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CROSS Safety Report

Missing fixing strap on Light Gauge Steel Framing System may affect fire performance

Report ID: 1141 Published: 26 August 2022 Region: CROSS-UK


Overview

A reporter has become aware of several cases where a critical strap has not been installed within a Light Gauge Steel Frame external wall system.

Key Learning Outcomes

For specifiers of products and systems:

  • Ensure that if a flat strap is part of a tested system then it is specified clearly and correctly

For suppliers:

  • Consider putting in place mechanisms that guarantee that all components of a system are included in shipments

For site engineers and contractors:

  • Any tested system should be constructed as specified
  • Ensure that where a standard product has options for its configuration, the correct one for the particular application has been selected, verified, and implemented

Full Report

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A reporter has become aware of several cases where a critical strap has not been installed within a Light Gauge Steel Frame (LGSF) external wall system. The strap is required to allow adequate fixings of the sheathing board and is required as a condition of the fire test.

The strap spans horizontally between the main studs and provides a fixing point for the sheathing board below the head track to allow for downward deflection. Horizontal straps are also required where there is an unsupported board edge. An example of such a system is presented in simplified form in Figure 1 below.

Image
Figure 1. A simplified illustration of a system in which the flat strap is indicated.

The reporter is concerned that if the strap is not included within the wall system, then the fire performance of the wall will not be the same as the one of the tested system and potentially will not perform as required in the case of a fire.

They consider that this can occur in cases where the external wall system is not specified correctly, the suppliers do not provide the necessary components, or when the contractors responsible are not installing the straps.

Expert Panel Comments

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Expert Panels 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-UK Expert Panels page.

Any time a wall construction needs to achieve a specific fire performance, and evidence of that performance is achieved through fire testing, then the actual construction needs to be in accordance with the system that was used in the test. Any deviation has to be assessed by a suitably competent professional in accordance with relevant technical guidance, such as the Passive Fire Protection Forum (PFPF) and Association for Specialist Fire Protection (ASFP) guide to undertaking technical assessments of the fire performance of construction products based on fire test evidence. This applies to all relevant fixing details, an example being screws holding plasterboard in place being at wider centres than those that were used in the test. If such changes happen then they might compromise the fire performance of the wall.

While not relevant to fire performance, it was also noted by the panel that the ability of the Light Gauge Steel Frame element to carry wind load seems critically dependent on the centres between fixings and these might have to be altered by omitting the flat strap. This is a consideration that should also be borne in mind.

This report also shows the general need for adequate drawings, competent installers, and suitable on-site checks. There needs to be adequate supervision of the works by suitably trained and experienced people, to ensure adequate quality control during execution, e.g., to ensure vital components of the system are not omitted. Care also needs to be taken so that the design specification and drawings include all necessary components to achieve the proposed system appropriate to its use and location. Manufacturers can assist by including in the product’s details any relevant design and construction procedures, but it should be borne in mind that it might be inappropriate to completely rely on the manufacturer’s instructions alone to ensure adequacy. It is the responsibility of the designer, site engineer, and contractor(s) to use all the information available in order to provide an appropriate solution. Such an approach can ameliorate any potential disconnect between the results of the fire test and the information provided by the manufacturers.

Further reading:

The Finishes and Interiors Sector (FIS) have published an industry alert (titled: Flat strap on through wall SFS external wall systems) related to this issue.

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