Experimental study on ultimate strength of bolted L shaped sleeve joints between CFS lipped channel sections

https://doi.org/10.1016/j.jcsr.2020.106022Get rights and content

Highlights

  • Bolted CFS L shape sleeve specimens subjected to combined bending and shear were studied.

  • Various parameters such as length and thickness of sleeve connector and span length were included.

  • The moment resistance ratio was proposed for evaluating L shape sleeved specimens under combined bending and shear.

  • Reliability studies were performed to determine the resistance and safety factors.

  • A simple design example was carried out to demonstrate the applicability of suggested design equation for two span purlin system.

Abstract

This study aims to understand the behavior of bolted L shape sleeve joint connecting two Cold-Formed Steel (CFS) channel sections. A total of thirty-two specimens were experimentally tested under three-point bending. Three different series namely, (A) control specimen with two different span lengths, (B) sleeved specimen with the flange of sleeve connector capping the compression zone, and (C) sleeved specimen with the flange of sleeve connector capping the tension zone were tested. The parameters considered in this study include (i) the span length (Lt), (ii) sleeve length to section depth ratio (β), and (iii) thickness of sleeve connector (LS). The experimental results revealed that the ultimate moment resistance of specimens with a 2.5 mm thick sleeve connector was 19 to 73% higher than 2.0 mm thick sleeve specimens. The moment capacity of series C specimens was about 90–103% of that of the series B specimens. Based on the experimental results, design predictor equations were suggested to determine the moment resistance of sleeve specimens with respect to β. In addition, reliability studies were done for test series B to determine the safety and resistance factors for allowable strength design (ASD) method and load and resistance factor design (LRFD) method respectively. The equivalent and design uniformly distributed load (UDL) was determined based on the combined effect of shear and bending.

Introduction

Cold-formed steel (CFS) channel sections are commonly used as secondary members such as purlins in roofs and as girts in the wall panels of an industrial building. In the modern roof constructions, various types of purlin systems are employed namely (i) single-span butt purlin system, (ii) double-span butt purlin system, (iii) single-span sleeve purlin system, (iv) double-span sleeve purlin system, (v) single-span lapped Z purlin system, and (vi) double-span Z lapped purlin system. In general, structural purlins run over multiple supports making them a continuous beam system. However, the support moment design and degree of continuity depend on the connection characteristics. The continuity of the purlin system has several advantages such as redistribution of bending moment from the span to supports and reduction in overall deflection limits. Overlap and sleeve joints between the CFS Z sections are commonly employed by the construction industry. The lap joint can be employed for connecting two Z sections due to its ease in installation and to improve the continuity at the intermediate supports. However, the geometry of channel (lipped) sections does not lend itself for lap-jointed connection and hence have to resort to sleeve connections to make it compatible. Therefore, the sleeve connection is the preferred feasible solution to improve the continuity between two-channel sections at the intermediate supports. In general, sleeve connections are easy to install and reduces the overall cost and time of the on-field construction (Yang and Liu [1], Gutierrez et al. [2]). In the past, Natesan and Madhavan [3] have studied the bolted channel sleeve joints between the channel purlin sections and proposed design equations for a similar system. However, L shape sleeve connections will minimize the field complications during installations (Fig. 1) when compared to the channel sleeve connection due to the absence of flange on one side (tension zone). In addition, the designer should ensure that the flange of the L shape sleeve connector should be placed in the compression zone (top for uplift load and at the bottom for gravity load) at the intermediate support as shown in Fig. 1. The strength and stability of the purlin section depend on the material properties, cross-section profile, lateral bracing system along the length of the purlin section and the connection behavior at support. A number of researches (Yang and Liu [1], Gutierrez et al. [2], Chung and Quinton [4], Yu [5]) in the past have studied the behavior of braced purlin system and suggested that the lateral bracings enhance the strength and stability of the purlin system. Similarly, Favero Neto et al. [6] studied the strength and stiffness of sleeve and lap joints between Z sections and observed a significant increase in peak load (10%) due to the provision of lateral bracing close to the connection. In the present study, the bracing is provided at a distance of Lt/6 for the comparison between the control and sleeve specimens.

Pham et al. [7] and Liu et al. [8] studied the interaction between the shear force and moment at the end of the lap with standard and slotted hole respectively. Pham et al. [7] suggested a linear interaction equation for unbraced members with lap joints subjected to combined bending and shear load. Liu et al. [8] proposed an interaction equation for lap joints with a slotted hole between the Z sections and determined the design point load. Moreover, previous studies have emphasized the need to determine the design point load for the lap-jointed Z sections (Ghosn and Sinno [9], Chung and Ho [10] and Ho and Chung [11]). Dubina and Ungureanu [12] obtained the design UDL based on the experimental and numerical studies of the multi-span purlin system using the interaction between moment and web crippling. Therefore, the current experimental setup is designed to simulate the two-span continuous purlin system, and finally, the design UDL is determined.

