Cross bracing in Compression

[JStephen]

A question for you folks.

Let's suppose you have a structure supported by steel columns with cross bracing in an X-shaped configuration between columns. The structure is loaded vertically and with a shear load at the top. Under what conditions is it permissible to neglect compressive stresses in the design of the cross braces?

If the cross braces are very thin (IE, rods with turnbuckles), I can see how they would never carry any compressive load, yet would not be damaged by a minor amount of movement in the compressive direction- they would just sag a little more.

Or, if the member sizes were comparable to the columns themselves, then the braces would be expected to carry part of the vertical loads as well as tension or compression due to shear at the top.

But at what point do you transition from one case to the other? Is it the whim of the designer? Or do you pretend that any member that is inadequate to carry its calculated compressive load just doesn't have any load? Is there any particular l/r ratio that you could use as a cutoff?

I'd be interested to know your thoughts on the topic, and would be especially interested in any references that might clarify this.

Design codes could vary, but AISC-ASD would be typical.

 

[Alekk]

I usually consider tension members only (but not under cyclic/seismic loads!). I Just check that member slenderness is between 100 (to buckle in copmpression) and 250 (not to yeld in buckling), but I've never find it in any design code.

Ale

 

[JAE]

You can check the member's compressive capacity against the load that would be delivered into it under each required load combination. You would then have three possible conditions:

1. The compressive capacity is lower than all the load combinations - i.e. it buckles way before any load combination is satisfied.

2. The compressive capacity is above some load combinations and below others.

3. The compressive capacity is above all required load combinations - i.e. it never buckles under all combos.

Since the wind/seismic load factors (or ASD combinations for W/E) don't vary all that much, condition 2 above would probably rarely occur...you'd be in either case 1 or 3.

For case 1 - you've got a weak compressive element that you can ignore. Case 3 - you'd better consider it as it WILL influence the behavior of other elements in the frame.

 

[Ibeam]

You're speaking about the stiffness matrix. If the design is by hand, then you do need to set some limitations as the previous posts imply. But it's all about deflection/flexibility/stiffness, something we usually let a frame program figure out. Probably most of us have solved a stiffness matrix by hand for a frame in school, but I doubt there's many that can do it later in life. I know I can't.

 

[Grizzman]

Ignore the compression strut, and assume simple span of the diaphram. The compression member will inevitably carry some compression, but by ignoring it you add a level of reserve resistance.
I generally design all "X"'s for the maximum force resisted by any brace. Occassionally, if the load is too evenly distributed, I'll increase the load by 15% or so. Of course I primarily work with low rise stuff, where a few pounds more steel won't break a project. If it gets to a point where it's a budget breaker, you need to use a frame program.
If you're facing an "X" and a moment frame along the same axis through a rigid diaphram, a safe design would be the "x" takes all the force and the moment frames prevents deflection of the diaphram.

 

[JStephen]

Thanks for the input, all. JAE, your approach to the problem seems very reasonable, and it appears the other responses could be considered as case #1 in your response.

The problem I ran into some years back was that treating the braces as tension only made them large enough that I didn't feel like I could neglect compression loading in them. In a single X-braced frame, the tension and compression in the two struts from shear are about equal. Once you start designing the braces for compression, the larger l/r gives a low compressive stress, which makes the braces much bigger still. And of course, when you make the braces bigger relative to the column size, even more of the vertical load is carried in them.

I've never seen this issue addressed anywhere, and it is nice to see that there is some variation in the approaches used here! Thanks again.