Underpinning @ elevator pit in existing building

[NinerStruct]

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?

 

[a7x1984]

Speaking to the reinforcing: For existing masonry walls without reinforcement, we design the underpinnings as 'mass' plane concrete wall/footings, so their thickness is usually greater than the masonry wall. Our design conforms to Chapter 22 (Plain Structural Concrete) of the ACI 318 and is usually justifiable based on low to nearly no moment at that new cold joint (in our cases), as well as the annoyance of having to install overhead epoxy dowels into the existing masonry. Your drawing shows 4 wythes of existing masonry (I assume it is unreinforced) - that is a pretty beefy wall to support a relatively shallow soil loading. However, if you do not think you can get an allowable cold joint bond strength greater than the allowable tensile strength of the masonry (30, 40 psi??)you may want to epoxy dowel small bars just to bridge the gap of the cold joint.

Let me know if I am assuming something incorrectly!

In Russia building design you!

 

[PEinc]

What you are showing is done all the time. The front edge of the existing footing looks small in your photo. It gets cut off after the underpinning is completed. Your underpinning is shallow and the existing building looks heavy. Therefore, the underpinning piers should be fine without any reinforcing steel or dowels. The underpinning piers should be large enough in plan to allow a worker to excavate in a timber shored pit. If you need more bearing area at the bottom of the pier, the pier can be belled. The depth of the increased pier width should be enough that there is no bending in the widened area. The main thing to remember is that the front face of the underpinning should be in line with the front face of the existing wall. You don't want the underpinning piers to encroach on the area required for elevator travel.

http://www.PeirceEngineering.com

 

[NinerStruct]

Thanks for the help a7x1984 and PEinc. I've come up with the following detail:

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?

 

[PEinc]

Underpinning is properly done in hand-dug, timber-shored pits. If there is any ground water, it should be pumped down before excavating under the building. Dry packing can be done with a damp, stiff mixture of sand and Type I cement, about 2:1 ratio of sand to cement. Strip the shoring boards between the underpinning piers when pouring adjacent piers.

http://www.PeirceEngineering.com

 

[NinerStruct]

I've been reading a few other threads regarding underpinning as well and PEinc had a nice explanation of the typical operation here:

http://www.eng-tips.com/viewthread.cfm?qid=54204,

but I'm still struggling with what exactly I should be directing and what I should let the contractor figure for himself that I review.

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?

 

[a7x1984]

"I've checked the wall for bending and the stress on the masonry is right at 30 psi"

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!

 

[NinerStruct]

The 30 psi is a conservative number using the bending moment from the soil only. I did not include any of the dead load to pre-compress the section.

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.

 

[PEinc]

You have relatively shallow underpinning with a heavy building. I don't see where you could have bending in the underpinning concrete unless the underpinning was much deeper and needed to be braced or tiedback. The net compression load on shallow underpinning should prevent tension in the unreinforced underpinning concrete. It is common to have have much less than 100 psi compression in a concrete underpinning pier. For example, if the footing had 4000 psf bearing, the underpinning pier could have about 28 psi compression (= 4000 / 144). If you had intermittent underpinning piers (say 4' lagged bays between 4' piers), that would give about 56 psi compression - still very minor compression per square inch of concrete and dry pack. The exact pressures depend on the footing area Per LF, original footing bearing pressure, and the total bearing area of the underpinning per LF.

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.

http://www.PeirceEngineering.com

 

[NinerStruct]

PEinc,

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-En..._1_1?ie=UTF8&qid=1363703300&sr=8-1&keywords=w

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?

 

[PEinc]

The 1975 Winterkorn & Fang edition has the big section on underpinning. That's the edition I have. The 1991 Fang edition has no underpinning. Buy the 1975.

http://www.PeirceEngineering.com

 

[NinerStruct]

Great! I really appreciate the help.

 

[PEinc]

I have the book shown in the picture from the Amazon link you posted.

http://www.PeirceEngineering.com