness of the composite beam to column connections. Here, the connection between the timber slabs across the column was provided by threaded rods placed inside the timber slabs and mechanically anchored in the pockets of cementitious grout. The STC subassemblies were subjected to a monotonically increasing push-down load to simulate behaviour of the nominally-pinned beam-to-column connections. The results of laboratory tests revealed that mechanically anchored rods can be used to effectively transfer the tensile force developed in the timber slabs of the STC floors and accordingly enhance the stiffness and hogging bending moment capacity of the simple STC beam-to-column connections. The effect of the number, size, and strength grade of the steel rods on stiffness, strength and ductility of the STC connections were evaluated experimentally and component-based method was employed to develop a simple mechanical model for predicting the bending moment capacity of the STC connections with mechanically anchored rods.

Steel-timber composite beam-to-column joints: Effect of connections between timber slabs

Studies of steel-timber composite (STC) connections and STC beams under sagging bending have been reported elsewhere in the literature using push-out and four-point bending tests respectively. However, the structural behaviour of STC beam-to-column connections under hogging bending moments (with the prefabricated timber slabs in tension) have hitherto not been investigated. In particular, the connection between the two prefabricated timber slab panels (across the column) has a major influence on the structural performance of a STC beam-to-column connection and is the focus of the current study. Eight full-scale STC beam to steel column cruciform specimens with different connections (half lap, single and double surface spline with timber and/or steel plate) for the timber slabs were fabricated and tested under a monotonically increasing downwards displacement, and these are described in this paper. The bending moment capacity, rotation capacity, failure mode, stiffness and ductility of the STC connections are evaluated and discussed. The composite steel-timber system exhibits both appreciable ductility and rotation capacity which fulfil the existing design requirements for semi-rigid composite connections in Eurocodes EC3 and EC4. Furthermore, the negative bending moment capacity of STC connections is significantly higher than that of bare steel connections without a timber slab.

Modelling of steel-timber composite connections: Validation of finite element model and parametric study

This paper investigates the mechanical behaviour of lap steel-timber composite connections using a 3-D continuum-based finite element (FE) model. In the FE models developed, the non-linear behaviour and failure of the timber is captured by a stress-based failure criterion formulated in the framework of continuum-damage mechanics. Yielding of the steel plates and fasteners (i.e. coach screws and bolts) is captured by an elastic-hardening plastic constitutive law. These FE models are validated by experimental (push-out) tests conducted on laminated veneer lumber (LVL)-steel and cross-laminated timber (CLT)-steel composite lap connections. It is shown that the FE models developed in the paper can replicate adequately the load-slip response and failure mode of the hybrid steel-timber composite connections tested. The validated FE model is used to undertake a parametric study that elucidates the influence of the yield strength and the length of the fasteners and the post-tensioning force in the bolts on the stiffness, load carrying capacity and load-slip behaviour of hybrid steel-timber composite connections.

Experimental and analytical behaviour of steel-timber composite connections

The mechanical characteristics of steel-timber composite (STC) connections play an essential role in the safe and economical design of hybrid STC structures and floor systems. Accordingly, this study investigates the load-slip behaviour of lap Laminated Veneer Lumber (LVL) timber-steel plate composite joints. Push-out tests on four different types of STC lap joints connected by coach screws (with and without a reinforcing nail plate), high-strength bolts and a combination of glued and screwed joints are reported, and the load-slip behaviour and failure modes of the connections are characterised. The use of a nail plate is found to be effective in reinforcing the timber and in increasing the stiffness of STC lap joints with coach screws, but they did not produce a significant improvement of their strength. A non-linear regression is carried out and an empirical load-slip formulation for STC lap joints with coach screws and high strength bolted connectors is proposed in analytical form. Furthermore, simple formulae for the strength and stiffness of LVL-steel lap connections with coach screws are proposed.

Load-slip behaviour of steel-cross laminated timber (CLT) composite connections

Connecting cross laminated timber (CLT) panels to steel girders using mechanical connectors (e.g. screws and bolts) is an attractive and novel method for developing a fully prefabricated and sustainable hybrid steel-timber composite (STC) floor that can also facilitate future dismantling and recycling of its structural components. Since the structural performance of composite floors (including STC systems) is significantly influenced by strength and stiffness of slab-to-girder composite joints, this study characterises the load-slip behaviour and failure modes of steel-CLT timber composite joints by conducting push-out tests on three different types of STC connections with high-strength bolts, coach screws or a combination of glue and coach screws. Furthermore, the effect of reinforcing the CLT slabs by using steel nail plates on the strength and stiffness of STC joints is evaluated. Empirical models for capturing load-slip behaviour of steel-CLT composite joints with dowel (i.e. screw and bolt) connectors are derived from non-linear regression of experimental data and simple formulae for the strength and stiffness of steel-CLT composite joints with dowel connectors are proposed. Moreover, 1D beam on inelastic foundation finite element (FE) models of the dowel STC connectors are developed and validated against experimental results.