Roof Bracing

[Alves21]

Hello Everyone.
I was asked to verify a design of a roof structure. Everything was fine till I started to check the braces. It was used a X bracing model and the bars where considered as tension only. But the profiles used where 3" starred angles (very common on industrial sites over here). For me it's very hard to consider such configuration working on as tension only model given that the profiles have a considerable rigidity when compared to rods or single angles and so X brace should be considered a tension/compression model. What is your toughts on this? Also should I worry about flexure of the bars given by self weight of the starred angles? Bellow I've atached a sketch of the situation.

 

[jayrod12]

We don't have dimensions to know for sure, but I feel that double angles that are as long as those appear don't have tons of capacity in compression compared to tension. I'd bet that the stiffer load path is through the beam/girt/purlin in compression to engage the other diagonal in tension. We idealize HSS bracing as tension only where I practice all the time. But we're non-seismic.

 

[Alves21]

Sorry, I forgot to mention. Those are 8m long. Also, I would have to add a gusset plate at intersection of the bars, wouldn't that afect the tension only hypothesis. Here is also a non-seismic área.

 

[jayrod12]

a gusset if anything would add a source of flexibility which in my mind, improves the tension only argument.

 

[GeorgeTheCivilEngineer]

I'd be OK with a tension only assumption. It is likely only an angle instead of a flat bar to reduce sagging or wobble.

 

[Alves21]

Wouldnt The gusset plate at intersection create a brace point to the braces thus reducing their buckling length and making them more susceptible to compression?

 

[GC_Hopi]

AISC had some recommendation for the maximum slenderness that braces should have as a tension only member.

Edit: Here is the discussion I was thinking of Link

 

[jayrod12]

What braces it at the gusset? the entire thing could buckle one way or the other, maintaining the 8m length.

There are some that indicate the tension diagonal braces the compression diagonal. Maybe that's true, but I haven't seen anything in writing to that effect.

 

[Alves21]

So we are assuming that a middle gusset plate like the one in picture wouldn't change the tension only assumption. It's not easy to my eyes because when using rods both bars are continuous and they can overlap one another without a middle connection.

 

[straub46]

If the angles were to buckle assuming compression, wouldn't the system revert back to a tension only system, with the struts carrying the full compression? Rather than decreasing the tension in the braces, just design for full tension assuming zero compression capacity in the angles.

A gusset plate doesn't do anything to help the out of plane buckling as we're looking at the connection. I would have no problem considering a 25' long 2L3x3 angle as tension only.

Go Bucks!