Hybrid Steel Trussed Concrete Beams Research Articles

Experimental characterization of friction properties of materials for innovative beam-to-column dissipative connection for low-damage RC structures

Low-damage design of structures in seismic-prone areas is becoming an efficient strategy to obtain “earthquake-proof” buildings, i.e. buildings that, even in the case of severe seismic actions, experience a low or negligible amount of damage. Besides the safeguard of human lives, this design strategy aims also to limit the downtime of buildings, which represents a significant source of economic loss, and to ensure an immediate occupancy in the aftermath of an earthquake. In this context, focusing on moment-resisting frames (MRFs), several solutions have been developed for the beam-to-column connections (BCCs) of steel and precast/prestressed concrete structures, but very few for cast-in-situ reinforced concrete (RC) structures. This paper focuses on a recently-proposed friction-based BCC for MRFs made with hybrid steel-trussed concrete beams (HSTCBs). The latter are made by a spatial lattice built using V-shaped rebars and a steel bottom plate, which eases the introduction of a friction dissipative device. HSTCBs are usually characterized by a small effective depth, which leads to a large amount of longitudinal rebars. The latter, together with a small-sized beam-column joint, make it potentially subjected to severe damage, which reduces its dissipative capacity. The shear force acting on the joint can be reduced by endowing the BCC with a friction device, with the aim of increasing the lever arm of the bending moment transferred between beam and joint, preventing the latter from damage. To evaluate the mechanical performance of the above connection, two experimental programs have been carried out at the Structures Laboratory of the University of Palermo. The first one focused on the characterization of the friction properties of two different materials (thermal sprayed aluminum and brass), by means of a linear dissipative device subjected to cyclic load. The second one tested a beam-to-column subassembly endowed with the recently-proposed connection in which the dissipative device was made with the best performing friction material tested before. The results of the cyclic tests are presented and commented, showing the promising performance of such connection in providing a low-damage behavior and a satisfactory dissipative capacity.

Innovative connections for steel-concrete-trussed beams: a patented solution

The most recent design strategies welcome the adoption of innovative techniques for seismic energy input mitigation, aiming to achieve high dissipation capacity, prevent the structure from collapse and ensure the serviceability of the construction. Friction damper devices have been widely adopted in framed steel structures for decades, while their introduction in different structural types is still under investigation. This paper presents the outcomes of innovative research supported by the industry and conducted on beam-to-column connections of RC structures in which the beams are Hybrid Steel-Trussed Concrete Beams (HSTCBs) and the columns are classical RC pillars. An innovative solution, recently patented, has been found for the mitigation of the effects of seismic cyclic actions on small-sized beam-column joints, typically characterised by a large amount of longitudinal reinforcement due to the small effective depth of the beam. This paper collects the main featuring steps of the innovative research, which has led to the patented solution. The calculation procedure for designing the proposed connection is shown, and the validation through 3D finite element modelling is described. For the structural analysis of the joint, several monotonic and cyclic simulations have been carried out with the scope of investigating different design moment values. The finite element results proved that the patented solution is effective in preventing beam, column and joint from damage and it is suitable for exhibiting adequate dissipative capacity ensured by a flexural behaviour dominated by wide and stable hysteresis loops.

Seismic Performance of Earthquake-Resilient RC Frames Made with HSTC Beams and Friction Damper Devices

ABSTRACT Seismic behavior of RC frames with hybrid steel-trussed concrete beams is affected by panel zone damage due to a large amount of longitudinal reinforcement. Here the seismic efficiency of innovative frames characterized by friction damper devices (FDDs) at beam-to-column connections is compared against traditional frame. Three configurations are investigated: FDDs alone; FDDs with column-to-foundation connections having preloaded threaded bars and disk springs; FDDs with self-centering friction devices. Non-linear analyses show that FDDs alone prevent plastic hinge formation at beam ends and beam–column joint damage. FDDs with self-centering friction devices effectively limit both peak and residual drifts, avoiding column base plasticization.

