Practical ways of achieving fire resistance of steel structures

1. INTRODUCTION

The mechanical properties of all common building materials decrease with elevation of temperature. Structural elements should possess appropriate fire resistance to resist total or partial collapse; in addition, fire resisting partitioning walls and slabs should resist flame penetration or excessive temperature rise on their unexposed faces in order to contain the fire in its original location. The fire stability of a structure is especially important and any failure of the structure in the fire zone should be gradual, involving large plastic type deformations. The parts of the building away from the fire should remain intact.

Fire resistance requirements are fixed by National Codes in terms of the time an isolated element should resist the action of a Standard Fire as defined by the heat exposure given by ISO834, (see Figure 1). Fire resistance times of 15/30/60/90/180 and 240 minutes are specified depending upon the type of buildings and number of storeys; these times can also be a function of the occupancy of the building and of the fire load.

Structural steel members will collapse in a fire when their temperature reaches a 'critical' level. This critical temperature varies according to the load conditions, the cold design theory adopted, and the temperature distribution across the section, which typically is in the range 500 to 900°C.

The fire resistance time is the time, in the standard ISO834 fire test, taken by the member to reach the critical temperature. This time varies according to the section size. In a building in which a real natural fire occurs, the heating rate is also influenced by the member location. The thicker the steel, the slower the heating rate, and therefore the greater the fire resistance time.

A fire resistance time requirement of 'O' does not mean the building will collapse. On the contrary, the fire resistance could be infinite, depending on the building type, natural fire behaviour, or other active measures implemented.

The heating rate is quantified by the Section Factor, known as the A_m /A ratio, where A_m is the perimeter of the steel member exposed to the fire and A is the total cross-sectional area of the section. Consequently, a heavy member with a low A_m /A ratio will be heated more slowly than a light member with high value of the section factor. Tables are published giving values of section factors for standard section sizes.

(Am/A dimension is m^-1)

For a member to fulfil a given fire resistance requirement, it is necessary to ensure that the temperature developed in the member at the required fire resistance time (taking into account its section factor and any insulation which may be applied) is less than the critical temperature necessary to cause failure (also known as the 'critical temperature').

For short periods of fire resistance (15 or 30 minutes) stability may be attained by unprotected steelwork. A conventional fire resistance time of 60 minutes may sometimes be obtained without applying fire protection by utilising the thermal and/or structural interaction between steel and concrete. For longer periods of conventional fire resistance time, the steelwork can be protected by applying an insulating material, by using screens. Composite steel-concrete structures can also exhibit significant fire resistance.

A brief survey of the simpler practical means of achieving structural fire resistance in steel structures is presented. It is important to recognise, however, that considerable research and development work (fuel loads based on natural fires) has been undertaken in Europe over the last 40 years. This work aims to optimise the process of the fire resistant design of structural steelwork leading to a better safety approach, regulation adaptation,  and potentially economies in construction. These 'fire engineering' methods have been introduced in Eurocodes and in National building codes to be applied at building project levels.