Behaviour of lightweight concrete slabs acting compositely with steel I-sections

Numerical evaluation for the effective slab width of continuous composite beams

The effective flange width is a concept proposed by various codes to simplify the computation of stress distribution across the width of composite beams. Questions have been raised as to the validity of the effective slab width provisions, since they have a direct effect on the computed ultimate moment as well as serviceability limit states such as deflection, fatigue, and overloading. The objective of this paper is to present results from an experimental and analytical investigation to determine the effective slab width in steel composite beams. The Finite Element Method (FEM) was employed for the analysis of composite steel-concrete beams having variable concrete flange widths. Results were compared to those from tests performed on eight beams loaded to failure. Beam test specimens had variable flange width and various degrees of composite action (shear connectors). The comparison presented in terms of the applied load versus deflection, and strain in the concrete slab show that the AISC-LRFD code is conservative and underestimates the width active. Based on a detailed parametric study an equation for the calculation of the effective flange width is recommended.

Element-based effective width for deflection calculation of steel-concrete composite beams

A composite section is made up of a steel girder and concrete slab connected by shear connectors. The shear lag phenomenon usually takes place in such a section and results in underestimation of stresses and strains at the web-to-flange intersections of the girder. With the introduction of the concept of effective slab width, the actual width can be replaced by an appropriate reduced slab width. The classical effective slab width definition does not take into account the strain variation through the slab thickness. More sophisticated definitions are introduced and used with finite element analyses. The method of finite element modeling is discussed, and the model is successfully verified with experimental results. Parametric study is conducted to investigate the effective slab width for both positive and negative moment sections. The effective slab width is computed and compared with the current AASHTO load and resistance factor design (LRFD) specifications. The results demonstrate that full width can be used as the effective slab width in the design and analysis in most cases for the design and analysis of both positive and negative moment sections. The current AASHTO LRFD specifications are found to be conservative for configurations with widely spaced girders, especially in negative moment sections.

Shear connectors are required to build composite (concrete and steel) beams. They are placed at the interface of concrete and steel to transfer shear and normal forces between two structural components. Such composite beams are sensitive to provide structural integrity when exposed to fire as they loss strength, stiffness, and ductility at elevated temperature. The present study is designed to investigate the shear resistance and the failure modes of the headed stud shear connectors at fire exposure and post-fire exposure. The study includes ordinary concrete and concrete with carbon nanotubes (CNTs) to build composite (concrete-steel) beams with structural steel. Experimental push tests were conducted on composite beams at ambient and elevated temperatures, such as 200, 400 & 600℃. Moreover, push tests were performed on the composite beams after being exposed to 200, 400 & 600℃. Push test results illustrated the reduction of ultimate shear capacity and stiffness of headed stud shear connectors as the temperature increased. Although similar values of ultimate shear were obtained for the headed stud connectors in both ordinary and CNT concrete, the CNT modified concrete reduced the concrete spalling and cracking compared to ordinary concrete and was observed to be effective at temperatures greater than 400°C. All specimens showed a lower shear resistance at fire exposures compared to the corresponding post-fire exposures. Moreover, numerical simulation by Finite Element (FE) analyses were carried out at ambient temperature and at fire conditions. The FE analysis results show a good agreement with the experimental results. In the experimental studies, failure of all specimens occurred due to shear failure of headed stud, which was later validated by FE analyses using ABAQUS.

This paper presents an experimental study on determining shear resistance of channel shear connectors in a solid concrete material slab under monotonic loading. The solid concrete slab is of different types of concrete including plain, reinforced and lightweight concrete. The channel shear connector length is varied as well. A series of push-out specimens were tested for this study. The results verified using channel shear connectors embedded in lightweight concrete in composite structures and shows that lightweight concrete has a slight effect on the ductility and load-displacement performance of the specimens. However, using reinforcement in concrete causes considerable increase in ultimate strength and ductility of channel shear connectors and using channels embedded in unconfined normal concrete has a brittle performance of them. In general, the channel shear connector showed a ductile behaviour.

