residential foundation question

[bigmikeshutt]

In a prior life I was a practicing Civil PE, but have moved on to other things. Unfortunately, I did not do much work in residential building, so I don't have a good working knowledge of the codes.

I've got what I think must be a very typical design change for a home. I am planning on removing a portion of a load-bearing wall (about 10') and replacing it with a beam. I am comfortable sizing the beam, but I am not certain about the implications to the foundation.

Assuming my home was build to code in the first place, I think the footing under a load bearing wall should be 18" wide. That assumes that the developer did not perform for a full geotechnical investigation and used the minimum soil bearing capacity of 1500 psf when designing the footings. I have verified that the exterior wall footings are at least 18" wide.

The soil around my home is a very firm in-situ sandy clay, which means its bearing is more likely in the 2000 - 3000 psf range.

I calculated that total (dead + live) load at each end of the beam is in the neighborhood of 3500 lbs. If I assume that the actual soil bearing capacity is 2500 psf and a safety factor of 2.5, that would mean that a ~4 ft2 footing should be sufficient for support at each end of the beam. That equates to a 2.7 ft section of the 18" wide existing footing should have sufficient bearing capacity for support of the beam endpoints.

That tells me that I don't need to worry about taking up the floor and enlarge the existing footing at the beam endpoints. Am I oversimplifying this or overlooking something obvious?

TIA

BMS

 

[bigmikeshutt]

After think about this some more, I think the more important thing being changed by creating an opening is the moment forces being applied to the footing. I think the overall load of the footing on the subsurface is mostly the same assuming the footing is fairly rigid.

Maybe a better way to ask the question is this:

The design of a continuous wall footing should take into account the possibility that there will be openings in the wall itself (i.e., doorways). Is there something in the code or a standard rule that determines how large an opening can be without changing the design of the footing to expand or deepen the footing width at the endpoints of the opening?

 

[DaveAtkins]

I don't think you have any problem, as long as the foundation wall above the footing is tall enough to allow the load to spread out 2.7'. If it is, bending of the foundation wall should be OK.

DaveAtkins

 

[dangnm]

Thanks Dave, that seems logical.

Can anyone offer an opinion as to whether a change like this will require certification from a practicing engineer? Assuming the answer is yet, is there a shorter span (i.e., 6 feet) that would not necessarily require an engineer's stamp? I would think the building code would indicate what changes require re-evaulation of the structural design and which ones don't.

 

[FSS]

Agree that footing should not be an issue since it is continuous under original wall. You will need engineered design for beam since you are modifying load bearing wall. Really no minimum length that gets away from this, especially since most jurisdictions will require permit for the work anyway.

 

[boo1]

In my area (Florida) permits are required for all work greater than $500. The generic non-engineered beam is 9'9" max.

Dave do you just consider the 45 degree projection or consider the rebar interation?