CROSS Safety Report
Glued on stainless steel panels fall
A ceiling in a retail setting was designed to be faced with reflective finish panels. The contractor proposed stainless steel bonded to MDF with folded edges and concealed mechanical restraint. The designer overruled the contractor and selected an adhesive-only solution which was implemented. Some months later, several panels failed, with some stainless steel facings falling to the floor.
Key Learning Outcomes
For designers and the construction team:
- Give attention to the design of ceilings and the safety critical aspects of their fixings and anchors
- Where possible, consider proven systems in preference to less tested designs
- Give consideration to thermal movements
- Pay full regard to the manufacturer’s recommendations and conditions when selecting and using adhesives
- Have in place an inspection and testing plan
- Changes to designs must be critically reviewed
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A reporter says that a ceiling in a retail setting was designed to be faced with reflective finish panels. The contractor proposed stainless steel less than 1.0mm thick bonded to medium-density fibreboard (MDF) with folded stainless steel edges and concealed mechanical restraint. Panel joints were specified as close fitting. However, the designer overruled the contractor and selected an adhesive-only solution. The designer also decided to increase the thickness of the stainless steel to reduce visually apparent distortions and have closer fitting panel joints.
The reporter says that within a few months of installation, a number of the stainless steel sheets debonded from the substrate and a small number of stainless steel facings fell (fortunately outside retail hours) with several others becoming partially detached.
a number of the stainless steel sheets debonded from the substrate and fell
It was determined that whilst the contractor had achieved adhesive bond over much of the panel area, the thermal expansion of the stainless steel had created a prying load at the joint edges that initiated the delamination. There was also doubt that the achieved bond was sufficient to hold the stainless steel, partly because the metal thickness had been increased. Such reliance on adhesives without secondary mechanical restraint is a potentially dangerous practice, particularly where materials with differing mechanical properties are involved, continues the reporter, unless specialist expertise is sought.
a designer insisted on a design solution against the contractor's advice and the result was a potentially dangerous outcome
The reporter asserts that designers should understand the constraints that materials impose, but, in this case, a designer insisted on a design solution against the contractor's advice and the result was a potentially dangerous outcome. The reporter goes on to say there was over-reliance on an adhesive bond where gravitation force and thermally derived forces were both acting on the bond. The designer changed the specification by increasing the thickness and hence weight of the stainless steel finish without, it appears, understanding the implications. The contractor used materials and adhesives that were untested in the proposed conditions and was not allowed to mechanically restrain the panels. Furthermore, the differing properties of the two materials selected and their complex thermal interaction was apparently not appreciated.
The reporter concluded that:
- Mechanical fixings may be required when adhesives are proposed, particularly in situations influenced adversely by gravitational and thermal effects.
- There should be adequate provision of tolerances for thermal expansion to prevent prying actions.
- Designers and contractors should only use systems proven appropriate for a particular situation and if not proven, they should be tested to establish suitability.
- When design changes are proposed, a critical review is required to ensure the changes can be accommodated without compromising integrity.
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Expert Panel Comments
An Expert Panel comment on the reports we receive. They use their experience to help you understand what can be learned from the reports. If you would like to know more, please visit the CROSS-US Expert Panel page.
The reporter notes that proven systems should be used in preference to less tested designs - this point should be emphasised. Proven systems have likely been developed to incorporate learning over a period of time and a number of applications. Less tested designs are unlikely to have the advantages that system development and time in use bring. The case highlighted in this report appears to be a less tested system. It appears that the design had not been considered and developed sufficiently to render the ceiling reliable.
proven systems should be used in preference to less tested designs
Stainless steels can have a coefficient of thermal expansion higher (and in some cases markedly so) than other steels, and therefore interaction with other materials becomes important. As in a case such as this, when subjected to a temperature change, stresses are produced in bonded materials if the materials have differing coefficients of thermal expansion. Furthermore, using a thicker stainless steel had the effect of increasing any force in the stainless steel which resulted from temperature change. This higher force will have increased the shear bond demand between the stainless steel and the MDF over that required for thinner stainless steel. Closer fitting panel joints could also have added to the likelihood of temperature changes causing material failure, and as the reporter has noted, consideration must be given to how thermal movements are catered for.
Choice and use of adhesives
It appears the bond between the stainless steel and substrate was not adequate. This could have happened because the choice of adhesive was not ideal, or due to a significant number of other reasons including contamination of surfaces, inadequate application technique, incompatibility between substrates and adhesive, and unsuitable environmental conditions. Any one or more of these, or other reasons, including those outlined by the reporter, could have contributed to the failures. The importance of paying full regard to the manufacturer’s recommendations and conditions when using adhesives must be stressed.
Robustness and redundancy
The need for mechanical fixings, as recommended by the reporter, must be given thorough consideration. Robustness should always be considered in design. The contractor originally specified concealed mechanical restraints. These fixings could, for example, be designed to provide some redundancy in the support system and prevent stainless steel panels from falling in the event that the adhesive bond fails. However, the choice of MDF could be questioned – this is very unlikely to be a robust substrate for such applications.
Consideration should be given to the required level of site control when installing systems to ensure that an appropriate level of confidence can be given to the likely performance in use. An inspection and testing plan should be used during and after installation to ensure adequate fixing. In the end, it is a matter of both adequate design and construction supervision to ensure that the work is to standard. Workmanship is often the weak point in failures, although in this case, the design which changed considerably from that initially put forward, has been questioned. The reporter is absolutely correct in saying that proposed changes to designs must be critically reviewed.
proposed changes to designs must be critically reviewed
CROSS has reported on a number of ceiling collapses including report number 854 Suspended ceiling partial collapse in 2020.