In this Newsletter we present a case study of a rainscreen panel failure, where the cladding panel dislodged due to inadequate vertical structural movement allowance. While helping hand brackets are a cladding subcontractor’s staple and preferred panel fixing arrangement, the security of the hook-on elements is reliant upon adequate engagement, without restrictions.
On the relevant building, helping hand brackets supported cladding panels weighing 30kg each and had been designed, developed and installed without issues. Some three years after practical completion (and in full occupation of the building), several cladding panels fell forward without warning, with the upper panel edge falling away from the façade line and the panel resting and retained by the lower helping hand connection, stopping them falling to the ground.
The cladding arrangement was a ladder of large format panels, with reasonably tight joints and the final (uppermost) panel of each ladder shared the framework of the ladder frame above. The panel failures occurred at floor levels, with panels beneath the floor junction being affected. The displacements occurred locally in clusters and at different levels on the building and offered no hint that these panel failures might be a repetitive fault. The cladding rails were fixed at floor levels, with movement joints at each level making for independent ladder frame assemblies.
Investigation of the brackets showed:
- No mechanical distress – the heavy cladding panels had not deformed the brackets themselves and were not deemed to have exerted excessive load onto the brackets.
- No physical external damage – the cladding panels had not been tampered externally, although a group of these panels had been previously accessed externally by an intruder.
- No clear indication that localised heat had changed the geometry of the cladding system.
- No clear indication that localised (or general) cold had changed the geometry of the cladding system.
- No differential structural settlement had occurred outside the structural engineers’ design predictions.
After three years, the end user changed their business model and reduced their space requirements, meaning departments moved. It was noted that during this move the department archive storage was relocated within the building. When assessed, it was determined that the applied live loads of the archive storage were significantly greater than the structurally engineered design loads for the typical office floors. In this instance the tenants had not restricted their storage vaults to the areas structurally designed to accommodate such. In addition, other tenants had occupied the building with their own archive vaults, these being in different locations across the building, again not respecting the loading limits.
The increased live loads made the floor slabs deflect by a greater amount than structurally predicted and whilst deemed structurally “safe”, the extra loads were causing floor edge beams to deflect more than anticipated. The deflection of the floor edges caused the cladding rails attached to them to drop locally at their respected load points, sufficient to exceed the bite dimensions required for the helping hand connections that bridged floor levels, causing the panels to open up, disengage and tilt. It was fortunate that the lower helping hand could accommodate the tilted loads and that the panels wedged within the joint.
Lesson learned: Consider the live loads applied to a building and avoid fixing regimes that cross over separate structural elements, unless adequate allowance has been made for all loading conditions.