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Temporary shoring from slab-on-grade

Temporary shoring from slab-on-grade

Temporary shoring from slab-on-grade

(OP)
What bearing strength is usually assumed for temporary shoring supported on existing slab-on-grade? In my case, the shoring is just a safety measure while some concrete chipping and replacement is carried out at delaminated concrete due to corroded rebar caused by leakage. The total deead load on the beam is quite large (about 190 kips, but I was going to design the shoring for about half of that becasue we are not chipping the concrete away from all the rebar.

The shoring posts would be supported from heavy timber bi-directional spreaders on top of the slab-on-grade. We know that the 30 year old existing building footings were specified to be founded on 10,000 psf soil about 5 feet below the slab-on-grade. We will likley have to get a geotechnical engineer involved, but I am just curious what bearing stress on the soil that engineers have assumed when supporting temporary shoring for garage repairs, off slab-on-grade. We generally specify for new buildings that the slab-on-grade be placed on soil capable of safely supporting 500 psf or 1000 psf, depending on the floor use, but that is for permanent loads.

Question:

What soil bearing pressure do engineers generally assume for design of temporary shoring supported from existing slab-on-grade?

RE: Temporary shoring from slab-on-grade

Here is one way to address your question. Have the geotech determine the allowable bearing pressure of the soil that supports the slab-on-grade. Then determine the allowable punching shear the loaded timber spreader on the slab-on-grade. IMHO, that is your answer. For the loads you are working with, a typical slab-on-grade will probably not contribute significantly to support from soil only. In fact, you may have to use larger than expected timber spreaders to keep from damaging (cracking) the slab-on-grade.

Unrelated to your question: Unless the chipping that will be performed is very minor, I suggest that you reconsider designing the shoring to support only half of the dead load. We can discuss that if you want...

www.SlideRuleEra.net idea
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RE: Temporary shoring from slab-on-grade

(OP)
Thanks Sliderule. What you recommend is what we are doing. Since we are putting five 8x8 timbers side by side, each timber 10 feet long, with transverse timbers on top at each line of shores, the only punching shear issue would be around the perimeter of the assembly of timbers which has a 30 square foot footprint on the floor. We will chech the punching shear stress around that.

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