T

where X is the column slenderness at ambient temperature. X is defined as

where Le and ry are the column buckling length and radius of gyration of the column cross-section about the relevant axis of buckling.

37.4.3.3 Steel Connections

The behavior of steel connections in fire is complicated due to complex temperature distributions in connections and connection interactions with the adjacent structure. Fortunately, since a connection is much "bulkier" than the connected members, its temperatures are much lower, and, if designed properly, it is rarely the weak link in the structure. Current design methods require that when fire protection materials are applied to a structure, the thickness of protection applied to a connection should be based on the thickness required for whichever of the members jointed by the connection that has the highest section factor Hp/A.

However, it should be pointed out that the above comments are related to steel structures in flexural bending under fire conditions, where a connection does not have to resist any tensile force. High tensile forces can develop in the connected steel beams to fracture connections, which was observed during the Cardington fire research on the steel framed building (Newman et al. 2000) when the connections were cooling, or found from investigations of the World Trade Center collapse (FEMA 2002) due to possible development of catenary action in the floor truesses at very large deflections. If the designer anticipates tensile forces to develop in the connected beams, it is important that their values are quantified accurately and connections are designed to resist such tensile forces.

37.4.3.4 Other Types of Steel Structures

37.4.3.4.1 Stainless Steel Structures

Due to architectural demand and superior corrosion resistance, stainless steels are becoming more widely used. Although their fire resistance is only a minor factor in determining whether to use stainless steel or not, stainless steel does have superior fire resistance to conventional carbon steels.

Baddoo (1999) gives some information on the strength retention factors of stainless steels at different elevated temperatures. Whilst conventional carbon steel loses about 50% of its strength at a temperature of around 600° C, the temperature that gives the same loss in the strength of stainless steel is much higher, at about 800° C. Also, the surface of stainless steel has a much higher reflectivity, hence the emmissivity (1 — reflectivity) of stainless steel is much lower. Typically, the emmissivity of stainless steel is about 0.3 to 0.4, compared to about 0.8 for carbon steel. This gives a much lower temperature in a stainless steel structure in fire. For realistic loading conditions, it is almost certain that stainless steel structures will be able to be engineered to provide sufficient fire resistance without the need for fire protection.

With suitable modifications to take into consideration the reduced strength and stiffness of stainless steel at elevated temperatures, the same design method for carbon steel may be extended to stainless steel structures (Baddoo and Burgen 1998).

37.4.3.4.2 Portal Frames

Portal frames are usually single storey buildings with a small density of occupants. Escape in the case of fire attack is relatively easy. Therefore, fire safety of a portal frame only becomes a requirement when the portal frame is adjacent to another building and it is necessary to prevent fire spread from the portal frame building to the adjacent building. Fire spread from a portal frame building is usually through collapsed walls; consequently, the structural safety requirement for a portal frame under fire attack is to ensure that the portal frame columns in the walls remain upright so that the walls are stable. The portal frame girders may be allowed to collapse.

The Steel Construction Institute in the United Kingdom has developed a design guide (Newman 1990) for portal frames in fire. Using this guide, portal frames are usually designed without fire protection.

37.4.3.4.3 Water Cooled Structures

The design of water cooled structures (Bond 1975) relies on the principle that boiling water has a very high value of convective heat transfer coefficient, some 3 or 4 orders higher than the convective heat transfer coefficient of air. If boiling water is replenished, heat on a water cooled steel structure is taken away and steel temperatures remain low. Typically, steel temperatures in a water cooled structure do not exceed 150°C. Therefore, structural stability of a water cooled structure is rarely a design issue. Design is mainly concerned with hydraulic calculations to ensure sufficient water supply and circulation in case of fire.

Water cooling a steel structure to achieve fire protection is expensive and because of this it is rarely used solely for the purpose of fire protection. It is usually combined with other functions. An excellent example (Bressington 1997) of recent application of this technique is in the roof truss of the cargo handling facility of Hong Kong Air Cargo Terminals Ltd. The steel structural roof truss is made of circular hollow sections and is used as the water distribution pipe for sprinklers. On operation of the sprinklers in the event of a fire, internal water flow through the steelwork members also provides sufficient cooling.

37.4.3.4.4 External Steelwork

In the case of a building enclosure fire, fire exposure on the external steelwork differs from that on the interior steelwork in two ways:

• The fire temperature to the external steelwork is much lower than that to the interior steelwork.

• The external steelwork may not be directly engulfed in fire.

These two differences ensure that temperatures in the external steelwork are kept lower than their failure temperatures so that fire protection is not necessary.

Law and O'Brien (1989) developed a design method for external steelwork and the design method in Eurocode 3 is based on their work.

37.4.4 Composite Steel/Concrete Members 37.4.4.1 Composite Slabs

Composite slabs are constructed from reinforced concrete slabs in composite action with steel decking underneath. The steel decking acts as support to the concrete during construction and is generally profiled to maximize structural efficiency. Composite slabs usually form the floor of a fire resistant compartment. Hence, they should meet all the requirements of fire resistant construction; that is, in addition to sufficient load bearing resistance, they should also have adequate insulation and maintain their integrity during fire attack.

Composite floor slabs are noncombustible and will not suffer integrity failure by burning through. However, the problem of integrity failure may occur at the junctions between a composite slab and other construction elements. Particular attention should be paid to the slab edges where large cracks may occur due to large rotations. It is important that reinforcement bars should be made continuous over the supports.

To check whether a slab can fulfill the insulation requirement, it is necessary to carry out a heat transfer analysis to determine temperatures on the unexposed surface of the slab. Results of this temperature analysis can then be used to determine the minimum slab thickness above which the unexposed surface temperature is unlikely to exceed the allowed values. For most applications where the required standard fire resistance rating does not exceed 90 min, the required minimum slab thickness will almost certainly be less than that required by other functions such as control of deflections. Hence, the insulation requirement for composite slabs is very rarely a problem in fire resistant design.

Uniformly distributed load

0 0

Post a comment