Underpinning @ elevator pit in existing building
Underpinning @ elevator pit in existing building
(OP)
I'm working on a project where we're putting an elevator in the corner of an existing brick building. The existing footings are not as deep as we had originally thought, based on what few existing drawings we had. So now it looks like we're going to have to underpin a portion of the existing walls.
On one side (north) of the elevator pit, the footing is ~4 ft higher than we were need for the pit, and on the adjacent side (east) the footing is ~1.5 ft higher. Below are section sketches of the elevator shaft.
http://i49.tinypic.com/54hm5k.jpg
The total length of underpinning is roughly 18-20 ft. I am planning on having them underpin in 4ft. sections, leaving a 3" gap between bottom of existing and top of underpinning, that is to be dry-packed with grout.
The existing footings also project about 4" into the elevator pit as well. I'm waiting to hear back from the architect on whether or not we have the clearance within the shaft to accommodate this, but if not, can the underpinning be poured as shown in the detail linked below, or does the lower portion need to be a separate pour?
http://i48.tinypic.com/34q8knt.jpg
I will likely tie the underpinning into the mat footing for the bottom of the pit, but should I provide any sort of reinforcing within the underpinning since it's retaining soil now?
On one side (north) of the elevator pit, the footing is ~4 ft higher than we were need for the pit, and on the adjacent side (east) the footing is ~1.5 ft higher. Below are section sketches of the elevator shaft.
http://i49.tinypic.com/54hm5k.jpg
The total length of underpinning is roughly 18-20 ft. I am planning on having them underpin in 4ft. sections, leaving a 3" gap between bottom of existing and top of underpinning, that is to be dry-packed with grout.
The existing footings also project about 4" into the elevator pit as well. I'm waiting to hear back from the architect on whether or not we have the clearance within the shaft to accommodate this, but if not, can the underpinning be poured as shown in the detail linked below, or does the lower portion need to be a separate pour?
http://i48.tinypic.com/34q8knt.jpg
I will likely tie the underpinning into the mat footing for the bottom of the pit, but should I provide any sort of reinforcing within the underpinning since it's retaining soil now?





RE: Underpinning @ elevator pit in existing building
Let me know if I am assuming something incorrectly!
In Russia building design you!
RE: Underpinning @ elevator pit in existing building
www.PeirceEngineering.com
RE: Underpinning @ elevator pit in existing building
http://oi49.tinypic.com/28lduop.jpg
We're not really adding any additional load to the existing walls, so I'm really just trying to match the width of the existing footing. I'm going to have the underpinning match the face of the wall, remove the portion of the footing that sticks out (4" or so) and have them add that width onto the backside of the underpinning to match the existing width.
I'm also going to include our standard structural notes that have verbage for surveying before during and after, and to go in 4 ft segments at a time, etc.
Is there anything else that would be pertinent that I am missing?
RE: Underpinning @ elevator pit in existing building
www.PeirceEngineering.com
RE: Underpinning @ elevator pit in existing building
I've checked the wall for bending and the stress on the masonry is right at 30 psi. We don't have any water issues, and I have notes for the dry-pack grout mix proportions, the maximum width of each underpinning section, and surveying existing walls prior, during, and after underpinning.
But should I be calling out the shoring methods and sequence that the sections should be poured or does this qualify as "Means and Methods" and I should just have the contractor submit their proposed method of shoring and sequence of pours for me to review prior to commencing the underpinning operations?
RE: Underpinning @ elevator pit in existing building
Niner: That seems awful close to the allowable working tensile stresses, depending on which mortar you are assuming (I imagine Portland type N). Did you include dead load to pre-compress the section in question? I use 80% of DL to help me sleep at night, along with my hot tea and milk.
At a minimum, sequencing and size of each sequence should be indicated. If you know who the contractor is going to be, and they are highly competent (as they SHOULD be), *I* see no reason to get into any additional M&M for a "small" job like you describe. I would only go further in assisting M&M if this were a larger job with concern that the M&M could affect the stability of the existing structure.
On a side note: Our M&M is explicitly excluded or included in the original proposal for the work. M&M can be much more costly due to the additional man-hours and liability taken to "instruct" contractors how to build something opposed to only specifying the end product.
In Russia building design you!
RE: Underpinning @ elevator pit in existing building
I will include a plan sketch with underpinning sections dimensioned and numbered for sequencing. I have a question to the G.C. as to whether or not his contractor has ever done anything like this. I am also meeting with them tomorrow to discuss any issues they might have.
Thanks again for all of your input 1984 and PEinc.
RE: Underpinning @ elevator pit in existing building
Don't let the contractor simultaneously work on piers that are closer than about 12' c.c. or you could overload the soil remaining between piers. This makes underpinning for a small elevator pit time consuming because you can work on only one pier at a time. But remember, the primary purpose of underpinning is to PROTECT the existing building.
Many contractors want to excavate for underpinning with an open, unshored excavation. With loose or cohesionless soils, you could undermine too much of the building if the pit sides slough to a slope. If the underpinning pit is deeper than 4 or 5 feet, then the pit excavation must be shored for safety and to meet OSHA requlations.
www.PeirceEngineering.com
RE: Underpinning @ elevator pit in existing building
I have read several other of your posts on underpinning and foundations and you recommend the Foundation Engineering Handbook by Winterkorn and Fang. It's also noted in several other threads (and a quick Google search) that this is a very expensive book. I found an older edition by Winterkorn in 1975 can be had for relatively cheap:
http://www.amazon.com/Foundation-Engineering-Handb...
So my question is, what is so much better in the '91 edition that commands such a higher price? And is there any reason that the older version would not be worth taking a look at?
RE: Underpinning @ elevator pit in existing building
www.PeirceEngineering.com
RE: Underpinning @ elevator pit in existing building
RE: Underpinning @ elevator pit in existing building
www.PeirceEngineering.com