Built-Up Column Bracing

[BouncedPod]

I am currently working on a project that involves removing the mid-height bracing of a glulam column. The column is 20' overall and attached/braced by the roof framing at the top, and has ceiling framing/bracing at 10' above grade. The project involves removing the ceiling bracing, and increasing the unbraced length from 10' to 20'. I am thinking about adding 2x6 nailers to the narrow face of the column to increase it's overall dimensions. I want to ensure that the existing column and nailers "act" together. I was thinking of sizing the connection between the nailers and the column using shear flow with a constant shear force of 2% of the axial column force. What does everyone else think of my approach?

Attached is a quick sketch of what I am thinking.

 

[JAE]

The NDS has in their appendix a reduction factor (for axial capacity) for built-up columns based on specific fastening requirements.
You might look at that and see what sort of correlation you can get with your own home-brew method.

The problem with your 2% method is that if the column starts to buckle, and the fastening you use can slip a bit to the extent that significant buckling mode deformation can occur due to nail slip, etc., then 2% probably isn't going to cut it.



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[BouncedPod]

I had initially started my design with NDS section 15.3.3 (nailed built-up columns), but interpreted the section to apply to columns where all plys are oriented in the same direction. I could add the nailers to the opposite sides of the column, but this will not increase d in KL/d calculation.

JAE, what do you think of applying 15.3.3 to columns where all plys are not oriented in the same direction?

 

[JAE]

Buckling can occur in either direction, right? The Kf factor applies to either direction depending on which controls FcE.

With plies in different orientations, I would suspect that a similar reduction in Cp occurs but perhaps not exactly what is shown.



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[JAE]

What you could do is ensure that you provide the same level of shear capacity in the fasteners between lams that is suggested by section 15.3.
Then use the associated Kf factor for nailing at 0.6....or perhaps 0.5 to be sure.

Then compare that result with what you come up with your own method for determining a possible shear value along the lams that you get from perhaps a second order analysis of an initially deformed column. This second order analysis might be difficult to achieve in terms of anything rational or accurate to reality, though.



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[dik]

You'll need a whole bunch of nails... best to use a quality adhesive... and you may have difficulty obtaining design values.

Dik

 

[JAE]

If anything, after thinking this through, the cross-ways orientation of your built-up column might actually improve the buckling resistance vs. one uniformly laminated.

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[BouncedPod]

Attached is my latest iteration.

The additional plys are attached to the existing column according to NDS 15.3, and the capacity of the built-up column was determined using Kf=0.6 (with Kf=0.5 there is also enogh capacity).

 

[JAE]

Looks stout.

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[KootK]

I don't love it. You've got that compound nail slip voodoo eroding your EI. You may have to make do with 16' pieces. Columns be kinda important. Your fasteners at the all important ends will be nail slip-o-palooza as your axial load attempts to enter and exit the reinforcing members locally.

I concede that you're playing by the book technically, so to speak. It's mostly a "guttural no" for me. I'd favor something similar to the detail shown below. Cumulative EI, no reliance on composite action. I know, it costs more.

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.