Light steel load-bearing frames are increasingly used in buildings of up to 8 storeys, for which longer periods of fire resistance may be required, The introduction of the new BS EN 1365 fire test regime has required an extensive programme of re-testing, as it is more severe than the former BS 476-20 test, due to the use of plate thermocouples in the BS EN test. Recent fire tests on load-bearing light steel walls show that 2 × 15 mm fire resistant boards fixed to resilient bars are necessary to achieve 90 minutes fire resistance. For floors, the fire performance is more sensitive to the particular details and boards used because, in fire, there is a tendency for the boards to bow or break away under their own weight, Furthermore, the load ratio acting on the joists is another variable, given that floor joists are generally not loaded to their full capacity when designed for control of deflections and vibrations. As part of the activities of the Light Steel Framing Group, a fire test to BS EN 1365 was carried out at BRE on 4th March in order to provide generic guidance for the light steel framing industry, The configuration of the light steel floor also satisfies the proposed Robust Standard Detail for acoustic performance in order that the information is widely applicable to practice. The floor configuration that was tested is shown in the figure. Its span was 4.5m and the imposed load was 250 kg/m2. The calculated load ratio for the floors joists in the tests was 0.3 using S350 steel to BS 5950-8, Using this load ratio, the test results may be extended to other joists sizes, loads and spans. The fire test supported the applied load for 93 minutes. Importantly, the joist temperature remained at less than 100C for over 80 minutes and so they would be essentially undamaged and re-usable, even after a severe fire. At 90 minutes, the joist temperature reached 280C, at which point the furnace temperature was over 1000C, The mean temperature of the upper surface was less than 60 C, which was well within the 140C limit, During the first 80 minutes of the test, the increased deflection of the floor was less than 5 mm, in addition to an initial 9 mm static deflection under load. Even at 90 minutes, the deflection had only reached 30 mm, which was well below the span/30 limit (or 140 mm) to the fire test standard. The information from this test will be used in the update of SCI Publication 129 Fire Resistance of Cold Formed Steel Members.
|
||||||||||||
© 2008, Innovation & Research Focus |
||||||||||||
For further information please contact Mark Lawson, SCI Professor of Construction Systems at the University of Surrey, Chair of the Light Steel Framing Group (01344 636525; E-mail: