Information about adding leg load?

[Kiri Robbie]

Where can I find information about the necessity to add 30% to the leg load?

[Dmitrii]

And this was my question too. I do add 25% when using so called "Area method" of leg load estimation because of that double bearers can rest on 3 supports in the worst case but I haven't seen it in the norms. Where cen I find it? Thank you in advance.

Edited December 14, 2020 by Dmitrii

[Evan Field]

For typical conventional slab formwork, you would allow for an additional 25% to the centre leg of a triple frame setup with a 2 span bearer (eg. located under slab beams) to account for the increased internal support load due to a 2 span beam. Typically under flat slabs, you would design bearers based on a min 3 span bearer which results in an additional 10% to the 1st internal support. 

[Dmitrii]

On 12/16/2020 at 11:26 AM, Evan Field said:

For typical conventional slab formwork, you would allow for an additional 25% to the centre leg of a triple frame setup with a 2 span bearer (eg. located under slab beams) to account for the increased internal support load due to a 2 span beam. Typically under flat slabs, you would design bearers based on a min 3 span bearer which results in an additional 10% to the 1st internal support. 

Thank you Evan. This exactly matches with what I have been doing.

[Justin Smith]

AS3610: 1995 has always had the requirement for a 1.3 (30%) increase in FW loads when designing for the strength limit state. AS3610.2 2021 DRAFT also has the requirement for 1.3 LF. This is over and above the 1.25 continuity requirements for a 2 span support. 

The 1.3 factor is essentially an adjustment between limit state and permissive stress and also based on field studies that showed loads were +/- 30-40% of what was predicted by area tributary methods for determining leg loads. The main reason is for timbers spanning over a support owing to small gaps between jacks and headers. Note that the factor does not need to be applied for single span systems (single joist span and single header span as is the case with most modular systems).