Behavior of Concrete/Cold Formed Steel Composite Beams: Experimental Development of a Novel Structural System
© The Author(s) 2013
Received: 5 January 2013
Accepted: 12 February 2013
Published: 29 March 2013
The use of light-gauge steel framing in low-rise commercial and industrial building construction has experienced a significant increase in recent years. In such construction, the wall framing is an assembly of cold-formed steel (CFS) studs held between top and bottom CFS tracks. Current construction methods utilize heavy hot-rolled steel sections, such as steel angles or hollow structural section tubes, to transfer the load from the end seats of the floor joist and/or from the load-bearing wall studs of the stories above to the supporting load-bearing wall below. The use of hot rolled steel elements results in significant increase in construction cost and time. Such heavy steel elements would be unnecessary if the concrete slab thickening on top of the CFS wall can be made to act compositely with the CFS track. Composite action can be achieved by attaching stand-off screws to the track and encapsulating the screw shank in the deck concrete. A series of experimental studies were performed on full-scale test specimens representing concrete/CFS flexural elements under gravity loads. The studies were designed to investigate the structural performance of concrete/CFS simple beams and concrete/CFS continuous headers. The results indicate that concrete/CFS composite flexural elements are feasible and their structural behavior can be modeled with reasonable accuracy.
The use of light-gauge steel (LGS) framing in low-rise commercial and industrial building construction has experienced a significant increase in recent years. In such construction, the wall framing is an assembly of cold-formed steel (CFS) studs held between top and bottom CFS tracks. The suspended floors are normally composite concrete/LGS decks spanning between load-bearing CFS walls. The composite floor system consists of a cast-in-place concrete floor supported by a corrugated steel deck. The decking is attached to the top chords of open-web steel joists by the means of stand-off screws. The stand-off screws serve as shear connectors that transfer shear stresses between the concrete slab and the top flanges of the open-web steel joists. The joist spacing in LGS construction can vary depending on the joist’s load carrying capacity, the building’s intended use, and the design requirements.
Since concrete/CFS composite beams have not been considered before by the engineering community as viable structural elements, current building codes do not provide provisions for the design and construction of such beams. Therefore, research studies were needed to assess the feasibility of developing this novel structural system. In response to this need, a series of experimental studies were performed on full-scale test specimens representing concrete/CFS flexural elements under gravity loads. The studies were designed to investigate the structural performance of concrete/CFS simple beams and concrete/CFS continuous headers. This paper presents results from two experimental studies and discusses the basic behavior of concrete/CFS composite flexural elements.
2 Concrete/CFS Composite Simple Beams
2.1 Material Properties
Nominal shear and flexural strengths for the beam specimens.
Measured concrete strength (ksi)
Nominal shear strength (kips)
Nominal flexural strength (kip-in)
2.2 Instrumentation and Test Setup
2.3 Experimental Results and Discussion
Summary of experimental results for the beam specimens.
Measured maximum total load (kips)
Average total end Slip (in)
3 Concrete/CFS Composite Continuous Headers
A total of four CFS wall with composite concrete beam test specimens were fabricated and tested until failure. The main purpose for the tests was to evaluate the structural performance of composite concrete/CFS beams when used as load-bearing headers over wall openings.
Each test specimen represented a 12 ft. long segment of a CFS wall frame with a 6 ft. long header spanning over a wall opening that was centered at the wall’s mid-length. The wall framing consisted of the following standard CFS sections: 600S162-68 (14-gauge) studs, 600T200-97 (12-gauge) top track, and 600T125-43 (18-gauge) bottom track. One 6 in. × 3 in. × 0.375 in. HSS king stud was used to support each end of the composite header. The wall frame was only 20 in. high to avoid premature buckling of the wall studs during load testing.
3.1 Material Properties
On the day of testing, the measured average concrete compressive strength values were 4.25 ksi for specimen I-1, 5.32 ksi for specimens I-2 and II-1, and 5.16 ksi for specimen II-2. The specified yield stresses for the CFS top track and the steel bars were 50 and 60 ksi, respectively; however, the specified values were not verified experimentally.
