Elsevier

Thin-Walled Structures

Volume 43, Issue 7, July 2005, Pages 1071-1090
Thin-Walled Structures

Analysis and design of lapped connections between cold-formed steel Z sections

https://doi.org/10.1016/j.tws.2004.11.005Get rights and content

Abstract

This paper presents an analysis and design method for lapped connections between cold-formed steel Z sections after careful calibration against test data obtained from a total of 26 one point-load tests on lapped connections. Based on the experimental observations on the lapped connection tests where combined bending and shear is always critical in the cross-sections at the end of laps, an analysis method is proposed to evaluate all the internal forces within the lapped connections. Once the co-existing moments and shear forces in the lapped connections are evaluated, the critical sections are readily checked against combined bending and shear using codified design rules. Moreover, design expressions are also proposed for the evaluation of effective flexural rigidities of lapped connections with various bolt configurations against practical lap length to section depth ratios. Consequently, the structural behaviour of lapped sections between cold-formed steel Z sections in terms of strength and stiffness is quantified rationally for general design. The research project aims to provide understanding to the structural performance of lapped connections between cold-formed steel Z sections, and hence, to develop a set of rational design rules for multi-span purlin systems with overlaps in modern roof construction.

Introduction

Cold-formed steel purlins are widely used in modern roof systems due to their high structural efficiency and buildability. The most common shapes of cold-formed steel purlins are C and Z sections, and the section depths typically range from 100 to 350 mm while the thicknesses range from 1.2 to 3.0 mm. Common yield strengths are 280 and 350 N/mm2, but recently, sections with yield strength up to 450 N/mm2 may be found in some propriety purlin systems giving improved load carrying capacities. At present, four different types of purlin systems are commonly found with different degrees of continuity:

  • (i)

    single span,

  • (ii)

    double span,

  • (iii)

    multi-span with sleeves, and

  • (iv)

    multi-span with overlaps.

In practice, multi-span purlin systems using cold-formed steel Z sections with overlaps are the most popular owing to their ease of transportation with effective stacking and high structural efficiency with high level of continuity between members. It should be noted that the load carrying capacities of these purlin systems depend on many factors, such as steel grades, section shapes and sizes of purlin members, restraints provided by attached roof sheetings and intermediate bracing members, and connection configurations at purlin-rafter supports.

Up to the presence, there is little guidance for engineers to assess the structural behaviour of cold-formed steel purlin systems due to the lack of understanding on lapped connections between cold-formed steel sections. Only simple and conservative design rules for purlin members are available, and thus, design-based purlin systems are often found to be very conservative with low market competitiveness. In fact, most modern roof systems with cold-formed steel purlins are developed through prolonged and expensive full-scale testing in order to acquire high structural efficiency.

Section snippets

Current design recommendations

In various codes of practice for design of cold-formed steel structures [1], [2], [3], [4], basic design methods are given for the evaluation of section capacities and member resistances against both local and overall buckling in typical sections. For connection design, these codes only provide design rules for the load carrying capacities of individual fasteners such as bolts, screws and welds while no guidance on the structural behaviour of connectors such as web cleats for simple

Objectives and scope of work

In order to improve the buildability of cold-formed steel structures, a series of research and development projects have been undertaken by the authors to study the structural behaviour of bolted moment connections between cold-formed steel sections. This paper presents the proposed analysis and design method [14] for lapped connections between cold-formed steel Z sections which forms a major part of an experimental and theoretical investigation on the structural performance of multi-span

Basic configurations of lapped connections

In order to establish effective bolted moment connections between lapped Z sections, two practical configurations for generic lapped connections are proposed after considering ease of installation as follows:

  • Config. W4

    • (a)

      Only the webs of Z sections are bolted together which are, in turn, attached onto primary structural members such as rafters through hot rolled steel web cleats.

    • (b)

      Six bolts per connection are adopted as the minimum configuration where four outer bolts are assigned to resist moments

Results of experimental investigation

Among all the lapped connections tested in Part I of the project [16], section failure in the cross-sections at the end of laps of the lapped connections under combined bending and shear was always found to be critical. Moreover, shear buckling of the section web at the ends of lap of the lapped connections was found to be fairly localized due to the restraining effects from both the lapped sections and the purlin-rafter connections, and the length of a typical shear buckling mode shape was

Distribution of internal forces within lapped connections

In order to assess the strength of lapped connections, it is essential to determine the internal forces within the lapped connections under external loads. Referring to the lapped connections with Config. W4 and Config. W6 as shown in Fig. 2, the following assumptions are adopted:

  • (a)

    The centre of rotation of the lapped connection, O, is coincided with the bolt group center.

  • (b)

    The magnitudes of bolt forces Fb and Fbm are proportional to the distances between the bolt holes and the centre of rotation

Design against combined bending and shear

Based on test observations, shear buckling of the section web at the ends of lap of the lapped connections was found to be fairly localized, and the length of a typical shear buckling mode shape was found to range from 0.8 to 1.25D, where D is the section depth. It should be noted that the strength prediction methods on shear capacities in both BS5950: Part 5 and Eurocode 3: Part 1.3 are technically similar, and thus, it is proposed to improve the shear capacities of the critical

Effective flexural rigidities of lapped connections

The effective flexural rigidity ratios of lapped connections αi and αf given in Table 2 are plotted in Fig. 5 for easy comparison. It should be noted that due to the scatterness of data, it is only possible to provide simple design expressions for both the maximum and the minimum values of αi and αf which are required in determining the maximum and the minimum moments at both internal supports and near mid-span regions for the design of multi-span purlin systems. After simple data analyses as

Conclusions

A rational design method for lapped connections between cold-formed steel Z sections is proposed after careful calibration against test data obtained from one point-load tests on lapped connections. It is proposed to assess both the moment and the shear capacities of the critical cross-sections at the ends of lap with allowances for the presence of bolt holes. Moreover, based on test observations, shear buckling is shown to be less critical in lapped connections due to the restraining effects

Acknowledgements

The project leading to the publication of this paper is supported by the Research Grants Council of the Government of Hong Kong Special Administrative Region (Project No. PolyU5040/99E), and the Research Committee of the Hong Kong Polytechnic University (Project No. G-W039). Moreover, the authors would also like to thank BlueScope Steel Asia Pte. Ltd, in particular, Mr John Kong and Mr Anthony Tan, for their support to the research project.

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