The LWF Blog

Fire protection requirements for structural steel design | Part One – Overview

April 7, 2014 11:22 am

Legislation and prescriptive guidance relating to structural steel can be onerous and difficult to work with for designers and architects in the UK and overseas. Deviation from such requirements has proved even more unpopular since the World Trade Centre disaster in 2001, which illustrated all too clearly the negative results of steel protection being inadequately applied and maintained.

However, there are performance-based opportunities through fire engineering consultation which are fire and construction safe and can be more cost-effective than the conservative, structural steel prescriptive guidance for some applications.

This short series of weekly blogs will look at the uses of structural steel in construction and how the study of loading, calculation of temperature assessments and other factors, performance-based solutions can be achieved.

Steel members are designed to carry dead loads (building weight, furniture, etc.) as well as imposed or live loads (temporary loads such as occupants, wind and snow). Calculations include safety factors to allow for variations in the properties of steel. Load design calculations are based on the strength of steel at ambient temperature, around 20°C. Building loads under fire conditions are considered to be lower than normal, due to the low likelihood of a fire occurring at the same time as high loads in the building. For example, it is assumed that occupancy will be reduced, as persons will have escaped.

Although the loadings are lower in fire conditions, so are the mechanical properties of structural steel. This metal begins to lose strength after its temperature has exceeded around 215°C, after which the relationship between loss of strength and temperature becomes linear – as the temperature of steel increases, its strength decreases, until failure occurs. 

The temperature at which a steel member is expected to fail is based on the residual strength of a member. This is the relationship between the load imposed on a member to the total capacity of the steel of which it is made. Where there is a greater degree of residual strength, there is generally a greater capacity for temperature rise in the steel member.

Other factors that determine when a steel member will fail are its shape and the degree of exposure to high temperatures. Where a steel shape has a high surface-area-to-volume ratio, it will become hotter – faster, because its exposure to heat is greater. There will also be less unexposed steel, to which heat can be conducted to lower the average temperature. Thicker, more solid members should take a longer time to heat to their limiting temperature than steel members with thin flanges or webs.

The degree of exposure to elevated temperatures may vary, according to the design of the building. The steel may be partially shielded where a beam is supporting a concrete slab, or is located inside an internal fire compartment wall or similar arrangement. This will reduce the amount of surface exposed to flames, so decreasing the amount of heat transferred to the member. 

It should be noted that, although a steel member may be thought likely to fail based on the assessment techniques above, these are based on test results on individual members. Interaction with other steel members in a steel frame or arrangement can provide support that may well extend the time until failure.

Applied steel protection

Several methods are used to protect steel members:

• concrete encasement
• concrete filling
• board systems
• spray-on systems
• intumescent paint
• water-filling

The design team will make their selection  based on the following factors:

• cost of the system
• ease or otherwise of installation
• aesthetic appeal
• applicability of the system to the structure

Product manufacturers and research groups have tested the protection systems above, such that an appropriate thickness of product is applied to give the required fire resistance. These products are usually tested in accordance with the ISO 834 standard fire curve, which gives a temperature increase over time that can be used in furnace testing. This allows products that have been tested in the same regime to be compared.

The series will continue with part 2 next week, looking in more detail at steel assessment methods, time equivalency and temperature assessments. In the meantime, if you have any queries about fire engineering solutions for your construction project, please contact Peter Gyere on 0208 668 8663.

Lawrence Webster Forrest (LWF) is a specialist fire engineering and fire risk management consultancy, which provides a wide range of consultancy services to professionals involved in the design, development and construction and operation of buildings.