STRUCTURAL DESIGN FOR ARCHITECTURE

Structural Design for Architecture

3.5 Performance of steel in fire

The performance of a building in a fire is an

important design consideration, the principal

concern being the safety of people who are in

or near the building at the time. There are

many aspects to this including the provision of

adequate means of escape and the control of

smoke. So far as the structure is concerned,

the principal concern is the prevention of

instability due to the lowering of the yield

strength of the material as a result of increase

in temperature. In the case of steel, serious

deterioration in strength begins at tempera-

tures of around 500 °C. The most vulnerable

parts of the structure are compression

elements and the compression flanges of

beams.

Most regulatory authorities specify minimum

requirements for performance of a building in

fire depending on the type of occupancy for

which the building is intended, the size of the

building, and the extent to which it is compart-

mentalised. So far as the structure is

concerned the performance is measured in

terms of a minimum period of fire resistance

(from 30 minutes to 4 hours, depending on the

size of the building and the type of occupancy).

Roof structures and single-storey structures are

normally exempted.

In the case of steel structures, two types of

strategy may be employed to meet the fire

performance requirements, namely the 'fire

protection' strategy and the 'fire resistance'

strategy. 'Fire protection' is by far the more

common. This involves the encasing of the

structure in a layer of insulating material in

order to reduce the rate at which the tempera-

ture of the steel increases, so that the critical

temperature is not reached within the required

fire-rating period.

Where the strategy is one of fire resistance

the objective is to minimise or entirely elim-

inate the need for protection. To comply with

fire regulations by this method the designer of

a structure is required to demonstrate, by

calculation or some other simulation method,

that the structure will not lose its integrity or

become unstable in a fire before the required

Fig. 3.22 The actions of the two principal types of bolt

used in structural engineering are illustrated here.

(a) The 'ordinary' bolt transmits load by bearing and shear.

This causes stress concentrations and is not particularly

efficient. Connections are simple and cheap to construct

by this method, however.

(b) The 'high strength friction grip bolt' clamps together

the elements being joined with sufficient pressure to allow

the load to be transmitted by friction between the

surfaces. This reduces stress concentrations and is a more

efficient way of transmitting load through the connection.

It is more expensive to construct, however, because both

the bolt and the elements being connected must be

manufactured to closer tolerances.

carry tension. Ordinary bolts are made in

several grades of steel and the two grades

which are most commonly used in the UK are

4.6 and 8.8. The first figure in the grade

number represents the tensile strength of the

steel in kgf/mm^2 X 10-1; the second figure is a

factor by which the first is multiplied to give

the yield stress of the steel.

Friction-grip bolts operate by clamping

components together with such force that the

load is transferred by friction on the interface

between them. Shear load therefore passes

directly from one component to the other and

is not routed through the bolts (Fig. 3.22).

Friction-grip bolts are also used to carry axial

tension through joints, however, in which case

the load passes along the bolt shank.

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