STABILITY ANALYSIS OF TRUSS

[teng22]

I am designing very big double-pitched roof truss (6"per foot pitch, 20 feet deep at midspan, span length is 90'-0&quot. We can't build this in one piece. So we plan to build two trusses with 10'-0" deep each and splice them in the field. Does anyone have an idea how to check lateral stability of these trusses (lateral bracing requirements, "K" factor used to design compression member, etc.).

Thanks

 

[ishvaaag]

Once properly joined you have a just a truss. There are conventional Ks stated and different values for chords and web members. For chords it is many times used Ky=1 on length between lateral braces, and Kx=1 and lengths between joints with web members. For web members going chord to chord K=0.8, some times even K=0.5 is allowed in the truss plane, and 1 out of it. The idea is the stiffer chords elastically delivering some fixity to the web members. In any case, K=1 is even more traditional for truss design, always for length between braces.

And of course, you may search for the recommended Ks in the texts or codes.

 

[Ron]

Why not just field erect the trusses using bolted connections? This would save transportion costs (oversize could be eliminated)and would, as ishvaaag said, just create a truss.

The entire truss could then be lifted in place by crane (might require two cranes).

Be sure to check your truss for the lifting stresses.

 

[Qshake]

Good Point Ron. Lifting stresses or, in general, construction loading/erection schemes are often overlooked and left to the contractor to make all the right choices with little design experience.

Do all parties involved and look at the entire project from design, fabrication, transporation and finally erection.