Truss Top Chord Lateral Support 1

[hoshang]

Hi all,
For a Pratt truss, lateral support is provided by a transverse U-frame consisting of the floor beam and truss verticals. What about a Warren truss? It doesn't have verticals.

 

[hoshang]

....

....

So how do these conversations with the architect go?
"Dear Mr. Architect, my sub-consultant who works for free at eng-tips.com suggested....."

Kudos to BA, man. Holy cow.

English is my second language. I wrote didn't, and accepted, I meant we discussed the matter at the beginning of the design, not during the discussion in the eng-tips forum. The whole discussion on this forum is about top chord lateral support and it came to the global truss so that the box support would simulate two roller support at the ends and one hinge support at midspan (as in the analytical model). If you notice I wrote in earlier posts that the spans were 90ft.
Hi BA,
Please notice I edited post #56.
Overall, how would you configure the box truss (as a support) to simulate roller support at the ends and a hinge support at midspan (as in the analytical model)? I mean assume you have a truss supported by two roller supports at the ends and a hinge support at midspan. How would you configure box trusses at the ends and midspan so it would simulate the support conditions in the analytical model (i.e., two roller supports at the ends and a hinge support at midspan)?

 

[BAretired]

English is my second language. I wrote didn't, and accepted, I meant we discussed the matter at the beginning of the design, not during the discussion in the eng-tips forum.

Your English seems fine to me. That is not the issue.

The whole discussion on this forum is about top chord lateral support and it came to the global truss so that the box support would simulate two roller support at the ends and one hinge support at midspan (as in the analytical model). If you notice I wrote in earlier posts that the spans were 90ft.

What makes you think that one hinge support at midspan is better than two?

Hi BA,
Please notice I edited post #56.
Overall, how would you configure the box truss (as a support) to simulate roller support at the ends and a hinge support at midspan (as in the analytical model)?

I wouldn't do that because it's the wrong approach.

I mean assume you have a truss supported by two roller supports at the ends and a hinge support at midspan. How would you configure box trusses at the ends and midspan so it would simulate the support conditions in the analytical model (i.e., two roller supports at the ends and a hinge support at midspan)?

It should be apparent in the sketch below that:

1. For simple spans, truss moments are much lower with two pinned supports.
2. For continuous spans, truss moments are much lower with two pinned supports.
3. With two pinned supports, moment is easily resisted by the X-braced frame of large width.
4. With two pinned supports, moment is applied to the box trusses which are not very stiff and bend over their height of about 20', causing instability of the structure. (Same for a line support.)
5. With one pinned support, moment is not transferred to a stiff frame, but may act almost as a true pin because of the frame stiffness.
6. With one pinned support, moment is not transferred to a box truss as suggested by OP. Horizontal forces or unbalanced gravity load cause instability unless the box truss is extremely stiff and fully fixed at foundation.

So, why would I choose a single pinned support?

 

[hoshang]

Hi BA,
Appreciate your help.
1- How would you configure a box truss to simulate a roller support in this image:

2- In the link below:

I captured these images:


The span is 27.75 m. How much pinned supports would you consider in an analytical model for this truss? One at each end or two at each end?

 

[BAretired]

Hi BA,
Appreciate your help.
1- How would you configure a box truss to simulate a roller support in this image:

A column hinged top and bottom simulates a roller support. That is not the issue and the architect has already approved that.
The architect has also approved a cross braced structure at midspan, but you are not content with that.
If you object to the diagonal braces, perhaps you would prefer a rigid frame adequate to resist horizontal forces parallel to the bridge. If so, problem solved.
My main concern is with seismic forces. I have no idea where this structure is situated, but we know that there has been a recent earthquake in Turkey, so be prepared to address that issue.

I captured these images:

Interesting video...Maybe we should go for a cable stayed bridge...(just joking)

The span is 27.75 m. How much pinned supports would you consider in an analytical model for this truss? One at each end or two at each end?

Hard to say what is intended here. The trusses are not symmetrical; on the right side there's no end diagonal, the top chord extends to an end vertical member which appears to align with the exterior of the supporting box truss. My guess is that it is intended to simulate a pin on the left and a roller on the right to permit temperature strain to occur easily, but it is an unusual detail. That would suggest two each truss top and bottom on the left. It is not clearly a roller on the right, but perhaps one top and two bottom each truss. I am tempted to say for the right end, one pin top and one pin bottom for each truss, but that leaves two legs of the box truss unattached. Let's just say it seems like a strange detail.

Others may see something that I missed in the photo; if so please contribute your thoughts.

 

[hoshang]

the top chord extends to an end vertical member which appears to align with the exterior of the supporting box truss. My guess is that it is intended to simulate a pin on the left and a roller on the right to permit temperature strain to occur easily

Can you elaborate more on the above?

That would suggest two each truss top and bottom on the left. It is not clearly a roller on the right, but perhaps one top and two bottom each truss. I am tempted to say for the right end, one pin top and one pin bottom for each truss, but that leaves two legs of the box truss unattached.

I can't get the above, can you explain it more?

 

[BAretired]

Can you elaborate more on the above?

No! Focus on your project, not on what someone else did!

I can't get the above, can you explain it more?

No! My comments related to the photo of the bridge being erected across a road, see below.
In any case none of that is related to your project, so forget about it.

I think that whoever designed that bridge was trying for a pinned support on the left and a roller support on the right, but I'm not sure. Kindly stop filling the screen with photos of extraneous bridge details by others. They have nothing to do with your project.

 

[hoshang]

A column hinged top and bottom simulates a roller support.

Which option do you mean?