At what point should you consider flecture in a footing?

[ridgeline]

I briefly looked in the ACI code to determine when to design for flecture in a shallow footing but havent found anything as of yet.

I would imagine you would begin adding flectural reinforcement when a minimum moment or stress is reached..

Any guidence would be appreciated. Thanks.

 

[BigH]

Try looking up Poulos and Davis' book on Elastic Solutions for Soil and Rock Mechanics. Section 12 on raft foundations. As example: The differential (say of circular footing) deflection of edge to centre depends on the relative flexibility of the slab - they given an equation (12.18). It appears that contact pressure is "uniform" as one would expect under a rigid foundation when the relative flexibility value is approximately 10. It is flexible say for values of about 0.5 or smaller with intermediaries of not really rigid - nor flexible in between.
K = [E(ftg)/E(soil)]*(1-mu(soil)^2)*[raft thickness/raft radius]^3.
Someone recently gave a download site for the book - I've found it again:

http://www.ce.ncsu.edu/usucger/PandD/PandD.htm

 

[JAE]

Footings may be designed for "Plain Concrete" provisions of ACI 318, (Chapter 22) as long as the moment is below a certain point. Once you violate that, you must consider the moment per Chapter 15.

 

[hokie66]

The OP meant "flexure" when he spelled it "flecture", which I imagine led to BigH's answer about "flexibility". Language and spelling are important in our work as engineers.

 

[msquared48]

In more active seismic regions, a minimum degree of flexural reinfoecing is required, regardless of the moment seen by the footing.

Mike McCann
MMC Engineering

 

[escrowe]

I always throw some in, just to feel better.

;]

Seriously, concrete likes to crack.

 

[Ron]

I always check it as JAE noted...we don't do many plain concrete footings anymore, so it is a consideration for anything beyond a cube!