Composite Truss
Composite Truss
(OP)
Guys,
Have anyone of you modeled a composite truss where the top chord is composite? Do you have any suggestions on how to model it in say ETABS or SAP?
The truss is 100' long and carries a roof top garden about 300 psf SDL.
Thanks
Have anyone of you modeled a composite truss where the top chord is composite? Do you have any suggestions on how to model it in say ETABS or SAP?
The truss is 100' long and carries a roof top garden about 300 psf SDL.
Thanks






RE: Composite Truss
I note that you have used the term composite truss before and wasn't sure exactly what that was. Is it like a composite beam i.e. a beam to concrete slab with composite action? except that its the top chord of the truss in composite action with the concrete?
Just want to be sure we are talking the same lingo !
Cheers!
RE: Composite Truss
Any references, reading materials in this regard is very much appreciated.
RE: Composite Truss
1. A continuous member with top flange encased.
2. A continuous member with metal sheeting and shear studs.
3. A typical truss with metal sheeting and shear studs (unusual case).
Also, how is the composite truss supported.
1. At ends of the top chord.
2. At ends of the bottom chord.
3. both
For a continuous top chord supported on its ends, I think the top chord (with the concrete)essentially is a beam for strength and flexural considerations. The remaining part (the truss members below) should be utilized for deflection calculation only.
RE: Composite Truss
A continuous top chord with studs + metal deck + concrete
Supported on the top chord at the ends, bottom chord braced at ends.
RE: Composite Truss
RE: Composite Truss
RE: Composite Truss
Dik
RE: Composite Truss
Also I downloaded the ASCE task committee document from 1996.
But I am looking at real world advice and tips from people who have actually designed these trusses.
Thanks
RE: Composite Truss
RE: Composite Truss
RE: Composite Truss
RE: Composite Truss
Technically, a starting point might be to go about it in a similiar manner to a composite steel joist (see http://ww
Calculate a moment capacity for the truss by summing moments of the forces in the tension member (bottom chord) and compression concrete block combined with some portion of the top chord (depending on the location of the PNA, shear transfer capacity of studs, etc).
Then design the truss web members for whatever shear you have just as you would a normal truss (the composite action is going to help flexure only i.e. top and bottom chords, it will not do anything to aid in the shear strength and therefore the web members can be designed using typical non-composite assumptions).
RE: Composite Truss
WillisV, thanks for your advice. This is my weekend exercise to figure this thing out.
RE: Composite Truss
RE: Composite Truss
1. The top chord is a relatively rigid beam (composit) supported on ends. Its deform-ability is much less than the free hanging bottom chord (no support at ends but braced for sway only).
2. The diagonals are pinned to the top and bottom chords, and follow the deformed shape of the top chord, however, the diagonals will not shorten or elongate.
3. Due to compability of displacement (the deformed shape of the bottom chord has to agree with the top chord), now the bottom chord(s) will experience forces carried through the diagonals. (Please note the bottom chord can be a continuous member, or formed by segments)
The analysis and design by hand will be quite tedious. The best and quick way to gain understanding on the behavior of this composite beam/truss is to model it on a PC with proper joint releases. Good luck, and have fun.
RE: Composite Truss
Thanks for your suggestions. I tried calculating the effective moment of inertia by using transformed area for the top chord and area of bottom chord. I reduced this by 15% to account for web axial deformations.
However, when i run the same in an analysis program, I am getting different results. Is there a suggested way to model the composite top chord? Once thats done, I guess the problem of modeling is solved. Any tips?
Thanks
RE: Composite Truss
RE: Composite Truss
Thanks for taking the time to help fellow engineers.
RE: Composite Truss
RE: Composite Truss
with bar joists - upside down - then pour in say 4'' of concrete which encases the top chords of the joists. Let it harden and ship to the site.
Sorry - cannot remember the names.. maybe Google??
RE: Composite Truss