Timber Beam in Torsion

[breaking_point]

Hey.

I have attached an image of my query. I have timber beams that support decking that is tied into the beams using lag screws that provide lateral restraint. Attached to the left support beam is a post at approx 1m c/c along the length of the beam. The design code requires me to check the post for a horizontal load at the top of the rail which is equivalent to 1kN. I know wood is weak in torsion as the force acts against the grain which is the weak section of wood. Would the attached decking provide adequate restraint to resist the torsion? Would I still need to design the beam to resist the torsion?

Thanks

 

[WARose]

Would the attached decking provide adequate restraint to resist the torsion?

Depends on it's flexural stiffness.

 

[jayrod12]

I would provide solid full depth blocking at each post for the first two joist spaces. This, combined with the decking, should resolve the torsion out adequately enough.

 

[JoshPlumSE]

Personally, I don't like loading wood beams in torsion. I always feel like the connections puts the beams in cross grain bending. Therefore, my tendency would be to connect the decking directly to the post as well. That way, the moment is taken out as a force couple. Beam gets a weak axis shear opposed by similar load going into the decking.

The Simpson detail looks pretty good to me. Plus the blocking seems to be the key to avoiding the cross grain bending.

 

[JoelTXCive]

Simpson has a connection set up for 36" rails and 42" rails. The attached literature is in pounds and feet, but I'm sure they have SI literature on their website.

See Attached PDF.

 

[breaking_point]

@WARose, Please could you elaborate? My thoughts were that the torsion would act around the centroid of the beam (where the bolts connect), and the top plane of the beam would try and twist left, while the bottom plane would twist right. I thought that the axial stiffness would prevent the top of the beam from twisting, as opposed to the flexural stiffness.

Cheers

 

[WARose]

My thoughts were that the torsion would act around the centroid of the beam (where the bolts connection), and the top plane of the beam would try and twist left, while the bottom plane would twist right. I thought that the axial stiffness would prevent the beam from twisting, as opposed to the flexural stiffness.

That isn't the way I see it. It almost certainly would try to (as drawn) bend the decking (as the whole section tried to rotate).

I like jayrod12's suggestion about blocking. (Lots of it too.)

 

[JoelTXCive]

Deck rail failures have been a hot topic lately. I believe all the prescriptive code compliant solutions have blocking in at least the external joist bay.

The AWC has a free document that has prescriptive code compliant setups for residential decks and posts. Even if your deck is for commercial loads; I would still start with their setup and then just tweak it.

Here's a link:

http://www.awc.org/codes-standards/publications/dca6

Also, attached is another article with connections (Structure Magazine July 2007)