Design of RC joints equipped with hybrid trussed beams and friction dampers

The challenge of this research consists in the first attempt to apply a dissipative friction connection to beam-to-column joints with semi-prefabricated Hybrid Steel-Trussed Concrete Beams (HSTCB) and RC pillars cast in-situ. Nowadays, HSTCBs are widely adopted in civil and industrial buildings and, therefore, it is required to evaluate their compliance with the capacity design criteria and their seismic energy dissipation capability. However, the design of the reinforcement of such beams usually lead to the adoption of large amount of steel within the panel zone which becomes potentially vulnerable to the effects of seismic cyclic actions and dramatically reduce the dissipation capacity of the entire structure. Therefore, the introduction of friction dampers in the HSTCB-to-column joints is investigated in order to evaluate the ability of the device in preventing the main structural elements from damage and limiting the cracking of the panel zone, thanks to the increase of the bending moment lever arm, which reduces the shear forces in the joint. Moreover, the proposed solution thoroughly investigates the connection between the friction device and the beam in order to ensure adequate strength and stiffness to the connection. The feasibility study is firstly conducted through the development of design criteria for the pre-dimensioning of the device and, successively, the proposed solution is validated through the generation of finite element models.

Review of Push-Out and Shear Response of Hybrid Steel-Trussed Concrete Beams

The hybrid steel trussed concrete beams examined in the present study are comprised of two principal components, i.e., a steel joist with inclined rebars, realized in industry, which is welded to a smooth steel plate and then embedded within the concrete cast in situ. The paper presents first the state of the art on laboratory tests and analytical modeling of the steel-to-concrete stress transfer mechanism investigated by push-out tests. Next, the most relevant scientific contributions currently available in the technical literature regarding experimental investigation on actual shear behavior are summarized and discussed. Lastly codes and analytical models are reviewed.

Stress transfer and failure mechanisms in steel-concrete trussed beams: Experimental investigation on slab-thick and full-thick beams

This study presents the results of an experimental investigation on semi-precast Hybrid Steel Trussed-Concrete Beams (HSTCBs) for analyzing the failure modes and the stress transfer mechanism between concrete and embedded steel elements (plate and truss). The available literature presents previous studies carried out by the authors mainly focused on the understanding of the local transfer mechanism by means of push-out tests. Conversely, in this paper original the results of laboratory tests conducted on six specimens of full-size HSTCBs subjected to four-point bending, with variation in the shear span, are reported on. From these results, the authors are able to assess the failure modes of each beam and, finally, the transfer length necessary to ensure a full-strength connection. In the experimental campaign, two beam typologies are considered: Slab Thick Beams (STBs) and Full Thick Beams (FTBs). Both global and local response are monitored, the former in terms of load-midspan deflection curves and the latter in terms of strain measurements from Strain Gauges (SGs) placed on steel elements and concrete. Finally, some of the most relevant analytical models available in the literature and/or deduced from codes are used for interpreting the load-carrying capacity of slab and full thick beams as well as the failure modes experimentally observed, i.e. failure due either to flexure or shear, or failure of connections.

Computational Study of Failure of Hybrid Steel Trussed Concrete Beams

AbstractThis study investigates the failure behavior of hybrid steel trussed concrete beams (HSTCBs) under three-point bending through a series of finite-element (FE) simulations. The FE model empl...

Cyclic Behavior of Composite Truss Beam-to-RC Column Joints in MRFS

In the present paper, the cyclic behavior of beam-to-column joints made up of Hybrid Steel Trussed Concrete Beams (HSTCBs) connected to Reinforced Concrete (RC) piers is investigated. HSTCBs are a typology of composite beam generally constituted by a steel truss with end zone added rebars embedded within a block of concrete cast in place. HSTCBs represent a structural solution for light industrialization in the civil constructions; the main advantages in their use are the higher construction speed with the minimum site labor, the possibility of covering wide spans with low depths and a final economical convenience. For these reasons, they are also increasingly adopted within seismic framed structures. In the present study, the examined joints are representative of framed RC structures subjected to seismic actions and designed according to the current Italian standard code. Cyclic tests are performed and interpreted by means of analytical and numerical models with the aim of verifying the strength capacity and ductility of the system, focusing on both the global behavior and the force transmission between steel truss and concrete in the joint region.

