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Re: Torsion on Wood Beam

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Wow, I was struggling with this earlier today.  I couldn't find anything in the NDS but found some information in the text book from Faherty and Williamson, "Wood Engineering and Construction Handbook" 2nd edition page 4.17.  Given a beam with depth 2a (in) and width 2b (in), the torsional stress is:

fs = Torque (in-lb) *(3*a+1.8*b)/(8*a*a*b*b)

The page refers to the "Wood Handbook" and gives the allowable torsional shear stress (Fs) as Fv for sawn lumber and the allowable radial tension stress for glulams (Fv/3?).
This reference shows you how to calculate the torsional stress but no guidance after that.

Having calculated the stresses, I guess you would need to use a form of the interaction equation:

   fs/Fs + fb/Fb + fv/Fv < 1.0  ????????????????

In my case, the archiitect wants to remove some columns that support trusses.  The bottom of the trusses is at 8'-9", so a GL6 3/4 x 18 centered under the support affects the height clearance.  I tried putting the bottom of the glulam flush with the bottom of the truss and side loading the glulam.  I was coming up with a much bigger glulam using the above approach.  As an alternative, I'm looking at using a W10 beam under the truss support or a laterally loaded HSS12x4.  I have to look at the building sections in more detail, I might be able to brace the top of the laterally loaded glulam up to the roof diaphragm.

On 5/9/2012 1:29 PM, Larry Hauer wrote:
Anyone know of some simple formulas, or reference, for designing an unbraced wood beam that has a torsional moment, (or twisting), at the center of the span as well as bending due to vertical loads on the beam?

Thanks in advance,

Larry Hauer, S.E.


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