Truss Design

[BAGW]

Hi all,

I am lost in the truss design concept. I am trying to design a warehouse consisting of trusses supporting the roof. Bay sizes are 50'x50'. The trusses at the column lines are going to be connected as they are erected. The trusses between the column lines, I am designing them in 2D with pin at one end and the roller at the other end. For the trusses at column lines, I am designing them as a continuous trusses (taking continuity into consideration for gravity as well as lateral loads). Therefore the trusses sizes at column lines are different than the intermediate trusses. Does this design concept makes sense?

Thanks

 

[JAE]

A couple of questions to better clarify your question:

1. Are the "trusses" actually pre-manufactured joists?
2. Are you making the column line trusses continuous to use as part of your overall wind resisting system? (i.e. moment frames)
3. Are you taking into account column stiffness in your analysis?
4. Are you alternating roof snow loads?



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[jayrod12]

It's entirely likely, as JAE is alluding to with questions 2 and 3, that the columns will have enough flexibility to be treated as pin-roller for that truss as well. Unless the columns are braced from lateral displacement at the top.

Generally, the trusses and joists I've seen at columns are the same as the others unless they are part of the lateral system, or the columns are not able to accommodate/allow some slight lateral displacement at the top.

What kind of displacement are you getting on your other truss runs that are considered pin-roller?

 

[KootK]

I agree with the previous posters. Unless the column line trusses need to perform an atypical function, like being the beams of a moment frame, it's likely simpler and cheaper to eliminate the continuity in the trusses across the columns. In many jurisdictions, the bottom chord connections to the columns are detailed to prevent chord sidesway (erection safety) but allow unfettered axial displacement.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[KootK]

On the other hand, if the column line trusses are part of the lateral force resisting system then I would expect them to be different from the other trusses as you've noted. The introduction of serious bottom chord compression load changes much, including bridging requirements.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[BAGW]

@ JAE,

1. Are the "trusses" actually pre-manufactured joists?
Trusses are not pre-manufactured joists. The trusses are built out of hold rolled sections.

2. Are you making the column line trusses continuous to use as part of your overall wind resisting system? (i.e. moment frames)
Yes. The lateral resisting system is truss moment frame. Therefore I am treating the trusses as continuous along the column line for gravity loads as well.

3. Are you taking into account column stiffness in your analysis?
Yes. I have a 3D analysis model developed.

4. Are you alternating roof snow loads?
Yes. I have considered pattern loading considered for continuous truss deign.

@ jayrod12

Look at my answers for 2 & 3. I have considered column flexibility in design of continuous trusses. Even with column flexibility it does not behave as simply supported truss.

At pin-roller trusses have vertical deflection of 0.32'' for dead + live
At continuous trusses (@ column locations) vertical deflection of 0.26'' for dead + live. Lateral deflection at top of column under dead + live = 0.1''.

 

[BAGW]

@ KOOTK: On the other hand, if the column line trusses are part of the lateral force resisting system then I would expect them to be different from the other trusses as you've noted. The introduction of serious bottom chord compression load changes much, including bridging requirements.

This is what I getting at. If the trusses behave like a continuous frame under lateral loads with serious BC connection, it will behave as a continuous truss under gravity loads too right? I want to design the truss at column lines as continuous frame for both gravity and lateral loads. Does this make sense?

 

[EngineeringEric]

This sounds expensive, but besides that...
If the load is different on truss 1 than truss 2 (ie different boundary conditions) then why would a different truss design be suspect. Truss 1 is BC all tension where Truss 2 is BC Tension and Compression.

 

[KootK]

This is what I getting at. If the trusses behave like a continuous frame under lateral loads with serious BC connection, it will behave as a continuous truss under gravity loads too right[/quote

Yes and yes.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.