CROSS Safety Report
Metal cladding panels blowing in the wind
This report is over 2 years old
A local authority was called to investigate a dangerous structure where metal cladding panels on the 10th floor of a student accommodation block were flapping in high winds.
Key Learning Outcomes
For civil and structural design engineers:
Selecting the correct fixings for the given loads and environment is important to ensure they perform as expected
Dynamic effects should be considered especially where vibrations due to wind are expected
Cladding design and installation should be given the same degree of attention as the primary structure during both design and construction to improve safety, reliability and longevity
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A local authority was called to investigate a dangerous structure where metal cladding panels on the 10th floor of a student accommodation block were flapping in high winds. The building had been constructed two years previously and had been subject to a building control check on both calculations and site work.
The cladding panels were of aluminium (3-4mm thick) which were folded at the edges and fixed into a supporting frame using 24mm long self-tapping screws fixed on site. There were a large number of fixings per panel, and subsequent investigation found the calculations to be adequate for standard design wind loading.
However, the cause of failure was identified as flexing of the panels under the fluctuating wind load which led to prying forces in the fixings for which they had not been designed. This demonstrates the need for fixings and systems to be checked for dynamic and quasi-dynamic forces, and that thin panel systems would be better stiffened in some way.
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There have been previous similar incidences and a very early CROSS report (report 7) cited a failure of cladding panels. The cause was that the self-tapping screws had simply vibrated out under fluctuating wind pressures. A low grade stainless steel had been used which would not accept self-tapping so the contractor pre-drilled pilot holes and the resulting slackness allowed the screws to unwind.
Vibration due to wind effects is well known and dynamic effects should always be considered, particularly where fixings are concerned. Risks can be minimised by education, good procedures and checking of designs. Fixing selection is often carried out by specialist contractors but these should be critically reviewed by the principal designers.
Vibration due to wind effects is well known and dynamic effects should always be considered, particularly where fixings are concerned
There is a strong argument for designs to be reviewed by an experienced engineer, with no connection with the design. This is not a compliance check but to spot conceptual errors, such as the importance of fixings and their interaction with associated components, and to bring to bear the benefits of experience.