Dangers of assumed design

24 Apr 2015

Modern design techniques offer advanced structural fire engineering and modelling which can benefit all concerned, with accurate prediction of the performance of steel for today’s modern buildings and the fire performance measures required to protect them. Here, Bob Glendenning of Sherwin-Williams Protective & Marine Coatings, highlights the dangers of over-assumptions to save costs which can result in compromised fire protection.

Modern design techniques offer advanced structural fire engineering and modelling which can benefit all concerned, with accurate prediction of the performance of steel for today’s modern buildings and the fire performance measures required to protect them. Here, Bob Glendenning of Sherwin-Williams Protective & Marine Coatings, highlights the dangers of over-assumptions to save costs which can result in compromised fire protection.

In scoping out designs for today’s modern complex buildings in our cities and where large numbers of people move about, design engineers have a host of considerations to take into account. The principle material used today is steel - now one of the key elements of any new building design - often due to the loads members can bear such as long span beams which require both strength and serviceability considerations.

With these parts of the building infrastructure already designed and prescribed, the fire engineer working with the designer can then advise on the most effective level of fire protection.

Traditional ‘prescriptive’ approaches use pre-defined limiting steel temperatures based on the individual parts of the steel structure, usually following recognised codes of practice.

These limiting temperatures are agreed in codes of practice with conservative calculations of maximum loads to ensure the 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.

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-sizefits-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.

For intumescent coatings, any compromise of the thickness could severely affect the level of fire protection where a steel section may carry a higher load than was allowed for by the fire protection expert and collapse 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 has been 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 much safer for all concerned including the building owners or managers, who are 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 safety and strength of the steel and of the fire protection coating, which enables fire safety services to enter a building, read the fire alarm data and enact a safe evacuation of any occupants from the building.

The potential level of savings when structural fire engineering is used genuinely can still be worthwhile - and more importantly safe.

As examples, using 75 per cent ambient utilisation of the steel beams only, with columns and brace sections remaining at 100 per cent, 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 responsibility of the fire protection expert to establish the correct level of steel ambient utilisation and with it the appropriate level of protection.

Working with designers, the fire performance expert can then agree the safe level of protection necessary for the sake of lives and property.

ENDS

Bob Glendenning is Manager, Fire Engineering and Estimation, Sherwin-Williams Protective & Marine Coatings, Europe, Middle East & Africa