New footings under existing foundation walls

s_rentmeester · 09-25-2008, 06:37 PM

Hello,

I have a 1917 bungalow (27' x 36', one story w/ finished attic). Recently started remodeling unfinished basement with future plans to add 6' addition to first floor kitchen and second floor addition. Doing these as three independent phases so that we can recuperate (mentally and financially) in between phases. Also, the current economy does not support the second floor addition. Do not anticipate doing the second floor addition until maybe 5 years down the road. And this is the source of my angst.

We spent almost 2 years think about the remodel and working with designer and engineer. We have well developed plans with full engineering. Engineering is based on current codes, but I fear codes will change before I begin the second floor addition (5 years down the road).

I cut into the basement slab on labor day weekend as prep work for pouring the new point load and shear wall footings that the engineer spec'ed. Opening the slab reviled no footings under perimeter foundation walls (pour 1917 portland cement walls). I spoke with the engineer and he was not expecting footings and put all new loads on point footings (2'-6" x 2'-6" x 12" thick w/ #4 rebar). So he was not concerned.

I'm concerned about changing code requirements that may come into play after I close up the basement slab. I'm thinking about pouring new footings under the existing walls. I already removed 20' of degraded slab along one wall and 12' along adjacent wall. When I do the kitchen addition, I will have another 20' of foundation open from the exterior, at which time I could add the remaining new footings to complete the perimeter of the second floor addition.

I would pour footings in 5' to 6' alternating sections and do two pours. During first pour I will embed rebar into the footing and leave sticking out, to be embed into adjacent footing during second pour.

Questions:
How wide of a footing? Foundation wall is 6" plus. Plans for finished basement spec a 6" stud wall on interior for insulation and drywall. I'm thinking footing should extend 6" on both sides of foundation and stud wall making a 24" wide by 12" deep footing.

What concrete mix should I use? I will have a cement company mix and deliver, but what do I order? Should the cement mix be slightly expansive? Is it possible to get a cement mix that expands slightly?

Am I completely crazy and going totally overkill? I like doing things right the first time and want to avoid future headaches. Thoughts??

btw - the soil is clayey sand and well drained. The house is complete level and shows no signs of differential settling.

thanks,

steve in portland, or

mudmixer · 09-25-2008, 07:14 PM

Your decription is difficult to understand despite spending a lot of time thinking and planning. - Possibly too close to the job.

I assume the new loads on point loadings were for interior posts or columns and not for the extisting exterior walls.

You cannot out-guess code changes, but what you are doing would not be effected by code changes unless they are earth-shaking and take many, many years. You have been busy using some time up trying to outguess the unknown.

Are you thinking you must support the existing exterior walls that have performed well and place new footings under the wall?

Regarding your questions. -

1. What foundation walls are you talking about and why a 6" wall for insulation where it is usually not needed? If it is an exterior wall it should be at least 8" wider than the exterior wall. If it is an interior wall you have no load on it and it can be on a normal floor slab or a locally thickened slab if a new slab is poured.

2. The strength of the concrete is not critical. An ordinary 3000 psi mix should be fine. Why would you want cement (concrete really) that expands. Are you ordering concrete now or just trying to think about what to order?

spaz2965 · 10-22-2008, 09:38 PM

Steve,

What you are doing is what we have done in past years on old buildings some I thought was a waste as it sat on solid rock, but we jackhammered out the rock and undermined the footers, 3500 footer mix is the concrete that I recommend and also use a vibrator as that works best (used on bridges) yes you footer should be 24" wide from what I gather you are doing, come up your pre-existing foundation at least 3" and when you go to pour your new slab drill 3/8" by 6" long rebar into the side of your new footers that will keep you basement floor from settling any, unless you floor is higher than you new footer which in new homes we put 2" of 3/4" stone over top of footer so that it doesn't nit to concrete and a expansion around the perimeter. Hope this is some help.

Joe~~

spaz2965 · 10-22-2008, 09:41 PM

Steve,

Also never do a whole section of wall this is done in parts as to if you undermined the whole wall you could have collapse and you don't want this.

911handyman · 10-27-2008, 09:34 PM

Hey there, this may help international residential code gets updated evey 3 years and will done i believe jan 2009. I would not be worried about the code for foundation changing, even if it did they did they would let u know before they approved your set of plans. i just recently built a house where the 2nd story addition was 2x6x14. it will also depend on lumber used to build the 2nd story. The best idea i ever had was to talk to inspectors and city engineers. They have done it before as well as want your project to come out a success. Their the experts let's use them. You will do fine you have started by asking. later

sandyedson · 12-01-2008, 11:44 AM

I have a 1908 house in Canada. We have recently taken old concrete out of the basement and dug down about 12 inches to get more headroom. When digging we saw that the foundation walls of the three story house ended at the original floor level. We want to pour new footings at 3 foot intervals in two pours which seems to be accepted standard here. But I am wondering about drainage along the new footings. We can't really dig from the outside because the walls are a ways underground but I thought there might be a way of draining the new footings from the inside.

Any thoughts?

spaz2965 · 12-08-2008, 07:05 PM

when you dig for new footing you will be to the outside of the foundation wall or you should be at that time you may put sections of pipe in and conect each peice as you continue to dig the will be 3' sections