Experimental Investigation of the Shear Response of Precast Steel–Concrete Trussed Beams

AbstractThe results of an experimental campaign of three-point bending tests on precast composite beams, named hybrid steel–trussed concrete beams (HSTCBs), are provided. HSTCBs are typically constituted by a precast steel truss embedded in a block of concrete cast in place. Two series of specimens were manufactured, designed such that shear failure would occur, and tested under positive and negative bending moment. The experimental results obtained showed that fragile shear failure occurred in almost all cases, evidencing the crisis of the compressed concrete strut involved in the collapse mechanism. Yielding of the steel members provided ductility to the system, especially in those cases in which the mechanical properties of the concrete were adequate and allowed the failure of the strut to be delayed. Some of the more accredited expressions for prediction of shear resistance available in the literature were used to assess the shear capacity and compared against those experimentally obtained for all spe...

Numerical prediction of the shear response of semi-prefabricated steel-concrete trussed beams

In this study, the shear behavior of hybrid steel-trussed-concrete beams (HSTCBs) realized with prefabricated steel trusses embedded into a concrete core cast in situ, is investigated by means of Finite Element (FE) numerical simulations.HSTCBs do not behave as classical RC elements nor composite beams. Up to now, there are not specific design criteria in the building codes and the calculation of this type of beams is conducted by means of design-by-testing procedures. The knowledge of the material behavior as well as the understanding of the interaction between materials in contact is the first requirement for the definition of proper design procedures and calculation methods for practitioners to be inserted in the international building codes.In the present study, the numerical simulation of the global beam behavior through the modeling of the local material response and contact properties, allow a detailed knowledge of the shear resisting mechanism aimed at the definition of simplified calculation formula.The accuracy of the numerical prediction is validated against the results of a reference experimental campaign of three-point bending tests with shear failure carried out by the author in a previous work.A first analytical approach for the interpretation of the FE results is conducted applying the variable strut inclination method currently prescribed in Eurocode 2 for classical RC beams. Conversely, in the paper it is shown how all information coming from the FE analysis are of paramount importance for the possible development of more proper simplified calculation methods of the shear capacity of this particular beam typology.

Validation of a Shear Model for RC and Hybrid Beams with Two Different Inclinations of Transversal Reinforcement

The validation of an analytical model recently proposed for evaluation of the shear capacity in Reinforced Concrete (RC) beams containing multiple inclination of transverse stirrups is presented. The model is a suitable extension of that currently proposed in Eurocode 2 for the evaluation of the shear resistance, and it is derived by means of the the variable-inclination stress-field theory based on Nielsen’s plastic approach. Experimental and numerical data available in the literature on Hybrid Steel-Trussed Concrete Beams (HSTCBs) are used for model validation and result discussion. Finally, also the comparison with a different analytical approach for the assessment of the shear resistance of HSTCBs is provided.

