Spread footing with bi-axial moment- Different method

[Robbiee]

Having read this thread,

http://www.eng-tips.com/viewthread.cfm?qid=236147,

I wonder if any one uses for analyzing spread footing subject to bi-axial moment the method of equivalent uniaxial eccentricity similar to that used for concrete columns subject to bi-axial bending moments.
I checked the footing indicated in the above noted thread using this method and found the max. stresses in the soil to be 1.92 ksf. Comparing to the other values provided in the thread that range from 1.88 to 1.93 the method, which is very simple to use, appears to be accurate.
Any comments are welcomed.

 

[ishvaaag]

More commonly the usual procedures from the books are used, yet I think that since different distributions are used (perfectly elastic reaction on a plane remain plane base, plastified zone for the displaced load on eccentricities), and one like in RC that also meets equilibrium, be it elastic or plastic, should be enough for most practical cases.

In any case what really proceeds is a soil-structure interaction, where the more difficult part to model is soil except you reduce it to a ballast modulus or elastic solid. This will give truly different reactions point per point to reactions derived from beam theory, but also to those other methods being used.

 

[AHaddad1]

Try a quick and Dirty way as a FIRST approximation for the footing size

P + 2*Mx + 2*My = 20kip + 2*88 + 2*4 = 204 kips

Assuming you have 2 ksf allowable bearing soil pressure (very low), the size would be approximately 100 SF or 10' x 10'

Or try the long hand way:
Obviously this is a very consuming iterative process to do by hand or even with a spread sheet with many cycles to achieve full balance with the tension springs released after each iteration (aka contact elements)

Or try getting a foundation software like PCAMAT or equal

 

[DaveAtkins]

I wonder why this question keeps resurfacing, when the chart in Teng is so easy to use.

DaveAtkins

 

[miecz]

DaveAtkins

I think it's because there is a general trend away from graphical solutions.

 

[KootK]

What's this Teng chart????

 

[Robbiee]

Kootenaykid,
See the thread I referred to in my original post.

 

[KootK]

Thanks Ailmar