Wood Floor Truss Spans

[mijowe]

My rule of thumb for a good span limit for plate connected wood trusses is 25'-0" when spaced at 24" oc. With options like increasing the wood properties, tighter truss spacing, double top and bottom chords, the spans can be increased. Several of these options begin to limit the flexibility of running MEP through the floor. I do not have access to truss software and getting manufacture feedback is sometime tough to get. The span charts i have are limited and span limits from chart to char vary greatly.

I am currently looking at a job with 28'-0" wide units (I prefer to span the joists side to side and not front to back but that is a topic for another thread). Although there is a possible line of bearing in the middle of the unit, i would like to span the full 28'.

I am interested in other peoples opinion on truss spans. Is the increase in cost of a longer truss span cheaper than an additional bearing wall (that is usually rated). Deflection and bounciness of the floor are also a concern.

In addition to typical dead loads 3/4" to 1" of gypcrete topping is part of the dead load.

 

[mmillerpe]

As long as you have room for a decent span to depth ratio you should be OK. Minimum depth should be on the order of L/20 for floor trusses. Have the truss designer design these for a tighter tolerance on deflection and install stong backs. Make sure the interior walls are isolated from the underside of the trusses so that they do not pick up loads, etc.

For a 28'-0" span I would go deeper than the minimim limit. I recently worked on a project with 24'-0" spans and used a 20" deep truss. The floor felt good. Going 28' I could see anywhere from 20" to 24" deep trusses.

I have to believe that the longer span with out interior bearing walls would be simpler and less expensive. This would also allow for getting the structure enclosed sooner. The contractor can come back later and set the interior walls.

 

[BAretired]

You don't provide enough information. It is possible to span considerably more than 25' with wood trusses. So far as the economy is concerned, I think you can discuss that with the contractor. There is nothing we can say that is relevant to your current design problem.

BA

 

[msquared48]

You are going to need a truss deeper than the standard residential structural depth of 10 to 12". More like 16 to 18" in depth due to the span and gypcrete.

Check in your Truss Joist catalogue or something similar. They have a lot of options that will work.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[msquared48]

If you use a central bearing wall, you will be able to use 10 to 12" deep joists. You will also need a strip footing or thickend slab under the bearing wall, but this is generally cheaper construction than longer floor joists that also either increase the height of the structure or lower the head clearance.

Your choice.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[mijowe]

Typically we use 18" deep trusses as a minimum, this allows reasonable room for MEP to get there ducts through. In a 28' wide unit even if i were to split the span down the middle i would use 18", so by reducing the span i am not lowering the building height. With unit mixes of various sizes and spans we keep the trusses the same depth so that the sill heights remain the same throughout the building.

I know that i can get over the 25' and have done so for isolated cases. In this particular building all of the units are 28' or 42' wide. With a lot of units i would like to clear span the 28' and eliminate the center wall.

According to the span charts I have, to span 28' I need to do some combination of decrease the spacing, increase the depth, or double up the chords, all of which will of course add cost and begin to limit the duct work running through the floor assembly.