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Attaching double timber bearers to metal posts

I am designing a 12mx6m hardwood decking under an existing pergola. The pergola has 3 90x90 metal posts along the 12m length. I have two main questions. I can use the span tables to calculate the FLW and thus the size timber bearers that I need but from what I read, they need to be double bearers.... so do I treat the double bearer as if it was a single piece of timber in the calculations eg, a double bearer made of two 190x35 timber beams is the equivalent to a single 190x70 bearer in the span tables?? And what if the double bearers are not nailed directly together but have spacers between them ... does this affect their FLW?

Also I need to attach the ledger to the metal posts but will also need more timber posts between the metal posts because of the large span of the deck. What is the best way to attach the ledger to the metal posts and the intermediate timber? as I can't notch into the metal posts as I would be able to for timber post. The ledger I am planning on is a single 190x70 softwood.

Thanks in advance

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Comments

MitchellMc

Welcome to the Bunnings Workshop community @liv. It's great to have you join us and many thanks for your questions.

I believe the second timber is required by AS1684 as a redundancy measure and to over-engineer the existing structure of the deck for safety reasons. When calculating the timber dimensions to be used as bearers on the deck, I would be of the opinion that you would calculate with single bearers. The second timber would be added as per AS1684 once you have already designed a structure which is sufficient to comply with the loads.

However, let me mention the fantastic @ProjectPete to see if he can steer us in the right direction.

Mitchell

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