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Underpinning a PEMB slab/footing

Underpinning a PEMB slab/footing

Underpinning a PEMB slab/footing

(OP)
Hi all,

I've got a project where a client is installing a sunken loading dock to an existing pre-engineered metal building slab in Austin Texas. The existing slab was cored out to determine depth. The perimeter beam is 38" deep by 18" wide, built in the 1940's. The new loading dock is 46" deep so some underpinning is required. The issue is that the new loading dock consists of 2 doors sandwiching an existing column, thus underpinning the footing will be required. Thankfully the column is a side column and not part of the main metal frame so the loads are relatively light, (25kips estimated).

My plan was to excavate sequentially furthest away from the existing column, install sacrificial metal columns under the existing exterior beams in 4'-0" increments towards the existing column. Once these columns are in place, the area underneath the footing will be excavated, the footing will have to temporarily span until a new footing can be poured into place. New concrete will then be poured underneath the existing foundation.

I've attached some drawings for reference. I'm wondering if there is a more effective/safer method for underpinning this column. My worries are causing settlement/cracking while these repairs are in place.

Thanks!

RE: Underpinning a PEMB slab/footing

Agree with you about the safety and settlement concerns. Instead of precast footing and sacrificial steel columns, suppose you excavate / shore in sequence (as proposed) but use excavatable flowable fill to support the perimeter beam. Could be used under Column "X", also.

If the existing soil to be excavated is reasonable good, the excavation under the perimeter beam is not too deep (say 2' to 2.5'). This excavation could have relatively vertical walls. Flowable fill that, by necessity, will be in the way of the new construction can be excavated after all perimeter support, including under Column "X", is in place.

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