Tension Chord Bracing (Built-Up Steel Truss)

[StrucPhanatic34]

Hi all

I am engineering a welcome center and in the entrance room, there is a high curved roof that is being supported by built-up steel trusses that span 68 feet long and are generally 30 feet apart.

I was wondering if there are any provisions on tension chord bracing for these long steel trusses? Any help would be greatly appreciated.

Thank you.

Struct Engineer

 

[Lion06]

There are no provisions in AISC for tension chord bracing (unless there is uplift and it is able to become a compression chord) and AISC is silent on the issue (they leave it up to engineering judgment.
I personally believe that tension chord bracing is a good idea and always use it (if calcs show it necessary). If you think about it, the only thing bracing the diagonals that are in compression are the top and bottom chords. The top chord is usually sufficiently braced, but the bottom chord also needs to provide adequate strength/stiffness to act as a brace for those diagonal compression members.

 

[SteelPE]

The only thing I have seen in regards to bracing is in appendix 6 of the AISC 13th edition. However, there is nothing there in regards to tension bracing.

Section D1 of the AISC does state that an “L/r preferably should not exceed 300” for tension members. This could be achieved by increasing the member size or some form of bracing… but as stated, it’s preferred but not required.

Most of the trusses I have designed are for roofs and therefore required some amount of uplift.

 

[LOKSTR]

StrucPhanatic34,

There is a very informative AISC engg. Journal by James S. Fisher ( 1983-Q3) on tension chord bracing. You can buy it or if you are a member you can download it free.



Lokstr

 

[WillisV]

This was discussed in some detail about 3-yrs ago with dissenting opinions - see thread507-124363

I see an awful lot of stadium trusses etc. with no tension flange bracing...

 

[Lion06]

It could be that the stadium bottom chords have adequate out-of-plane strength/stiffness without the bracing.

 

[JKStruct]

I've certainly used it in certain situations... think standing on a suspended jump rope.

 

[Lion06]

Just out of curiosity, how is failing to brace the bottom flange at any location any different than failing to torsionally brace a beam at its ends?
While I may agree that the buckling length of an individual web member would be the actual length (k=1), that still doesn't address the idea of global stability (i.e. LTB of the section).

I'm thinking of sidesway web buckling in a steel beam. I have this paper (I'm attaching it) that talks about how sidesway web buckling manifests itself and how a compressive stress field in the web acts as a column and is dependent upon the tension flange to brace it out-of-plane.

I'm failing to see the difference between this and bracing the tension flange of a truss.