Bolting two PSL beams

[AlexWong122]

Hi All,

Is it practical to bolt two PSL beams? Due to large span and large force from column upstairs, a single 7” x 11 7/8” PSL is not enough, therefore I am thinking to use two PSL beams instead of steel beam.

Alex

 

[KootK]

Depends...

1) If most of your load is coming in from the top and can be delivered to both pieces, then I'd say you're good and the bolting might even be nominal.

2) If most of your load is being delivered to the top of one of the PSL, then you're probably good there too and I'd design the bolting for the load that you need/expect to be transferred to the unloaded piece.

3) If a substantial part of the load is being delivered to only one side of only one PSL, then I share your trepidation. Even then, if you wanted it badly enough, you could probably get this done convincingly with wood screws angled up into the unloaded piece.

 

[XR250]

I'm worried about Koot's #2 and #3 due to the final width (14" ?) Seems like you might get into a torsion issue.

 

[KootK]

Seems like you might get into a torsion issue.

4) You have this with all side loaded built up members to some degree. We either wish the condition away via judgment, engineer it explicitly, or avoid it.

5) A simple way to handle it here might be to deal with the torsion by splitting it into a bi-moment couple: extra downward on the loaded piece some upward on the unloaded piece.

6) For #2 above, I would submit that would be little different from a side loaded single PSL. And surely we're comfortable with that?

 

[skeletron]

Watch out! In my limited experience those PSL beams are a pain to drill through. I've had to take a handful of calls from crews spitting fire after seeing the requirement for multiple through bolts in a similar size beam. I'm not sure what the issue is for certain (hardness of the product, availability of the beam, size of the hole) but I'd check with some contractors about their preference too...

 

[jayrod12]

Hardness of the product would be my guess... have you ever tried manipulating that stuff? It's hard on saw blades and drill bits.

 

[KootK]

If you have access to the construction team, maybe query them on what they dislike most: steel or PSL fastening. Head the griping off at the pass.

 

[XR250]

) You have this with all side loaded built up members to some degree. We either wish the condition away via judgment, engineer it explicitly, or avoid it.

I guess my fear is the aspect ratio - 12" tall, 14" wide. Most side loaded members are much narrower and sometimes have other things framing into the sides.

 

[KootK]

I don't disagree. It's just something requiring designer attention. Note that if one follows the bi-moment approach to torsion that I mentioned, combined with predominately one side face loading, the two plies would be little better than one anyhow.

 

[XR250]

Bi-moment - oh, got it. One acts in reverse bending. Depends if that end is held down sufficiently, but yea, I could be on board with that.

 

[KootK]

Depends if that end is held down sufficiently

If it needs much, it'll not prove a good solution.

 

[dhengr]

Presumably, some of that ‘bi-moment approach to torsion’ will take place as the combined, two piece member, takes the one sided loading, in acting together. That’s just the shear stresses due to torsion working on the two members, on one side or the other, up or down. But, don’t forget that the reason for adding the second, side-by-side, member was because the single member was overstressed already, in bending and torsion. So, adding more bending stress to the first member because the second member acts in upward bending, to overcome some torsion, really does not solve the problem. You really do have to fix the two members together in some way, then their combined bending and torsional properties and strength are truly improved. That is, some sort of a shear flow on the vert. faying surface btwn. the two side-by-side members.

 

[KootK]

But, don’t forget that the reason for adding the second, side-by-side, member was because the single member was overstressed already, in bending and torsion.

It was not forgotten.

Note that if one follows the bi-moment approach to torsion that I mentioned, combined with predominately one side face loading, the two plies would be little better than one anyhow.

Unless the member is predominately top loaded, the two ply solution will likely not bear fruit.