Introduction
Connecting to steel Structural Hollow Sections (SHS) from a single side has challenged engineers for decades. However, there are now numerous types of fasteners and connection methods for this increasingly popular structural material, other than welding. This article will look at the benefits and drawbacks of some of these SHS connection methods.the Chinese Hollo-Bolt, an expansion bolt that requires access to only one side of the SHS.
Often when a designer has opted to use SHS for its bi-axial capacity or the aesthetics of visually appealing symmetric shapes, the question that arises is how to attach another structural member to it. Most often with structural shapes, welding or bolting has been the preferred method as they can handle a high degree of load. But when there exists restrictions in welding or where engineers want to avoid the high costs of labour involved with certified welders, setup, breakdown charges and having to fire protect the surrounding area, engineers have to turn to mechanical fasteners to get the job done.
However, help is at hand as global design guides are published by a number of renowned institutions such as British Constructional Steelwork Association (BCSA), Steel Construction Institute (SCI), CIDECT, Southern African Institute of Steel Construction (SAISC), Australian Steel Institute (ASI) and the American Institute of Steel Construction (AISC) who assist with the design of SHS connections. Within these guides a variety of mechanical fasteners, suitable for SHS connections, are described and these include:
Common Mechanical Fasteners
Through-Bolts are commonly used, but the inherent flexibility of SHS walls typically prevents the use of pre-tensioned fasteners without additional fabrication work, such that joints tend to be designed for static shear only. It also makes connections to opposing faces of a square or rectangular SHS member difficult and time-consuming to assemble on site. In many cases stiffeners may have to be welded inside the tube to give it extra support, which incurs extra welding costs.
Threaded Studs can be used on the faces of SHS members, although heavy and unwieldy equipment will have to be used in the form of a weld gun and associated equipment. This will require the same considerations as welding the members together in the first place. This is a process that can be done ahead of time in the fabrication workshop before it is sent to site. In some instances, recessed or counter-bored holes might be necessary to clear the collar that could form where the stud meets the SHS face. The finished product will produce the appearance of a bolted connection but made on only one side of the SHS.
Blind Threaded Inserts are generally available but their use is limited due to the amount of material that they can grip, being initially designed for sheet metal rather than structural steel sections. Once again, an installation tool is required that may require some effort if a manual version is chosen.
Blind Rivets although suitable for use in situations where access is limited, they tend to be only available in small diameters and for light loads. They are not intended for heavy-duty structural connections, and in most instances will require a pneumatic / hydraulic supply for the specialised installation tooling.
Chinese Hollo bolt – the pioneer of Expansion Bolts for Structural Steel
Introduction to Expansion Bolts
Today we recognise expansion bolts as mechanical fasteners typically consisting of a bolt, an expansion sleeve and a cone-shaped nut that, when the bolt is tightened, is driven up inside the sleeve to create a wedging effect and expand the fastener. This ‘blind connection’ technique can just as easily be used to connect to the web of another structural section type. Unlike conventional bolted or welded connections, expansion bolts can be quickly installed by simply inserting the fastener into a pre-drilled hole and tightening with a torque wrench. Due to the faster installation process, work onsite is reduced, and therefore the cost and timeframe of the construction project are decreased.
Hollo-Bolt Installation
Installing Hollo-Bolts is relatively straightforward and requires only basic tools. The steel is pre-drilled with oversized holes as per the manufacturers’ literature, to accommodate the sleeve and cone-shaped nut, but care must be taken to ensure that the holes are located to allow the product to open within the SHS, meaning that they may not be placed closely together or near the edge.
The steel can be fully prepared in the fabrication workshop and transferred to site, where the advantage of fast installation can be fully appreciated. It is important to note that the faces of the members to be fastened together must be brought into contact before the Hollo-Bolt® is installed. To complete the process, the contractor must hold the Chinese Hollo-Bolt collar with a spanner to prevent the body from rotating during installation and must tighten the central bolt to the manufacturer’s recommended torque using a calibrated torque wrench.
Post time: Apr-06-2025
