NDS Section 3.3.3.5 Flatwise use bending 1

[jreit]

Hello. I usually don't design with wood so I'm a little out of my depth here. I have a timber lagging design that I'm doing a review on and the EOR is checking the bending stress for a 3x4 roughcut beam. However, he's using the actual span of 6'. From NDS 2015, Section 3.3.3.5 it appears that the bending moment should be calculated using an effective span length, le, rather than the span (lu) of 6'. When I made the comment, he stated that this effective span length does not apply for flatwise use.
I can't find anything in the code that explicitly states that.
9.4.1(a) states the design span for flatwise bending should be the distance between supports. It doesn't say effective span length which is what 3.3.3.5 refers to.
I wanted to check what the normal practice was before replying to him.

 

[Guest090822]

jerit - The EOR is correct, the effective span is not used when dealing with flat use. See section 3.3.3 (2015 NDS). The effective span is used when determining the beam stability factor, CL. If the depth (d) is less than the breadth (b)then no lateral support is required and CL = 1. If you think about it then it makes sense since forcing a timber member in a flat-use scenario forces it to bend about it's smaller dimension. It's predictable so the beam stability factor doesn't apply.

 

[jreit]

Yes I understand now. Thank you for explaining it.