CROSS Safety Report
Metal cladding panels fail and fall to ground
During high winds, a number of metal cladding panels failed, became detached from a tall commercial building in a major UK city and fell to the ground. Subsequent surveys found a number of other panels were loose and could also become detached.
Key Learning Outcomes
For architects, engineers, and other cladding specifiers:
- Cladding design and installation should be given the same degree of attention as the primary structure to improve safety, reliability, and longevity
For civil and structural design engineers:
- Give attention to the whole design of cladding systems and the safety-critical aspects of their fixings and anchors
- Select the correct fixings for the given loads and environment
- Dynamic effects should be considered especially vibrations due to wind
- Wind can promote metal fatigue
- If possible, attend site to inspect the installation of cladding systems and their fixings
For construction professionals including cladding contractors:
- Adequate quality assurance and competent supervision can help to ensure that cladding systems are installed in accordance with the design
- Where complex cladding systems are being installed it is good practice that specific training for installation is provided
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During high winds, a number of metal cladding panels failed and detached from a tall commercial building in a major UK city and fell to the ground. Fortunately, no one was injured, but the incident could have caused serious injury or fatalities. Subsequent surveys found several other panels were loose and could also become detached.
No specific cause has been determined but it appears there was a flaw in the design, manufacture and/or installation of the sheeting. The slotted holes for the fixings were very close to the edge of the panel. The panel material was relatively thin. The failed panels exhibited failure of the panel between the fixing hole and the edge as shown in Figure 1 below.
The reporter says the panels were designed and manufactured by a large international cladding firm that carries out extensive work in the UK. The concern is therefore as to if these panels, or panels of similar design, are being used elsewhere. It also raises the question of whether adequate inspection of the works was carried out on site.
The reporter is concerned about public safety and the potential for fatalities. The reporter cites the following issues for consideration by those involved in manufacturing, specifying, supplying, and fixing cladding:
- Consider robustness in design
- Consider tolerances in manufacture and installation
- Undertake adequate site checking of works
- Consider the potential for very high wind loading
- Undertake adequate periodic inspection during cladding life
The reporter confirmed that the manufacturer has been made aware of the failures.
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Noting the position of the failed connection illustrated, the reaction on fixings mid-way along panel edges is commonly and grossly underestimated by the practice of dividing the total load by the total number of fasteners. If mid-span fixings fail, then other fasteners become overloaded. Such over-simplification is sadly common among inexperienced designers of cladding.
Expert Panel Comments
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The image suggests that the root cause of failure was an inadequate hole edge distance which would suggest one or more failures in specification, design, manufacture or indeed fixing of the sheeting. A specification failure could have occurred if the sheeting was used for an application for which it had not been designed – perhaps a location subjected to more severe wind loading than envisaged. An inadequate hole edge distance could leave insufficient material to resist the forces exerted by the fixing, whereas manufacturing errors or indeed tolerances could have placed the hole too near the edge. It could also be the case that an incorrect fixing or insufficient fixings were used which promoted local failure of the metal sheeting.
The failure could simply be a result of material overload due to inadequate hole edge distance, however, dynamic effects should not be ruled out. Thin sheets can vibrate, wind being one of the forcing mechanisms. Sheeting has very little inertia and almost no structural damping. Whilst fatigue from gust buffeting appears extremely unlikely, wind can in some circumstances induce dynamic effects in thin members for which they and their fixings may not be designed. If a panel oscillates (vibrates) in wind then it can build up a large number of cycles quickly. Moreover, it might oscillate in low winds and so be subject to oscillation bouts more often than otherwise considered. Fatigue may or may not have contributed to the stress overload. Cladding is often tested in a rig for water ingress, perhaps in some circumstances, it would be appropriate for the testing to include dynamic wind testing of the system including the panels and their fixings.
Vibration resistant fasteners
A generic hazard sometimes overlooked when considering the design or selection of fasteners is loosening under vibration. It is not unknown for threaded fasteners when subjected to a critical vibration to unwind. In one case, a whole section of tunnel lining fell out. The cause was the self-tappers unwinding themselves as an effect of pressure pulses from traffic going down the tunnel. Vibration resistant fasteners that won't work loose should be specified where necessary. CROSS Report 461 Metal cladding panels blowing in the wind details failure of cladding on a tall building brought about by the loss of fixings under vibration due to wind effects. Selecting the correct fixings for the given loads and environment is important to ensure they perform as expected.
Failures may be linked to manufacturing rather than design. Holes may have been made in the wrong place or manufacturing tolerances may allow holes to be made too close to the edge. Construction and manufacturing tolerances do need careful consideration to ensure all components meet stress and strain limits at adverse tolerances. Manufacturing processes can also play a part - slotted holes may be punched which can leave a hard brittle edge with micro-cracks and lower fatigue resistance.
Clearly correct site fixing of sheeting must also be assured. If the number of fixings completed correctly is less than required, then local failure at one or more fixings would be possible. It is essential that there is adequate supervision of subcontractor activities to ensure satisfactory execution of works. Arrangements for the inspection and acceptance (including inspection and testing plans) of sub-contractor packages should be carefully considered and agreed upon. Consideration should be given to the provision of independent supervision for all aspects of the construction process for safety critical fixings. Where complex cladding systems are being installed, it is good practice that those undertaking the installation have specific training for the system to be used.
provision of independent supervision for all aspects of the construction process for safety critical fixings
Where bespoke cladding systems are proposed, there are well established test procedures that can be used - see for example the work of the Centre for Window and Cladding Technology (CWCT). The effects of wind, water and impact loads can be assessed. Some tests can give information about short-term structural performance, perhaps by highlighting obvious oscillations.
CROSS recommends that cladding design and installation be given the same degree of attention as the primary structure during both design and construction to improve safety, reliability and longevity.
References that readers may find helpful include: