J. Cent. South Univ. (2013) 20: 3689−3696 DOI: 10.1007/s11771-013-1897-9

Feasibility of developing composite action between concrete and cold-formed steel beam

S. O. Bamaga, M. Md. Tahir, T. C. Tan, S. Mohammad, N. Yahya,

A. L. Saleh, M. Mustaffar, M. H. Osman, A. B. A. Rahman

Construction Research Centre (UTM-CRC), Faculty of Civil Engineering, Universiti Teknologi Malaysia, Skudai 81310, Malaysia

© Central South University Press and Springer-Verlag Berlin Heidelberg 2013

Abstract: Cold-formed steel structures are steel structure products constructed from sheets or coils using cold rolling, press brake or bending brake method. These structures are extensively employed in building construction industry due to their light mass, ductility by economic cold forming operations, favorable strength-to-mass ratio and other factors. The utilization of cold formed steel sections with concrete as composite can hugely reduce the construction cost. However, the use of cold formed steel members in composite concrete beams has been very limited. A comprehensive review of developments in composite beam with cold formed steel sections was introduced. It was revealed that employing cold-formed steel channel section to replace reinforcement bars in conventional reinforced concrete beam results in a significant cost reduction without reducing strength capacity. The use of composite beam consisting of cold-formed steel open or close box and filled concrete could also reduce construction cost. Lighter composite girder for bridges with cold-formed steel of U section was introduced. Moreover, types of shear connectors to provide composite action between cold-formed steel beam and concrete slab were presented. However, further studies to investigate the effects of metal decking on the behavior of composite beam with cold-formed steel section and introduction of ductile shear connectors were recommended. Key words: cold formed steel; beam; composite; shear connection; thin-wall; composite girder

1 Introduction

Cold-formed steel (CFS) structures are steel structure products made from sheets or coils by cold rolling using press brake or bending brake method. Currently, cold formed steel sections are extensively used in construction industry. Three primary areas of load-bearing CFS applications are framing, metal buildings, and racks [1]. The general use of cold formed sections as primary members of light steel framing requires a more simplified design process appropriate to their applications as beams, floor joists, columns, stud walling, members of roof trusses and sub-frames. Structural use and understanding of the behavior of structures made in this way advanced during the Second World War and the first design specification for this type of construction was produced in the USA in 1946 [2].

Standard specifications (e.g., North American specification, AS/NZS 4600 and Eurocode 3) for design of cold formed steel members and connections have been established to satisfy the needs of design and information construction and accommodate the extensive use of CFS members in construction industry. Due to the thinness of

CFS materials (usually ranging from 0.4 to 6.4 mm), instability of buckling is the major difficulty in designing CFS members when subjected to compression [3−4]. Therefore, stiffening such members by edge stiffeners is a common practice to improve their strength and toughness.

Lightness, high strength and stiffness, pre-fabrication ease, mass production, fast and easy erection and installation and non-combustibility as well as the efficiency in cost, material and energy savings encourage the designers, builders and companies to use CFS members for residential construction [5]. However, the use of these members in composite concrete beams has been limited [6]. The use of CFS sections with concrete to form composite beams can be a new alternative solution to replace hot rolled steel and reinforced concrete in small to medium sized buildings [7].

In composite beam, the concrete provides the compressive strength, fire resistance, and floor surface, while the steel possesses high tensile strength which provides good ductility. The design of composite beam depends primarily on the shear transfer mechanism provided by shear connectors between concrete slab and

Received date: 2012−09−03; Accepted date: 2012−12−10 Corresponding author: M. Md. Tahir, Professor; Tel.: +60−75531616; Fax: +60−75576841; E-mail: mahmoodtahir@utm.my

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steel beam. In composite hot rolled steel concrete beam, the shear connection between steel beam and concrete slab is usually accomplished by welding the headed stud to the top flange of steel beam; however, this is not practiced in CFS sections. Thus, the challenge in design and execution of composite beam with CFS section is to provide such devices that could produce the composite action, increase the strength capacity and meet the ductility requirements of shear connection. In this work, a comprehensive review of the use of cold formed steel sections in composite beams was presented. 2 Previous studies 2.1 Composite reinforced concrete beam

