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Truss Design Question

Truss Design Question

Truss Design Question

(OP)
I am currently working on the design on a cantilevered truss for an outdoor canopy.  All members of the truss are to be the same size and based on my loading I have determined that the bottom chord is going to be the critical member in compression.  Is there a good reference out there to help me determine the effective length I should be using to size the member?  Right now I am looking at the bottom chord as a cantilever and therefore getting a K = 2.  This is resulting in a very high KL, which is marking me select a member with a large r to get KL/r <= 200.  This approach seams to be too conservative but I can't find any guidance to tell me I can assume otherwise.

Also, as of now - there is no bracing out of plane for the bottom chord, the architect wants to minimize the amount of steel seen.

Any help you guys can provide me would be appreciated.

Thanks.

RE: Truss Design Question

Im guessing this is a steel truss because you mentioned steel once. I ususlly use the SJI specs when designing a steel truss. They are included in most joist manufacturers catalogs. If you dont have access to this, check the SJI website. They use some different requirements than AISC.

RE: Truss Design Question

When an architect imposes constraints (such as no visible bottom chord bracing), then why be hesitant to be conservative? It was his call. Not knowing your detailed truss design, a double angle with relatively long horizontal legs will give you the larger r required, at the best economy.

RE: Truss Design Question

(OP)
Thanks guys for the response - I know I didn't give too much detail on the situation I am looking at...so here is more detail on the truss.  This is a steel truss and is going to be left exposed; therefore the architects would like it to be constructed of pipe or hollow steel sections.  Yes, it will be a steel truss (that was a bit of important information I left out!) and the cantilever span is about 45 ft.  At this point I have settled on giving the architect the design without bracing the bottom chord.  I think he is going to be surprised at the size that will be required.

Just to check on my thought process...since I have no bracing in the bottom chord and the truss is cantilevered - my KL will be twice the length of the length of the bottom chord I am considering.  I am looking at the bottom chord somewhat in "sections"...for example, my largest loaded member I have a smaller KL since this largest compression force only occurs in the final member in the bottom chord.  And my smallest compression force has the largest KL, since when that load is on the farthest member in the bottom chord - that bottom chord could buckle all along the length - in theory.

Like I said - I want to thank you again for your responses...this is the first time I have tried to design a truss and I feel that my thought process is correct, but always feel better to hear others opinion of something.

RE: Truss Design Question

If British codes are ok where you work, next time, use BS 449 or it's latest version, which, I think, is BS 5950 (or something like that. Their approach to the design of trusses is the most striaght forward I have ever seen. Limitations to slenderness ratios for conpression members are very clear. Once your specified member dimensions meet the specified limit (which is 180 or thereabouts), you move on to the tables 17 (? check the code. I have not used it for a while now) and corresponding allowable stress in compression wrt the slenderness ratio previosly calculated. Compare this with your simple F/Anet and voila!

Regards,

John Audu
Nigeria

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