Rectangular Pile Cap for Round Steel Tank

[structec]

Hi,

I have to design a pile foundation for a water tank dia. 23ft x 38 ft high. The piles will be driven H-piles. The pile cap will be about 27ft x 27 ft square with 9 piles to start with. Is the strut & tie model to design the pile cap applicable in this case? My thought is that it is not since it is a tank with uniform pressure across the bottom. Should the pile cap be designed as a conventional 2 way flat slab with columns? Please confirm.

 

[ishvaaag]

It depends on your structural scheme. If the piles are required because of the poor bearing capacity and high compressibility of the upper layers of the ground, assuming supports just at the piles will be a better assumption than letting any part of the load be transmitted directly to the soil.

However you really can exact some relief from direct support by just assuming springs of the proper constant in the direct support area; for the piles you would assume far (but not wholly, if floating piles) stiffer springs or just unyielding support. This of course I wouldn't do if separation of the soil under the supporting slab is potentially expected.

 

[swearingen]

For such a small foundation, you should absolutely assume that the soil will pull away from the foundation eventually. All tank foundations I have designed assume this. Most that I have designed have been in very poor soil, so I didn't have much to work with anyway.



-5^2 = -25 ;-)

http://www.eng-tips.com/supportus.cfm

 

[asixth]

The strut tie would be applicable for a 9 pile pilecap supporting a central column which needs to distribute the central column load to the piles.

For a water tank with uniform pressure the pile struts are going to develop a rather uniform compressive field as the load distributes thru the cap to the piles.