CFS section and concrete as composite beam was first introduced by NGUYEN [6]. Commonly, on-site construction of concrete beams require installation and removal of formwork and shoring thus increasing the cost of construction. Therefore, the author conducted an investigation to reduce the cost of cast-in-place concrete beams construction by replacing the conventional reinforced steel rebar with equal cross sectional area cold formed steel lipped channel section (Fig. 1). The CFS lipped channel section was designed to work as formwork supporting the fresh concrete during construction. It also functioned as reinforcement in composite reinforced concrete beams, which significantly reduced the cost of construction. The author tested a total of 32 full size CFS-concrete beam specimens to determine the bending and shear capacities and study the behavior of tested specimens under a combination of shear and bending stresses. To improve the bonding between CFS channel section and concrete, round embossments (diameter is 38.1 mm, depth is 6.35 mm) were formed in their webs. This kind of embossment serves as a connection mean to provide composite action between steel and concrete and the forces are transferred from concrete to steel through such embossment. In addition, the lipped channel provides confinement to concrete in tension which also enhances the bonding between the CFS channel and concrete. It also improves the uplift force that can split the steel and concrete.

A four-point load bending test was conducted to determine the bending capacity of four composite reinforced concrete beam specimens. Theoretically, the author used the ultimate strength concept in reinforced concrete design to determine the bending capacity of beam specimen. The author introduced the formula for composite steel-deck slab provided by American standard to determine the ratio of steel for the tested composite beam. The experimental results turned out to be compatible with the initially predicted ultimate

Fig. 1 Sketch of proposed composite section bending capacity of the tested beam, i.e., within the range of 0.981 to 1.055.

Single concentrated load at the middle span of the tested beam was applied to determine the shear capacity. The concrete and CFS channel worked compositely and the load was supported by both concrete and steel channels. At the advanced stage of loading, the failure was initiated by the loss of composite action due to the vertical separation, resulting in disengagement between concrete and steel channels, and additional load was carried by concrete until the diagonal tension cracks were formed. The results of 16 tested beam specimens showed good compatibility (within ±15%) with the predicted values.

Same configuration of three-point load test was used to study the behavior of 12 beam specimens under combined shear and bending stresses. The results of tested specimens and computed values of bending and shear capacities varied from 0.691 to 1.073 and 0.695 to 1.030, respectively, which indicated that the capacities were reduced when beam members subjected to large combined bending and shear stresses. The author developed preliminary conservative interaction equation, however more experimental work is needed for the purpose of validation. 2.2 Thin-walled composite-filled beam

Thin-walled composite-filled beam is a cold formed open steel box section with an infill of concrete. The strength capacity of such beams is limited by the compression buckling capacity of the steel plate at the top of the open box section. As an attempt to improve their capacity, HOSSAIN [7] studied the behavior of thin-walled composite-filled beam using four strength-enhancement devices mentioned below (Fig. 2).

1) Cold formed steel open box section (OS).

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Fig. 2 Proposed strength-enhancement devices: (a) OS; (b) WE; (c) WER; (d) RC; (e) CS

2) Cold formed steel open box section with welded extension (WE): The welded extension plates have the same thickness as the steel box section, they were holed to allow for easy casting of concrete. The holes had varied diameter, spacing and layout, to study their influence on the strength enhancement. The depth of extension plates was about quarter or half of the depth of steel box section.

3) Cold formed steel open box section with welded extension and rod (WER): Same as WE with additional rods having diameters of 6 and 10 mm and spacings of 100 and 200 mm to study their influence on the strength enhancement.

4) Cold formed steel open box section with reinforced concrete (RC).

The results of four-point load test showed that the failure of OS beams occurred due to the lateral buckling of free open top flanges causing separation of steel from concrete. This result is expected since the top flange of steel section is under compression and the infill concrete does not provide stiffness to the flange due to the weakness of composite action between steel section and concrete that lies only on the surface bonding. The strength enhancement devices in WE and WER beams improved the lateral buckling capacity by stiffening the top flange of steel section and preventing separation of steel from concrete and thus essentially improved the strength capacity of beams. It was observed that the strength of WE beams increased when welded extension

plate depth increased. However, the strength and ductility of WE beams were higher compared to OS beams. WER and RC beams showed higher strength capacities than respective WE beams but lower ductility. The RC beam showed higher strength capacity due to the fact that the concrete was reinforced as compared to infill concrete in OS and WE beams. The welded rod in WER beam provides more stiffness to extension plate and top flange of steel section as well as more confinement of the infill concrete. Based on the experimental results and mode failure (either by yielding steel or buckling), the author developed an analytical model to predict the flexural strength of thin-walled composite-filled beam (either partial or full shear connection) and elucidate the behavior of tested beams. The developed analytical model adhered to the experimental results within the range of 1.02 to 1.15. Moreover, guidelines and considerations were presented regarding the use of this model, optimal size and layout of holes in welded extension plates in WE and WER beams as well as premature failure due to welding operation.

