Piled Rafts 1

[slickdeals]

Please excuse my ignorance on the subject. This is the first time I am dealing with piled rafts. I have a few questions for which I would appreciate some direction/references.

1. Thickness of the raft:
I have seen calculations where the thickness is determined based on the most heavily loaded column (by checking the punching shear capacity).

However, if a building column has a pile right beneath it, will you still determine thickness based on that column's load? I don't see how a column can punch through in this situation. The thickness can be determined by the punching shear capacity based on pile strength and diameter.


2. Is it common to pick a certain spacing for the piles on a equally spaced grid or is it typically clustered under heavy loads (columns, core walls etc)

3. When you have tall buildings with lot of bending moment in core walls, do you still assume the raft to be rigid and try to distribute the loads to piles much further away from the cores? If so, how do you determine what thickness makes it rigid?

I have seen rules of thumb saying 80-100mm per story (ie) a 10 story building will have 0.8m to 1m thick.

Any other tips/pointers?

 

[paddingtongreen]

I only considered a raft if I needed to evenly distribute the piles. In that case, punching shear was the first consideration in the thickness, but usually the bending moment controlled. If I could concentrate enough piles under each column, I used piled foundations and a pile supported, if piles were necessary, slab in between.

I think that you have to "feel" whether the thickness is in proportion. Sometimes, you make it thicker to add DL, concrete is cheap.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[slickdeals]

@Paddington: Thanks for the reply.

1. Did you size the raft thickness based on the punching due to the heavily loaded columns?

2. When you say, bending moment controlled.....could you explain what you mean?
Do you mean that not all piles get equally loaded and a result the thick raft spans as a beam over the piles?

I am visualizing the most heavily loaded piles settling relatively more than lightly loaded piles. As a result, the raft undergoes bending? Is that a correct interpretation?

Thanks.

 

[ishvaaag]

Some beautiful info

http://www.seags.ait.ac.th/78-84 H.G. Poulos - Piled Raft Foundations for Tall Buildings.pdf

http://www.coffey.com/Uploads/Documents/Non-linear Analysis of Piled Rafts_20110321174809.pdf

http://sydney.edu.au/engineering/civil/publications/phd_thesis_chow.pdf

http://www.civil.iitb.ac.in/~dns/IACMAG08/pdfs/M31.pdf

http://www.gnpgeo.com.my/download/p...led Raft Foundation on Soft Ground(Final).pdf

http://www98.griffith.edu.au/dspace/bitstream/10072/20881/1/52507_1.pdf

http://gashydrates.iqsoft.co.in/200... raft foundations using infinite elements.pdf

On the matter, if the piles stay floating, you will need exceptionally high mat or mat plus basement rigidity to indent the settlement behaviour; i.e., for every class of foundation, except local effects for shallow, or through piles the loads being discharged directly to rock, you will see very similar settlements for every kind of competent foundation, as a virtue of dissemination of the same loads from the same points in the underlaying soil. Since entering the soil-structure interaction, one may need a very complicated model of the subsoil plus piles to get the most of the thing; other than that just get approximations from more simple approaches.

 

[paddingtongreen]

@slick,
A raft with evenly spaced piles is much like a raft on grade, upward pressure all over, and downward force locally at the columns. If you invert it, it looks like a plate, supported on columns, supporting a more or less uniform load.
You really have to check punching shear wherever it can occur. Clearly, the piles have a much smaller shear area than the column as well as a smaller force, check both.
You must check diagonal shear, punching shear and bending moments wherever they can control the design

Michael.
Timing has a lot to do with the outcome of a rain dance.