CROSS Safety Report
Blind bolt query
This report is over 2 years old
Further to report 71 regarding ‘blind bolts’ a query was received as to how to include an appropriate clause in a specification for structural works.
Key Learning Outcomes
For civil and structural design engineers:
Connections can often be the weak link in structures and attention to detail is required to ensure what is designed can be fabricated
Careful consideration is required for connections, particularly at interfaces between different materials. The role of tolerances should not be overlooked.
Helpful guidance can be found in the alert that has been published by the Standing Committee on Structural Safety (SCOSS) - The selection and installation of construction fixings
The Steel Construction Institute has published a document AD 276 entitled 'The use of Toggle Bolts' which provides advice on these type of fixings
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Further to report 71 regarding ‘blind bolts’ a query was received as to how to include an appropriate clause in a specification for structural works. Clarification was requested on what type of connector does the report actually refer to as the description ‘blind bolt’ can mean a number of things e.g.:
Sleeved bolt with expanding mechanism at the end
A bolt with the locking pin mechanism
A threaded stud or section of threaded rod that is welded to one element in order to receive say an endplate of another element to be fixed with the nut
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The reports that have been received by CROSS are about bolts with a locking pin mechanism in a slot in the shank of the bolt. There were two separate problems. In the case that you refer to in report 71 the bolts fractured due to incorrect hardness.
In another case bolts failed in shear because the joint between two steel members was in the plane of that part of the shank of the bolt where there was a reduced section due to the presence of the slot. Problems with fixings generally have been publicised more recently in CROSS Newsletter No 10 on ceiling collapses, and by the Construction Fixings Association (The Structural Engineer 6 May 2008).
An industry wide alert has been published by the Standing Committee on Structural Safety (SCOSS) on 'The selection and installation of construction fixings'. Where possible, fixings should comply with a British Standard or another recognised code. The Steel Construction Institute has a document AD 276 entitled 'The use of Toggle Bolts' which give some advice, while The Construction Fixings Association publishes material on fixings to concrete, brickwork, blockwork, and stonework.
Concerns with fixings can arise if they are selected fairly far down the construction chain and are not given the attention that they deserve. Another issue is that specified fixings are replaced by other types on site, again without an appreciation of their importance, possibly for reasons of cost or convenience. The best practice is to make sure that the engineer designing a structure chooses fixings that are up to the job and then checks that they are properly installed on site.
Another issue is that specified fixings are replaced by other types on site, again without an appreciation of their importance, possibly for reasons of cost or convenience