Wood and Steel Strap Columns

[rudy3663]

Hello All,

I have this design problem: a roughly 20' tall 5-1/8 x 5-1/8 GL column supports a ridge beam. The GL column is undersized for buckling and needs to be braced or built up in the field. (The column sits in a wall with lots of windows and is supported in the plane of the wall at about 6' intervals. The out of plane direction is the problem.)

A coworker suggested using continuous Simpson steel straps on each side of the column, with the theory being that the straps are strong in tension. I am not comfortable with this solution and haven't been able to find anything on designing built up columns with steel straps.

Anyone have any thoughts or advice?

 

[MiketheEngineer]

I don't see any real help here. The buckling problem will still exist on one side or the other.

Can you install a knee brace from ridge to column?? Or add an interior sister column that looks "nice"?? Or use a 5 x10 (dimensions??) that looks "nice" Or do it anyway and cover with drywall.

 

[Lion06]

I can't say it's wrong, but it just doesn't feel right. Before the column buckles, both straps will be in compression. When the column tries to buckle, one fo the straps will be in tension as the member deflects sideways. I'm not sure I see the strap restraining that or how to quantify it if I did think it would.
Instead of straps, what about 1/4" steel plates? They will actually provide some capacity that you can count on and calculate.

How close is the column to working? Can you beef up the properties or section a little to get it to work?

 

[msquared48]

The size of the glulam column seems odd, but that's another matter.

Only two options are possible at this point since the column and windows are apparently in place:

1. Install either an interior or exterior knee brace (if the ridge beam extends far enough outside of the window wall) of sufficient length to satisfy the column instability. The Architect will love this one unless you can arrange some form of a hammerhead knee-brace detail. Might work actually. And...

2. Install another interior and/or exterior column to build up the section in the weak direction, thru-bolting to gain some composite action. The result may look massive Architecturally relative to the rest of the wall, but that's the result of the undersizing in the first place.

What are the lateral deflection ratios in the window wall system currently at the column?

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[a2mfk]

If you have the space, what about a small steel channel lag screwed to the side of the column to provide out of plane bracing only. Then you can reduce your unbraced length until your column is OK, you won't likely need it to be full height...

Or take it out and put in a HSS.

The other suggestions are fine, your problem sounds like it is architectural more than structural.

 

[hokie66]

Doesn't the column have to be reinforced for bending due to wind load as well? Depends on how the rest of the wall is framed, but combined bending and compression could control.

 

[rudy3663]

Thanks for all the thoughts, everybody.

I should have mentioned that we were able to brace the column with horizontal beams at about the half way point, at a floor diaphragm level. The tall wall is two-story, so the beams will be exposed.

I was mainly curious about the straps, though.

Thank again.

 

[a2mfk]

Since the OP was about the straps idea, I think I can see how this may work in theory, but I would need testing to believe it. But its not much different then using carbon fiber to reinforce the skin of concrete..

 

[bradleyelwood]

A theory says that you can treat a strapped wood column like a composite beam with E, I, etc for wood and for steel. Then use the equivalent E and I for the column loading.... However, a simpson tie is usually designed for tension. Between nails there will be a radius of gyration of 0.288 times the thickness. If the thickness is maybe 16 gage, 0.060 inch you have an "r" of about 0.015 inches. (there would be a short column between nails of about l/r = 100 if the nails are on 1.5 inch centers.) Can you really depend on a sillion nails? What happens if a few nails are not set perfectly...

Be careful about mixing wood and steel, especially reinforcing one side of a wood column with steel. Wood shrinks and grows with moisture content (to a lesser extent in the longitudinal direction) and an assmetrical reinforcement may be a curved column in a few years.