HOSSAIN [8] studied the effects of using lightweight volcanic pumice concrete as infill concrete together with closed section (CS) along with the afore- mentioned sections. The result revealed that the CS beam exhibited better strength capacity than WE and OS beams except WER beam. In addition, similar behavior was expressed by beams filled with normal and volcanic pumice concrete with higher strength and better

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deformation capacity than beams filled with normal concrete. In conclusion, the lightweight volcanic pumice concrete could be used in thin-walled composite-filled beams with more satisfactory performance as compared to normal concrete. 2.3 Cold formed steel box section composite-filled

beam A total of 12 CFS box section composite-filled

beam specimens were tested in laboratory of Faculty of Civil Engineering, Universiti Teknologi Malaysia (UTM) to study the behavior of tested beams under bending and shearing [9]. The composite beam consisted of CFS box section filled with normal concrete. The box section was formed using two 1 mm thick CFS channels, spot welding at every 500 mm and metal strap fastened with tek screws on flanges. Beam specimens with 1 000 mm length were used for shear test while beam specimens with 2 000 and 3 000 mm length were used for bending test. Two control specimens were tested without infill concrete for comparison purpose.

The results of four-point load test revealed that all 2 000 and 3 000 mm tested beams showed elastic behavior with a very limited plastic deformation before failure and failed abruptly by breaking at the middle span, whereas the 1 000 mm tested beam failed at the support where the concrete cracked and split off pushing out the steel box section. The beam specimens without infill concrete were excessively deformed as load increased and lateral torsional buckling occurred at the failure spot. No deformation occurred at any part of the beam specimens was reached when the limiting value of deflection, in accordance with British standard [10]. Thus, it was concluded that the allowable load capacity of tested specimens was governed by limiting value of deflection given by BS5950, not the maximum failure load.

2.4 Composite girder with cold formed steel U section In bridge plate girders, the girder is usually formed

using multiple pieces of steel plates that require more welding and fabrication operations resulting in cost increments. Cold formed steel U section fabricated from one steel sheet was proposed [11] to replace the conventional plate girder, thus significant reduction in cost of fabrication and construction could be achieved. The proposed composite beam was designed compositely with the reinforced slab using studs at middle span area where the concrete was poured into the steel section and prestressed at the intermediate supports by prestressed concrete bars to increase the strength capacity to resist the maximum bending moment at the support. Three models were tested (Fig. 3). The first model presented The positive moment area where the girder model consisted of 6 mm cold formed steel of U section and reinforced concrete slab connected to the upper flanges with studs, the second model presented the negative bending moment (hogging moment developed at the support) area where the concrete is poured inside the cold formed steel of U section, reinforced concrete slab connected to upper flanges and prestressing process was performed using two prestressed bars. The third model also presented the negative bending (hogging moment developed at the support) area where extra plate was welded at the same level of the upper flange between U section webs, and then the concrete was poured into U section, the concrete slab in this model was ignored while it was in tension zone. The test specimens in the second and third models were turned over and then loaded from the lower flange side.

The test results revealed that all three models behaved as composite girder. The first model failed by collapse of concrete slab at ultimate load indicating that the connection between steel girder and concrete slab is strong enough to provide composite action with no failure at shear connectors. However, the stiffening of the

Fig. 3 Sketch of proposed composite girder section: (a) U-1; (b) U-2; (c) U-3

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steel flange provided by the concrete slab was lost due to crushing of concrete, resulting in steel flange tend to buckle. The second and third models failed by buckling of steel flange and lower part of web at the compression zone. The concrete slab in second model was cracked transversely and the cracks became wider as load increases, resulting in reduction of composite action between steel girder and concrete slab. The reduction in the stiffness of steel flange became evident as flange buckled at ultimate load. The second model exhibited the best strength capacity along with good deformation capacity while the first and third models showed the same strength capacity with very good ductility for the third model compared to the first model where the applied load during testing sharply decreased after the concrete slab collapsed. Based on the experimental results and Bernoulli-Euler principle, the author proposed a design calculation method for the proposed composite girder U section of cold formed steel beam. It was found that the calculated and experimental values were compatible with 5% higher value for first model and 9% and 3% for the second and third models, respectively. It was concluded that the proposed composite section could be classified as compact section. Hence, plastic design could be utilized. 2.5 Concrete cold formed steel track composite beam

