Construction fixings and anchors in fire scenarios 31 July 2020

By John Muir, technical manager, Construction Fixings Association

In light of recent tragic events such as the fire at Grenfell Tower in London, UK, the construction industry has had cause to look at how the buildings we construct perform in a fire.

The investigation and subsequent findings of Grenfell Tower have highlighted many issues that need to be addressed, including performance of products, and associated testing regimes, and related legislation.

This article will not attempt to address the many broad issues associated with this, but will look at one very important, if less visible, element of the construction. The method of connecting a building component to the structure is generally achieved by fixings and anchors, and it is vital that these fixings can maintain their integrity and hold components in place for a defined period of time in a fire.

The approach to fixings design in a fire situation appears to be dealt with in one of two ways at present. Firstly, the fixings are relatively ignored for purposes of fire, which could lead to fire rated components collapsing through fixing failure before their rated resistance time. An example to illustrate this would be a compartment floor or wall fixed to the structure with an unsuitable fixing. This could fail within a timescale less than the fire rating for the compartment element and so lead to the compartment not meeting the required fire resistance duration. A different scenario could include a sprinkler system pipework support failing – so the active fire protection is not fully engaged.

Alternatively, the other diametrically opposed approach is to ban all anchors of a certain type, such as resin anchors, through a total lack of understanding of how anchors perform in a fire situation.

In a fire, temperatures can exceed 1,000°C (the ISO 834 standard temperature curve used for fire testing reaches 1,200°C at two hours), and at these temperatures, structural materials such as concrete and steel undergo major changes to their mechanical properties.

These changes apply to concrete, structural steel, nuts and bolts, and therefore also to anchors, and have the effect of reducing the strength of the steel, and subsequently the performance of the anchor. In fire testing of fixings, failure of the anchor threads is the most common mode of failure. In addition, in the case of resin anchors, it is also possible that the resin to concrete bond can be the limiting failure mode in a fire.

What is vital for selection of fixings for use in a fire situation is to use the correct information, obtained from a valid approval system and carry out a design based on the correct design standard.

Areas that should be look at and considered include:

Legislative background
Under the Construction Products Regulation, to allow affixation of CE Marking, construction products need to comply with a harmonised EN or alternatively obtain a European Technical Assessment (ETA) in compliance with the relevant European Assessment Document (EAD) and associated EOTA Technical Reports.

An EAD assesses the performance of a product against its essential characteristics. In the case of EAD 330232-00-0601 (mechanical anchors in concrete), these include fire resistance values for steel failure and pull-out failure in tension, and steel failure in shear.

Testing
To achieve an ETA, an anchor has to to be fire tested to the requirements of EAD 330232-00-0601 and EOTA TR 048 (detailed tests for post-installed fasteners in concrete). These tests are carried out to EN1363-1, using the STC (standard fire curve), and provide fire resistance durations as classified by EN13501-2. Values for characteristic resistance at 30, 60, 90 and 120 minute fire duration.

EAD 330499-00-0601 (bonded fasteners for use in concrete) does not include fire resistance as an essential characteristic for resin anchors, and so these must be dealt with by different means. All major manufacturers, however, provide performance data in fire conditions – tested in accordance with the requirements of EAD 330232-00-0601 and EOTA TR 048.

Design
On deciding the duration required for fire resistance, the design of the anchor is carried out in accordance with EN 1992-4:2018 (design of fastenings for use in concrete).

Anchor design to this standard involves calculating the design resistance (e.g NRd or VRd) of the fixing for a number of failure modes in tension and shear at ambient temperature. The lowest calculated design resistance value for tension and shear is used for the design.

In the case of fire design, the characteristic resistance (e.g NoRk or VoRk ) value used in the ambient temperature (cold) design is substituted by the characteristic fire resistance (e.g NoRk.fi(xx)) value for the required duration (30, 60, 90 or 120 minutes) and the partial safety factors for fire, in all calculations. These values are all provided in the relevant ETA and Declaration of Performance.

As outlined above, all construction materials reduce in performance in a fire, including anchors, so the values for NoRk.fi(xx) do drop off substantially over the period of a fire.

This being the case, it is important that the design of the anchorage and component is a holistic one.

To provide protection to a structural member of steel or reinforced concrete, measures such as encasement in plasterboard or fire board, or coating with intumescent material, can be adopted.

It is important that the exposed anchor is also protected in the same manner and thus provide the same degree of increased fire resistance.

Conclusion
Having looked at the detail of fire testing and design, it is very clear that the correct design for the integrity of fixings associated with a fire rated application are treated as comprehensively as any other element.

Where fixings are involved in a fire application they need to be selected and designed by obtaining the fire resistance data from the ETA or other authoritative sources, and the design carried out fully in line with Eurocode 2 (EN 1992-4).

When this process is carried out, and in conjunction with other fire resistance measures as described above, the designer can specify the fixing with complete confidence in the chosen fixing’s ability to achieve the fire resistance requirement.

www.the-cfa.co.uk

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