Punching shear pile cap. perimeter encloses many piles

[sharp910sh]

I have pile cap, huge pile cap 88.4m by 28.4m 7m deep. 28 2.5m diameter piles at 7.5 m spacing. the 1.5d perimeter encloses 4 piles! does that mean i include the 4 pile loads in the punching shear check. or should i just include the one pile, and the shear stress at 1.5d is just due to the single pile. in reality that isn't the case. the shear stress at 1.5d perimeter will be due to whatever is within the 1.5d, i.e the 4 piles inside the perimeter.

hmmmm


any ideas guys?

 

[ron9876]

Be sure you check beam shear.

Probably can't fail in punching shear from a single pile. I see it as similar to headed studs. You can check a single pile but a more realistic approach would be a group of piles with the punching shear perimeter.

 

[sharp910sh]

Cheers. I see eurocode allows you to check a reduced perimeter to only include 1 pile. i have checked one way shear, (beam shear) which makes sense.

i shall use a reduced perimeter, which of course will result in an increase in shear stress! sya hello to shear links. 7m long!

 

[hokie66]

I would call this a mat foundation on piles, rather than a pile cap. But whatever you call it, I think this should be done using a strut and tie analysis. It will not work as a flexural element, so should not be analyzed as such.

To support the top reinforcement, you will need some substantial bars anyway, so they will by default serve as stirrups.

 

[slickdeals]

Just out of curiosity, what is this supporting? A 7m thick pile piled raft.....a slender supertall?

It’s no trick to get the answers when you have all the data. The trick is to get the answers when you only have half the data and half that is wrong and you don’t know which half - LORD KELVIN

 

[asixth]

That is a massive cap. Are you designing the foundations for the Lotte World Tower?

It would be worthwhile looking at some case studies for raft foundations for supertalls like the burj or similar. Some of them use a combined rock bearing-pile bearing system.

I would think that beam shear would be critical because the pile reactions to the underside of the raft will begin to overlap and one-way shear becomes the failure mechanism.

 

[sharp910sh]

The pile cap is supporting a 1500m long, 700m main span cable stayed bridge. On of the corner piles is heavily loaded, 50,000kN compared to 30,000kN for the other piles. Therefore punching shear of the corner piles needs to be checked. I have checked a reduced periemter to enclose just one pile. it seems critical. Very high shear stress 1.25N/mm^2.

I did toy with strut and tie, but for so many piles and load cases. it seems very complicated!

the problem is thes say eurocode and bs 5400 is based on building slab design. so it doesnt really work. i have yet to see a bridge pile cap fall down though!