High Standards

Changing approaches to the fire safety design of high rise buildings should not lower safety standards argues Niall Rowan

In the event of fire, the structural integrity of buildings should avoid premature collapse thus ensuring time for the occupants to escape and for the fire service to obtain access. In most high rise buildings, the loadbearing function is provided by a steel framework to which the rest of the building is attached.

Fires in buildings regularly exceed 1,000oC within a relatively short period of time, yet at a temperature of between 500 to 600oC mild steel loses approximately half its strength. Consequently, it is necessary to limit the temperature rise so that sufficient strength remains in the structural frame.

Traditionally ‘fire protection’, comprising non-combustible boards and/or cementitious sprays, was applied to insulate the frame. These materials continue to be used, usually where the structural frame is largely hidden or is in a harsh environment. However, recently, reactive coatings (intumescent paints) have become more popular as they can be quicker and easier to apply and have better aesthetic properties.

Whatever type is used it is evaluated by testing individual structural members in special furnaces. The temperatures reached are then analysed and predictions made (by calculation and other methods) of the amount of material needed for the almost infinite number of sizes and shapes of steel sections available.

Conservative approach?

Whilst this approach is sound, it is conservative and will not always provide designers with the most cost-effective solution for protecting the structural steel frame of a modern high rise building. Existing design codes result in structural frames that can be over-protected. In addition this approach does not take into account that when connected together into a frame, a real structure will behave much better than the individual elements.

The use of newer design codes and fire engineering techniques for structural frames has significant implications, not least because these rely more upon fire protection products being correctly specified, installed and maintained. The ‘redundancy’ provided under the old codes has been ‘engineered out’ and consequently it is vital that the fire protection provided is fully compliant.

New codes

The design of steel structures in fire is covered in BS 5950: part 8 and the structural Eurocodes, EC 3-1.2 and EC 4-1.2, designated EN 1993-1.2 and EN 1994-1.2. Using these designers can exploit the properties of structural steel to its maximum capacity. Previously, the height/span and cross-sectional shape of sections used in buildings was dictated by applied loads appropriate to the type of building occupancy (office, shop, residential) and the application of defined safety factors. Generally, this provided a conservative working stress within the members of approximately 50% of that which would induce their failure.

This approach provided a maximum allowable temperature that could be tolerated when exposed to fire, assuming a standard load utilisation for all members. A single limiting temperature (550oC columns, 620oC beams) became the target for fire protection products applied to structural steel members.

However, in the new design codes, the designs of the individual component members are optimised so that the stress in any single member may be higher or lower than conventional design approaches. This requires a more comprehensive analysis of the insulation capability of the fire protection material over a range of temperatures, resulting in a range of minimum thicknesses varying with temperature.

The most common method of determining the required fire protection thickness is to use product-specific tables in the Association of Specialist Fire Protection (ASFP) Yellow Book – Fire Protection for Structural Steel in Buildings, 4th edition. The limiting temperature for a particular member is found in Table 1 of the ASFP Yellow Book, which is identical to table 8 of BS 5950: part 8.

Cellular beams

Many modern structural steel frames use cellular beams i.e. beams with holes. The driver for this is that by passing essential building services through the beams (instead of below them) a significant reduction in storey height is realised. This can result in one extra storey for every ten, with the associated extra value that will provide to the owner of the building.

The individual design of each member will dictate at what position and at what temperature structural failure is likely to occur. It is therefore essential that in order to provide an adequate level of fire protection an associated critical temperature resulting from the design is also determined.

Products are subjected to a special test protocol given in the ASFP Yellow Book. The data is then used in a structural model which generates tables of limiting temperatures dependant on the specific beam geometry. The required fire protection thickness is determined from product specific thickness tables as before.

Structural frames

There are several commercially available computer software applications that allow experienced fire engineers to analyse the way that structural frames behave in fire. These demonstrate that much of the fire protection required by traditional codes may not be necessary to prevent the building from collapse due to sharing and transference of loads through the frame. Such an analysis should still result in a building which will give occupants sufficient time to escape before collapse, although greatly increased distortion will mean that it is likely that the building will still need to be demolished and rebuilt.

Implications

The increased use of fire engineered structural frames means that the remaining fire protection will be relied on like never before. Yet, when we examine passive fire protection in buildings there are issues that give significant cause for concern. In the rush to take advantages of all the freedoms that the new codes and fire engineering techniques give us, we are not taking into account the problems of existing buildings and how those problems may lead to an unacceptable risk. Existing buildings may be ‘over- engineered’, but they have pitfalls and problems that compensate for this including badly specified and improperly installed fire protection.

What we are doing now is removing the over-engineering (or robustness) and not replacing it with anything. So the fire- engineered structural frame may well prove to be a problem if much of the reduced fire protection that is specified does not function, or is not installed properly.

What needs to be done?

To ensure fire protection of steel members, structural design drawings and associated schedules of the various members should specify:

• The required period of fire protection, e.g. 30/60/90 minutes for each member.
• The temperature for design (if known) which shall not be exceeded by the critical part of the member within the specified fire protection time.

Otherwise the guidance given in the ASFP Yellow Book and BS 5950: part 8 should be used and, in the case of cellular beams, the limiting temperatures from product-specific tables of limiting temperatures.

Most fire protection products are subject to comprehensive testing and assessment. To prove the consistency and reliability, specifiers should require that the fire protection manufacturers hold third party product conformity certification as provided by Certifire, LPCB, IFC, UL and others, as required by the appropriate authorities.

However, the fire performance of the product is not assured unless it is applied correctly in accordance with the manufacturer’s instructions. Specifiers should also ensure that the application of fire protection to structural steelwork is only undertaken by third party certificated contractors.

The number of fire-designed structural frames and fire- engineered buildings is increasing across the world but those taking over such buildings often have very little knowledge in this area.

It is imperative that if we are to increasingly use engineering techniques to provide fire safety, we must have total confidence the passive fire protection measures specified are installed and maintained correctly. Changing approaches to fire safety design should not mean lowering fire safety.

Niall Rowan is the Technical Officer of the Association for Specialist Fire Protection. The ASFP strives to promote excellence in the design and installation of passive fire protection products through high quality and technical expertise, and by providing education and training. The Association has recently released a new Guide to Inspecting Passive Fire Protection for Fire Risk Assessors. For further information please visit the ASFP website, www.asfp.org.uk.