Truss tension chord bracing 5

[MIStructE_IRE]

Forgetting roof trusses which are subject to stress reversal/wind uplift - I would have always braced the bottom (tension) chord Of a floor truss, at least at a couple of locations, so that the internal compression members are restrained - or at least held by a bottom chord capable of laterally spanning between the restraint points.

For me, if the bottom chord has no restraint then the Effective length of the internal diagonals Is unquantifiable.

However I’ve recently seen some trusses where the bottom chord is not restrained And I doubt very much that the bottom chord can withstand any lateral kick.

What are your thoughts? Random photo below just for reference.

 

[Agent666]

I believe its a valid concern.

If you search google for 'importance of tension chord bracing', there is a good paper that AISC published which comes up in the initial result which addresses and discusses these effects. Same applies to steel or timber in my view.

 

[r13]

I think bottom chord bracing is a good practice, and I provided it on all truss projects.

 

[human909]

Oh dear... I feel an urge to buy into this discussion with some more modelling... Except I really don't have the time.... But...

EDIT: Ok that was quicker than I though. Yep a quick buckling analysis showed a significant when from an buckling load factor of 5 to 13 just but a single mid span restraint on the tension span. This was for 1mx1m panels 8m long. Not too hard to bump up a few member sizes and bump up the loads and make buckling of the lower flange to be the primary failure mode...

 

[KootK]

1) Not that you asked but this is the paper that introduced me to tension chord bracing: Link.

What are your thoughts?

2) For very light, top chord supported truss of the sort that would normally be called open webbed steel joists in north america, I do feel that the bridging is important for this reason, among others. And the Steel Joist Institute prescriptive provisions on bridging seem to be mostly in line with my expectations in this regard. I like to see, at a minimum, two lines of bracing near the ends of these kinds of trusses, sometimes more for longer spans.

3) Curiously, I've seen numerous examples in the wild of short floor joists trusses with only a single line of bracing at mid span. This flies a bit counter to the voice of my intuition which tells me that you'd at least want two lateral bracing points on the bottom chord so that that bottom chord can brace the compression webs in a spanning, girt-ish fashion like the bottom flange of a steel beam. With just one brace point at mid-span, it feels like the bottom chord would just want to spin around that. As I'll get into below, there are mechanisms of tension flange buckling resistance other than the girt-ish one and I recognize that they increase capacity and would sometimes be helped by a single line of bracing. Still, I like the girt-ish mechanism for ease of evaluation if nothing else.

4) There are some trusses that simply need to not have their tension chord braced for architectural reasons. Exposed steel trusses over conference centers, sexy heavy timber trusses... stuff like that. They're out there and seem to work. In studying these situations myself, I've come to the conclusion that the self weight of the bottom chord itself is often adequate to brace the tension flanges of heavy trusses. Additionally, where a concrete slab is in play and/or the top chord is an HSS etc, torsional stiffness can help out a fair bit with the bracing function.

5) I've come to view the question of truss tension chord buckling as quite analogous to wide flange beam lateral torsional buckling. Rather than ask "why does a tension chord buckle?", I ponder "why doesn't a tension flange buckle?". The answer is, of course, that wide flange beams generally have a much more favorable combination of torsional stiffness and code mandated rotational support at the ends. It's an interesting parallel regardless however. I've also considered this explicitly for beams used as josit substitutes where you basically top flange hang your beam to match the depth of your joists seats and, thus, rob it it of much of it's normal end rotational restraint (snipped below).

And I doubt very much that the bottom chord can withstand any lateral kick.

Technically, the bottom chord doesn't need to withstand a lateral kick from a stability perspective. Rather, it only need to return to it's original position when you're done kicking it.

 

[university_professor]

check longitudinal shearing.

 

[MIStructE_IRE]

So if we agree that the bottom chord doesn’t need to withstand lateral kick, the effective length of the strut highlighted below would be difficult to calculate correctly since it could in theory ‘whip’ laterally? I do get what you’re saying that its in tension so could in theory pull itself back into line... but I’m not sure how I could argue that in court if I did have an issue.

Taking the example of a non pre-tensioned bar as the bottom chord - Historically, in the late 1800s over here we would have seen countless iron bar trusses which have no bottom chord restraint. I always wonder how you could really justify any assumed effevtive length of the internal diagonal strut.

Point A below of course is restrained in position and direction, point B however is not...really..kinda..sorta..

 

[MIStructE_IRE]

Also - just to note.. the example image above is purely for reference. This thread has nothing to do with composite trusses or longitudinal shear.

 

[KootK]

So if we agree that the bottom chord doesn’t need to withstand lateral kick, the effective length of the strut highlighted below would be difficult to calculate correctly since it could in theory ‘whip’ laterally?

It's super easy to calculate. K = 1.0; KL = compression web length. Using bifurcation methods, it's a binary thing: either the panel point kicks out... or it doesn't. The reality is more complicated of course.

I do get what you’re saying that its in tension so could in theory pull itself back into line...

I don't believe that I did say that. The caternary argument certainly holds water but only if the caternary itself makes its way out to some point that is itself laterally stable. The stability mechanisms that I'm proposing for most of these situations would be:

a) the restoring force created by bottom chord self weight for heavy trusses and/or;

b) a torsional connection to a supported CIP slab where the details of that would make sense.

 

[Triangled]

KootK
tension chord bracing
nice paper, thx for posting

 

[KootK]

You're most welcome Triangled. Now go forth and fret over arcane technical matters that do not slow down you competitors!!

 

[MIStructE_IRE]

That’s exactly it Koot. I’m on annual leave at the moment with too much time to think about nonsense that doesn’t seem to bother other engineers.

 

[KootK]

My first boss and mentor was a bona fide Mensa genius and, perhaps, simultaneously the laziest and most effective structural engineer that I've ever seen practice the dark art. He had a fascinating theory of structural engineers that I still subscribe to:

Level 1: so ignorant that they don't even know how bad they are. These guys either drop out early in terror / inadequacy or become principals by thirty because this makes for easy, profitable project management and smiley faced clients.

Level 2: smart enough to see the truth of things but not quite savvy enough to set aside the low risk stuff for the sake of their own personal gain. These guys spin their wheels in painful mediocrity and drop out mid-career if they don't go the distance. Principles instead of Principals.

Level 3: smart enough to see the truth of things and savvy enough to not be bogged down by it. Easier said than done of course. These guys will generally be principals, entrepreneurs or, later, owner's reps / developers.

All of this speaks to the most general of general trends of course.

My boss naturally fancied himself a level three -- and I believe it. Goodness knows I've known enough successful level one's. I was probably a level 2.10 when I first got the speech; I fancy myself maybe a level 2.70 now. Once you cross the threshold into level two though... there's just no going back and level three becomes the only oasis in which to find any peace without just up and switching careers. This dovetails back in with your "Are You a Real Engineer?" thread. I wonder if dentists struggle with this kind of thing. At my next gingival scraping, I'll be sure to ask my guy if he fancies himself a "Real Dentist" or just a hygienist herding administrator.

 

[human909]

5) I've come to view the question of truss tension chord buckling as quite analogous to wide flange beam lateral torsional buckling.

I agree completely, it is nearly completely analogous. Which is where my first comment fits into the picture. I think we have had a fair bit of discussion on LTB. This is yet another example why just bracing the compression flange only gets you so far, twist restraint is important.