The Data Gap - Why Timber Infilled Steel Floors Remain a Fire Engineering Challenge

Anyone who has tried to justify the fire resistance of a floor constructed from structural steel beams with timber joist infill will recognise the challenge. It is a construction type found throughout churches, halls, converted commercial buildings and refurbishment projects, yet published fire testing covering this specific arrangement remains limited.

A common construction with limited published evidence

Steel beams supporting timber joists, with timber boarding above and a lath and plaster or plasterboard ceiling below, are a common form of construction in older buildings. They regularly appear in projects involving changes of use, extensions and refurbishment.

The challenge arises when the floor is required to demonstrate a 60 minute fire resistance period, or another specified performance under Approved Document B or BS 9999.

Current guidance expects fire resistance to be supported by recognised fire test evidence, engineering assessment or other accepted methods of demonstrating compliance. While there is extensive published testing for protected steelwork and traditional timber floor constructions independently, there is comparatively little published evidence covering hybrid steel and timber floor assemblies as complete systems.

Why this is difficult

Steel and timber behave very differently when exposed to fire.

As temperatures increase, steel progressively loses strength and stiffness, while timber chars at a predictable rate and retains a residual structural section. Fibre cement boards, plasterboard systems, calcium silicate boards and intumescent coatings each contribute differently to the overall fire performance of the floor.

The challenge is not understanding how each individual product performs. Manufacturers invest heavily in testing and certification for their own systems.

The difficulty arises when those products are combined into a construction that has not itself been tested.

At that point, it should not be assumed that because each individual component performs well, the complete floor assembly will achieve the required fire resistance period.

Engineering judgement still requires evidence

One of the biggest misconceptions in fire engineering is that an engineer can always produce an engineering judgement.

In reality, engineering judgement is still based on evidence.

It should be supported by published fire tests, recognised engineering assessment methods, established engineering principles, manufacturer data and, where appropriate, analogous test evidence. Without a sufficient evidence base, the correct professional response may simply be that there is not enough information available to support the proposed solution.

Saying “I don’t know” is sometimes the most technically robust answer an engineer can give.

Developing a proportionate solution

Where sufficient supporting information exists, an engineering assessment may draw upon:

• Steel section factor calculations and critical temperature analysis.

• Established timber charring rate calculations and residual section analysis.

• Published manufacturer data for fire protection systems.

• The known performance of relevant tested floor or ceiling systems.

• A documented engineering assessment explaining how the available evidence has been applied.

This often produces a more proportionate and technically robust solution than simply specifying the heaviest protection system available.

However, where sufficient supporting evidence does not exist, designers may need to adopt fully tested and certificated proprietary systems, even if they are more complex or less practical to install.

An industry wide challenge

The construction industry is making greater use of hybrid structures, while many existing buildings already contain combinations of steel, timber and modern fire protection products that were never tested together.

This creates a gap between modern design challenges and the available published evidence.

Manufacturers continue to expand their testing programmes, but there remain situations where neither the designer, the manufacturer nor the fire engineer can identify a directly applicable tested solution.

Those situations require careful professional judgement, honest acknowledgement of the available evidence and, occasionally, acceptance that further evidence is required before a conclusion can be reached.

Final thoughts

Good fire engineering is not about finding a certificate to fit every situation, nor is it about making assumptions where evidence is lacking.

It is about understanding the limits of the available data, applying recognised engineering principles and recommending solutions that are technically defensible.

Sometimes that means developing a robust engineering assessment.

Sometimes it means adopting a tested proprietary system.

And sometimes, the most professional answer is to acknowledge that further evidence is needed before a conclusion can be reached.