Fire Performance of a Laterally Loaded Light Timber-framed Compartment

摘要

The New Zealand Building Code (NZBC) deemed to satisfy solution for houses and small multi-unit dwellings requires external walls within 1 m of and at angles less than 90° to a property boundary to be fire-rated to a minimum 30-min fire resistance rating (FRR). The NZBC also requires structural building systems to remain stable during and after fire when subjected to a uniformly distributed horizontal face load of 0.5 kPa ‘in any direction’. This research investigated the fire performance of laterally loaded light timber-framed compartments, to assess their suitability under the NZBC requirements for residential buildings.The research involved a full-scale standard furnace experiment and a full-scale compartment fire experiment. The building design was based on common New Zealand residential building construction for a light timber-frame building with compartment dimensions of 4.33 m × 3.35 m and stud height of 2.4 m. One of the 4.33 m walls was 30-min fire-rated and the other building elements were of typical non-fire-rated construction. Internal wall and ceiling linings were 10 mm thick standard grade plasterboard, except for the fire-rated wall which had 10 mm fibre-reinforced plasterboard on both sides of the timber framing. The external cladding and roof consisted of light-weight sheet materials except for the fire-rated wall which had no additional covering. The fire-rated wall was subjected to a lateral load applied at the top plate equivalent to a 0.5 kPa face load. The roof truss system was an integral component in providing lateral support to the fire-rated wall and each roof truss spanned between the fire-rated wall and the parallel 4.33 m wall. Each roof truss was designed with a splice in the centre of the bottom chord, connected by toothed metal connector plates.In the furnace experiment the compartment was heated to the ISO 834 standard time-temperature curve, failure of the roof truss system at the truss connector plate was observed after 30.5 min, causing failure of the roof resulting in lateral deflection of the loaded fire-rated wall. It was found there was non-uniform temperature distribution in the compartment. An analysis of the failure taking into consideration temperature distribution in the compartment suggests that the roof truss system with splice supporting the fire-rated wall would fail in lateral stability after 26 min if the furnace had been driven to achieve the ISO 834 time-temperature relationship at ceiling level. An analysis of expected performance of an unlined compartment predicts a lateral stability failure time of 19.5 min if exposed to the ISO 834 standard time-temperature curve.The compartment experiment was a natural fire with a fixed initial ventilation and fuel load consisting of wood cribs. The roof truss design incorporated specific protection of the splice using blocks of timber. The wall system failed in lateral stability after 28 min and before all the fuel in the compartment was consumed. Applying a time equivalence method suggests that the fire-rated wall restraint system performance would be the equivalent of 33.5 min in a standard fire resistance test for the compartment. Comparing the results for Experiments #1 and #2, the protection added to the splice improved the performance of the lateral load restraint system from 26 min to 33.5 min. An analysis of expected performance of an unlined compartment predicts a lateral stability failure time of 26 min if exposed to the ISO 834 standard time-temperature curve.