Blind bolted T-stub connections to unfilled hollow section columns in low rise structures

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Abstract

This paper presents the results of an experimental program investigating blind bolted T-stub connections to unfilled hollow section columns in the tension and compression regions under static loading. The T-stub connection is proposed as an alternative connection to the welded connection currently used as a moment connection for unfilled hollow section columns in the Australian construction industry. The flexural performance of the T-stub connection, in terms of stiffness, has been evaluated in accordance with EC3 classifications and was found to behave as a semi rigid connection under serviceability loading. Results from the experiments have been compared with existing theoretical models that predict the maximum load the flexible column face can carry in the tension region before yielding occurs, and good agreement has been achieved. Furthermore, the experimental results have been used to validate a detailed three dimensional finite element (FE) model which simulates the proposed connection. The developed FE model in turn has been used to perform a sensitivity analysis which is presented in this paper.

Introduction

Hollow sections are generally more efficient as structural members than conventional open sections due to their superior torsional rigidity and hence resistance to flexural-torsional and torsional buckling modes. In practice, welded connections are used when rigid or semi-rigid connections are required between beams and hollow section (HS) columns. Welding is necessitated due to lack of access for installing conventional bolts. However, welding on site is subject to weather constraints and in general requires highly skilled labour, making it unattractive in Australia and similar economies where labour costs are high. An example of a moment connection currently used in Australia is shown in Fig. 1. This connection was designed by Australian Tube Mills [1] and is part of an integrated framing system for house construction. The connection is achieved by shop welding angles to the side walls of the HS columns, with the beams bolted to the angles on site. As with all welded connections, the configuration is restricted by the arrangement of pre-welded components, leading to the need for tight installation tolerances on site. The restrictions and limitations imposed by welding are likely to be overcome by the development of alternate connections using blind bolts. The blind bolts require installation from one side only, hence they enable bolted connections to be utilised with hollow sections.

Several studies have been conducted on different blind bolted structural connections. Simple beam to HS column connections can be achieved by direct blind bolting to the column wall. However, due to the inherent flexibility of the HS column face, developing a flexurally rigid blind bolted connection proves to be a challenge. One of the most common types of blind bolted moment-resisting connections is the extended end plate connection. Several researchers have investigated the behaviour of such a connection with different types of blind bolts. Mourad [2] and Ghobarah [3] were among the early researchers to investigate the behaviour of the blind bolted extended end plate connection using the Huck High Strength Blind Bolt (HSBB) for square hollow section (SHS) column sizes of 203 to 254 mm wide with thicknesses of 9.53 to 12.7 mm (width to thickness ratio, b/t=16 to 27). From their test results, connections to unfilled HS columns were in the semi-rigid range of the EC3 classification [4] whereas connections to the concrete-filled HS columns displayed remarkable improvements in stiffness and could be classified as rigid according to the EC3 specifications.

France et al. 5., 6., 7. conducted extensive investigations on the behaviour of simple and moment-resisting end plate connections that used the Flowdrill blind bolting system for 200×200 SHS with thicknesses varying from 8 to 12.5 mm (b/tratio=16 to 25). From the studies of France et al. [6] on extended end plate connection, it was concluded that concrete filling the HS is the most effective means by which the connection strength and stiffness can be increased, followed by increasing the tube wall thickness, and lastly by increasing the steel grade.

While France et al. [6] and Mourad [2] demonstrated that the use of concrete infill is very efficient in stiffening the connection further, this is not considered to be practical in low rise construction due to the use of small column sizes as well as the need to limit the number of trades on site. Hence the discussion in this paper is limited to the use of unfilled columns. Larger scale moment-resisting connections to concrete-filled HS columns using Ajax blind bolts are already under investigation as part of a companion project by Yao et al. [8].

In this paper a T-stub connection (with T-stubs connecting the top and bottom flanges of the beam to the column) is investigated for smaller columns than the ones previously tested by other researchers. These smaller size columns are typical of those used in low rise residential and commercial structures. The connection relies upon commercially available Ajax ONESIDE blind bolts, a different type of blind bolt connector to those investigated previously. The paper aims to investigate the behaviour of the connection and explore parameters that affect its stiffness. Design models for strength and stiffness of the T-stub connection are also developed. Based on the experimental results, the behaviour of the connections in terms of stiffness has been assessed according to EC3 classifications and the strength of the connections has been compared with established design models.

