foundation stiffness
foundation stiffness
(OP)
I am designing a spread footing for a large fan. The vendor has specified a required foundation stiffness of 1,000,000 lb/in. I have never designed a foundation with a stiffness requirement. Any guidance would be appreciated.






RE: foundation stiffness
RE: foundation stiffness
If not, you may try to deliver the stiffness by using a piled footing or mat. You normally will be then in a very bad soil, that you may need to bridge through the piles to some tip resistance. Piles working by tip resistance to rock or stiffer soil are better to the stiffness purpose, because floating piles in a very bad soil will go with the soil under the load, at least far more than those firmly set at their tips.
If you can't use but floating piles, try to ascertain what the force-deformation for the piles will be from your geotech, specifying the pertinent previous and on works tests. Also, if the asked stiffness is also wanted in tensile action (the footing separating from the soil), piles are inmediately of order, and then you also shall specify the required tests to get the geotech "warrant" to the required reliability.
RE: foundation stiffness
RE: foundation stiffness
I would model it in FEA, with my footing supported by appropriate springs. If i get less than 1" of displacement aynwhere, or 20 KSF (again very high) then i would consider this requirement met.
This does seem like an odd way of specifying, yet there are likely other factors, maybe dynamic or something, that led them to this number.
I'm no geotech though, so there might be something i am overlooking, its just my 2 cents.
RE: foundation stiffness
If you looked at it as a whole: yes. But if this thing is [for example] supported on piles that socketed into rock, the flexural stiffness of the supporting mat may not provide that level of stiffness at certain locations. It may be important to ask the fan manufacturer about that.
Another thing to consider here is: with just about any foundation for a fan I've done, I've been forced to do a dynamic analysis for the unbalanced load. He may want to ask the manufacturer if this needs to be considered here and (if so) what the unbalanced forces are. (With a lot of those guys, you can't get that info; therefore you are forced to assume [in the case of a fan] an eccentricity for the rotating mass.) And if you have to do a dynamic analysis, it will then be necessary to get dynamic spring constants from the geotech.
RE: foundation stiffness
In any case, in the simple way of tackling the problem, just a matter of juggling the data and formulae.