Mat footing idea/question
Mat footing idea/question
(OP)
This might not be original and it might not be the best idea ever. Just wanted some opinions.
I have a site that got a pretty bad soils report and a rigid mat was recommended. Pretty light loads, one story office building. 100 psf floor load. Line loads less than 1200 plf. Point loads less than 20 kips. Seismic design category B. Will be using a mat 12" thick, roughly No 5 top and bottom around 16-18" o.c.
Getting to the point now... Frost depth is 30". Owner doesn't want to step down into the finished floor. I worry a little bit about doing a turn down as I don't want to have the possibility of an increased bearing pressure occurring at the turn down. Has anyone used a trench of 57s to get below the frost depth at the perimeter? Seems to be a descent idea, the only real concern I have is with scour. A rigid insulation detail would be provided just inside the trench interior of the building.
Thanks.
I have a site that got a pretty bad soils report and a rigid mat was recommended. Pretty light loads, one story office building. 100 psf floor load. Line loads less than 1200 plf. Point loads less than 20 kips. Seismic design category B. Will be using a mat 12" thick, roughly No 5 top and bottom around 16-18" o.c.
Getting to the point now... Frost depth is 30". Owner doesn't want to step down into the finished floor. I worry a little bit about doing a turn down as I don't want to have the possibility of an increased bearing pressure occurring at the turn down. Has anyone used a trench of 57s to get below the frost depth at the perimeter? Seems to be a descent idea, the only real concern I have is with scour. A rigid insulation detail would be provided just inside the trench interior of the building.
Thanks.






RE: Mat footing idea/question
Hence in my view adding some hypotheses may deal with your problem with scarce structural change.
RE: Mat footing idea/question
RE: Mat footing idea/question
A rubble foundation is nothing new, just haven't seen one on a "new" set of plans before. I've never done a rigid mat for an entire building before, so I was curious how everyone else handles the frost depth at the edge. I've only done them supporting a group of columns/line loads interior of the building.
RE: Mat footing idea/question
DaveAtkins
RE: Mat footing idea/question
RE: Mat footing idea/question
DaveAtkins, thanks. Yes, I suggested this, but the owner doesn't want to step down into the finished floor, which would be approx. 30" (frost depth) - 12" slab = 18" or two steps down into the building. Plus, then you have short CMU stem wall with waterproofing to hold back soil. Makes his building look shorter from the road and all the other complaints one could think of.
Specifically, does anyone see a big problem with the 57 trench? Like I said, I only see a problem with possible scour.
RE: Mat footing idea/question
Also, I don't see an issue with having a "turndown" at the perimeter like you originally suggested. This will likely improve the mat's performance and make it behave more like a rigid body. Let's say the bearing stress is exceeded below the ribbing. As soon as the soil would begin to fail the stresses would redistribute by either having the ribbing span over a localized failure location or by additonal stresses being transferred to the 12" slab immediately adjacent to the ribbing.
RE: Mat footing idea/question
RE: Mat footing idea/question
RE: Mat footing idea/question
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http://www.supergrund.se/files/document/tjale.pdf
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http://www.huduser.org/publications/pdf/frost.pdf
http://abecsouth.org/Design_Tab_02_03_30.pdf
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http://www.styrotech.com/products_general.html
RE: Mat footing idea/question
You are correct that cold can still transfer through the 12" mat if the mat is exposed. This will not create a heave issue since the heat transfer will need to freeze the mass of soil below the mat. However, if exposed, this could cause cold spots in the floor, sweating of the concrete, flooring issues, etc. This is why it is better to have a floating slab on grade with expansion felt between the frost wall and the SOG.