Timber Framed Carport - Pier Foundation Design

[JohnnyB_]

Hello -

I am providing the structural design on a small timber framed carport with a pier foundation, see attached. The structure will eventually have a roof on it. My client has already secured a swell test for the area based on the cabin she has built on the property. The swell test shows a swell magnitude of 7.7% and a swell potential of 3600 psf at a surcharge load of 100 psf. The bearing capacity of the clay is around 2000 psf. Generally in the area, on a structure that would have full time occupancy with standard finishes, a 3 foot over excavation for the foundation is recommended.

I think this sounds like overkill for the size and style of carport that my client is looking for and I don't want to create a huge cost of construction if it isn't warranted. The location has a high water table in the spring and early summer months so wetting of the soil is likely. I was planning on a big foot style foundation or just poured piers but I would like to hear the groups discussion on whether or not the style of structure and foundation requires the over excavation.

Thanks in advance.

 

[EcoGen]

In all honesty, I'm not familiar with swell tests and how that affects your foundation calculations. But regardless, shouldn't a geotechnical engineer/report determine what exact soil improvement or modification is required to obtain the required 2,000 psf? If a geotech report/testing is an overkill for this residential project, then you could be relying on local jurisdiction recommendations. Which is where you might be getting the 3 foot over excavation value from. I would be hesitant to specify anything less without a geotechnical engineer's approval.

EcoGen Consultants LLC
Structural Engineers
ecogenconsultants.com

 

[JohnnyB_]

Thanks for the reply EcoGen. That will probably be the way I go. One reason I am asking is due to the attached drawing showing a 3 foot over excavation for the cabin foundation and the porch footing bearing on native soil. My client sent me these engineering drawings from her cabin.

 

[bhiggins]

3 ft seems pretty light and reasonable for expansive clays. If you design anything less then there is a chance the structure will move. Would you be willing to design a structure that will move around?

 

[JohnnyB_]

bhiggins, I would but not sure exactly what that implies. Would you mind elaborating?