Do we need our foundation underpinned?
We have a 1925 cape which we will be tearing the roof of in order to add a full second floor. The current structure has a shed dormer on one side and the original roof plane on the other. The existing foundation is 26 X 42 and the walls are 8 inches thick with a lot of aggregate (if that matters.) For the foundation is a most 4 feet below grade. There is one crack in the foundation in the middle of one of the long walls that appears to have been there for a long time. The chimney (not represented in the attachment) is on the front exterior of the structure.There is no footing under the existing foundation walls. One hundred twelve linear feet of the existing foundation will carry the new second floor. There will be a new pour of 10 X 23 that will also carry the weight of the part of the roof.
What is the best way to determine if the existing foundation walls will need to be underpinned in order to support the additional weight? Does the structural engineer begin by assessing the amount of weight added by the new second floor or performing a soil evaluation? Can a soil evaluation be as simple as digging down to the bottom of the wall or does it require performing ‘soil boring’ tests? The structural engineer we have been referred to also owns a helical drilling company (different phone numbers ring into the same office.) Seems like a conflict of interest. What are the different methods of shoring up the foundation? Can any of it be performed from inside?
People are starting to throw around large numbers that are wreaking havoc with our budget and our appetite for this project! Any help would be greatly appreciated. Attached is a rough outline of the foundation.
Tree
Parkinson’s Law: ‘Work expands so as to fill the time available for its completion.’
Replies
You can't rely on advice gotten free on the internet from people who cannnot see the specifics of your situation, and mostly aren't qualified to rule on it even if they could see it. Get a referral to a structural engineer you are comfortable with and get a foundation design. You will probably need engineered plans anyway to get a permit for what you want to do.
Your point is well taken. We are having structural plans developed. The strength of putting this out in the FHB forum is that people who have more knowledge and less direct interest in our project will have an opportunity to really kick this around.
Thanks,
TreeParkinson's Law: 'Work expands so as to fill the time available for its completion.'
How do you know there's no footing?
I have dug it out. there is not footing. In the location that I dug there seemed to be a lot of rubble like bricks and stones under the wall.
Parkinson's Law: 'Work expands so as to fill the time available for its completion.'
Edited 10/19/2005 7:21 pm ET by TreeMcGee
Edited 10/19/2005 7:21 pm ET by TreeMcGee
Just curious, but have you dug it out on the inside as well.....you didn't mention in your first post what kind of floor is in the basement....it's possible that rather than the traditional "T" shaped footing, yours is more like an "L"....
As far as finding lots of rubble at the bottom of your test hole, that's quite common..back before well engineered drain tile systems were used, lots of the left over junk stone went into the backfill......
The floor is poured concrete so I havent investigated the shape of the inside of the foundation wall. My procedure for checking the foundation wall was to dig to the bottom of the wall and then because the soil was so full of rubble, I dug the rest with a pinch bar. When I was able to get the bar a couple of inches under the wall I assumed there was no footer or at least not the 'T' shaped footer with which I was familiar. This is the first that I have heard of an 'L' shaped footer. Are they common? Is their use regional (more common in the mid-west vs. east?)
Thanks,
TreeParkinson's Law: 'Work expands so as to fill the time available for its completion.'
Hello:
First the earlier relies contain both good advice and question. You first need to resolve the "no footing" question. Often these period bldg were constructed without separate footings, but the foundation walls were (brick or stone) 16-24" thick so they acted to distribute the loads. A modern footing design is like an inverted "T" with the bottom hoiz. section being the footing and the vertical being the "stem wall." In a typical rsidence the footing width is 16-24". It is odd to hear of a 8" stem wall without a footing. What is the construction of the found. wall (conc., brick, stone)?
Typically, a soil engineer will only need to use a small probe rod. This is a steel rod of known dia. and area that is hit with a weight (like a mini pile driver). Based on the amount of penetration of the rod, the bearing capacity of the soil is easily computed. Soil samples/auger seem extreme to me.
I have worked in the eastern seaboard converting early 1900's hotels into condos. Since Cap Cods were/are popular in New England near the ocean, here are one issue for informational purposes I ran into:
Brick: Masons used soil from beach in mortar mix. Due to salt content, walls had to be repointed (mortar would come out w/ pencil eraser).
I supect, however, you will have no problem. The largest load being added in a second storey addition is the "occupancy" or floor load, but in this Cape Cod design you essentially already have that load. The slightly higher walls are marginal.
I failed to mention the foundation is poured concrete. The boards that were used for the forms were recycled as the subfloor for the first floor. I know this from looking up when in the basement. Love that New England Yankee spirit, "Don't waste a thing, that stuff cost money!"
I like the idea of the rod and known weight. Makes sense to me and I will certainly challenge the boring test.
There is consensus around your point that the new floor will not substantially increase the load on the foundation. However, our house is about a quarter mile down the road from a remodeling project that was condemned as they were beginning exterior finish work. The circumstances are fodder for another thread but suffice it to say that the foundation was cinderblock supporting a one story structure that had been relocated from elsewhere in town many years ago. Its in a wetlands (literally) and after the teardown, people began to come out of the woodwork to tell the homeowner that they had had problems with the contractor. So even with all of these outlying issues, people are nervous about projects that involve adding floors to existing one story structures. Unfortunately, we are on a non-conforming lot. It really would be less money to raze the structure and begin again but its not an option. If this underpinning thing is the only way to move forward we may be forced with a decision to either redesign or clean up and move. Neither are heartwarming thoughts. Parkinson's Law: 'Work expands so as to fill the time available for its completion.'
I spoke with the architect regarding your soil compaction testing method. Although he agreed it was a viable test he felt it would be more appropriate in regions where it was likely that the soil contained fewer rocks. Your thoughts?
TreeParkinson's Law: 'Work expands so as to fill the time available for its completion.'
My house built in 1927 has no footings. On the backside of the house part of the wall wasn't even below grade. I added footings to a couple sections at the back of the house, where it was settling. We don't get much frost here, I think it's only 18" that is required, it was still a shock to not find footings though. My neighbours houses from the same era don't have footings either, although, I think at least their walls are below grade a couple of feet.
True...hitting rock makes a difference but given the test includes a rod only about 3' long, any soil engineer worth his salt will know when a rock os hit and move the rod over. You will be getting multiple probes anyway at footing level and then an average for design purposes.The lack of separte footing may not be an issue in any case. The NAHB has been very active in eliminating enginners/architects from the housing industry, so we end up with rules of thumb and very large safety factors. I personally prefer to engineer my projects because the engineering cost is more than made up in construction savings.In your case, 2 floors + roof (simplified):
Depth not shown, but assumming 16' span on fl/rf members.
LL=30 psf fl and rf , DL = 15 psf rf and fl.
TL=3x(30+15)=135 psf.
Reaction TL = 8' (1/2 span) x 135 =1080 #/lin ft
Wall=8psfx16ft=128 # /lin ft
Found Load=1080+128=1208#/ft
An 8" wide wall=96 sq in/ft or .67sq ft/ft
Required Soil brg value = 1028/.67=1803 psfThis is a low value and likely explains the absence of a formal footing. I put up an addition on my home 11 yrs ago w/o footing, only a 6" wide stem wall. I only needed a 4" wide footing but pouring 6" is more practical.
Given that the UBC req 16"w x 8" h footings, imagine the money saved by some simple calcs?
Edited 10/21/2005 6:44 pm ET by panchovilla