Expert: Arshad Khan - 2/19/2010

Question
QUESTION: I am working on a project to move a number of separate business units to a single site. The site is already built and need to verify that the existing concrete slab is strong enough to take the weight of the injection moulding machines to be moved there.

The machine characteristics are:
Maximum weight = 6150Kg
Number of feet = 6
Size of each foot = 150mm dia

Can you tell me what the kN/m2 I need the concrete slab to be able to take please?
And also what would be the minimum specification of concrete I would need the building to have to support this weight of machine?

Many thanks in advance.

ANSWER: Hi Rex,

If the weight is evenly distributed amongst the 6 feet, then the static pressure under each foot is around 0.5 N/mm2, which any good concrete will be able to support. However, whether the floor slab is good enough depends much on the quality of the supporting ground, as well as on the thickness of the slab and the amount/location of the rebar.

Assuming that the slab is 200mm thick, then the ground immediately below should be able to safely bear a pressure of around 150-200 kN/m2 to avoid any settlement or distress to the concrete.

All this assumes that there is no dynamic loading, as this will increase all the loading/pressure figures.

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

QUESTION: Thank you for your 1st reply and since asking that question I've been given a little bit more detail about the location. They are:
Core sample detail:
0-165mm Reinforced concrete floor slab with no steel reinforcements found
165mm Visqueen
165-300mm Crushed rock (No fines)
300-580mm Brown silty sand
580-1100mm Light brown sand

Also the machine stands on 6 feet that are positioned in 2 rows of 3 down each side of the machine evenly positioned over a 4m length and the rows being 0.75m apart.

Many thanks,

Rex

ANSWER: Hi Rex,

If the slab is only 165mm thick, then the subgrade will need to support more than 200 kN/m2. Without knowing the standard of compaction, etc, the only way that you can determine capacity is to do a test loading of 1000 kg over an isolated plate of 150mm diameter and see how the slab behaves over 24 hours.

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

QUESTION: Thank you for a fast response once again.

The slab has been in place for over 20 years I believe so would it be fair to assume that the subgrade will by now be very compact? Or is that a naive question?

If the subgrade is compact would the slab be OK for the weight of machines I have please?

Many thanks, Rex

Answer
Hi Rex,

Yes, your assumption could be correct, but this depends on the loading the slab has been subjected to. Normally the maximum load in a warehouse would be about 7.5-10 kN/m2, but you are applying a pressure (localised) of between 200-400 kN/m2, so as you will appreciate there is a big difference!

You could mitigate the potential problems by using steel spreader beams under the feet, if that is practical.