We have a FRL 90/90/90 wall which needs to go to the underside of roof but has a break where a glulam beam runs through. The detail is similar to Figure 5.38 in the WS TDG 37C guide. The wall below and above the beam must achieve the FRL 90/90/90. A few questions: 1. What details do we need to include to ensure the beam is achieving the FRL? 2. We have a large cavity, do we need something in the cavity to prevent the spread of fire?
In Fig. 5.38 of TDG 37C timber members are protected by fire-resistant plasterboard. The glulam could be designed to AS 1720 Part 4, Fire resistance for structural adequacy of timber members, where members are oversized to provide sacrificial material that allows them to retain structural adequacy during a fire event. However, this would result in a large beam to cope with a 90/90/90 FRL. In your attached detail the glulam appears to be protected by plasterboard encasement. With regard to the wall cavity, cavity barriers are required to prevent the spread of fire via the cavity, which in your case is a large one of 277mm. Your attached detail is somewhat beyond the scope of our Technical Design Guides, and our advisory service, and it is difficult to gauge its relationship to the rest of the building. Note the sidebar to Fig. 5.38 in TDG 37C to the effect that Evidence of suitability must be obtained to demonstrate compliance with the required FRL We suggest you discuss this further with a fire engineer.
Could you advise what would satisfy this clause in AS1684? - 7.2.3: Where ceiling battens are used, the size and fixings shall be appropriate for the mass of the ceiling material used, to provide a flat finish to the ceiling. The standard specifies batten sizes based off 12kg/m2 but does not specify a connection. Would a nailed connection suffice or would it have to be screwed?
AS1684 is specific on ceiling batten sizes but, as you say, doesn't specify how to fasten them. If the ceiling battens are part of a trussed roof system, providing bottom chord restraint, the truss company should be able to provide connection details. Installation guides for products such as Gyprock assume the plasterboard will be direct-fixed to the underside of ceiling joist or trusses, in which case the plasterboard has the same restraining effect as ceiling battens at 600mm centres. Unfortunately we have no other data on fixing ceiling battens, but no doubt a consulting engineer could advise.
There is no standard time limit on the allowable weather exposure of timber framing. Perhaps moisture content has exceeded 20% for periods of time, with periods of drying in between, since it would be unusual to record such high levels of moisture for a continuous 12 month period. It might also be the case that the high moisture levels are only present in surface layers, rather than throughout the depth of the timber. In any event, there is not necessarily any harm done by a prolonged period of 20% moisture content. Loss of structural integrity (decay) can be checked by probing with an instrument such as a screwdriver, paying particular attention to joints. This topic is explained in more detail in our Technical Design Guide no. 12 titled Impact and Assessment of Moisture-affected, Timber-framed Construction. A copy can be downloaded from our website via this link https://www.woodsolutions.com.au/publications. If there are any glued products such as LVL’s on site, these are made with a Type A water-resistant glue bond as used in Marine Grade plywood. However the producers of such products may have their own recommendations on weather exposure.
