Built-up wood beam section moduli

[btomcik]

Please pardon for the elementary question here I have on section modulus.

On a particular project I am working on, there is a timber built-up beam I am analyzing for capacity. Basically it is built-up out of three pieces of wood - a large rectangular beam, and then located on each side of the large beam towards its bottom is a smaller 2x section.

With the built-up area the neutral axis is closer now towards the bottom of the section. There are two section moduli that differ in value - S-top = I/c-top, and S-bot = I/c-bottom. S-top is the smaller of the two values due to the larger distance of the top of the beam to the neutral axis. In checking bending stress in the beam fb = M/S, I believe I need to be using the smaller of the two S's since that will provide the larger bending stress to compare to F-prime-b.

Can someone please confirm? Thank you very much.

 

[JAE]

You are correct. Use the smaller S.

 

[woodman88]

In my personal opinion you can not connect the three members together to use that approach. Unless you do like a LVL and pressure glue them together before applying any loads.
For nail/bolt/screw connected buildup member. I would calculate the maximum load and the deflection of the largest member. Than see how much load the lesser member(s) will take to match that deflection. Than add the loads for the maximum capacity of the buildup member.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[msquared48]

Woodman's answer is conservative.

Mike McCann
MMC Engineering