Continuous Footing for Garage

[Einsteim]

Situation: We have designed a residential garage slab with a reinforced turn down edge (24" below grade)under the garage door. Code official requires a continuous strip footing & foundation wall under garage door.

Worth fighting for?

 

[BAretired]

Only if you disagree with him.

BA

 

[Einsteim]

Thanks BA. I kinda thought it might be obvious that I disagreed. My client also disagrees with wasting money on concrete that isn't needed. I was really looking for opinions on my argument or if other jurisdictions around the country have similar requirements regarding these foundations.

 

[Lion06]

So the turned down slab edge is acting as your foundation wall and footing? The walls for the garage attach to the turned down slab? How wide is the turned down slab edge at the 24" below grade elevation?

 

[mmillerpe]

Does the rest of the garage use a foundation wall and footing? If so, you are only looking at minimal cost for additional concrete.

If the entire foundation uses a downturned slab refer the code official to details R403.1(1) and R403.1.3.2 for typical down turned slab details.

 

[Einsteim]

No, the turn down edge is only supporting the slab itself and the door. The walls (up to the garage door jamb) is supported by traditional strip footing and foundation wall -see attached, this may help.

 

[msquared48]

I believe that the minimum width for a bearing wall footing is 12" and for the walls next to the ends of the garage doors, they are technically bearing walls if there is a gable end truss over the garage door. If the minimum footing width is 12", then the stem wall will have to be that thick too for a monopour condition. Are you really going to save concrete?

Seems like a small amount of concrete to get into a pissing match over. I'd say that it is a small price to pay to avoid ruining a good relationship with the local jurisdiction. Just do it their way and move on.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[msquared48]

OK..

Another issue here with that plan is if the 2'- 3 3/4" center wall is a shear wall or not. I would assume probably not.

So. Then they are concerned about spreading of the endwalls in a seismic event and want a tie beam to limit that. From the plan, this does not seem like that should be an issue.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[Einsteim]

No, the center wall isn't a shear wall.

 

[Einsteim]

..and I agree msquared, the added concrete volume isn't worth the battle here, but as you can imagine, the cost to provide the labor to construct a strip footing and foundation wall is FAR greater than with the turn down edge. Keep in mind, there are multiple residences we are constructing, not just one. So, cost is certainly worth the battle in my opinion, especially if the code official doesn't have a legitimate reason for it.

 

[BAretired]

Firstly, I don't think a building code official should interfere with your design unless he wants to assume responsibility for foundation design or unless he sees a clear violation of the building code. Officials in my location never become involved in structural design.

On the other hand, you have not given us any reason why you prefer your detail over his other than volume of concrete. We don't know if you are in a seismic zone, the depth of frost penetration or the soil properties.

Your detail would not work well in my locale because of frost heave but that may not be a consideration for you. For the small amount of concrete involved, I would likely have gone with the footing and wall as it is easier to form than the downturn. In my locale, the bottom of footing would have to be at least four feet below grade.

Alternatively, a grade beam 8" x 16" spanning across the door openings would be an option. The grade beam would require a 4" or 6" deep frost cushion below it and would certainly have more reinforcement than you have detailed in the downturn. The top of grade beam would be at the underside of slab. If the driveway is concrete, I would want it to bear at least 2" onto the grade beam and be tied to the garage floor slab but not to the grade beam thus allowing the driveway to heave freely. A couple of inches of rigid insulation under the driveway for 2' from the grade beam is a good idea to reduce frost penetration.

If a building official told me he wanted a different detail, I would ask him to kindly seal and sign the foundations plans.

BA

 

[JAE]

If you are up north - then just make sure that you are lower than frost.

This turn-down is NOT a foundation - that is your "out". It is not supporting anything and is only serving to stiffen the edge of a slab. Therefore the code requirement for a strip footing doesn't apply.

 

[paddingtongreen]

The code is a "model code". It is not enforceable unless states, cities and municipalities adopt it as part of their codes. They usually have some change or input, frost depth is a prime example; these changes are made on the recommendation of the code official. Much is left to the code officials judgment as to whether a design is in compliance or not. The code official does not design, he simply will not sign off until he gets the type of design he wants.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[a2mfk]

JAE hit it on the head, you could replace the whole slab with brick pavers. Then what would the code official say? Slabs-on-grade such as yours are non-structural, the thickened edge is just a little insurance against cracking due to settlement (and most driveways don't have them)...

I am like BA, code officials don't say much in Florida unless you do something really obviously out of the ordinary.

 

[Einsteim]

Thanks, I agree with JAE and a2mfk. The slab isn't a foundation and doesn't require a foundation. I thought I was actually being a little conservative with the turndown edge. This is no different than the slab they will pour in the driveway, it doesn't have a foundation wall either, but will be accepted by the BO..thanks again people.