In designing today’s complex modern buildings, through which large numbers of people pass, design engineers have myriad considerations to accommodate.Steel is now a key element in any new building design partly because it meets strength and serviceability requirements demanded by long span beams.
With such parts of the building infrastructure already designed and prescribed, the fire engineer and designer can then advise on the most effective level of fire protection.
Traditional ‘prescriptive’ approaches use predefined limiting steel temperatures based on the individual parts of the steel structure, usually following recognised codes of practice.
Agreed in codes of practice with conservative maximum load calculations, these limiting temperatures ensure that relative passive fire protection – including intumescent coatings, which offer resistance to fire – is more than adequate.
Some of those in the supply chain may question why these steel parts – whether a beam, column or brace for example – would be overly-specified and under-utilised in terms of their load bearing capacity in their ambient design state.
Dangerous assumptions
In reality, this performance-based approach allows designers to account for different applied loads being used in various parts of a building for a diverse set of reasons, rather than the ‘one-size-fits-all’ prescriptive approach which assumes loads and tolerance.
The trend to assume loads well under the reality of performance-based modelling on each section of steel in today’s complex buildings – thus creating savings for the project in fire protection – is dangerous indeed.
Any compromise of the thickness of intumescent coatings could severely affect the level of fire protection where a steel section carries a higher load than allowed for by the fire protection expert and collapses under stress when subjected to a real fire as the section properties change.
The risk of collapse is relative to how far the original assessment was made below the real applied loads and – with the subsequent protection also reduced accordingly – this type of scenario offers little reassurance of what will happen in reality and is unsafe.
It is more time-consuming – but still potentially cost-saving for the project – to apply accurate calculations of each key section of a building. But it is also much safer for all concerned, including the building owners or managers liable under the Regulatory Reform (Fire Safety) Order 2005.
In principle, if these separate steel sections are under-utilised in all calculations, they should be safe in a real fire where adequate fire protection has been designed and applied.
This guarantees the steel’s safety and strength and the fire protection coating, which enables fire-safety services to enter a building, read the fire alarm data and enact a safe evacuation of building occupants.
Worthwhile
Deployed competently structural fire engineering is worthwhile and more importantly safe.
Imagine that there is 75% ambient utilisation of steel beams only, with columns and brace sections remaining at 100%. There are savings at relevant levels depending on the scale of the project.
In reality, a single level of utilisation throughout is not seen but for demonstration purposes this has been adopted here.
For a project specifying 60-minutes of fire resistance to building design code BS5950-8, the thicknesses of the intumescent coating are generally low so an assumption of 75 per cent has reduced the total theoretical volume required by nine per cent from 27,300 litres to 25,000 litres.
Further savings can be made for example where a project requires 90 minutes of fire resistance to a building designed to Eurocode 3 &4 (1993-1-2 and 1994-1-2 for fire design).
Here the thicknesses are more significant and savings are higher at 17 per cent, with a reduced volume from 58,000 litres to 48,000 litres. When achieved as a result of genuine, professionally practised fire engineering, this could result in thickness reductions of hundreds of microns.
For a further project example, where thickness ratings are near the highest available, with 120-minute fire resistance to building design code BS5950-8, the savings rise again at 23 per cent, falling from 46,000 litres to 35,300 litres.
This demonstrates how, using modern fire protection design, savings can be made when used professionally and can play a major part in delivering a safe, cost-effective project.
It is the fire protection expert’s responsibility to establish the correct level of steel ambient utilisation and the appropriate level of protection.
Working with designers, the fire performance expert can then agree a level of protection safe enough to protect lives and property.
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