Live load reduction for warehouse slab

Hi,
I'm working on the warehouse slab design which live load 5t/sq.m. Due to the soil condition is not good, I decide to use piles to transmit load from slab to the good soil below and I designed the floor slab as flat slab. I got the suggestion from my senior colleague that when we design the floor slab we use 100% of live load (5t/sq.m.) but when we calculate the load on pile, we use only 75% of live load (3.75t/sq.m.). He said from statistic the warehouse has never been used their full live load so we can reduce the live load. But I think it's not consistent for the slab design and load on pile calculation. Where can I find the code or reference for this reduction or you guy has any sugeestion?
Thanks,
Argadoo

 

[ishvaaag]

If your slab on the ground has some stiffness and is fixed to the piles another consideration than estochastical distribution of the load intervenes, since in such in the end piled-mat piles and mat will be sharing the load.

It is typical in piled-mats to share the load in around halfs, say 40% to 60%, and the proportion even varies with time with the progressive settlement and aging of the concrete. In such case the advice of your colleague would be normally warranted even from merely a structural standpoint.

What above is merely to reckon that some loads will be passed directly to the ground through the slabs even for modest soils.

UBC 94 1606 expressly indicates that for storage areas where LL>4.79kN/m2 no LL reduction is allowed.

If some reduction is to be allowed due to the expected average allocation of the loads you should be able to ascertain of what really the project estimates the loading will be, or some code statement such that above.

 

Thank you ishvaaag,
I have a further question, when we design flat slab,the thickness of drop panel will be about slab thickness/6. So if the flat slab is supported directly by plies, the thickness of pile cap will be slab thickness/6 or more? and how deep of pile be embedded in the slab?

Argadoo

 

[ishvaaag]

If for single piles you may add cube pile caps to widen the introduction of the forces, diminishing the punching problem.

Other than that, normally you will count with just the depth of the slab aa the punching area, or the depth of the slab where it is constant. Respect how much penetration is required, just enough to get bearing and leave enough cover for the bottom main rebar in the mat on the ground, say 6 to 8 cm, or the cover the specification demands.

I have practiced all that here said with in-situ augered-concreted piles with no other problem than some misplaced piles causing some additional embedded beams on the ground work.