Fig. 2a shows the shear force and bending moment diagram for a two-span continuous beam/purlin system (AISC [13]) subjected to UDL. The point of inflection (zero bending moment) was positioned at a distance of 0.25 L from both sides of the middle support. The parabolic bending moment profile due to UDL exists between the points of zero moments. The parabolic bending profile is treated approximately as a linear bending moment profile (Natesan and Madhavan [3], Favero Neto et al. [6], and Liu et al. [8]) for an applied point load at the mid-span to investigate the bolted L shape sleeve connection. The experimental setup to generate the linear bending moment profile and constant shear force of a single specimen is shown in Fig. 2b. In general, the design of the purlin section depends on the maximum value of gravity load or uplift. However, both the load types should be taken into account for the design of the L shape sleeve connection to avoid the failure of connection since the flange is absent on one side of the L shape sleeve connector. Liu et al. [14] have studied the sleeve connections between the sigma sections with the sleeve flange placed only in the compression side and did not include the effect of sleeve flange in the tension zone to account. In the current study, full-scale experiments are conducted to assess the effect of the sleeve with its flange capping the tension side and compression side separately on the moment capacity.

Cold-formed steel C and Z sections are used as purlins in the structural steel construction. However, a review of past literature studies clearly shows that the design provisions for bolted sleeve connection between CFS channels are quite scarce. The aim of the present research is (i) to investigate the performance of bolted L shape sleeve connection between CFS channel sections with varying thickness of sleeve connector, length of sleeve connector, and span of the test specimen, (ii) to compare the strength under two different sleeve configurations; test series B with flange of sleeve in compression side, and test series C with flange of sleeve in tension side, and (iii) to suggest a design equation to determine the moment resistance of L shape sleeve joints between two C sections using bolts in multi-span purlin system. In addition, this study would help improve the design guidelines of sleeve connection under gravity load and uplift load.

Section snippets

Test configuration

In total, thirty-two specimens were prepared for three different test series such as series A, series B and series C. In series A, two control specimens were tested for two different span lengths of 2.4 m and 3.6 m. To fulfill the structural requirements such as strength and serviceability (Ho and Chung [15]), the length of the tested specimens were chosen based on span length to section depth ratios of 32 and 48. Therefore, the length of purlin was obtained as 4.8 m and 7.2 m for the channel

Results and discussion of experimental results

This section includes discussion on moment-rotation plots, failure modes, comparison of moment resistance between the sleeved specimen and its corresponding control specimen. In addition, the equivalent and design UDL determined based on the interaction between shear and bending moment are also included. The observed experimental strength (load) and calculated moment of all the tested single specimens (Half the magnitude of pair tested value) of test series A, series B, and Series C are

Reliability analysis for the suggested moment resistance (Mns) of the L shape sleeved specimens in test series B

The design moment resistance of the bolted L shape sleeve specimens can be calculated using the resistance factors. These resistance factors shall be obtained through the reliability analysis of the suggested Eq. (3) (the nominal moment resistance of sleeve specimens). The reliability analysis could be dealt by one of the two approaches: (i) determining the AISI reliability index for a prescribed safety factor in ASD method and a resistance factor in LRFD and LSD methods, or (ii) determining

Combination of bending and shear forces

It can be observed that the failure of the sleeve specimens initiated due to local buckling in the compression flange of the sleeve connector along the loading line. This could have occurred due to the action of combined bending moment and shear force since none of the sleeve specimens in test series B have failed due to plastic deformation of the bolt hole and the same is confirmed by the comparison between the bolt forces (Fb) and bearing strength (Pnb) of the bolt hole (bolt force is lesser

Summary and conclusion

In this investigation, the performance of bolted L shape sleeve joints between two CFS channel purlin sections was investigated for various parameters such as span length, thickness and, length of L shape sleeve connector. In addition, the effect of providing the sleeve flange at the compression and tension side and stiffening of the compression zone using bolts was also examined. In total, thirty-two specimens were tested under three-point bending configuration. From the present study, the

Acknowledgments

The first author would like to acknowledge the fellowship from the Ministry of Human Resource Development (MHRD), India. The authors would like to gratefully acknowledge Pennar Engineered Building Systems Ltd., Hyderabad for their help in fabricating the test specimens required for experimental investigation.

Conflict of interest

None.

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