There are basically 3 means of connecting steel hollow sections, which include:

    1. Welding
    2. Through-Bolting, and
    3. Blind-Bolting

Welding is usually restricted to workshop joints and applied mainly in the fabrication of trusses, lattice girders and sub-assemblies.

For more information on this go to Welded Joints

Site welding should be avoided – it’s costly, often difficult and there can be no guarantee of the weld quality, except in circumstances where quality control can be implemented.

Through-Bolting makes use of standard nuts and bolts and is probably the most economical in terms of fabrication and erection. Such connections make use of End-Plates, Fin-Plates and Angle-Cleats which protrude beyond the outer surfaces of the hollow section and are tightened in the usual manner.

These are ideal for site connections as they generally provide easy access and are, to a great extent, quite forgiving - in that they allow for the use of shim packs and any slight misalignment can be rectified by the judicial use of a podger.

Structural Detailer-Bolting Options_1a

Other means of using through bolts include:

  • Drilling a hole through both faces of the supporting member and inserting a long bolt. For wide hollow section profiles, it may be necessary to weld ferrules to prevent the walls of the profile being distorted by tightening the bolt. Using a thicker wall would alleviate the problem but it will significantly increase the cost to what is already a costly connection.
  • Cutting a hand hole and closing it with a bolt-on cover-plate is also an option, but it will weaken the profile significantly and should only be used under the direction of the responsible engineer.

Blind Fastening requires access from only one side of the joint and results in a neat compact connection which can enhance the aesthetic qualities of hollow section profiles. They do require a high level of workshop accuracy as there is little room for any adjustment on site as rectification work can prove difficult and costly.

There are a number of options for blind fastening, the first being to drill and oversize hole in the supporting member and then insert and weld a nut into the hole. The connection is made by offering up the incoming member and tightening the joint from the outside.

Another method is to drill and tap a hole into the supporting member, making the connection as before. This method however will only work if the wall thickness is sufficient to provide sufficient load resistance. There is a process developed by Flowdrill - This process yields a threaded hole that has dimensions and hardness properties similar to that of a heavy hex nut. (For more information on this see the link at the bottom of this post)

Alternatively, you could use one of the proprietary fasteners available.

Structural Detailer-Bolting Options_2

Proprietary Fasteners

There are a variety of proprietary fasteners available which come in many shapes and sizes – there is no empirical way to determine load values for these products, so it’s necessary to refer to the manufactures literature where the published values have been established by testing and statistical analysis.

This has led to the publication of International Code Council’s (ICC) evaluation reports. These independent reports verify third party testing and provide applicable load values that are consistent with the intent of the International Building Code (IBC).

The following is a reference to some of the proprietary fasteners available:

Hollo-Bolts by Lindapter - The connection is made by inserting the Hollo-Bolt into the pre-drilled hole of the supporting hollow section. As the bolt head is tightened, the cone is pulled up the bolt thread, causing the sleeve to expand until the cone locks the sleeve against the hollow section's inner wall.

For more information on Hollo-Bolts visit the manufacturers website at:

Box-Bolt – which is also an expansion wedge bolt which in similar in operation to the Hollo-Bolt

For more information on Box-Bolts visit the manufacturers website at:

Blind-Bolt – The proprietary Blind Bolt fastener is an internal toggle bolt fastener which is inserted into the pre-drilled hole. Once placed, the toggle is rotated inside the hollow section forming a firm lock – the bolt is then tightened from the outside.

For those of you confused by this rather vague description, visit the manufacturers website at: where all will become clear.

Ajax One-Side Fasteners – A special installation tool inserts both the head of the bolt and the split washer through the bolt hole and then swings the split washer into place between the bolt head and the inside face of the member, allowing tightening only from one side.

For more information on Ajax Fasteners visit the manufacturers website at:

Huck Blind Fasteners – Huck fasteners require a special tool to install. The nose assembly of the tool grips the pintail and pulls the head toward the sleeve, causing the sleeve to expand on the blind side of the hole. This pulls both sides of the joint together and breaks the pintail flush with the head.

For more information on Huck Blind fasteners visit the manufacturers website at:

Flowdrill Process – This is a process that creates a threaded hole in a hollow section wall.  The process forces a hole through the HSS wall that forms an interior truncated hollow cone and hardens the steel.  A tap is used to create the threads after the drilling is complete.  The process yields a threaded hole that has dimensions and hardness properties similar to that of a heavy hex nut.

For more information on the Flowdrill process visit the manufacturers website at:

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