Interface between new/existing structure 1

[GalileoG]

Hello friends:

I am trying to come up with some solutions for this design. I am designing a new elevator structure with a deep pit on the side of an existing building. I have attached a sketch showing the new and existing structures, and their relationship.

Considerations/Challenges:

-The new structure interferes with part of the existing perimeter spread footings (three in total).
-Soil excavation necessary to construct the new elevator pit is within the zone of influence of the existing spread footings.
-Openings through the existing wall is required.

Some questions I am struggling with:

Should I move the new structure away from the existing building and bridge the two together at door locations? This would eliminate the interference with existing spread footings and allow an excavation shoring wall to be installed between new/existing structures to protect existing spread footings. Bridging the new structure to the existing structure at openings will make a more complicated structure, and I am not sure how I would detail the wide gap between new/existing structures.

OR

Abut the new structure to the existing structure, as shown in the sketch. Support existing footings with micropiles to allow for interfering part of footing to be cut and also to allow for excavation adjacent of footing without risk of undermining it. How would the concrete be formed next to the existing wall? How can the formwork be removed? How should I detail this small gap?

Also,

As the new openings are wide, I want to design the new structure to support the existing structure at openings. Any ideas on best way to do this and the sequence of this work?

Any feedback is appreciated.

 

[JAE]

One solution:
1. Excavate down to the bottom of the existing footing - but not lower than that
2. Chip away two or three sections of existing footing that extends out from the building face. This might be a 12" wide section for each.
3. Install two or three helical piles with brackets attached to the side and underside of the footings to underpin them. (not sure how many are required so I guessed at 2 or 3. Usually about 4 to 6 feet apart depending on the load).
4. Once the helical piling is installed, continue excavation down to the level required for your elevator pit.
5. Construct the pit base mat and walls. These probably would encompass/surround the helical brackets but you still may have to move the shaft out a bit to ensure that coverage.

Another solution:
Check the existing foundation and see if the limited width of the undermining excavation is limited such that the existing foundation and foundation walls could possibly span that excavation despite being undermined. From your sketch it appears that the existing foundation is simply a small stem wall so I doubt this would work but you could check.

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[GalileoG]

Hi JAE,

Thanks for your response.

The first solution you proposed is the one I am currently sitting on. However, I was thinking of installing a transfer beam that is doweled through the concrete pier with a micropile on each end of the beam (2 total) with a slight batter towards to the existing building. This way, I do not rely on the existing footings, and the micropiles would not encroach into the envelope of the new elevator structure, it would be under the footprint of the existing building. By the way, the existing ground level slab is suspended, and so the soil under the existing ground slab can be safely excavated up to the underside of the footing without shoring, as you suggested.

Do you think the existing building in this area would need to be vacated during micropile work and subsequent partial footing removal and excavation for pit? I don't think so. I would say yes if it was conventional underpinning.

Also, how much of a gap between the new and existing walls would I need for formwork? And can this formwork be erected without transferring fluid pressure to existing brick wall? How would this formwork be removed at the interface and how would I detail this gap for the permanent condition?

Regarding your other solution, I ran numbers and this was not possible.

 

[CANPRO]

If you poured a new wall tight to the existing, you'd never get all of the formwork out, just at the openings. If you want the new structure tight to the existing (reasonably tight, probably a 1"-2" gap), precast might be an option.

How long is the interface between the two buildings, and how much of that is opening? You could put a traditional steel angle in to support the brick, and another angle on the opposite side to support the existing concrete wall (where there is one), and then embed an angle in the new concrete (be it CIP or precast)to line up with the angles in the existing....then you can weld a tie-plate between all the angles and tie it all in laterally. The existing brick and concrete would have to support its own weight...the new tie plate might take some vertical load from the existing.

If you cut the outstanding leg of the spread footing, how are you looking for bearing pressure there under realistic (current) live and dead loads? Maybe you can cut the footing flush with the foundation wall and pour a new wall/footing beside - install a bunch of dowels between existing and new foundation walls. Given the nature of the pit construction, the new foundation wall/foundation could likely be made to the take the eccentricity.

How tall is the existing foundation wall? The way its drawn in your sketch, it looks like you could have some lateral earth pressures to deal with on the existing foundation wall during construction.