Introduction

Connecting to steel Hollow Structural Sections (HSS) 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 HSS connection methods and compare them against Lindapter’s alternative solution, the Hollo-Bolt®, an expansion bolt that requires access to only one side of the HSS.

Often when a designer has opted to use HSS 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 labor 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 HSS connections. Within these guides a variety of mechanical fasteners, suitable for HSS connections, are described and these include:

Common Mechanical Fasteners

Through-Bolts are commonly used, but the inherent flexibility of HSS 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 HSS 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 HSS 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 HSS face. The finished product will produce the appearance of a bolted connection but made on only one side of the HSS.

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.

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.

Early Lindapter Product Developments

Lindapter was at the forefront of early expansion bolt product developments, one of the first such fasteners to become widely available was the Lindibolt® a threaded stud expansion bolt, launched by Lindapter in 1948. The Lindibolt® was designed to provide a threaded stud that protruded from the HSS to provide an attachment point for lifting equipment during marine salvage operations; Lindibolt® was a huge success and, being still available, remains a popular component within the offshore industry to this day.

Although initially developed in the 1950s, the widespread usage of HSS did not start until the mid-1960s and since then, its popularity has continued to increase, with many of today’s contemporary designs making a feature of exposed structural steel profiles to enhance the aesthetics of a building. During the 1980s, various shapes and sizes of HSS became available and engineers needed a more versatile connection. This led to Lindapter developing a connection product in 1986 that had a fairly similar design to today’s Hollo-Bolt. This product was a two-part assembly consisting of a slotted cylinder and cone, the latter threaded to receive a standard high tensile fastener. while popular, system installation was not always easy; errors could arise when inserting the product into the tight ±0.2mm tolerance hole. Working overhead and/or with the larger sizes of square and rectangular HSS, the required hammer blows could cause chord face flexure and spring-back, sometimes producing incorrect installation of the product and possible hole damage. The use of podger spanners to align beam end plates and column hole centres during steelwork erection was restricted, as the product could easily be displaced.

Modern Day Hollo-Bolt

The knowledge gained by Lindapter from several decades, combined with continuous direct contact with evolving on-site practices, formed the basis for the current designs of Hollo-Bolt® expansion bolts that are designed to satisfy the needs of steelwork contractors in terms of ease and speed of installation. They also provide some tolerance to site abuse, for example hole size and misalignment, providing the construction industry with a robust yet aesthetically pleasing system. Hollo-Bolts are manufactured in carbon, alloy or stainless steel and are produced with a variety of finishes, meaning that they can be used in almost any application: Zinc Plated (JS500) for standard use, Hot Dip Galvanised (HDG) where a more robust product is needed, and Stainless Steel for the most demanding environments.

Hollo-Bolts are suitable for structural connections due to their capability to resist loading in both tension and shear. For example, the ³⁄₄" Lindapter Hollo-Bolt has a safe working load of 7868 lbs in tension and 8992 lbs in shear. Product sizes range from  ⁵⁄₁₆" to  ³⁄₄" and the total material thickness clamping range is from  ¹⁄₈" for the smallest diameter to 3 ³⁄₈" for the largest diameter.

Research & Development

Lindapter has continuously developed the Hollo-Bolt® range to meet the diverse requirements of structural engineers and architects. For example, there is now a flush fit head variant that leaves no protrusion above the surface of the steel section. Recent performance optimisations for structural connections include a mechanism that compresses during installation and provides a High Clamping Force, resulting in a more secure connection.

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 HSS, 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 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.

Designing with Hollo-Bolts and Approvals

When Hollo-Bolts are used for structural steel connections then they are proprietary products, and the Lindapter catalogue or datasheets must be referred to for the strength of each specific Hollo-Bolt. It is the designers responsibility to check the suitability of the HSS member from the applied loads imposed on it.

The Green Book – simple joints to Eurocode 3 and other design guides published by leading authorities include details and methodology for the use of expansion bolts in steel connections and the shear and tensile resistances. 

Hollo-Bolts have been independently verified by various approval bodies including the TUV in Europe and have gained CE mark approval which required an independent certifying body to oversee the testing of the Hollo-Bolts with the results used to calculate characteristic resistances. Those resistances are used when designing a connection to Eurocode 3.

In addition to these European approvals, Hollo-Bolt® (Hot Dip Galvanized) is approved by North America’s leading approval body, the International Code Council Evaluation Service (ICC-ES), for use in resisting wind loads and for use in all Seismic Design Categories A through F, in compliance with the International and Los Angeles Building Codes.

Summary

As this article has shown, there are many options available when connecting to HSS and therefore, the designer or engineer will have to carefully select the most appropriate connection method for their application. The relatively recent acceptance of HSS as a construction material and subsequent increase in usage for structural frames has led to the development of innovative structural fasteners such as the Lindapter Hollo-Bolt® as a true alternative to conventional methods such as through-bolting or welding. Likewise, the trend for construction developers and contractors to build structures in incredibly short timeframes has also contributed to an upsurge in demand for Hollo-Bolts for structural steel connections, as it is specifically the speed of installation that is the fundamental benefit of the Hollo-Bolt®.

When engineers are faced with a situation that requires a construction project to be completed within an extremely short timeline or if there are restrictions in welding, a designer will need to look at alternative connection methods. Hollo-Bolts for structural steel are the perfect choice for making structural connections to HSS. With their capability to handle much higher loads than other single-sided fasteners, such as rivets, and the fact that they can be installed onsite using standard hand tools, unlike threaded studs, you may find yourself taking a serious look at Hollo-Bolts.