<p>Steel and concrete composite structures are widely used in both bridge and building structures due to their numerous advantages against conventional solutions. However, the design and analysis of continuous composite beams are rather complicated due to their different behaviour in positive (or sagging) and negative (or hogging) moment regions. Moreover, in the regions of negative moments,</p><p>e.g. at the internal support regions of continuous members, a large part of the composite beam section is subjected to torsional moment under certain loading conditions and curved composite structures or flyovers, thus the bottom flange and the web are susceptible to local instabilities and the concrete slab are easily to crack during its service life. In this study, a straight steel-concrete composite beam was tested to investigate their mechanical performance under pure negative bending and combined negative bending and torsional moments. Two-point loading condition with and without load eccentricities were applied to create necessary loading conditions. Load and deflection relationships, normal strain on the bottom flange of the steel main girder, and shear strain of the stud shear connectors at different sections were recorded in the test and reported in this paper. The results indicate that the classic beam design theory can be used for predicating the behaviour of a composite beam under pure negative bending moment. In addition, the failure mode and shear strain of shear connectors of the test specimen under combined negative bending and torsional moments were also discussed.</p>

Adopting hybrid cellular I-sections can favor optimizing steel consumption, as the higher steel grade is localized only in the compression tee. In addition, continuous steel-concrete composite cellular beams allow the construction of longer spans due to the combination of higher bending stiffness of the cellular I-section and its connection with the concrete slab, further the continuous of the beam, which provides a better distribution of the bending moment diagram. However, due to the hogging moment on the support regions, the steel-concrete composite beam can present lateral instability of the compression flange accompanied by web distortion, named Lateral-DistortionalBuckling (LDB). No investigations have evaluated the behavior of steel-concrete composite beams with hybrid cellular I-section under hogging moment. This way, the present study aims to assess the behavior of the structure in question using a nonlinear finite element model via ABAQUS software. The investigation consists of verifying the beams' failure mode, considering plastic behavior or LDB, and the effectsof different combinations of steel grades for the hybrid I-section, and further comparing the use of hybrid and homogeneous steel profiles on the composite beams.

The composite slim beam has become popular throughout Europe in recent years and has also been used on some projects in China. With its steel section encased in a concrete slab, the steel-concrete composite slim beam can provide the floor construction with minimum depth and high fire resistance. However, the design method of the T-shape steel-concrete composite beam is no longer applicable to the composite slim beam with deep deck for its special construction, of which the present design models are not available but mainly depend on experiences. The elevation of the flexural stiffness and bending capacity of composite slim beams with deep deck is rather complicated, because the influences of many factors should be taken into account, such as the variable section dimensions, development of cracks and non-linear characteristics of concrete, etc. In this paper, experimental investigations have been conducted into the flexural behavior of two specimens of simply supported composite slim beam with deep deck. The emphases were laid on the bonding force on the interface between steel beam and concrete, the stress distribution of beam section, the flexural stiffness and bending capacity of the composite beams. Based on the experimental results, the reduction factor of equivalent stress distribution in concrete flange is suggested, and the calculation method of flexural stiffness and bending capacity of simply supported slim beams are proposed.

The paper shows the shear-lag effect in slabs of twin-girder steel-concrete composite decks due to the main prestressing techniques such as support settlements, bonded cables inside the concrete slab, and external slipping tendons. The analysis is performed by means of a beam model taking into account the loss of planarity for the slab cross section, the flexibility of the shear connection, and the time-dependent behavior of the concrete. A linear elastic behavior is considered for the steel beams and shear connection while viscoelastic behavior is assumed for the concrete slab. A parametrical analysis is carried out on realistic bridge decks for each kind of prestressing technique, by varying the span length and the girder spacing. The distribution of longitudinal normal stresses in the slab is shown for each prestressing technique and the effective slab widths are then evaluated and compared with those suggested by ENV 1994-2 for different span lengths and for different spacing of the beams.

