Combine Pier and Beam and Slab on Grade for Residential Addition

[8thStreet]

Hi

I am working on a house addition/renovation that is pretty common in my area and my question is regarding combining pier and beam foundation with a slab on grade foundation. The existing 1920 (or so) one-story pier and beam house is gutted and remodeled and a new addition is added on the back (typically 2 stories). The existing houses are usually in pretty poor quality and require new concrete spread footings, but does it make sense to do the addition as a slab on grade?

For most of my career, I have always heard the advice that if part of the structure is pier and beam, then you should design the addition as pier & beam to limit differential movement between the new and old. This is based on keeping the bearing conditions (depth and area) the same so if the house shifts (we are on mild to moderate clays) that it all moves together.

What is the issue with doing a slab on grade addition on the back of the house? In some cases, it is the only option because of how the grade slopes, that it doesn't make sense to build peir and beam because it would be so low to the ground, or you would have to remove soil and have some sort of perimeter retaining just to get the wood clearance.

What are you thoughts about long term performance of the combined foundation systems? Good idea?

I am thinking about doing deep perimeter grade beam at the addition and new concrete footings that are at the same bearing depth.

Thanks.

 

[calvinandhobbes10]

By "Pier and beam" do you simply the existing house has a crawlspace?

Differential settlement concerns usually apply to pile-supported portions adjacent to shallow-footing portions. In my areas, the shallow footing portion usually has an overall lifetime settlement limit of 1".

Crawlspace first-floor framing can be over shallow footings or over piles. 1920's--I'm certain you have shallow footings and that most of the settlement has happened by now. Given this, SOG/spread addition shouldn't be a problem settlement-wise. Pay attention to how the interface is detailed.

 

[XR250]

Your new addition will likely be bearing on deeper, more suitable soils anyway. so you will likely have differential movement regardless of the foundation type. We are in an area of highly plastic clays but typically build the additions on much deeper footings than the original as the geotech won't pass it. Yes, we have differential movement, but we warn the customers about it up-front. We have not figured out a way to prevent this. Usually the differential movement is not that bad.

 

[oldestguy]

If thee is concern with differential settlement between new and old, why not disguise the joint in some way to hide any such movement, as with moldings in walls and ceiling. At floors I'd tie together. Slight floor "tilt" is usually not noticeable.

 

[8thStreet]

Thanks for the responses:

I am proposing to do a foundation replacement on the "pier and beam" beam portion with concrete footings founded about 3 feet below grade. The new addition will be designed as a slab on grade, due to site restrictions and grading, but I will extended the exterior grade beams to the same bearing depth.

I am curious about how significant the movement between the two portions will be. I am going to press the architect to provide a break in the finishes at the joint between the two foundations to accommodate some movement.

@Calvin: The house has an existing crawl space with shallow concrete footings with wood floor framing. Since we have high plasticity clays, the footings have the potential to heave and settle since they are founded in the active zone

@XR250: How do you warn the clients about the movement? I typically explain the issue and many homeowners know about foundation problems because they are common in my area. Do you have them sign some sort of "understanding" or release of liability?

Thanks.

 

[XR250]

I usually tell them and add a note covering my ass on the plans