WEHBE et al [12] conducted a research to investigate the feasibility of connecting the concrete slab to cold formed steel (CFS) track to work compositely in light-gauge steel construction. Such idea significantly reduced the cost of light-gauge steel construction by avoiding the use of heavy hot rolled steel angles and hollow section steel tubes which are usually welded to the top of CFS load bearing walls to function as load distribution members. In this research, the CFS channel section was used to serve as tension reinforcement, the composite action was provided by stand-off screw. The effects of thickness of CFS channel and number, spacing and configuration of connectors and pour stop engagement on behavior of proposed beam were investigated. The results of experiments showed that the CFS channel and concrete slab behaved as compositely with less slippage and higher strength capacity when two connectors were used as compared to one connector. However, it was found that the strength capacity of tested beams was not well improved when less than 150 mm longitudinal spacing between connectors was employed. It was found that when the CFS track thickness increases, the stiffness and strength capacity increases. The most thicker CFS track showed better stiffness than others in the elastic stage. However, it showed unexpected results in the non-linear stage, where the ultimate strength capacity of the composite section was lower than the

sections with thinner CFS tracks. This is due to the lower tensile strength of the most thicker CFS track as compared to the other CFS tracks, resulting in a much higher rate of softening. It was observed that the specimen with pour stop engagement showed significant improvement in strength capacity of the proposed beam by 22.5% as compared to specimens without pour stop engagement. 2.6 Conventional composite beam with cold formed

steel section Conventional composite beam usually consists of

hot rolled steel section and concrete slab. The design of such beams depends primarily on the shear transfer mechanism provided by shear connectors. Thus, attention is given to the design and execution of shear connectors to provide the maximum shear connection capacity and required ductility.

In small to medium size buildings, the use of hot rolled steel sections is not economical (e.g., cost of cutting operation, waste material and increased labor). In addition, the extensive use of CFS members nowadays is limited to non-composite members design, resulting in the use of bigger size of sections. Therefore, it is an economic solution to replace the hot rolled steel section with CSF, in small to medium size buildings, to function as composite member with concrete slab. Thus, smaller CFS section could be used to avoid wastage of hot rolled steel sections and related additional cost could be minimized.

HANAOR [13] studied the composite action of slab comprising of cold formed steel I-sections and concrete using two types of shear connectors. The first type consisted of a short length of channel of the same dimensions as the beam section, with its web connected in two different ways (self drilling screws and welding) to the top flange of the beam parallel to beam length. The second type of shear connector was the one considered suitable for certain types of composite deck slabs. The deck consisted of wide channel sections with holes in their webs, through which the concrete penetrated. The vertical webs formed the shear connector and the connection to the beam was achieved using self drilling screws. The mechanism of shear connectors is to resist the longitudinal shear force by its end bearing which is similar to the channel shear connectors in composite hot rolled steel concrete beams. The lips of shear connectors could provide enough resistance to uplift forces. The results revealed that the welded channel connectors exhibited the best performance and attained greater capacity compared to the self drilling screws. The welding provided strong connection between shear connectors and steel beam, and the flanges of shear connectors were strong enough to resist the longitudinal

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force. Thus, the failure of the specimen was first initiated by buckling of steel section followed by concrete crushing and welding failure. It could be deduced that the weakness parts in screwed shear connectors are the screws themselves. As the applied load increases, the screws tend to tilt and shear off at ultimate load or pulling-out with deck buckling for the second type of shear connectors.

