How to analyze plywood over SIPS panel 1

[MillR]

I have to determine the capacity of a floor that consists of a new layer of 3/4" T&G over an old SIPS panel. The building is being converted from residential to office space and the extra floor live load has to be addressed. The remodel is already done, building red-tagged, and I am now justifying the as-built.

My first thought was to find the properties of the new total section (convert the moment of inertia, etc), but this neglects the shear deflection of the core and may be unconservative (this may be why SIPS uses tested values). I also considered looking at the individual rated spans and just superimposing them, but this neglects that the bottom face of the SIPS panel is taking all the compression from bending.

Actually, my very first plan was to tell the contractor to add some beams, but he'd like to know it doesn't actually work, first. Any thoughts?

 

[civilperson]

What is SIPS panel?

 

[COEngineeer]

I forgot, I dont have my ibc. So you are bumping the load from 40 to 50 psf? Or is it 100 psf?

 

[MillR]

SIPS are structural insulated panels - an OSB sandwich with a foam core.

The live load is going from 30 psf to 50 psf.

 

[civilperson]

Is the Tongue and grove plywood considered composite with the structural insulated panel, applied with adhesive or a pattern of fasteners to insure composite action? As you said, the bottom face has no additional strength for bending so will probably be the limiting element in tension in the middle of a span. The negative moment across the supports will use the new plywood as the tension element if composite action is assumed.

 

[MillR]

Yes, the plywood is glued and nailed to the SIPS panel. And using a transformed section works, even for the bottom face.

However, since both pieces are rated products rather than calculated sections, that transformed section calculation uses alot of guesses and completely neglects the shear deflection of the foam core.

 

[COEngineeer]

When did IBC have 30 psf? Since it is sip floor, I just assume it is not that old. Well, the sip just span from spline to spline and I assume the splines are 4' O.C.

I would draw a section of the spline+ 7/16 osb on each side of the foam + 3/4 new plywood on top of it (4 ft wide). Just assume everything is DF #2. If you go this direction, I have a feeling the inspector would want to see the shear flow calc on the new plywood.

 

[dik]

and often a partition load of 20 psf...

Dik

 

[MillR]

The load was 30 because it was a mobile home, is 50 now because it will be offices. The new walls have been built and they are permanent, no moving partition type walls, and line up with beams below.

I only have to justify it to myself - not much building department second guessing in this area. In any case the shear flow between new and old won't be a problem, it's well attached. But because I don't have a feel for SIPS panels, I'm worried about overstressing the bottom plywood and worried about excessive deflection I can't predict.

 

[COEngineeer]

I meant, I thought it should have been 40 psf.

 

[DonPhillips]

IBC allows 30 psf for sleeping areas of residential occupanies.

Another option is to adjudicate the floor loading to a lower occupancy. Office occupancy at 100 sf per person may be reudced to say, 200 sf per person. The building would have to be posted for the reduced occupancy but seldom do the number of occupants reach what Chapter 10 Means of Egress allows. Just an idea if you are short a couple of psf in your analysis.

Don Phillips

http://worthingtonengineering.com