Foundation - Flexible or Rigid?

[mar2805]

Hi floks!
What defines the foundation to behave as a flexible or as a rigid?
Ive seen dozens of articles where people call foundations - flexible, semi-flexible, semi-stiff, stiff....and this applies to strip foundations, rafts....
Ive always thought of strip footing (under the wall), whoes 45 degree disperse load line doesnt pass thru the bootom, as a stiff foundation.

If I imagine this same strip footing placed under 3 columns (spaced few meters away and acting as 3 point loads) I would say that the foundation would behave a flexible since it will start bending.
But this "flexible term" would again depend on the b x h dimensions and the E modulus of the strip.
Looking the at the same strip with 3 point load, strip with a height of 1m would be considered stiff compared to a strip of a height =0.5m.
Loads should also be considered in this classification...
Thank you.

 

[JedClampett]

You need to get a beam on elastic foundation textbook. As I remember, there's a ratio made up of the soil stiffness and beam stiffness. There's a lot of hyberbolic functions involved. Above a certain ratio, the beam is considered rigid compared to the soila and the bearing pressure is equal everywhere. And when it's below a certain ratio, the beam acts as a multispan beam with x supports (three in your example) with higher bearing pressures at the supports. The goal was to make it one or the other.
Now I'm guessing that a decent computer program could analyze the in-between cases. But you still need soil stiffnesses which are not always provided by the Geotechnical Report.

 

[FixedEarth]

Roark's formulas has the flexibilty equations. Joseph Bowles Foundation Anlaysis & Design book has a good section on mats. This flexibilty factor is a function of mat geometry, E, I and vertical subgrade modulus. It is important, however, to match the flexibilty or rigidity of your mat with the anticipated soil movements.

The following link has a spreadsheet that will calculate what you are asking.

http://www.steeltools.org/resources/viewdocument/?DocumentKey=5f2a8623-bb3e-4af9-b68a-6a66de397757

http://www.soilstructure.com/

 

[mar2805]

Thanx a bunch!
This is great!