Piled Raft Slab
Piled Raft Slab
(OP)
Can anyone help me with my first attempt at designing a piled raft foundation for a 30 ft dia. 60 ft high grain storage bin. The structure weighs 28,000 lbs and the product weight is 2,640,000 lbs.
We had designed a slab on grade (See fig 1). But our Geotech warns that frost heave could be a problem here in Canada. Insulating the slab is possible but difficult and not really a good option.
Does the pile configuration in Fig 2 make any sense?
We would like to ensure that all pimes hav close to the same load.. What order of slab thickness have you found works in a situations similar to this? Should we not spec a void form under the slab?
The available pile drilling equipment is set up for 24” dia. piles and can drill to 24 ft.
We had designed a slab on grade (See fig 1). But our Geotech warns that frost heave could be a problem here in Canada. Insulating the slab is possible but difficult and not really a good option.
Does the pile configuration in Fig 2 make any sense?
We would like to ensure that all pimes hav close to the same load.. What order of slab thickness have you found works in a situations similar to this? Should we not spec a void form under the slab?
The available pile drilling equipment is set up for 24” dia. piles and can drill to 24 ft.






RE: Piled Raft Slab
RE: Piled Raft Slab
The distribution of the loads to the piles thru the raft will be dependant of the slab stiffness, pile settlements and span lengths. For this configuration I think you would get an even distribution to the piles.
I remember reading a paper in the ACI a while back which recommends that a span to depth ratio less than 2 can be assummed rigid. In the days of FE packages you can assess this distribution thru an FE model.
RE: Piled Raft Slab
I contrarily even extended 20 inches the mat to get a more favourable distribution, I always endear regularity over economy. Just as a device to place the loads well in the FEM program I retained the rim at 20 inches thick whereas the 30 ft diameter central part I took to be 22 inches thick. Load is applied at service level and -with all criticable aspects of the model- the principal stresses at the bottom of the piled mat -that correspond to the tie part of one strut and tie model, atop are lower- show to be very moderate. Hence you may likely work at any reasonable thickness ... I probably would decide between 20 and 30 inches thick after examining lateral displacements and some tumbling irregularity etc.
RE: Piled Raft Slab
Thanks so much for your reply. One question ..when you say "span" (in span to depth ratio ) do you mean the clear space bet ween piles?
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Thanks to ishvaag .. What do you mean by " "extended 20 inches the mat"?
Regards
RE: Piled Raft Slab