new lvl floor joists on existing wood stud wall...concrete strip footing needed?

[Jmeng1026]

I am working on a project where the owner is looking to add a 2nd story to their existing one story house. The house was built in 2001 but the owner does not have the original plans.

22' long lvl floor joists will be installed (16" o.c.) and they will bear on an existing 2x4 wood stud wall that sits on an existing 4" concrete slab with infloor heat.

Would the slab would have to be cut and a concrete strip footing installed under the wall or would the 4" slab be able to handle the addition load? There would be approximately 750 pounds every 16" on top of the wall. The wall is approximately 12' long.

 

[phamENG]

The slab is rarely the problem - it's the subgrade below the slab. No way to know how well it was compacted. Best bet is to install a footing.

Why LVL floor joists? That seems a bit...extreme.

 

[JAE]

"technically" by the code the minimum footing thickness is, I believe, 8".

The slab could be checked for punching shear but as phamENG noted, the subgrade will affect how that load is spread out over an area (width along the wall) that affects in turn the shear and bending in the slab.

If the subgrade is infinitely stiff rock, then all the load simply goes though the slab directly and into the rock, no slab shear and no slab bending.

If the subgrade is mush, mashed potatoes, etc. then the slab will work very hard to spread the load out, with lots of downward deflection under that load and lots of cracks, failure.

Install a footing please.

 

[RontheRedneck]

LVLs are awfully expensive to use as floor joists. Is there a reason for that?

 

[Jmeng1026]

The span is 22' and with 2x12's there will be too much deflection and they will have a bouncy floor.

 

[StrucPatholgst]

But why LVL's. Why not I-joists, or truss joists. 22 foot long LVLs are going to want to twist. Plus you'd probably need 11-7/8 LVLs, and they'd would still probably deflect a half inch under load.

 

[RontheRedneck]

At a 22' span I wouldn't want to see any floor member less than 14" deep.

I'joists or wood webbed floor trusses would be a lot less expensive than LVLs.

 

[hokie66]

Before cutting out a slab with infloor heat, I would want to be sure it is required. Probes along the wall by a geotechnical engineer can give you parameters for deciding. My own home was extended upwards in the manner you described, without new footings. I did have advance knowledge that the site was cut from very rocky terrain.