Expert: Dan Griffin - 10/2/2007

Question
QUESTION: I am planning on building an addition onto one side of my two-story 24'x 36' garage/shop. The foundation for the existing building is an 8" thick poured concrete wall on a 1'4" wide by 8" high spread footing. The addition will be one story with a mono truss design roof extending from the existing building. The bottom chord of the tall end of the mono trusses will rest on a new studwall/girder. My question is: Do I need to pour another foundation footing and wall right next to my existing garage/shop foundation footing and wall to carry the additional load of the trusses and studwall/girder or simply tie-in to my existing foundation wall somehow? What do you recommend?

ANSWER: Samuel,

The answer to this question has to do with the bearing capacity of the soil where the footing is sitting.  This varies by location.  I think you local jurisdiction will expect an engineers stamp to approve the additional load.  Using a code book allowance of 1,000#/SF soil bearing allows your existing footing to carry 1,333#/SF.  YOu would need to calculate the weight of half of your existing structure and half of the proposed new structure.  Without knowing what your loading or materials are, I think you will be pushing the limits.  Please work with a structural engineer in your locale, we will both sleep better.  If the decision is to increase the footprint, I expect he will require pinning the footings together so they function as one.  Here is a short treatise: <http://www.concretenetwork.com/concrete/footing_fundamentals/footing_dimensions. >

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QUESTION: Good idea about contacting the structural engineer - I have one local that I have worked with before. But just to take this a little further before contacting him: Based on a geotechnical report, prepared a couple of years ago for the property where the addition will be built, the native soils have a bearing capacity of 3,000 lbs/sf.  Based on the attachment that you referenced, for a one-story wood frame type construction, the width of the footing needs to be 8 inches. How does that translate to my current 1'4" wide by 8" high spread footing which is currently loaded with a 2 story wood frame building? Could I avoid the use of the existing foundation altogether by simply provide a footing and foundation wall for three sides of the addition and then use foundation blocks and posts (on 8 ft centers) to support the girder and trusses? Please advise and thanks for your help.

Answer
The fact that you have 3,000# soil helps a great deal.  Remember that a typical footing has a more or less uniform loading, think of a stud wall applying the load every 16".  If you take out a huge number of studs for a major opening, all the load is transferred through the framing to point loads at each end of the header.  I'm not sure that I completely understand your foundation block/girder reference.  If you mean that you would use spot footings under vertical posts to carry a girder (header) that is carrying the new roof, remember that the cumulative square foot of spot footings must carry the total load of 1/2 of the new roof which will be minimal unless you have substantial live load for some reason.