LAKKAVALLI and LIU [14] studied the behavior of composite slab joists consisting of cold formed steel C-sections and concrete. They investigated the capacity of four shear transfer mechanisms, including surface bond, pre-fabricated bent-up tabs (Fig. 4), pre-drilled holes, and self-drilling screws. Four different types of mechanisms for shear connectors were employed on the surface of the flange embedded in the concrete. The first mechanism is the surface bond where the longitudinal shear force is assumed to be transferred by shear and friction resistance between concrete and flange surface of steel beam, this mechanism is usually very weak. Therefore, the surface bonding to develop composite action with profiled steel sheeting is neglected. The second mechanism is pre-fabricated bent-up tabs that resist the longitudinal shear force by its end bearing resistance. The third mechanism is pre-drilled holes shear transfer where the dowels of concrete formed through the pre-drilled holes resist the longitudinal shear force in specimens. The last mechanism is self-drilling screws, the screw resists the longitudinal shear forces in a similar way as welded stud shear connector. The effect of thickness of cold formed C-sections on the capacity of composite slab joists were also investigated by using two different thicknesses, i.e., 1.905 and 1.524 mm. The results showed that the pre-fabricated bent-up tabs shear connectors provided the best performance followed by drilled holes in the embedded flanges and self-drilling screws. The results indicated that owing to the variation in CFS thickness, the ultimate capacity of composite slab joists increased significantly when thicker C-sections were used. This is due to the contribution of large end bearing area of pre-fabricated bent-up tabs and the self-drilling screws become more stiffened due to the depth contact between screw and the flange of steel beam. Also, the results showed that the ultimate strength capacity of pre-drilled hole shear connector increased when the spacing between holes decreased from 200 to 150 mm. In contrast, the ultimate strength capacities of bent-up tabs and self-drilling screws decreased when spacing decreased from 200 to 150 mm, the authors attributed the decreasing of ultimate strength to the overlapping of longitudinal shear stress fields created by the knife action of bent-up tabs and screws as the spacing decreased, resulting in weakening of the concrete between the connectors. The failure mode of most push

test specimens was initiated by a longitudinal crack that appeared in the concrete around the lower end of the concrete column and gradually extended, causing failure at ultimate load. For full-scale beam tests, the failure occurred when the transverse cracking in the concrete had progressed through the bottom of the concrete slab and extended over the entire width with crushing of the concrete in the bearing area and excessive yielding in the bottom flange of steel beam. The inspection of specimens showed that the pre-fabricated bent-up tabs and pre-drilled holes transfer enhancements are unaffected indicating that these enhancements are strong enough to provide composite action between steel and concrete with no sign of failure. The self-drilling screws showed considerable rotation around its base which explained the lower strength capacity as compared to pre-fabricated bent-up tabs and pre-drilled holes transfer enhancements. The deformation capacity of self-drilled screws is higher than the pre-fabricated bent-up tabs and as high as pre-drilled holes. However, all tested transfer enhancements failed to met the ductility requirements of Eurocode 4 where the characteristic deformation capacity of shear connector must be at least 6 mm.

Fig. 4 Pre-fabricated bent-up tabs shear connector

IRWAN et al [15−17] studied the shear connection

between CFS sections and concrete using shear transfer enhancement called bent-up triangular tab shear transfer (BTTST) (Fig. 5) where small triangular tab formed in the top flange of cold formed section and then bent up to desired angle. Compressive strength and modulus of elasticity of concrete, cold formed section strength, dimensions and angle of BTTST and cold formed section thickness were investigated. For comparison purpose, specimens using shear connector proposed in Ref. [14] were prepared and tested. BTTST is assumed to resist the longitudinal shear force by its end bearing area and shear area (which is the contact area between BTTST and top flange). It was found that when the bearing or shear area increases, the capacity of BTTST increases. Thus, the capacity of BTTST increases as thickness of steel beam, angle and dimensions of BTTST increase. The failure

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mode of push test specimens with BTTST shear connector was initiated by diagonal cracking at the concrete slab edges followed by concrete crushing, the diagonal cracking is due to the transverse direction of BTTST resistance. For full-scale beam tests, specimens with thicker steel beam were failed due to concrete crushing while the failure mode of thinner steel beam was governed by fracture of steel beam. The results of push out tests also showed that the strength capacity of BTTST shear connector increased when concrete compressive strength increased. Also, the strength capacity of BTTST shear connector was higher than LAKKAVALLI and LIU’s transfer enhancement capacity [14]. This was expected since the end bearing area of BTTST was larger and the shear area was contributing to the BTTST strength capacity while it was not happened in LAKKAVALLI and LIU’s transfer enhancement. However, the ductility of BTTST connector was insufficient to meet the requirements of ductility as per Eurocode 4. Based on the results of 28 push test specimens, the authors developed an equation to predict the ultimate shear strength of BTTST connector. The experimental results were compatible with the predicted values using the developed equation with an average of 1.14.

