CROSS Safety Report
Nuts falling from tension glazing system
This report is over 2 years old
Overview
A reporter raises concerns about the fixings used in a tension glazing system.
Key Learning Outcomes
For civil and structural design engineers:
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Fixings between glass and other materials can be critical and often flexibility between them is required to allow for different expansion rates and other minor movements
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Adequate fixings should be used and where vibration is a potential risk, design measures to stop loosening should be carried out
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The Construction Industry Research and Information Association (CIRIA) guidance provides general advice on glazing at height – Guidance on glazing at height (C632F)
For the construction team and clients:
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Glazing should be installed as per the manufacturer’s instructions. If there are any uncertainties, seek advice for the supplier.
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Regular inspections to check safety-critical fixings of glazing units are beneficial to ensure fixings have not become loose
Full Report
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A reporter says that they were asked to look at an office building containing glass curtain walling where a steel nut had fallen from the walling into the atrium area below. The walling comprised up to four storeys of large glass panels supported by secondary steelwork running vertically. The secondary steelwork was formed from tension rods arranged in X-shapes stacked vertically and screwed in to cast components.
These cast components in turn connected to clamps to the corners of each glazing panel. The frictional properties of nuts, says the reporter, will depend on the materials and finish so for example polished stainless could have very low friction compared to standard nuts. They suggest that the friction between nuts is not something structural engineers normally consider even when different materials are used.
In this case a liquid locking compound had been used on the threads to aid to aid security. The nut which had fallen was an 'acorn-shaped' locknut for the clamps into the glass. The nuts are much larger than a normal nut with a pointed end (the thread stops within the nut). Eventually if not arrested, the nuts falling would have led to the very large glazing panels falling out.
The inspection revealed that some of the locknuts, some clamping nuts, and also some of the secondary steelwork rods had worked loose, presumably under wind induced vibration. There was a maintenance requirement in the operation manual that the rods (only) be visually inspected for looseness and a suggestion that the system requires annual adjustment. Although this had not been carried out, a visual inspection for looseness, says the reporter, would in any event be inadequate.
The inspection revealed that some of the locknuts, some clamping nuts, and also some of the secondary steelwork rods had worked loose, presumably under wind induced vibration
They consider the design inadequate for its purpose. Structural members should not come undone under vibration (BS5950-1:2000 cl6.1.7.1), and this should apply equally to secondary members. If there is any such possibility, then the O&M manual and the H&S File should be much clearer on the matter. The reporter is concerned that there are many similar buildings which may be suffering from the same issues.
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Expert Panel Comments
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This is a problem which is not commonly appreciated and about which there is little technical information. In certain circumstances fastenings and fittings can work loose under even modest vibration. The vibration source may be wind fluctuation or from traffic or other source.
In mechanical engineering, various proprietary retention devices are common place but not so much so in structural engineering. Where vibration is a potential risk, design measures to stop loosening should be carried out. These include:
Spring Washers
PAL Nuts (fitted on the bolt thread after the nut and is screwed up tight against the nut)
Lock Nuts (with the thinner lock nut going on first)
Nylon insert lock nuts can also be effective and add a degree of damping to high frequency movement. Preloaded bolts can be a good answer but these require a significant construction control requirement.
Reliance on maintenance procedures for structures can be uncertain as these may not be carried out properly. It is usually better to ‘design out’ problems in the first place. The consequences of a nut falling are not trivial. Items falling from height have caused severe injury and missing nuts may result in a loss of member stability leading to progressive collapse.
Reliance on maintenance procedures for structures can be uncertain as these may not be carried out properly. It is usually better to ‘design out’ problems in the first place
In addition to the factors related to vibration there is a key issue associated with load reversal in terms of the loads experienced by a bolt group in the connection itself. If the loading actually experienced by the group moves from one vector direction to its (near) opposite, then slip will conceivably occur on every load reversal.
The forces involved are then quite capable of not only causing this repeated slip but also eventually loosening every fastener assembly in the bolt group and then providing a force to not only drive the nuts off the bolts but to drive the bolts out of their holes. BCSA are considering this matter with a view to publishing an Advisory Desk Note.