Steel Truss

[spats]

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?

 

[MikeHalloran]

For one thing, you're missing actual centerlines anywhere near where the C/L COL. and C/L VERT. annotations appear. ... or anywhere.

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

 

[BAretired]

Very strange design.

BA

 

[msquared48]

I think that the "X" should have been in the middle and the current X bay with just one sloping rod. A drafting error?

Can you verify it in the field?

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[spats]

The trusses are truly built this way, and have been in service for over 10 years.

 

[msquared48]

Unreal...

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[woodman88]

The webs are mainly needed to transfer shear from the top chord to the bottom chord. Under most loading conditions you will have vary little shear transfer in the mid span of a parallel chord truss. This is why wood floor trusses can be design without a center diagonal web. The design of the top and bottom chord at the opening does need to be able to resist the moments cause by the opening.

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.

 

[spats]

I was hoping there would be words of wisdom out there, but pretty much all I've gotten so far is sympathy.

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.

 

[msquared48]

Spats:

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

 

[BAretired]

Well, you said it was a pipe bridge so I suppose the load could have been almost perfectly uniform and the pipe might have been able to bridge the central panel points in such a way as to avoid putting shear in the unbraced panel. Nevertheless, I consider it bad design and would add a diagonal member in that panel in addition to any other measures which might be required to sustain the new loads your client is contemplating.

BA

 

[msquared48]

Oh, and thinking about it, and they do make wood attic trusses with the center half or third open for storage, and they work, but they must be designed for such.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[woodman88]

It is a standard truss configuration. Not done too often as it will have reduced load/deflection carrying capability. And yes there is some shear transfer in the opening by moments in the chord. But the center third of the truss does have limited shear to begin with so the moment forces are smaller than at the ends. And yes the cost of the truss will increase for the design of the opening. Which is why there are so little attic trusses used in residential buildings.

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.

 

[bridgebuster]

Mike - it depends how you look at it. Looking at it with the text right side up, it's a form of a Pratt; flip it 180, and one side sort of looks like a Pratt and the other side sort of a Howe. Throw in part Vierendeel. It resembles type of truss found in industrial plants. Weird nevertheless.

 

[woodman88]

spats on your "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."

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.

 

[spats]

Woodman,

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.

 

[woodman88]

spats,

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.

 

[spats]

Peace, Woodman. I'm a lover, not a fighter. Sorry if I offended you.

 

[woodman88]

spats - I am at peace and hope you are too. I afraid that your "blow the whistle" statement cause me to think that you were looking for a fight. I merely wanted to ensure that you did your engineering "due diligence" before that.

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.