# Behavior of Concrete/Cold Formed Steel Composite Beams: Experimental Development of a Novel Structural System

- Nadim Wehbe
^{1}Email author, - Pouria Bahmani
^{2}and - Alexander Wehbe
^{3}

**7**:31

https://doi.org/10.1007/s40069-013-0031-6

© The Author(s) 2013

**Received: **5 January 2013

**Accepted: **12 February 2013

**Published: **29 March 2013

## Abstract

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.

## Keywords

## 1 Introduction

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.

^{5}/

_{16}in. diameter stand-off screw commonly used in composite deck construction and which could also be used as a shear connector in concrete/CFS composite beams.

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

^{5}/

_{16}in. stand-off screws for providing composite action had to be examined. Two groups of concrete/CFS composite simple beam test specimens, referred to as Group 1 and Group 2, were built to represent two different stand-off screw configurations. Three identical specimens of each configuration were constructed to verify repeatability of test results. The two cross sectional details of the test specimens are shown in Fig. 4. Each beam specimen was 9.75 ft. long and consisted of a 6 in. wide by 8 in. deep concrete section on top of a CFS track. The concrete section represented the slab thickening on top of a CFS track of a stud wall. The track was a C-shape section with 6 in. deep web and 2 in. wide flanges. The track thickness was 14-gauge (0.068 in.). According to the Steel Stud Manufacturers Association’s (2012) (SSMA) cross section designation, the track thickness used in this study corresponds to CFS Sect. 600T200-68. When composite action is provided, the track acts as the tension reinforcement in the composite beam section. For a concrete strength of 3,000 psi and steel yield stress of 50 ksi, the maximum and minimum tension steel ratios in beams allowed by the ACI code (American Concrete Institute 2011) would be 0.0220 and 0.004, respectively. Considering the given track cross sectional areas and an effective tension reinforcement depth

*d*, measured to the centroid of the track, of 8.43 in., the tension steel ratio of the beam specimens would be 0.0141. This steel ratio is well within the ACI allowable limits. Stand-off screw connectors were used to provide composite action between the CFS track and the concrete. The stand-off screws were placed in one row for the specimens in Group 1 and in two rows for the specimens in Group 2. In both groups, the center-to-center screw spacing along the beam’s longitudinal axis was 6 in. It should be noted that no shear reinforcement was placed in the concrete. The test specimens were designated as 1-GA14-X and 2-GA14-X for specimens in Group 1 and Group 2, respectively, where GA14 denotes the track thickness of 14 gauge and X is the specimen number (1, 2, or 3) within the group.

### 2.1 Material Properties

Nominal shear and flexural strengths for the beam specimens.

Specimen ID | Measured concrete strength (ksi) | Nominal shear strength (kips) | Nominal flexural strength (kip-in) |
---|---|---|---|

1-14GA-1 | 3.52 | 5.70 | 214.9 |

1-14GA-2 | 3.52 | 5.70 | 214.9 |

1-14GA-3 | 4.26 | 6.27 | 218.2 |

2-14GA-1 | 4.15 | 6.18 | 217.6 |

2-14GA-2 | 4.15 | 6.18 | 217.6 |

2-14GA-3 | 4.22 | 6.23 | 217.9 |

### 2.2 Instrumentation and Test Setup

### 2.3 Experimental Results and Discussion

Summary of experimental results for the beam specimens.

Specimen ID | Measured maximum total load (kips) | Average total end Slip (in) | Failure mode |
---|---|---|---|

1-14GA-1 | 8.33 | 0.26 | Flexural-shear |

1-14GA-2 | 8.33 | 0.22 | Flexural-shear |

1-14GA-3 | 8.05 | 0.17 | Flexural-shear |

2-14GA-1 | 13.29 | 0.04 | Flexural |

2-14GA-2 | 12.25 | 0.03 | Flexural-shear |

2-14GA-3 | 12.25 | 0.03 | Flexural |

*q*, of elastic beams:

*V*is the shear force,

*Q*is the static moment of the area above or below the horizontal shear plane, and

*I*is the transformed cracked moment of inertia. The computed shear force carried by a screw was found to be equal to 0.0645 kips per 1 kip of the applied shear force,

*V*. The force in the screw was also determined experimentally by measuring the strain in the CFS track at two predetermined reference sections in each shear span. Knowing the strain at a reference section, the corresponding tension force in the track was computed at that location. The force per stand-off screw was then determined by dividing the change in the tensile force by the number of standoff screws between the two reference sections. Plots of the theoretical and experimental force per stand-off screw versus the applied total load are presented in Fig. 9. The results indicate a very good agreement between the theoretical and experimental values until the point of first measured relative slip between the concrete and the CFS track.

