Steel Truss
Steel Truss
(OP)
I am working on a project where I'm reviewing an existing pipe bridge for some new loads. The bridge sections are essentially trussed-up boxes, with trusses top, bottom and both sides. An elevation of one side of one span of the bridge is shown in the attached.
My concern is no diagonals to take shear near the center of the span. The truss isn't even symetrical. Does anybody ever design like this? What's the justification? If there is any shear whatsoever within that panel, then the truss panel would have to work like a vierendeel. Obviously it can't, with only double webs and WT chords. And why the "X" in one adjacent panel? An "X" is adjacent to every truss panel in every truss that's missing a diagonal. What am I missing here?
My concern is no diagonals to take shear near the center of the span. The truss isn't even symetrical. Does anybody ever design like this? What's the justification? If there is any shear whatsoever within that panel, then the truss panel would have to work like a vierendeel. Obviously it can't, with only double webs and WT chords. And why the "X" in one adjacent panel? An "X" is adjacent to every truss panel in every truss that's missing a diagonal. What am I missing here?






RE: Steel Truss
At least the splice is not in the middle.
Maybe it's a test, to see if you'll actually raise a flag and say "This ain't right".
I'm guessing it was drawn by someone who doesn't understand how trusses work.
Mike Halloran
Pembroke Pines, FL, USA
RE: Steel Truss
BA
RE: Steel Truss
Can you verify it in the field?
Mike McCann
MMC Engineering
http://mmcengineering.tripod.com
RE: Steel Truss
RE: Steel Truss
Mike McCann
MMC Engineering
http://mmcengineering.tripod.com
RE: Steel Truss
Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.
RE: Steel Truss
Woodman, the chords do have to resist bending in the open panel, but only because there is shear there. Without shear there is no bending.
I'm about to blow the whistle on this, and would defintely welcome any insight that someone may have that I'm possibly missing here.
RE: Steel Truss
This is a mix of a Verendiel (sp) truss and either a Warren or Pratt configuration, can't remember right now.
I would throw it on the computer and see what happens. If the panel points at the top and bottom chords at the box section are laterally braced, it could work (but I would never do it :) ).
Mike McCann
MMC Engineering
http://mmcengineering.tripod.com
RE: Steel Truss
BA
RE: Steel Truss
Mike McCann
MMC Engineering
http://mmcengineering.tripod.com
RE: Steel Truss
The truss shown in the OP should carry the loads it was designed for.
Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.
RE: Steel Truss
RE: Steel Truss
I think that any engineer who can not run the truss design in a frame program (RISA 3D for example) to check the design, as is exists, should not be doing engineering. I hope that before you do "blow the whistle" that you do a proper design check or you may very well be "blowing the whistle" on yourself.
On the other hand, If you do not like the current configuration of the truss. You may add the diagonal web to increase the truss strength. So that your new loads may work.
Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.
RE: Steel Truss
Evidently I rattled your cage by pointing out that it took shear in the panel to produce moment. No need to get nasty with me. Analyzing the truss is easy, but doesn't provide insight, which is what I'm looking for. Bridgebuster said it resembles an industrial truss. Evidently it's been done more than a few times, and there has to be some logic to it.
By the way, as far as analysis is concerned, it's pretty simple. Whatever kind of loading you want to apply,the bending moment in the chords will be the panel shear times the panel width (10'), divided by four. It wouldn't take much panel shear to fail a WT7x15 chord.
RE: Steel Truss
Whatever.
But please, if you have a problem with the existing truss do a design check of it. Especially if “...as far as analysis is concerned, it's pretty simple.“ If your calculations shows that it fails. Show us and ask for opinions on your calculations. At least then you would be getting engineering answers and not just opinions.
Or just repair it as you think fit.
Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.
RE: Steel Truss
RE: Steel Truss
Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.