Expert: Dan Griffin - 12/19/2007

Question
QUESTION: We have a machine of 20 ton(20000kg)rested on 10 pads.
the dia of pad = 185mm.
i want that what should be the structure of floor so that it can bear this load (ie thickness of concrete and ratio of sand, cement etc, net of steel, thickness of brick blast). Waiting for your reply

ANSWER: Mr. Ghani,

I do a  bit better in American dimensions, but can translate fairly well.

There are several big issues.  Does the machine deliver vibration or impact to the floor like a punch press?  What is the bearing capacity of the soil under the slab?  What are the overall dimensions of the machine?  Does the machine depend on the floor or the floor's mass to maintain tolerance or rigidity?

As an illustration, I set a Japanese multi chucker machine in a valve plant that needed a heavy floor mass to provide rigidity and accuracy.  We excavated an area about 30 feet x 40 feet to a depth of 4 feet.  We isolated this machine base away from the rest of the existing floor for vibration isolation.  The bearing soil was a heavy dense sand/shale capable of over 3,000 pounds per square foot.  There were three simple rebar mats in the design.  We used 4,000 pound concrete and provided over 60 belled anchoring points as required.  These design parameters were provided by the machine manufacturer.  The mass was required by their engineering department.  The heat of hydration of the concrete the next day was staggering.  I always felt there should have been more provisions for cooling the mass concrete.
You have a machine that weighs 40,000 pounds resting on a total pad for which I have no information on a soil I do not know.  If it were a 10'x 10' slab it would need to deal with 400 pounds per square foot - no big deal.  If it were on a 4'x 5' slab you're at 2000 pounds per square foot - doable if the subsoil can take it.  Smaller than that starts to get questionable.

This is an absolute guess, but I would head toward a rebar fill in .20% range in two separate mats in a slab 2' thick using a concrete that would provide test cylinders in the 4,000 pound range that will require 7 to 8 sacks of Portland per cubic yard.  I do not understand the term brick blast.  This assumes you have a soil capable of bearing 3 to 5 thousand psf.

---------- FOLLOW-UP ----------

QUESTION: Thanks Graffin,
the Machine is blow molding and machine has a small vibrations,no imapct loading.
I dont know the soil bearing capacity. the soil contains clay, peat and a small content of sand.
Machines dimensions are 24' x 8' x 13'(l x w x h)
Brick blast means broken red bricks which is used before concrete in floors.
Some of our collegue said that every foot bears 20 tons is it true? but we divided the load of 20 tons on ten pads and get 2 tonper pad.
Kindly tell us the steel structure too.

Machine is rests on the pads.

Answer
The machine will create about 208 pounds per SF, if the concrete is 1' thick it will add another 150, so about 300 to 400 pounds per square foot.  Almost any soil will give you 1000 pound load bearing, unless it is a swamp.  Dry stable clay that will not get wet is ok, sand is ok, the peat thing worries me.  How far down would you need to drill or dig to hit true bearing soil such as bedrock or shale?  Do you have a high water table?

I would pour the slab at least 26 x 10 which is just 1 foot over size around.  It would be better to go 32 x 16 to provide a 4 foot perimeter.  I think the 1 foot slab is minimal and would suggest 18".  I would tie up 2 mats of #6 or #7 rebar with bars each way on 12" to 16" centers.  Hold the bottom mat about 4" above the bottom subgrade.  I would keep the top bars covered with about 4" of concrete.  I would pour 4 to 5,000 pound concrete with a water/cement ratio around .45.  If you have a way to hold water on top of the slab after initial set to aid in curing, I would keep it under water for at least 3 days, 7 would be better.