3.2 Instrumentation and Test Setup
3.3 Experimental Results and Discussion
The average load-carrying capacities for the single-point and the two-point load specimens were 35 and 45 kips (22.5 kips/point load), respectively. The load-deflection curves for the two-point load specimens were very identical. However, the load-deflection curves for the single-point load specimens indicate that Specimen I-1 was stiffer than Specimen II-1 up to the formation of first negative moment crack. This anomaly was the result of the test procedure; the testing of Specimen I-1 was interrupted at a load of 14.7 kips due to malfunctioning of the data acquisition system. The specimen was unloaded then the test was restarted from a zero load. Since the CFS studs and tracks are connected by means of self-tapping screws, the first loading excursion allowed for any slippage at the screwed CFS frame joints to take place. Thus, the second loading excursion did not include the softening effect resulting from slippage of the jointing screws.
Building codes specify a deflection limit of L/240 under the combined service dead and live loads, where L is the span length (International Code Council (ICC) 2012). For the 6 ft. header considered in this study, the deflection limit would be 0.3 in. The measured average total load at 0.3 in. was 22.3 and 27.6 kips for the single-point and two-point loading, respectively. These load values correspond to the total service load limit that can be applied without exceeding the deflection limit of L/240.
The theoretical flexural and shear strengths of the concrete/CFS headers were computed in order to determine the ultimate load carrying capacity of the headers under single- and two-point loads. Under negative bending, the flexural reinforcement consisted of two #4 top bars. Under positive bending, the flexural reinforcement was assumed to consist of the CFS track. Flexural capacities were determined from moment-curvature relationships for a header section under positive and negative bending moments. The computer program XTRACT (Imbsen Software Systems 2000) was used to develop the moment-curvature relationships. The theoretical flexural capacities of the concrete/CFS beam in positive and negative bending were 32.8 and 15.4 k-ft., respectively. The total nominal shear strength of the concrete/CFS header was calculated by adding the nominal shear strengths of the concrete, wire-mesh, and CFS track. Fully composite action between the CFS track and concrete was assumed in computing the shear strength. The nominal shear strength of the concrete and the shear reinforcement were calculated according to the ACI code (American Concrete Institute 2011) provisions for shear, and the CFS shear strength was determined using the AISI (2007) and the AISC (2008) provisions. The effect of the variation in the wire mesh size on the overall shear strength was insignificant. The nominal shear capacity of the composite section was approximately 22 kips.
Comparison of experimental and theoretical results for the header specimens.
Theoretical & numerical analysis
Experimental test results
Ratio of experimental to theoretical
Total load (kip)
Total load (kip)
Based on flexure
Based on shear
Flexural & shear
Flexural & shear
4 Summary and Conclusions
A series of experimental studies were performed on full-scale test specimens representing concrete/CFS beams and headers gravity loads. Continuity at the concrete-CFS interface was provided by 2½ in. × 5/16 in. stand-off screws. The studies were designed to investigate the structural performance of concrete/CFS simple beams and concrete/CFS continuous headers.
Concrete/CFS composite beams can be designed for ductile flexural failure.
The use of stand-off screws as shear connectors is feasible for providing composite action. When adequate number and spacing of stand-off screws are furnished, the CFS track acts as tension reinforcement under positive bending and the concrete/CFS composite beams can attain their full flexural capacity.
When premature relative slip is prevented, the flexural response of concrete/CFS composite beams can be predicted with good accuracy.
The ACI expression for the effective moment of inertia may result in underestimation of I e at lower moments and overestimation of I e at higher moments.
The ACI provisions for flexural and shear strengths in beams can be applied to fully composite concrete/CFS beams.
The AISI equations for the bolt bearing capacity in bolted connections can be used for evaluating the bearing capacity of the stand-off screws in the CFS track.
Concrete/CFS composite headers are feasible to construct using stand-off screws to provide shear continuity at the interface. When adequate number and spacing of stand-off screws are used, concrete/CFS headers can achieve their full composite strength.
The structural behavior and strength of concrete/CFS composite headers can be modeled with good accuracy.
Funding for this study was provided by NUCOR Corporation in Norfolk, Nebraska. The findings and conclusions of this study are those of the authors and do not necessarily represent the opinion of NUCOR Corporation.
This article is distributed under the terms of the Creative Commons Attribution License which permits any use distribution, and reproduction in any medium, provided the original author(s) and the source are credited.
This article is published under license to BioMed Central Ltd.Open AccessThis article is distributed under the terms of the Creative Commons Attribution License which permits any use, distribution, and reproduction in any medium, provided the original author(s) and the source are credited.
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