Three-Dimensional FE Modelling of Simply-Supported and Continuous Composite Steel-Concrete Beams

New methods to provide shear strength on the cold-formed steel (CFS) lipped c-channel section composite beam encased with concrete partially is proposed using rebars embedded in concrete. The development of research on the CFS composite beams technology with partial encasement shows that the section of the composite beam encased with concrete partially can provide ductile flexural action for the composite beam. The application of profiled metal decking slabs in the composite beam is becoming increasingly popular compared to solid slabs. However, it has a detrimental effect on the structural behavior of a composite beam. This research is aimed primarily at presenting the behavior of the rebar shear connector to evaluated ductility, shear capacity, and modes of failure. Two samples using a rebar 12 mm in diameter in the profiled metal decking slabs and solid slabs were tested using the standard push-out test till failure. For the composite beam design, the proposed shear connector embedded in the slab concrete and the CFS encased with concrete partially used the test results’ shear capacity, greater than the values as proposed by section 3.1, BS 5950. Due to rebars shear-off, the solid slab specimen failed with the highest load of 489.6 kN while the Profiled Metal Decking Slab specimen failed due to the formation of the cracks on the PMDS at 421.1 KN ultimate loading.
 HIGHLIGHTS
 
 A new system of the rebar shear connectors for the Cold-Formed Steel (CFS) composite beam partially encased in concrete have been developed
 The main purpose of push-out testing is to investigate the shear capacity and ductility of the proposed shear connector failure modes and load-slip connections
 The shear capacity of the test results is greater than the values as indicated by BS 5950 Part 3.1
 The proposed rebars as a shear connector for partly embedded CFS can be used in the construction
 
 GRAPHICAL ABSTRACT

The use of steel–concrete composite beams in which wide-flanged steel beams act compositely with a concrete or steel–concrete composite slab above is well established in building construction. However, for composite beams with low degrees of shear connection between the beam and slab, significant additional deflections can occur due to slip in the shear connectors. A design formula is presented for the effective stiffness of composite beams taking account of the stiffness of the shear connectors, which is compared to measured deflections of six symmetric composite beams and an 11 m span composite beam of asymmetric profile. It is shown that the comparison is good when using a shear connector stiffness of 70 kN/mm for single shear connectors and 100 kN/mm for pairs of shear connectors per slab rib. The results of push tests on a range of slab profiles confirmed these initial elastic stiffnesses. To ensure that the slip at the serviceability limit state does not lead to permanent deformations of the composite beam, it is proposed that the minimum degree of shear connection should not fall below 30% for unpropped beams and 40% for propped beams of symmetric cross-section.

Elastic behaviour of a tapered steel-concrete composite beam optimized for reuse

This research studies the effect of high temperature which is reached to 600 °C onstructural lightweight and normal weight concrete. Lightweight concrete mix designedaccording to ACI committee 211-2-82 with mix proportion 1:1.12 :3.35 by volume .Thewc ratio equal to 0.5 by weight and cement content 550 kgm3. Mix proportions ofnormal weight concrete were 1:2:3 by weight with cement content 400 kgm3 and samewc. The design compressive strength at 28 days of normal weight concrete (NWC) andlightweight concrete (LWC) were 34.7 MPa and 22.62 MPa respectively. Compressivestrength tests were performed on 100 mm cubes exposed to high temperature 100,200,400and 600 °C. The normal weight concrete and light weight concrete test specimens wereexposed to high temperature for 10 minute suddenly at the required degree. Moreover,light weight concrete test specimens tested after graduate exposure to high temperaturereaching to the required degree with and without drying to examine the effect of moisturecontent.The results indicated that the structural lightweight concrete exhibits approximatelysimilar compressive strength loss compared to normal weight concrete up to 600 °C at 28days in graduate exposure .The percentage of reduction on compressive strength was30% in lightweight concrete compared to 28% in normal weight concrete at 600 °C .Insudden exposure to high temperature ,the opposite behavior was noticed .The percentageof reduction on compressive strength was 64.4% in lightweight concrete at 600°C .Drying of lightweight concrete specimens before graduate exposure to high temperaturessignificantly reduce the loss of compressive strength.