4 Ways to Join Steel with One-Sided Access
Blindbolt UK | 21st December 2021
Several techniques can be used to connect steel components together when access is only possible from one side – typically connections to structural hollow sections or when access to both sides of a connection is limited for some other reason. Welded connections may be used, but can be costly when preparation, provision of safe access and remedial work after welding is required. This article discusses the use of four connecting devices which may be considered as an alternative to welded joints.
Drilling and subsequently tapping holes to create a threaded socket in the supporting steel member is one solution, which may be appropriate for fasteners acting as dowels (i.e. subject only to shear). Standard nuts are generally around three times stronger than normal steel plate or section, so if the fixing is in tension, there is an increased risk of thread stripping in the weaker material with the tapped hole.
Some engineers insist on a minimum thickness of 16 mm before considering a tapped hole, and may prohibit the approach if the fastener is in tension. Tapped holes are obviously formed in precise locations, so some of the tolerance associated with ordinary bolts in clearance holes (usually 2 mm larger than the bolt) has been lost.
A blind rivet may be used for one-sided applications, but their use is limited in constructional steelwork applications. Rivets may be found in other applications, typically connecting relatively thin sheets to supporting members.
A tubular rivet with a central pin is inserted into a hole drilled through the elements to be connected, and the central mandrel pulled back through the tubular rivet with an installation tool. A bulbous end on the mandrel splays the rivet body and forms the rivet head on the inaccessible side of the connection before fracturing under the applied tension. Rivets are often manufactured from aluminium, but can also be obtained in stainless and carbon steel. Blind rivets are intended as a permanent solution – disassembly involves drilling off the rivet head.
Hollo-bolts are an expansion anchor with a tubular external sleeve and threaded internal bolt. As the bolt is tightened, a cone-shaped nut forces the legs cut in the sleeve to deform, expanding on the blind side of the connection. Tightening to a recommended torque is important, to ensure the legs have deformed sufficiently. The bolt head is turned whilst holding a collar formed as part of the sleeve.
Hollo-bolts have an excellent shear resistance, as the shear plane passes through both the bolt and the sleeve. The sleeved arrangement means that relatively large holes are required in the joint, which increases the minimum distance between fasteners. Joints assembled with Hollo-Bolts may be disassembled, but this involves prying the assembly out of the hole, forcing the deformed sleeve legs back together.
Hollo-bolts are available with a range of visible head types and with a range of corrosion protection coatings. Stainless steel assemblies are also available. Hollo-bolts are suitable
BlindBolts have a gravity-operated anchor which initially sits within a slot machined in the bolt shank. Once the bolt is inserted in the connection and rotated, the anchor on the inside of the connection rotates to be perpendicular to the axis of the bolt, allowing the nut to be simply tightened from the outside. Identification marks and a setting tool are available to demonstrate the anchor has correctly rotated. BlindBolts are designed to transfer load in shear and bearing (Class A in EN 1993-1-8) and applied tension (Class D) as non-preloaded assemblies. BlindBolts are used in standard oversize holes and may readily be removed. Like Hollo-Bolts, BlindBolts are suitable for joints in constructional steelwork.
Are Blind Bolts the Right Fastening Solution to Join Steel with One-Sided Access? Find Out by Checking Out Our Guide
Would blind bolts be the right fastening solution for you? Learn more by checking out our helpful guide here.
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