*α*is a coefficient for conversion of units (= 1 for US customary units with

*t*in inches),

*d*is the nominal bolt diameter,

*t*is the uncoated steel sheet thickness, and

*F*

_{ u }is the tensile strength of the steel sheet. When hole deformation is not a design consideration, the nominal bearing strength is determined using the following equation:

*C*is a bearing factor obtained from AISI Table E3.3.1-1 and

*m*

_{ f }is a modification factor for the type of bearing connection in accordance with AISI Table E3.3.1-2. Since only the yield strength,

*F*

_{ y }, of the CFS track was measured experimentally,

*F*

_{ u }was assumed to be equal to 1.25

*F*

_{ y }when calculating

*P*

_{ n }. For the CFS track and stand-off screw used in this study, the nominal bearing strength would be 2.21 kips when hole deformation is limited and 2.55 kips when hole deformation is not a design consideration. Based on the experimental results shown in Fig. 9, the total applied load corresponding to stand-off screw forces of 2.21 and 2.55 kips would be approximately 12.0 kips. It should be noted that the theoretical flexural strength of the test specimens is attained at a total applied load of approximately 12 kips (corresponding to a moment of 216 kip-in). At the strength limit state, the hole deformation would enhance the system’s ductility by increasing the beam deflection without significantly affecting its strength. Therefore, Eq. (3) should be adequate for selecting the size and spacing of the stand-off screws.

*I*

_{ e }, that can be determined using the following empirical equation:

*M*

_{ cr }is the cracking moment,

*M*

_{ a }is the maximum service load bending moment in the beam,

*I*

_{ g }is the gross moment of inertia based on the concrete section, and

*I*

_{ cr }is the cracked moment of inertia. The cracking moment corresponds to the modulus of rupture as determined from Eq. (5):

*I*

_{ e }. The experimental load-deflection results were used to back calculate values for

*I*

_{ e }at different applied loads using the elastic beam load-deflection relationship. The elastic modulus of the composite section was assumed to be equal to that of the concrete,

*E*

_{ c }, as given by the following ACI (2011) expression:

*M*

_{ a }) in Fig. 10. Also shown are the theoretical

*M*

_{ cr }and the best fit line for the experimental

*I*

_{ e }. The ACI based

*I*

_{ e }was approximately 220 in

^{4}(Imbsen Software Systems 2000). Up to approximately 3.2 times the cracking moment, the ACI expression results in lower effective moment of inertia than the experimentally based value and, therefore, higher estimates of the deflection. For

*M*

_{ a }above 3.2

*M*

_{ cr }, the ACI based

*I*

_{ e }becomes higher than the experimental

*I*

_{ e }. At an applied moment of 180 kip-ft, the ACI based

*I*

_{ e }is 1.42 times the experimental

*I*

_{ e }. Hence, further investigation is needed to derive an appropriate expression for

*I*

_{ e }for concrete/CFS composite beams.

## 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.

^{5}/

_{16}in. stand-off screws placed in two rows and spaced at 6 in. on center over the opening length. The selection of the stand-off screw size and arrangement was based on the results of the composite beams study described in the previous section. The concrete in the beam was reinforced with two #4 longitudinal top steel bars that were placed 1 in. below the top of the beam. The top reinforcement was needed to provide flexural strength in the negative moment regions over the king studs. Shear reinforcement was provided in the form of a single layer of 3 × 3-W2.1 × W2.1 or 4 × 4-W2.1 × W2.1 mesh. Although the 4 × 4-W2.1 × W2.1 mesh was theoretically adequate for the shear reinforcement, the higher shear reinforcement amount provided by the 3 × 3-W2.1 × W2.1 mesh was also used to compare the performance of the two shear reinforcements in the event of development of premature wide shear cracks in the concrete. Figure 11 presents the cross sections of the Concrete/CFS composite test specimens. Based on the shear reinforcement, the four specimens were divided into two groups with two specimens in each group. In each group, one specimen was subjected to a single point load, while the other was subjected to two point loads. The two loading schemes were selected to represent actual loading conditions in the field where either one or two joists could potentially be carried by the header. The specimens were labeled using a Roman numeral followed by a single-digit number to reflect the shear reinforcement, and loading pattern. The Roman numerals “I” and “II” indicate shear reinforcement consisting of 3 × 3-W2.1 × W2.1 and 4 × 4-W2.1 × W2.1 wire-mesh, respectively. The numbers “1” and “2” represent single-point and two-point loading, respectively.

### 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.

Specimen | Theoretical & numerical analysis | Experimental test results | Ratio of experimental to theoretical | Failure mode | ||
---|---|---|---|---|---|---|

Total load (kip) | Total load (kip) | |||||

Based on flexure | Based on shear | Theoretical | Experimental | |||

I-1 | 35.0 | 44.2 | 35.9 | 1.03 | Flexural | Flexural |

II-1 | 35.0 | 45.4 | 34.6 | 0.99 | Flexural | Flexural |

I-2 | 42.0 | 45.0 | 44.2 | 1.05 | Flexural | Flexural |

II-2 | 43.0 | 43.6 | 46.2 | 1.07 | 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.

- 1.
Concrete/CFS composite beams can be designed for ductile flexural failure.

- 2.
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.

- 3.
When premature relative slip is prevented, the flexural response of concrete/CFS composite beams can be predicted with good accuracy.

- 4.
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. - 5.
The ACI provisions for flexural and shear strengths in beams can be applied to fully composite concrete/CFS beams.

- 6.
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.

- 7.
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.

- 8.
The structural behavior and strength of concrete/CFS composite headers can be modeled with good accuracy.

## Declarations

### Acknowledgments

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.

### Open Access

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 Access**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.

## Authors’ Affiliations

## References

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