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]

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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?