Light Gage Steel Studs - Bearing on Plywood

[DEL2000]

We have a multi story Light gage steel stud building, where we break each level and run the plywood over the top plate, similar to how a wood building would normally be framed. The joists ledger into the side of the studs. This is a 4 story building in a seismic zone, so the loads in the studs get really high - so high in fact that it may overload the allowable plywood bearing strength (either 340 or 370 psi, I can't remember for sure, but not very high). For loads on the area of a stud (1.625*5.5 in^2), I am already to 500 to 600 psi in a great many places, and a few places under stacked headers, it calcs out to 700 or 800 psi.

My question is two fold: 1) Do most engineers run the plywood between the different wall levels, or do they do a true balloon framed construction and leave the plywood out of the mix altogether? 2) If you do run the plywood between the levels, how much can you count on the sill track to spread the load out?

By the way, what would the consequence of a bearing failure in the plywood be? Would the building simply compress and settle down, much like happens in a wood building, or does the crushing cause serious consequences for the building that you would want to stay away from all together. By the way, this building is a condominium, on the west coast, so any advice would be greatly appreciated.

 

[apetr26542]

From my experience with wood structures, we detail platform construction not balloon. The floor joists sit on the bearing wall below, the plywood runs over the joists and the new wall above sits on this. The bottom sill plate will spread out the load somewhat. With wood anyway if its a 2x4 or a 2x6 flatwise you will get some distribution. You might not get as much with a metal stud sill. I would treat it almost like a section of footing, take your stud load, divide it by the length between studs and see if the sill can handle the imposed bending flatwise. This might justify your higher bearing stresses. Also we are on the east coast (fortunately).

 

[DEL2000]

Thanks for the response. I checked out this already, and even a fourteen gage track doesn't spread the load out very well, based on its moment capacity. But even still, I've designed heavily loaded multi-story buildings before with studs bearing on plywood, and I've never heard anything else about them. I've seen other drawings designed by other engineers with heavily loaded studs bearing on plywood. I think I'll try asking with some of the light gage steel groups around to see if they have looked at this.

 

[jheidt2543]

Compare the crushing strength of the plywood with the crushing strength of a 2x member, I think you'll be surprised.

For Group 1 plywoods, for compression in the plane of the plys, Fc = 1,550 psi and Bearing on the face perpendicular to the plane of the plys Fct = 340 psi.

For Construction Grade Southern Pine Fc = 1,100 psi and Fct = 345 psi.

These are older values based on AITC 2nd Edition tables.

 

[JAE]

Don't forget your lateral load path.

The plywood MAY be part of your lateral force distribution system and the edge of this sheathing may need to drag forces into the end closure track of the supported joists at the exterior face of the framing.

If you terminate the plywood at the inside face of the stud wall, the sheathing may not have any collector element to drag into the wall below.