The Ajax ONESIDE blind bolting system used in this paper is relatively new compared to other blind bolting systems in the present market. The ONESIDE bolt, as shown in Fig. 2 comprises of a circular bolt head (A), internal folding stepped washer (B), sleeve (C), external solid stepped washer (D) and nut (E). A special installation tool (F) is required to insert the blind bolt and tighten it. The main difference between the ONESIDE bolt and a standard 8.8/S bolt is the oversized bolt hole required to allow the folding washer (a washer split in half) to be inserted inside the hollow section with a special tool during installation.

The procedure used to install the ONESIDE bolt is illustrated in Fig. 3 [9]. Initially the bolt assembly is placed on the installation tool (3a). The bolt and the folding washer are then inserted through an oversized bolt hole; the folding washer is folded to clear the bolt hole (3b). Then, the insertion tool is rotated to unfold the folding washer and pulled back so that the bolt head and folding washer are bearing on the inside face of the tube wall (3c). The solid washer and nut on the external face of the tube are then pushed along the shaft of the tool and the nut is tightened (3d).

Section snippets

Design concept

The T-stub should be designed to transfer a limited amount of tension force in order to ensure that the column face deformation will not exceed 1% of the column width at the serviceability limit state and will be no more than 3% of the column width at the ultimate limit state as recommended by the International Institute of Welding (IIW) [10]. These deformation limits are likely to govern the maximum design force in the T-stubs rather than strength considerations. The experimental study

Test specimens

In total three T-stub specimens were tested and are described in this paper. Test specimens were constructed to simulate the tension (S1 and S2) and compression (S3) regions of a proposed T-stub connection. Details of the specimens are shown in Fig. 4. All specimens were identical except that bolt sleeves (component C in Fig. 2) were only utilised in specimens S2 and S3. All specimens were comprised of unfilled SHS columns with a cross section of 150×150×6 mm and endplate of 10 mm thickness.

Tension region

At low levels of applied load, the endplate started to deform upwards at the intersection with the T-stem, and the endplate separated from the tube face. As the load was increased, a larger separation was observed; the deformed profile of the endplate became a double curvature profile due to the clamping action provided by the snug-tight bolts. The bolts tilted away from the T-stem indicating that the bolts were subjected to lateral bending and shear as well as tension. There were significant

Finite element (FE) analysis

A three-dimensional detailed FE model using the general purpose software ANSYS to represent the blind bolted T-stub connection to an unfilled SHS column. The FE model takes into account material and geometric non-linearities. Complex contact interactions between the following elements were also included: endplate and tube face, external bolt washers and endplate, internal bolt washers and interior tube face. Additional contact surfaces were included for specimens S2 and S3: bolt sleeve external

Design model

In this section simplified design models are formulated for the T-stub connection in terms of strength and stiffness. In terms of strength, the possible failure modes for the T-stub connection are yielding of the column face, bending failure of the endplate, and bolt fracture. These are in addition to possible failure modes of the connection between the T-stub and beam (e.g. bolts in shear, ply bearing and tearing) which are well codified; hence they are not discussed here). A designer would

Summary and conclusions

T-stub blind bolted connections to unfilled SHS columns have been tested separately in both tension and compression. The experimental results were then used to validate detailed FE models. In addition design models were examined for application by designers. The findings from this work can be summarised as below:

  • (1)

    A new blind bolt, Ajax ONESIDE can be used to develop alternative bolted connections to welded connections in residential or low rise commercial frames utilising hollow section columns.

Acknowledgements

This research is supported by the Australian Research Council, Linkage Project No. LP0669334. The authors would like to acknowledge the invaluable contribution of the industry partners Ajax Engineered Fasteners and Australian Tube Mills. The authors also wish to thank Mr. Grant Rivett and Mr. Huang (Jack) Yao for their assistance in the tests and Mr. David Heath for conducting the photogrammetry surveys.

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