Standard fire tests have shown that structural members which are not fully exposed to fire can exhibit substantial levels of fire resistance without applied protection. Methods have been developed using this effect to achieve 30 and 60 minutes fire resistance. Where higher periods of fire resistance are called for, reduced fire protection thicknesses can be applied to the exposed steelwork since the heated perimeter is less than that for the fully exposed case
There are four common ways in which this principle can be used:
Block-infilled columns - (Figure 34) 30 minutes fire resistance can be achieved by the use of autoclaved, aerated concrete blocks cemented between the flanges and tied to the web of rolled sections. Longer fire resistance periods are possible by protecting only the exposed flanges.
Web-infilled columns 60 minutes fire resistance is obtained when normal weight, poured concrete is fixed between column flanges by shear connectors attached to the web. The concrete is retained by a web stiffener fixed at the bottom of the connection zone. The load carrying capacity of the concrete is ignored in the design of the column but in fire, as the exposed steel weakens at high temperatures, the load carried by the flanges is progressively transferred to the concrete. This provides stability in fire for periods of up to 60 minutes. The connection zone at the top of the column is protected along with the beam.
Shelf angle floor beams - (Figure 36) are beams with angles welded or bolted to the web to support the floor slab. This protects the top part of the beam from the fire while the bottom part remains exposed. Fire resistance increases as the position of the supporting angle is moved further down the beam and fire resistance periods of 60 minutes are achievable in some instances.
Slim floor beams - (Figure 37 & Figure 38) In the UK there are two main slim floor options.
The first, known as SLIMFLOR, comprises a column section with a plate welded to the bottom flange to support deep steel decking, or in some circumstances pre-cast concrete slabs (Figures 37 and 38). Almost the whole section is protected from the fire by the floor slab and periods of fire resistance up to 60 minutes are achievable without protection to the exposed bottom plate.
The second option also used deep decking but removes the support plate by using an asymmetric beam (Figure 39).
This eliminates welding but retains the easy assembly and the 60 minute fire resistance properties of the original design. This system has been patented by Corus under the trade name SLIMDEK. The shape of the asymmetric beam is uniquely designed to give optimum performance in fire. A thick web / thin flange configuration gives maximum capacity under the non-uniform temperature distribution at the fire limit state.