Portal Frames with Grade Beam vs. Stem Wall

[medeek]

Here is a detail for a recent project showing some braced wall and portal frame details on top of a stemwall foundation:

After spending some time working through Chapter Thirteen of Terry Malone's book I have tended to shy away from using thickened edge slabs at portal frames and have gone with a stemwall and standard cont. footing instead on most of my residential projects. My thinking is that you cannot get enough stiffness and strength from a shallow thickened edge slab foundation as compared to the depth/section you can achieve with a stemwall foundation.

However, I now have client who does not like the stemwall option and would prefer a more shallow grade beam at the garage doors. As this one is a little new for me I am wondering if there are any caveats I should be aware of and if there is any sort of industry standard or specific software utilized when it comes to the design of grade beams. I will probably end up writing my own spreadsheet for this sort of thing but I always like to have a way of checking my calculations when possible.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[jayrod12]

18" is shallow. I would tell the client that you aren't comfortable going less than that.

I normally have a 24" stemwall/gradebeam and at the garage door I drop it to 18.

 

[KootK]

You don't actually require any grade beam at all unless it's for frost protection. If the numbers worked for both upwards and downwards loads and shear, you could just use shallow pad footings. Of course, the added weight of the stemwall/grade beam often helps to make the numbers work.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[medeek]

The frost line depth is only 12" in our area. The frost line depth is not my concern in this situation I am wanting more section in the foundation to provide bending capacity from the moments induced from the portal frames (ie. Figure 13.8 Malone's book). I don't see why a grade beam can't work it just need to be substantial enough to handle the moments.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com