footing sizing question

[darcar]

Our plan reviewer wanted me to post this to the forum and get everyones feedback

According to Section R403 and Table 403.1 and assuming the soil bearing capacity is 1500 psf and we have a 2-story plus a full basement residence with at least 8 foot height to the ceiling, let us not just consider the width of the structure that the foundation is supporting, my understanding is that the foundation width to this situation shall be no less than 23-inches considering that the 8” concrete basement wall has a considerable amount of weight that contributes to what the foundation supports.

 

[KZQuixote]

I think the determination hinges on whether or not the basement is a story. Here's a definition offered by Montgomery County, MD:

A basement is considered a story and defined as, that portion of a building below the first floor joists where at least half of its clear ceiling

height is above the average elevation of the finished grades along the perimeter of the building. A cellar is not counted as a story and

defined as, that portion of a building below the first floor joists where at least half of its clear ceiling height is below the average elevation

of the finished grades along the perimeter of the building.

So I'd go with 15" as the minimum footing width if the basement/cellar was more than half buried and 23 if it was more than half way out of the ground.

Bill

 

[Francis Vineyard]

Assuming a 23 inch wide footing, according to the commentary it’s using the definition of story, not the story of grade plane.

The question that follows if this is the case with 3 stories above grade plus a basement; does it now require an engineer design?

 

[FredK]

Also table 403.1 limits the floors to 16 ft trib area (32 ft x 32 ft) square home with roof trib area limited to 20 ft (40 ft x 40 ft). So if you meet that then you can use the table if not get engineering.

 

[peach]

doesn't really matter if the basement is a story or not.. the load all comes from above.. it could be a crawl space... the footings need to carry the load imposed on them (the floor and roof loads above).

 

[darcar]

This scenerio is a 2 story home with a full basement 85-90% below grade. Is the basement considered a "story" in calculating the footing size?

Sure the footing will be supporting the weight of the poured fnd wall but it does not include a floor load as it does subsequent stories.

What if the basement wall were a wood foundation?...far less dead load to factor in.

We're trying to stick to the tables until the design goes above and beyond.

 

[darcar]

also, at what point would a crawl space be considered a "story"?...