Blind bolted moment connection to sides of hollow section columns

https://doi.org/10.1016/j.jcsr.2011.06.007Get rights and content

Abstract

This paper presents the results of an experimental programme for a new blind bolted connection to unfilled hollow section (HS) columns under static loading. In this connection, channels with side plates connect beam flanges to the side face of HS columns. This is a simple alternative connection to typical face connections, i.e. the extended endplate connection or the T-stub connection (with T-stubs connecting the top and bottom flanges of the beam to the face of the column). The channel side plate connection was found to be much stiffer than a typical face connection and has the potential to achieve a rigid connection status for braced frames according to the EC3 classifications. Comparison of the test results and three dimensional finite element (FE) modelling indicates that the FE analyses can be used to predict the connection behaviour with sufficient accuracy. The preliminary investigation for the channel side plate connection shows promising results and forms the basis for the development of other blind bolted side connections for applications in the low rise construction industry.

Research highlights

► Blind bolted side connection developed as an alternative to a typical face connection. ► Side connection found to be much stiffer compared to a typical face connection. ► Has the potential to achieve rigid status for braced frames based on EC3.

Introduction

The application of hollow sections in low rise structures has traditionally been hampered by access constraints to provide for a fully bolted connection. This may now be overcome by using a blind bolting system requiring access to one face only. The most common types of blind bolted moment connections are the extended end plate connection and the T-stub connection which are connected to the face of the column, also referred to herein as face connections. Recent studies by Lee et al. [1] show that increasing endplate thickness for a typical T-stub connection can evidently enhance the stiffness of the connection. However this is only true up to a certain endplate thickness beyond which the stiffness of the connection is dominated by the flexibility of the column face. The inherent flexibility of the hollow section (HS) column face renders the face connection semi-rigid.

Early investigations of framing connections between I-beams and SHSs were carried out by White & Fang [2]. Five different types of simple connections between I-beams and SHSs were tested. White & Fang [2] found that the large deformation of the column face would reduce the stiffness of the subassemblage significantly. A suggestion was made to concentrate welding near the corners of the HS to avoid excessive deformation and hence enhance the stiffness of the connection.

Giroux and Picard [3], [4] built on the concept of transferring loads from the beams close to the corners of HS columns to prevent excessive column face deformation. They developed the coped strap angle connection as shown in Fig. 1 which connects I-beams to the sides of a SHS column. It was found that the strap angle connection was not as stiff as a fully butt welded connection between I-beams and H columns but the connection had considerable stiffness and was able to carry the full plastic moment of the connecting beams with adequate rotation capacity.

In recent years Kumar & Rao [5] developed a RHS (rectangular hollow section) beam to column connection. The RHS beam was bolted to a channel and the channel was welded to the sides of the RHS column as shown in Fig. 2. As blind bolts were not used in this connection, a web opening was introduced to provide access to tighten the nut of a conventional bolt inside the RHS beam. This highlights the convenience of using blind bolts. The channel connectors transferred stresses from the beam flanges to the column webs directly. The connection was totally independent of the column face thus avoiding flexible column face deformation.

Even though connecting to the sides of the HS has the added advantage of increasing the strength and stiffness of the connection, there are no such standard connections developed or in use in current practice. To date, there has been no research reported on blind bolted connections attached to the sides of a HS column. This paper explores the feasibility of connecting to the sides of a HS with blind bolts as an alternative to the T-stub or extended end plate connection in order to achieve a connection with a higher stiffness compared to typical face connections.

The channel side plate connection proposed in this paper (refer Fig. 4) incorporates a SHS column, open section beam, side plates which are cut from standard flats and channels which are cut from the same type of SHS column. Channels are first connected to the top and bottom flanges of the open section beam with standard structural bolts. Straps or side plates are then bolted to the legs of the channel sections with standard structural bolts on one side. On the opposite side, the straps or side plates are first blind bolted to the SHS column. The blind bolt heads are inside the SHS tube. Finally the various components are brought together and bolted in place (refer Fig. 3). This configuration uses only bolts on site with no need for welding at all. All bolts must be fully pretensioned to achieve good initial stiffness for the connection.

In order to assess the feasibility and performance of such a connection, a full scale connection test was carried out as outlined in Section 2, with the results described in Section 3. An FE model of the connection was developed to predict the behaviour and also undertake a sensitivity analysis as detailed in Section 4. Finally a design model was developed for this connection to estimate its strength and stiffness as illustrated in Section 5.

Section snippets

Test specimen

A full scale beam-to-column channel side plate connection was developed and tested as described in this section. The specimen length was determined based on an assumed contraflexure length of 0.15 L for a 6 m beam length. In practice under gravity loading, the joint is subjected to shear and moment.

Details of the specimen are shown in Fig. 4. The specimen comprises a Grade 300, 310UB32 universal beam (approximately 310 mm deep, 32 kg/m section) connected to a 150 × 150 × 6 mm Grade 350 SHS column. The

Specimen behaviour

Loading was gradually applied to the test specimen at the end of the cantilever beam up to failure. As the load was increased, the beam started to tilt in the direction of the applied load. Slip occurred at an applied load of approximately 35 kN as can be observed from the graphs in Fig. 6, Fig. 7, Fig. 8. However, the connection started to become more flexible at a load of approximately 20 kN. At higher loads, the column started to deform in bending; bearing of the beam flange against the face

FE model setup

A three-dimensional finite element (FE) model was created using the general purpose software ANSYS to represent the tested connection. The FE model takes into account material and geometric non-linearities and complex contact interactions between the various elements. Surface to surface contact elements with friction coefficient, μ, of 0.15 (which is the commonly adopted μ for painted surfaces) were employed in the FE model. This was also validated by separate friction tests.

Taking advantage of

Design model

In this section, simplified design model for the channel side plate connection in terms of strength and stiffness are discussed. A sensitivity analysis to investigate the influence of connection stiffness on an overall frame stiffness is also carried.

Summary and conclusions

A new bolted moment resisting connection to hollow section columns utilising blind bolts and side plates has been proposed and tested. Detailed FE model and design model have been developed for this connection. The findings from the test and FE modelling may be summarised as follows:

  • 1)

    The results show that the channel side plate blind bolted connection has the potential to be classified as a rigid connection for a braced frame system according to the EC3 specifications, depending on the stiffness

Acknowledgements

This research is supported by the Australian Research Council (ARC Linkage Project No. LP0669334). The authors would like to acknowledge the invaluable contribution of the industry partners Dr Saman Fernando from Ajax Engineered Fasteners (the inventor of the original blind bolt technology) and Mr. Hayden Dagg from Australian Tube Mills. The authors also wish to thank Mr. Rodney Wilkie and Dr. Huang (Jack) Yao for their assistance with testing and Mr. David Heath for assisting with the

References (13)

  • J. Lee et al.

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

    J Constructional Steel Res

    (2010)
  • R.N. White et al.

    Framing connections for square structural tubing

    J Struct Division, Am Soc Civil Eng Proc

    (1966)
  • Y.M. Giroux et al.

    Moment connections between wide flange beams and square tubular columns

    Can J Civil Eng

    (1976)
  • Y.M. Giroux et al.

    Rigid framing connections for tubular columns

    Can J Civil Eng

    (1977)
  • S.R. Kumar et al.

    RHS beam-to-column connection with web opening — experimental study and finite element modelling

    J Constructional Steel Res

    (2006)
  • Ajax Engineered Fasteners

    Australia

There are more references available in the full text version of this article.

Cited by (49)

View all citing articles on Scopus
View full text