Fig. 5 BTTST Shear connector

The problem arised from pre-fabricated bent-up tabs

[14] and bent-up triangular tab shear transfer [15−17] is that, the longitudinal shear force is not resisted solely by the transfer enhancement. The researchers did not take into account the portion of the top flange between each two transfer enhancements (Fig. 6) as an additional transfer enhancement might be resisting the longitudinal shear force. Bearing this in mind, the strength capacities of pre-fabricated bent-up tabs and bent-up triangular tab shear transfer may be reduced. Future work of push tests on similar specimens after removing the pre-fabricated bent-up tabs and bent-up triangular tab transfer enhancement are very much recommended to determine the contribution of the top flange portion between each two transfer enhancements to the resistance of longitudinal shear force.

Fig. 6 Top flange portion between transfer enhancements

3 Discussion and current research

Twenty years back, NGUYEN [6] stated that the use of CFS members in composite concrete members was very limited. The statement still holds true, even though the cold-formed steel members as non-composite are being extensively used in light industrial, residential and commercial buildings [5]. The thinness of CFS members that poses difficulties in application of conventional shear stud connectors or other known welded shear connectors might be the reason for the limited use of CFS members in composite concrete members. In addition, the lack of specifications with reference to special considerations and guidelines for design of composite concrete members with CFS sections is also a contributory factor.

The feasibility of using CFS sections to reduce the cost of construction and improve the construction methods along with maintaining same strength capacity of reinforced concrete beams has been proven [6−9], where CFS sections such as channel, open box and closed box section could be used as formwork during casting and as reinforcement at service stage. Also, lighter composite bridge girder is introduced using cold-formed steel U section [11]. This could considerably reduce the design and construction cost of bridge and improve the erection techniques.

Attempts have been carried out to introduce new shear connectors that could be suitable to provide composite action between CFS sections and concrete slab in conventional composite steel-concrete beam [13−14, 16]. The introduced shear connectors are easy to fabricate and possessed higher strength capacity when compared to natural shear bond between concrete and flange of CFS section. All previous researches were used double CFS lipped channel sections oriented back to back with top flange embedded to concrete slab.

However, the common practice in composite construction is to use metal decking to cast floor slabs. The metal decking can be either parallel or perpendicular

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to the beam at the floor. Therefore, the need to study and investigate the effects of metal decking on the behavior of composite concrete beam with CFS section is recommended. Also, since the previously introduced shear connectors [13−14, 16] are impractical in the presence of metal decking, the introduction of new shear connectors that could be pertinent with metal decking is essential.

Currently, a project research is being conducted at Construction Research Centre at Universiti Teknologi Malaysia (UTM) to investigate the structural behavior of composite beam with CFS section and proposed shear connectors. The composite beam consists of two lipped C-channel CFS sections connected back to back to replace typical hot rolled I-section, normal concrete slab of grade C30 and transverse profiled steel decking. Innovative shear connectors are proposed to connect the steel beam and the concrete slab which is expected to form a composite beam.

Three types of shear connectors are proposed in this work. The first type of shear connector comprises of hot rolled plate (6 mm) embedded to the steel beam web by bolting, small bar (12 mm diameter with 55 mm length) fixed perpendicularly through a hole at the center of the plate portion embedded to the concrete slab to resist uplift force presented. The second type of shear connector is an angle obtained from same CFS sections that formed the steel beam, for this type of connection, the shear connector either comprises of two angles oriented back to back or one angle installed to the steel beam web with bolts. Standard headed stud with 16 mm diameter and 76 mm height is used to investigate its strength and ductility when welded to thin CFS section flange. Standard push test in accordance with Eurocode 4 full-scale beam tests are performed to investigate the capacity and ductility of shear connectors and the behavior of proposed composite beam. It is expected that, the composite beam would show higher capacity than non-composite beam and could be concluded that the first and second type of shear connectors will exhibit more ductility and capacity than standard headed stud. The proposed composite beam can be fabricated at the factory and then transferred to the site. Thus, reduction in labor and avoidance of congestion on site are expected to be achieved. 4 Conclusions

1) The use of CFS section with concrete as composite beam attracts attention of the researchers.

2) Efforts have been made to use and to validate the use of CFS section as replacement for the conventional reinforcement bars in conventional reinforced concrete beams, composite-filled concrete beams, composite

bridge girders and conventional composite beams. 3) Generally, the adequate strength capacity could

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(Edited by DENG Lü-xiang)