Bearing stresses on 7-ply formply over shoring loads

[Ingenuity]

BACKGROUND: I have a complicated structural slab repair underway. 8" thick suspended PT slab to a mechanical room (boilers, chillers, etc)that has severe corrosion damage, such that we are doing full-depth removal and replacement, under load/service. The boiler sits on the damaged slab slab area and water-softening system daily rinsing (thick NaCl brine dumping on the slab for 10+ years in a very hot/humid room) is the primary cause of corrosion, along with lack of maintenance of any waterproofing etc. etc.

Anyway, the boiler is about 30,000 lb and supported on steel skid beams onto the structural slab, via 4 reaction points total. So each support point takes approx 7.5 kips.

We have a bunch of shoring under the boiler whilst concrete demolition was undertaken and when the new slab gets placed we need to form up the new slab soffit, install new PT, and new rebar. Obviously the shoring has to remain in place until the slab is cured and re-stressed, so I have small 8" high sacrificial W4 steel stubs bolted to the top of shoring legs. That means the formwork/formply has to be cut around the in-place shoring.

I am thinking of an alternative of bearing the W4 stubs (with 5"x 5" bearing plates T&B) on 7-ply MDO formply and so the load path would be boiler load > steel W4 > plyform > shoring. So with a 7500 lbf load on W4 with a 5" sq bearing plate bearing on ply that results in 300 psi bearing stress. APA recommends an allowable bearing stress (compression stress perp to face of plies) of 360 psi which is derived on a displacement limit of 0.04".

The numbers work, but would you transfer the load through the plyform, or stay with steel-to-steel transfer?

 

[Tomfh]

How do you install the plywood between the shoring and the stubs?

 

[SlideRuleEra]

I would use Option 1 for four reasons:

1) If Option 2 is used, compression of 0.04" doesn't sound like much. Say the bolts connecting the shoring are 1/2-13 (coarse thread), 0.04" is a half-turn of the nut. I would not want the connection to loosen under full load.

2) Using Option 2, there is the possibility that the shoring member picks up unintended slab loading in addition to boiler load.

3) Steel shoring typically has a safety factor of 3. Could argue that introducing formply into the load path reduces the safety factor to 1.2 (360 PSI_Allowed / 300 PSI_Applied).

4) Assuming the 30 kip boiler load is precisely split into four loads, each 7.5 kips, is optimistic.

http://www.SlideRuleEra.net

 

[Ingenuity]

SRE - all good points. We will stay with option 1

Tomfh: we would have to transfer load to skid beams temporarily then switch back after plywood ‘sandwich’ between W4 and shoring, repeat for each location, sequentially.