In this Newsletter we present a case study of another rainscreen panel failure. A folded aluminium cladding system that relies upon panel adjacency support failed unexpectedly. Two panels fell to the ground from height. The first panel failed and an adjacent panel fell quickly after. A third panel was left in a dangerous condition and was removed to make safe. The weather conditions were not extreme, with reported light winds at the moment of failure. It was noted that the panel appeared to have disengaged before its failure and was ‘flapping’ in the breeze.
The panels were inspected for damage, as were the parts remaining on the building. On inspection there were very little deformations at the panel connections and could not be attributed to the panel failure, with most damage being the result of the panel falling to the ground. The two panels had signs of prying damage to their lowest connection overlaps. Panel fixings showed signs of pull-out failure, with a number of individual fixings being pulled through the panel body. It was noted that on the first panel that fell, there was a single panel connection with two overlapping drilled holes, clearly where a panel correction had been made on site.
The structural design of the overlap makes assumptions on the strength of the fixings and the resistance of the overlap. The shared connection overlap relied upon a 12mm bite for effective load transfer. This would take into account thermal expansion and installation tolerances. The installed panel had clearly become disengaged from its neighbor suggesting that the required overlap had been compromised, and the panel was suspended on its single fixing line. A critical review of the calculations revealed that the design did not take into account the failure of an adjacent panel, where the panel fixing nearest to the missing fixing is required to accommodate double the load. In this instance it was the lowermost fixing that, with the double drilled hole, had effectively lost this fixing strength. Load was now being shared/transferred to the next fixing along the panel.
These effects alone should not have been sufficient for a complete panel failure, but they were. Even with disengagement, the two panels should have been able to remain in place, albeit hanging loose. The failure occurred from a site tolerance error causing the panel to be at the point of disengagement, which occurred under a combination of temperature and wind deflections. Disengagement overloaded the already compromised panel fixing arrangement, causing panel failure. This was evident by scrape marks on the disengaged joint and pull out distortion across fixing connections.
Review of the structural calculations revealed that the design had not taken into consideration the possibility of the panels disengaging and for one panel to attract the entirety of its load down its fixed side. It was found that the fixings would be at the limit of their structural capacity under normal circumstances (which is not desirable), but in this case, this deficiency was further compounded by the loss of a fixing through the site corrections.
Lesson learned: When designing connections allow for redundancy. When checking on site, confirm that fixing methods that rely on overlap sharing have sufficient bite that allows for building movement, wind deflections and thermal expansion. Panel overlap can be easily checked by measuring vertical joint widths.