Slab on ground foundation design (WRI methodology) 1

grb22 · Jul 23, 2018

Greetings. I am working on a slab on ground foundation design for a smaller 1-story house and have a question that I hope some of you might help with. This is a personal project and not for pay or a client, but I am still committed to meeting code and engineering principle requirements.

The code in my build area is IBC 2012, which further references WRI Slab on Ground Foundation Design for calculating moments/shear/etc. I have been studying WRI, along with the BRAB #33 report that WRI was based from, for some time now. It is fairly straight forward, with one exception. That being in the design example shown in Appendix B of WRI. In the example, when determining the beam steel, the ultimate moment (Mu) for the beams are calculated based upon assumed steel configuration and moment arm based on the depth of beam. This is easy enough, but then the Mu value is divided by a factor of 1.6 to determine the 'working' or 'design' moment? I assume this factor relates in some way to the cantilever section or beam length/configuration, but cannot determine why or where this factor came from.

Anyone familiar with the WRI design procedure and can help answer this, I would certainly appreciate it.

Thanks in advance.

JoelTXCive · Jul 23, 2018

I am familiar with that example. This is my interpretation.....

Please note that the example is based upon the original 1981 design document. The design document was updated in 1996, but they did not update the example.

In appendix B, they first solve for moment demands of 106 kip ft on the long side and 132 kip ft on the short side. This demand is at service loading levels.

They then make a steel selection of six #5 bars and solve for a capacity that they call Mu= 213.9 kip ft. (in 1981, this is Allowable Stress Ultimate Capacity, not LRFD Ultimate Factored Load)

They then reduce their ultimate capacity of 213.9 by a factor of 1.6 to see what the acceptable service loading would be. (213.9/1.6 = 133.7 kip ft) They then compare the 133.7 kip ft capacity to the 106 kip ft service loading and deem the design acceptable.

Attached is a memo I put together a while ago comparing the 1981 to 1996 design procedure. I also tried to replicate some of their equations. I was successful on their moment calc, but I never figured out their deflection calc.

grb22 · Jul 23, 2018

I see. They are just removing the load factoring in order to adjust the ultimate load calculation back to a service loading condition for comparison. Makes sense now. I had pulled out my old Reinforced Concrete Design textbook trying to figure this out, and now remember reading about the load factor relationship between Mu and service loading (although, I think it was referred to as nominal loading in the textbook). It was something along the lines of 1.7DL + 1.4LL to determine the factored load; the 1.6 used in the WRI example is how that was accounted for. They just used a single load factor instead of a weighted average of DL and LL.

Thanks so much, I appreciate the explanation.

And your attachment was excellent. Good stuff!

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Slab on ground foundation design (WRI methodology) 1

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