Designating Cross Bracing on a Space Frame

[TobyGA]

I have a question about cross-bracing on a space frame, i.e. vertical diagonal bracing for the columns of my space frame. I am desiging a pipe rack for an industrial facility and am modeling it as as a space frame on STAAD-Pro. I have got some questions about the cross-bracing.

Here's my thing. I was told that if I just ran my space frame as it is without specifying special provisions for the cross-bracing, then:

(i) STAAD would analyze the cross-bracing for both compression and tension and STAAD would select unreasonaly large or members, or only large or heavy members would work for those cross-bracing members, and

(ii) my cross-bracing would fail in compression as it has a lower slenderness ratio. (Note: The limit of KL/r for compression is 200, while it is 300 for tension.)

QUESTION#1:
Don't the cross-bracing carry *BOTH* compression and tension, depending on where the loads, such as wind loads, are coming from? If the cross-bracing do carry *BOTH* compression and tension, then why can't I design those cross-bracing as *BOTH* compression and tension?

I was told that I should specify all my cross-bracing as MEMBER TENSION, so that the members would be more reasonably sized. I know there are certain "special" commands that I need to input into STAAD when I have members that are designated as MEMBER TENSION (such as SET NL f1, REPEAT LOAD, CHANGE, LOAD LIST n1 TO n2, etc.) and I am willing to do those, but I *NEED* to understand the rationale behind my actions.

QUESTION#2:
If I do designate all my cross-bracing as MEMBER TENSION, what happens to the compressive loads acting on the cross-bracing? It is not as if all the compressive forces would disappear altogether if I specify all my cross-bracing as MEMBER TENSION only?

(Note: Members designated as MEMBER TENSION can only carry axial forces, and not moments or shearing forces. They are, essentially, like a piece of steel spaghetti or a piece of steel rope.

QUESTION#3:
I was also told that one way of getting around this problem is to use only *one* bracing on my pipe rack, instead of the cross-bracing. This would essentially *force* or *trick*STAAD into designing the single bracing as a "tension-only* member? Did I get this right? Later, I was to get the axial load for the single bracing from the STAAD output and use it to design the other bracing member. Essentially, I would still end up with a cross-bracing, but I would have gotten STAAD to do it without my having to input the MEMBER TENSION command? Does this make sense and does it work? If it works, can someone please explain why this is so?

Would appreciate comments or answers from experienced members who have come across this problem before and are very experienced with the design of cross-bracing members and understand very well how the MEMBER TENSION command works.

Thank y'all and have a great weekend.

TobyGA

 

[JMorrison]

Yes, if you run Staad without the MEMBER TENSION command, you will have forces in both diagonals - this isn't a mistake, and it is a valid design methodology. The decision whether to use C/T bracing or Tension-only bracing is made based on engineering judgement. You can refer to pretty much any structural analysis book for the concepts involved.

As far as your other questions, when operating under a MEMBER TENSION loop, Staad detects members that have compressive loads under that load case, sets them to inactive, and reruns the analysis. This is the analog to the assumption that these members are buckled and therefore ineffective under compression loads. Just using a single diagonal to start with is an appropriate solution, and one that was used quite often back before we all got these fancy computers *grin*.