Pre-Engineered Wood Trusses @ 2'-8" and TJI's 4

[ThorenO2]

I have a 90x60 flat roof restaurant with a 40' span and (2) 25' spans. Since the ARCH is going with 3/4" plywood (instead of the customary 5/8"), is there a benefit to a 2'-8" c/c max. truss spacing, particularly for the long span or are there pitfalls? It seems like cutting down the pieces & parts will simplify the construction and produce some economy. For the short spans, is it inefficient to specify roof trusses at that length when TJI's @ 24" c/c max. may do the job better? Or can I delegate to the manufacturer/truss designer to elect which member ( joist type, depth & spacing ) to submit for these spans based on the loads given, including parapet drift? My intent is to detail the project with conventional "WOOD TRUSSES @ 2'-8" C/C MAX.", but enable the option for the Owner to purchase Red-Built metal web trusses, TJI's or otherwise, which I'd be able to approve. I appreciate any insight in preparing an efficient design for this type of roof. Thanks,

 

[Celt83]

double check the diaphragm requirements this may drive the truss spacing down to 24" o.c. regardless of if the sheathing can handle a 32" o.c. spacing for gravity loading.

My Personal Open Source Structural Applications:

https://github.com/buddyd16/Structural-Engineering

Open Source Structural GitHub Group:

https://github.com/open-struct-engineer

 

[BAretired]

There could be an advantage to using a 32" spacing, assuming that the diaphragm requirements are satisfied. If studs are spaced at 16", trusses align with every second stud. If additional studs are needed to carry the roof load, double studs may be used under each truss.

BA

 

[XR250]

If you have a sheetrock ceiling, the additional effort required for back-framing may not be worth the truss cost savings.

 

[Triangled]

from the viewpoint of the framing members themselves, generally speaking, spreading them out is cheaper.
XR250 points out ramifications in other trades.
Fire Sprinklers is another trade which may be affected.
I understand your terminology TJI to mean solid web I-Joist, whereas "Red-Built metal web trusses" are open web trusses. Each has their sweet spot of economy, but I'd get other trades in. Regarding Fire sprinklers, magic words are "concealed spaces" as in, is additional fire sprinkling needed or not.

"I delegate to the manufacturer/truss designer to elect which member" and they will naturally provide the very most economical (cheapest for them) system they can (with little or nor regard to other trades), which is not necessarily bad. If an RTU is added, or a replacement is increased in weight sufficiently to require reinforcements, "Red-Built metal web trusses" are Very Much more expensive to reinforce than solid web I joists. It is generally very economical to add a note that says "in additional to all other loads in these plans, truss supplier shall provide that each joist/truss/(whatever) shall support an additional 500 LBS (or something)Dead Load at any location along the top chord. They may require panel point loading..... but you can negotiate that with them ....

good luck