Elsevier

Engineering Structures

Volume 56, November 2013, Pages 805-822
Engineering Structures

Experimental and computational study of the vertical shear behaviour of partially encased perforated steel beams

https://doi.org/10.1016/j.engstruct.2013.04.025Get rights and content

Highlights

  • Comprehensive study on the behaviour of ultra shallow floor beams – composite beams.

  • Experimental work and computational finite element analyses on USFBs.

  • Sensitivity study with various constitutive models for the concrete material properties.

  • Design method for simply supported USFBs with any degree of shear connection.

  • Superior structural performance because of their low cost in manufacturing and usage.

Abstract

A comprehensive study has been undertaken by the authors to conduct advanced analysis and enable design tools for innovative Ultra Shallow Floor Beams (USFBs) in buildings. In the USFB, the concrete slab lies within the steel flanges and is connected to the slab through the web openings, providing enhanced longitudinal and vertical shear resistance. There are additional benefits in providing increased fire and buckling resistance to the steel beam. In this study four specimens of symmetric steel–concrete composite beams with large circular web openings in the steel section and low concrete grade were tested under static monotonic loading. One of the specimens was from a lower quality of concrete and was tested in order to further investigate the failure mechanism and the actual behaviour of the concrete confinement. The load carrying capacity of the perforated bare steel beam is also presented for direct comparison. For the computational approach to the problem, a three-dimensional (3D) Finite Element (FE) model was created, employing solid elements with material, geometrical and interfacial non-linearity. Two-dimensional (2D) FE contact models using shell elements were established to examine the steel–concrete interface condition.

The results show that the FE models are able to satisfactorily predict the load carrying capacities and the crack patterns of these new composite beams against the Vierendeel failure mechanism. A sensitivity study of material models and contact strengths using various constitutive models from the literature and the dominant parameters which affect the structural behaviour of the USFBs, are presented and discussed. Furthermore, the FE models provide detailed information on the structural behaviour of the confined concrete between the flanges and the section of concrete that passes through the web openings, as this is of paramount importance for the load carrying capacity and the failure mode of the USFBs.

The comparison between the experimental and computational results leads to useful conclusions. The results for the composite beams show a significant increase in vertical shear resistance, even though mechanical shear connectors were not used. A previous design method is presented and modified to be able to be used for the load carrying capacity prediction of this new composite structural system. The results compare very well and the shear enhancement demonstrated in this study is now used in design practice.

Introduction

Structures such as high-rise buildings benefit from shallow flooring systems since the floor-to-floor height is a significant factor. The fact that a conventional composite (concrete slab sits on top of steel beam) beam is deeper than a reinforced concrete beam is a strong disadvantage. Hence, in several situations it is important to reduce the overall structural depth of the floor using partially encased composite beams [1]. These fully composite beams also have other advantages such as increased fire resistance, load carrying capacity, local buckling stiffness and dramatic increase in the bending stiffness compared to conventional beams. Moreover, a lower construction cost compared to the reinforced concrete is achieved by using partially encased composite beams eliminating the construction time and amount of formwork and scaffolding [2], [3], [4], [5].

Perforated steel beams are also widely used nowadays, replacing plain (solid-webbed) beams, while integrating services such as electric wires and hydraulic pipes. Tests on short-span composite plate girders with web openings were initially carried out by Narayanan et al. [6] and Roberts and Al-Amery [7]. These tests showed that the shear strength of a composite plate girder is significantly higher than that of a steel plate girder alone, if adequate shear connectors are provided in the composite girder. In addition, the composite action under predominantly shear loading depends on the tensile or pull-out strength of the shear connectors. Analytical models including the contribution from the slab were proposed for determining the shear strength of composite plate girders. Experiments conducted by Clawson and Darwin [8] and Donahey and Darwin [9] indicated that the behaviour of composite beams with web openings is largely controlled by the shear–moment ratio at the web opening. Darwin and Donahey [10] proposed an equation to express the ultimate shear–moment relationship for composite beams with web openings.

In order to minimise the structural depth of the composite sections, steel perforated beams are designed to act compositely with floor slabs lying within the steel flanges. The analysis that has been performed and presented herein, together with the experimental programme carried out by Tsavdaridis [11], is the first such work on shallow light-weight floor beams, and has resulted in a better understanding of the failure mechanisms and the ultimate shear capacity.

Whilst numerous research papers were found in the literature review regarding conventional composite flooring systems with the use of plane and perforated steel beams support the floors from below as well as partially encased composite beams with the use of plain steel sections, only the last decade very limited study has been carried out on partially encased composite beams with the use of various steel section profiles with web holes [12], [13].

Comparing conventional composite flooring systems and partially encased composite perforated and non-perforated beams it is seen that the concrete between flanges in the latter case increases the bending stiffness and reduces the vertical displacements. Despite the advantages in terms of structural behaviour and cost, the behaviour of encased perforated beam is not entirely understood yet.

Section snippets

New composite flooring system

For conventional composite floor beams or down stand composite beams, the thickness of the flanges increases with the increase in span. Consequently, the steel sections are often heavier than needed [14]. The Ultra Shallow Floor Beam (USFB) is a new type of composite floor beam, which is fabricated by welding two highly asymmetric cellular tee-sections together along the web. Profiled steel decking or precast concrete floor units sit on the bottom flange, as shown in Fig. 1, Fig. 2. The top and

Aim and objectives

The main purpose of this study is to compare the conventional composite beams, using perforated beams, instead, and allowing services to pass, with the use of the innovative USFB. Using USFB, the need of shear connectors is limited, the structural depth is minimised and a light-weight system is achieved. Consequently, the span can be increased, hence fewer columns are required and free of column areas can be constructed as well as the concrete provides fire-resistance. In addition, these

Experimental work

Four USFBs were tested in this research programme. The web opening diameter, do, is equal to 0.76 h. For small web opening diameters, for instance 30% of the beams depth, it is easy to show that a load path of 45° between flanges, transfer the load across the web opening. However, for larger web openings the load path is not so clear. All material and specimen tests are conducted in the Engineering Department laboratories at City University, London.

Proposed design method evaluating vertical shear strength

The experimental programme and the non-linear FE analyses showed that the concrete infill in the perforated sections and the composite action enhance the vertical shear strength of the USFB. Liang et al. [25] proposed a design method for the vertical shear strength of simply supported conventional un-perforated composite beams (where the concrete slab sits on top of the plain steel beam) with any degree of shear connection, β. This method is modified herein to include USFB sections. Comparison

Conclusions

Overall, the USFB offers lower structural depth inversely to conventional composite beams, where the concrete slab sits on top of the plain (or perforated) steel beam. The decrease of the structural depth for every floor, and the ease of construction for large spans, as heavy propping is not needed, makes USFBs worth studying. Flexural tests were conducted to evaluate the structural behaviour of the proposed composite beam using symmetric steel section with circular web openings. The perforated

Acknowledgements

The results from this research study are incorporated in ASD Westok’s Ltd. (2010) design software for USFBs (USFB-AutoMate v1.0) developed and certified by the Steel Construction Institute (SCI). The authors would like to thank the ASD Westok group for the supply of the steel perforated specimens and the SCI for the approval of the experimental setup, structural arrangements and geometrical configurations of the specimens.

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