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At what point should you consider flecture in a footing?

At what point should you consider flecture in a footing?

At what point should you consider flecture in a footing?

(OP)
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.

RE: At what point should you consider flecture in a footing?

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
 

RE: At what point should you consider flecture in a footing?

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.

 

RE: At what point should you consider flecture in a footing?

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.

RE: At what point should you consider flecture in a footing?

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

RE: At what point should you consider flecture in a footing?

I always throw some in, just to feel better.  

 ;]

Seriously, concrete likes to crack.   

RE: At what point should you consider flecture in a footing?

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!

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