Structural issues with cladding

A reporter who investigates cladding failures says that they are normally asked to establish the cause of the failure and comment on the roles of the designers and contractors. This report highlights some of the issues they have encountered.

The designer often fails to adequately consider the combined effect of construction tolerances with thermal and moisture movements. This can cause the following design issues which may result in cladding failure:

• Failing to provide movement joints, causing materials and fixings to become overstressed

• Insufficient allowances at movement joints causing either the joints to close and impose stresses on the cladding and fixings, or to open excessively causing restraint fixings to disengage

Cladding fixings sometimes rely on a series of hooks attached to the structure onto which the cladding is hung. When this is the case, it is vital that the hooks and the corresponding fixing points on the cladding panel are aligned so that they are properly attached.

The reporter has encountered cases where the required attachment was not properly achieved. They advise that hook systems like this should either be fully manufactured off-site. If hooks are installed on site; this should be done using templates to ensure the correct location of the fixings. In both cases, they say that a rigorous checking procedure should be in place.

The interaction between cladding panels and the supporting structure is sometimes not well understood, says the reporter.

For example, if stiff cladding panels, such as precast concrete, are fixed to the edge of a concrete floor slab which spans parallel to the panels, the panels may act compositely with the slab when it is under loading if the fixings provide restraint against vertical movement. This can overload the fixings and cause them to fail, causing panels to fall off in extreme circumstances.

To overcome this issue, and as recommended in relevant guidance such as BS 8297:2017 (Code of practice for Design, manufacture and installation of architectural precast concrete cladding), the support system for panels can be positioned at two points only. This can ensure that the panel does not act compositely with the floor slab and provides a predictable distribution of loads onto the supporting structure.

The attachment of fixings into heavy panels is sometime inadequate. An example the reporter encountered was when fixings for heavy concrete cladding panels consisted of cast-in anchors located very close to the top of the panels. In some cases, this resulted in no reinforcement between the cast-in anchors and the top of the panel.

This issue can arise if the reinforcement for the panel is designed by a structural engineer, and then subsequently, the fixings for the panel are designed separately by the precast supplier, without review by the structural engineer who originally designed the reinforcement.

The reporter says that every element of the cladding system should be designed, all the way back to the structural frame.

An example of where this might get overlooked is in the design of bespoke metal cladding systems. Although the cladding itself may be robust, the fixings can rely on lightweight aluminium sections concealed within the cladding and connected to the cladding using stainless steel screws.

The reporter has come across cases where the lightweight aluminium sections were substantially under-designed. This can cause the fixings to fail and cladding panels to fall. They recommend that the aluminium sections, along with the screws, are checked to ensure that they can resist the applied loads.

An issue that can affect all types of cladding, but particularly bespoke systems, is a failure to fully design all the details and interfaces.

The reporter says that this can result in ad-hoc design being carried out by the installers on site without a full understanding of the engineering requirements. Such ad-hoc design is not reviewed by the original designer and is often not recorded on as-built drawings.

An often overlooked element of a cladding system is the copings on the top of parapet walls. However, these are often subjected to the most severe wind loading on a building.

The reporter has encountered issues where the design of copings on parapet walls was left to a specialist sub-contractor who did not have the required competence to understand the required load resistance or the need to provide for thermal movements. The reporter recommends that the design of such elements should always be reviewed by the project’s structural engineer.

Although not strictly cladding, the reporter states that shop signs are often installed in an ad-hoc fashion without any engineering input. Such signs, which can weigh several hundred kilograms, can be inadequately fixed and/or subject to deterioration of the fixings due to the use of unsuitable materials. This can result in the signs falling from buildings.