Can BlindBolts be Used as Preloaded Fixings?
Blindbolt UK | 16th November 2023
Preloaded fixings are used in non-slip connections. Although the fixings are installed in clearance holes (usually 2 mm larger than the bolt diameter), the bolts are preloaded so that the shear load is transferred by friction between the clamped plates, not by the bolts bearing on the edge of the hole. The preload means that the joint does not move at all – described as “non-slip” in BS EN 1993-1-8.
Why Preload to Stop Slip?
For the vast majority of connections, a very small movement is of no significance, and most movement will take place during erection and the construction loads, as the bolts move to the sides of the clearance holes. This initial movement is sometimes referred to as “shakedown”. Under subsequent load, there may be a very small amount of additional movement as the bolts bear on the steel plates, which deforms the plate and the bolts very slightly.
In some situations, non-slip joints are recommended – if slippage would lead to deflections which might lead to additional forces as well as looking unsightly. Site bolted connections within large trusses are often non-slip, as movement within the joints may lead to significant deflections – for the truss and everything it supports. Splices in moment-resisting connections are sometimes designed as non-slip, to ensure the splice maintains the member alignment.
The Preload Force
The preload force introduced into a fixing in a non-slip connection is considerable. As an example, the minimum preloading force for a Property Class 8.8 M20 bolt is 137 kN. This is only a little less than the design tension resistance.
A common misunderstanding is to assume that if the joint is then loaded, the bolts will receive even more tension, and may fail. In fact, as preload is applied, the connected plates are compressed. When the joint is loaded, the compression is relieved – the bolt does not experience any significant additional tension until the applied force exceeds the preloading force.
The high preload force means that the bolt will have been strained beyond its elastic limit, which means that discarded preloaded bolts should never be reused – once they have been preloaded it is likely that they will have less reserves of ductility.
Fixings suitable for preloading are usually a little different to ordinary fixings. Preloaded bolts are usually manufactured to a different standard, and often have rather thicker nuts. A preloaded bolt may be used as an ordinary bolt, but not the other way around.
The special fixings suitable for preloading used to be known as “High Strength Friction Grip” or “HSFG”, but are now known as preloaded assemblies, used in Category B and Category C slip-resistant connections according to BS EN 1993-1-8.
Applying the preloading
There are a number of ways to apply the correct preload. A calculated torque may be applied to the fixing, or a specified number of turns after initially tightening the bolt, or by the use of special equipment.
The shear force which can be transferred by a preloaded bolt depends on the preload, but also on the coefficient of friction between the mating surfaces (often called the “faying” surfaces in the UK). In design codes, coefficients are given for various surface finishes, ranging from bare steel to different coatings. The highest coefficients are with bare steel, so usually the faying surfaces are protected and have no coating. If faying surfaces are coated, the friction coefficient is much lower – the coating may have to be removed if the designer has assumed bare steel.
Other Reasons to Preload
Non-slip connections are less prone to fatigue, so preloaded assemblies are generally used in bridge steelwork and similar structures. If a structure is subject to vibration, some designers require the fixings to be preloaded to stop the fixings from working loose over a long period of time.
The Cost of Preloading
Specifying preloaded assemblies and non-slip joints will lead to a more expensive joint than a joint with ordinary fasteners. More work is required in the workshop to prepare the faying surfaces and then to mask them until the steelwork is ready to be erected. The fixings themselves are more expensive than ordinary bolts. Installation takes longer, must be carefully controlled and requires special equipment. Despite these additional costs, non-slip connections should certainly be specified when the consequences of slip must be avoided.
Preload and BlindBolts
BlindBolts are not intended for use in non-slip connections – they have been developed and tested as non-preloaded bolts for use in clearance holes – Category A connections in BS EN 1993-1-8. Because of the reduced cross-section of a BlindBolt and the additional material removed for the pivot of the toggle, a BlindBolt will have different behaviour at high strains, compared to an ordinary bolt.
A small preload is introduced when BlindBolts are tightened to the recommended torque given in the technical data sheets. This preload is not significant – it is much less than the preload required for a non-slip connection. The tightening torque of an M20 BlindBolt produces a preload of approximately 12 kN.
BlindBolts may be used in joints where vibration is expected. Instead of preloading the fixing, a nut with a nylon insert, or the use of a second lock nut is recommended.
Alternatives to preloaded assemblies
If slip is to be avoided, the obvious alternative is a welded joint. A second alternative is the use of close tolerance holes, which are drilled just very slightly larger than the bolt, meaning there is no clearance which would allow movement. The lack of clearance also means a lack of tolerance, so joints made in this way are either formed in the workshop, marked, disassembled and then re-assembled on site, or on-site the joint is assembled and temporarily held whilst the holes are drilled through all the plates in situ.