Fire Protection: Water Cooling. Fire Resistance of Steel Structural Elements

Water cooled structures in steel-framed buildings. A closed circuit cooling system is created by connecting the upper column ends to header pipes from an overhead reservoir. The cooling medium flows to the lower column ends, which are connected to distributor pipes that lead to a riser pipe back to the overhead reservoir.

Two circuit systems must be provided following the general structural arrangement of the building. In some cases, building regulations demand that, in the event of the destruction of a structural member, for example, as a consequence of an explosion, the overall structure must remain stable (3). For this kind of catastrophic loading case (i.e. for the failure of every second support), a design stress of 90% of the yield point value is used as a basis for structural calculations.

Typically, four 3m³ overhead tanks (i.e. 12 m³ of water), are sufficient to counteract a normal fire of 90 minutes duration, involving a spread of fire to two floor levels. On the basis of expert opinion, this also gives a safety margin of almost a third in respect of the available water.

Where the structural columns are outside the building, freezing of the cooling water is prevented by the addition of potassium carbonate in a 33% solution, lowering the freezing point to -25°C. Internal corrosion of the columns of the circulation pipework and of the tanks is prevented by the addition of sodium nitrite to the cooling liquid.

A good example of the use of water cooling is the ten- storey building in Karlsruhe for the Landesanstalt für Umweltschutz (Federal Institute for Environmental Protection). It has (12 + 12) x 2 = 48 steel columns, which are supplied with cooling water circulation such that the 12 + 12 columns are alternately connected to separate water circuits. The two circulatory systems of the front and rear elevations are separate.

Very high temperatures have also been measured on the steel structural elements due to normal warming by the sun in summer. In one instance, following an increase of 30°C, the approximately 33m long outer columns of the building expanded vertically by about 12 mm, resulting in displacements of the supports for the continuous, multispan structural frame. This factor had to be taken into account in the design. Since differences in density of the cooling medium occur due to warming, not only by fire but also through solar radiation, a natural circulation of the coolant takes place and the columns which are heated by the sun are cooled.

A favourable effect here is that each of the four cooling systems has columns on both the north and south side of the building, so that a temperature equalisation can take place. Column temperatures of -15°C and +50°C were therefore taken as the basis for calculation. Without the equalisation through the cooling medium, values of around -25°C and +80°C would have had to be assumed in demonstrating structural integrity.

Fire resistance of steel structural elements. The fire resistance duration of structural steel elements for a prescribed level of fire intensity is dependent on the rate of heat increase and the respective critical temperature of the element. The temperature of a steel member increases more rapidly as the ratio of the surface exposed to the fire increases in relation to the steel cross-section. Large steel cross-sections heat up at a slower rate given the same depth of coating, the same material and equal fire surface coverage, and therefore have a greater resistance to fire than smaller cross-sections.

An important influencing parameter for the heating up process is therefore the section factor Hp/A (i.e the ratio of the heated perimeter to nominal cross-sectional area. The characteristics of the coating material are also decisive to this heating up process, as is the adhesion of the coating to the steel surface. The heating up period can be calculated or obtained from fire tests in accordance with relevant standards.

Steel components can fail if the 'critical steel temperature' is reached on critical cross-sections. The fire resistance period is therefore dictated by the time taken for the component to be heated up to this critical steel temperature.

The relationship between section factor, depth of coating and the duration of fire resistance of steel columns and steel girders has been investigated for various types of covering. The results are widely available and should be considered in the light of the possible fire risks associated with the proposed building.