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Deflection of built-up LVL column, weak-axis

Deflection of built-up LVL column, weak-axis

Deflection of built-up LVL column, weak-axis

(OP)
Continuing with my wooden elevator shaft design project but seemed worth it to make another topic.

As would be expected my guide rail supporting columns are deflection controlled. I'm preliminary going to use a 3-ply LVL column built-up with the rail bracket bolted to the flat of the LVLs. However, my worst deflection will now be in the weak-axis of the built-up column. So, my question is how do I account for the interaction of the plys with the screws for the deflection? I imagine the slip to be relatively negligible but not non-existent.

With a 11' column and a 0.125" deflection limit I want to get this as precise as practical.

Maine Professional and Structural Engineer.
(Just passed the 16-hour SE exam, woohoo!)

RE: Deflection of built-up LVL column, weak-axis

Use more lag screws or bolts than usual!.........or, more to the point.........prestress! plys together to prevent sliding (think of High Strength bolted joint in steel).

However....there may be practical problems with potential for small lateral movement of connection at each end of the column, for wood especially.

Considering very small allowable deflection........how about steel column?

John F Mann, PE
www.structural101.com

RE: Deflection of built-up LVL column, weak-axis

I would design the quantity of fasteners for the computed horizontal shear and just assume it acts as solid piece at that point. will likely get you close enough. You could also just use a solid piece (as long as you don't have to install it)

RE: Deflection of built-up LVL column, weak-axis

(OP)
They're trying to avoid steel for cost and ease of installation. For now I believe it is doable with wood and like will be significantly cheaper. However, first sign of this becoming impractical and I have no problem switching to steel (and have told them as such).

Good idea with the prestress to get some clamping friction on the plys. Where the rails bolt to these wood columns I'll have them install them with through bolts and a plate washer on each side. This should allow me to develop a fair amount of prestress in the bolts. I'm also going to have them glue each ply together and wont skimp on the number of connectors, as you said. I'm just hoping someone has a reference so that I can quantify the degree that fastener slip between plys affects deflection.

As for the column ends, I will likely have a number of simpson clips connecting to the floor and ceiling framing. The actual rail loads are only 1,000 pounds at most, so I don't need much to keep the posts anchored. The posts will also have continuous plywood framing connecting all the floors inside the shaft.

Maine Professional and Structural Engineer.
(Just passed the 16-hour SE exam, woohoo!)

RE: Deflection of built-up LVL column, weak-axis

(OP)
XR250: The horizontal shear is almost non-existent so the prescribed minimum fasteners from the LVL manufacturer and the NDS controls (and I plan to conservatively use more than that). But, yes, otherwise I plan to just consider it a solid piece and ignore fastener slip between the plys. I'm also using a reduced E value and plenty of conservative extra moment of inertia to make sure this doesn't get fouled up. No sense putting in a $100,000 elevator only to be burned because I tried to save $100 of LVL.

As for a solid piece, I need the E value that an LVL provides, dimension lumber is just too flexible for the span (without making a huge wall thickness). A single ply 3.5" LVL isn't deep enough, so I need 2 or more plys.

Maine Professional and Structural Engineer.
(Just passed the 16-hour SE exam, woohoo!)

RE: Deflection of built-up LVL column, weak-axis

Glue is a great idea IMO. Alternately, can you just screw laminate a bunch of 1.75 x 7.25 LVL together to form the same section? That way, you wouldn't have a lamination interface perpendicular to the load.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Deflection of built-up LVL column, weak-axis

(OP)
KootK: By screw laminate, you mean rotate the interface 90 degrees? The problem is I would then be bolting the elevator guide rails into the interface between plys, rather than onto the flat and engaging each ply. In addition, my impact load on the rail provided by the elevator company is either 90% in the horizontal x-axis or 90% in the y-axis, so no matter which way I orient the plys there will still be a load case where the lamination interface is perpendicular to the load.

Maine Professional and Structural Engineer.
(Just passed the 16-hour SE exam, woohoo!)

RE: Deflection of built-up LVL column, weak-axis

There are solid, engineered lumber members which are wider than 3.5" (as I recall, they are called Parallams).

DaveAtkins

RE: Deflection of built-up LVL column, weak-axis

(OP)
Dave: Good point, Parallam (Weyerhaeuser's PSL brand) come in 7" widths. However, as a guess I imagine these widths are custom order, which would make me rather use steel since time is somewhat a factor. Good point, though. If I can't use an LVL comfortably then I'll keep PSL's in my back pocket if the contractor gets grumpy about using steel.

Maine Professional and Structural Engineer.
(Just passed the 16-hour SE exam, woohoo!)

RE: Deflection of built-up LVL column, weak-axis

That was what I had in mind TME. You could bolt a plate to the post and then connect the rail to that. It might be a good idea no matter what you use to mitigate cross grain bending. Not sure if you can weld in this environment.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Deflection of built-up LVL column, weak-axis

(OP)
KootK: Well, the rail comes with a fairly large plate that bolts to the supporting member. I can weld in the environment but not while the metal is in contact with the wood, I don't think we could get away with that without a huge fire hazard.

It is also worth pointing out that the 1,000 pound load I'm designing for is the impact load on the upper floor only. The actual service loads are 260 pounds at most, so 25% lower and hopefully will only require a single ply (haven't gotten there yet in my design).

Maine Professional and Structural Engineer.
(Just passed the 16-hour SE exam, woohoo!)

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