Analytical evaluation of steel–concrete composite trussed beam shear capacity

A calculation method for the prediction of the shear resistance of precast composite beams, named Hybrid Steel Trussed-Concrete Beams (HSTCBs), is herein proposed. HSTCBs are constituted by a prefabricated steel truss embedded within a concrete matrix cast in situ so that, after curing, the two materials work together in the mechanical response of the composite structural element, the steel truss behaving as reinforcement of the beam. The proposed analytical model is developed on the basis of the results of a reference experimental campaign of three-point bending tests available in the literature, carried out on specimens of HSTCB designed in order to attain a shear failure. Furthermore, the proposed analytical formulation is supported by the outcomes of the finite element modeling available in the literature concerning the abovementioned reference tests. The model presents an additive formulation in which the shear resistance of the HSTCB consists of the contribution from concrete and shear reinforcement, being the diagonal rebars of the steel truss regarded as the stirrups typically present in Reinforced Concrete (R.C.) beams. Finally, also the contribution of the steel plate is taken into account in the resisting mechanism. Both reference experimental and numerical results are employed for the validation of the proposed analytical expressions and, with the aim of widening such a validation, also further experimental data available in the literature are taken into account, dealing with bending tests with shear failure on a similar typology of HSTCB.

FEM analysis of push-out test response of Hybrid Steel Trussed Concrete Beams (HSTCBs)

Aiming to investigate the steel truss–concrete stress transfer mechanism in Hybrid Steel Trussed–Concrete Beams (HSTCBs), a three-dimensional (3D) nonlinear Finite Element (FE) model is developed. The constitutive relationship of the steel composing the plates and the rebars is modeled by means of a quadri-linear law, while the concrete behavior is defined by means of a Concrete Damaged Plasticity (CDP) model, suitable for modeling concrete and brittle materials. Two main failure mechanisms are considered, namely the tensile cracking and the compressive crushing. In order to accurately grasp the complicate dowel and bond phenomena arising at the steel–concrete interface, a 3D solid model is realized to account for the actual contact surfaces between the truss and the concrete. The behavior of HSTCBs constituted by either deformed or smooth steel diagonal bars is investigated, and four models for steel–concrete interface are analyzed. The model is calibrated on some of the existing experimental push-out tests available in the literature and, afterwards, it is used to simulate the slip–load curve of a number of push-out tests in order to individuate the main geometrical and mechanical parameters influencing the maximum load that can be transferred from the steel plate to the concrete core. The FE simulations are also used to develop a first interpretation of the maximum load carried by the steel-to-concrete connection by means of analytical regression of the results numerically obtained, taking into account the geometrical configuration of the beam and the range of the geometrical and mechanical parameters.

Stress transfer mechanism investigation in hybrid steel trussed–concrete beams by push-out tests

Results of push-out tests carried out on Hybrid Steel Trussed–Concrete Beams (HSTCBs) before and after the concrete casting are presented and interpreted. Firstly, in order to check the ability of weldings before casting, tensile tests were performed on specimens reproducing different types of welded joints. Simplified design formulae were used to predict their ultimate strength. Secondly, results obtained by push-out tests on specimen representative of the beam before and after the concrete casting are presented and discussed. Finally, simplified analytical models proposed by the current European building code were adapted to the specific typology to roughly predict the ultimate strength obtained by push-out tests on specimens complete with concrete casting.

Experimental test results vs. analytical prediction of welded joint strength in hybrid steel trussed concrete beams (HSTCBs)

The aim of the paper is to provide practical guidelines for the design of welded joints of steel truss for encased composite steel–concrete beams. The results of experimental tests are presented and interpreted. The steel truss is made of a steel plate acting as the bottom chord, made of S355 structural steel, coupled with steel rebars which form the upper chord, and steel inclined web rebars (V-reverse) welded to the two chord elements, both of them made of steel B450C. Butt welded joints of web bar to bottom steel plate and fillet welding joints of inclined web bars to top chord bars were tested, as they are different from the ones which are normally used due to the coupling of different steel types and complex geometry. For the prediction of strength of fillet welding joints, two analytical expressions are derived: the first is based on a local failure criterion and the second on a limit domain of the welding section. Comparison with experimental data proves the equivalence and reliability of the proposed indications. Guidelines for choosing the effective dimensions of the fillet weld able to ensure a required strength are provided. For validation of the suggested guidelines, a high precision digital scanner is used to determine the geometrical characteristics of the fillet weld.