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904 Structural issues with cladding

Report ID: 904

Published: Newsletter 58 - April 2020

Report Overview

A reporter who investigates cladding failures shares common issues they have encountered.

Report Content

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. Issues they have encountered include:

1. Design of movement joints: the designer failing 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:

a. failure to provide movement joints, causing materials and fixings to become overstressed;

b. 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.

2. Tolerances for cladding hooks: when cladding fixings rely on a series of ‘hooks’ attached to the structure onto which the cladding is hung, 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, or if installed on site, 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.

3. Interaction between cladding and supporting structure: 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 and installation of non-loadbearing precast concrete cladding), the support system for panels can be positioned at two points only so that the panel does not act compositely with the floor slab and provides a predictable distribution of loads onto the supporting structure.

4. Fixings for heavy cladding panels: 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, resulting in no reinforcement between the cast-in anchors and the top of the panel in some cases. 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.

5. Design of whole cladding system: 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, which 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.

6. Design of details and interfaces: 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.

7. Copings on parapet walls: 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.

8. Shop signs: 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, which can result in the signs falling from buildings.

 

Comments

This is an interesting report highlighting a number of themes:

• The complexity in reality of what might seem relatively straight forward items.
• The need for industry feedback on real behaviour.
• The anticipation of modes of failure.
• As always, fixings are a vital component in any system.
• The need to assure that real installation matches anticipated design.
• The recurring theme of danger at the interfaces of responsibility.

The report highlights the trend for design being split into multiple packages and passed down the contractual chain with no one seemingly responsible for how the final product performs. This has many similarities with report 911 both in terms of the risks associated with failure and the lack of design control.

Several recent cladding failures have illustrated concerns with:

  • inadequate design;
  • inadequate specification;
  • inadequate / unsupervised installation leading to missing components or inadequate installation of fixings;
  • failure of the fixings exacerbated by the use of hidden fixings which cannot be inspected, and
  • inadequate assurance checks, where primary structure is given consideration, but secondary structural elements may
  • be overlooked.

CROSS recommend that cladding design and installation is given the same degree of attention as the primary structure during both design and construction to improve safety, reliability and longevity. It is really important that a single entity (or chartered engineer) should have overall control of the design of the cladding system including its interfaces with the support structure and to assure the ability of the structure to support the applied loads.

Where bespoke cladding systems are proposed, there are well established test procedures that can be used. See for example the work of CWCT (Centre for Window and Cladding Technology). These test wind, water and impact loads and can help to flush out problems even if the principles are similar to cladding used on other projects. Other sources of information are the CFA (Construction Fixings Association) and BS8539:2012 (Code of practice for the selection and installation of post-installed anchors in concrete and masonry).

Fixings for signs can be safety critical. For example, there was a fatality in 2015 after a sign fell on to a shop worker. A report by the HSE said an original “much lighter” sign had been fixed to the building prior to 1980 using woodscrews and a vertical stud. It is believed the sign was made unsafe by the addition “of a larger and much heavier sign”, also installed prior to 1980, but the installers relied on the fixings of the original sign.

 

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View other CROSS reports published in Newsletter 58


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