Need to know how strength relates to thickness 1

[somguy2u]

i tried to search for this for a while and i didn't find much.

Problem:

"management" purchased a 66,000 lbs 250 ton mechanical press and said put it somewhere by next week.

I am pretty sure the required slab depth is 12" of concrete. the depth of concrete where they want it is a drastically short 3.5". being a 27 hit per minute press i would assume that the vibrations and the shock load are going to be pretty drastic. is there anything i can do with the floor short of cutting it up and re-pouring it?

I have been playing with the bearing stresses and i have devised away to get the bearing stress down more than %50, but i have no idea if this is going to be enough.

i know absolutely nothing when it comes to concrete honestly, all i know is what's out there isn't very thick and it's rebarred.

what sort of distances do i need from separation lines?

 

[msucog]

besides the load of the machine itself, you've got the dynamic loads to consider (as suggested in your post). for machines like this, i've seen the foundations (designed by others) be 4' thick and a few feet larger on each side than the footprint of the machine so as to dampen the "bang bang bang!". i've never done the math, but i vaguely remember something to the effect that you're trying to have the mass of the foundation to be near 1x the anticipated dynamic load. i'm not positive, but i don't think the "250 ton" was the number to consider...it was smaller than that but it seems like the size of the machine was a good bit larger than the actual dynamic load considered. i think the dynamic load for my scenario was 75,000 pounds. you might get by having it on the slab...for a while anyway until it stops holding then i suspect it might go downhill quickly from there.

i'd suggest cutting out the slab and having someone check the underlying conditions. on the project i mention above, they cut the slab out )~8" thick) and we found tires and such. the slab had been holding up well even though it was heavily loaded with rolls of sheet metal...but in my opinion, you've got two different animals when dealing with "static" loads versus dynamic loads. the underlying subsurface conditions influence the overall performance of the subgrade in addition to looking at bearing capacity (shallow groundwater being detrimental to the subgrade under dynamic loading for instance...sort of like tractor trailers drying over a pavement that has a bird bath due to the grades...the subgrade is fine to begin with but the loading occurs under a psuedo saturated conditions so that spot usually fails first).

that's my opinion and observations for what it's worth...

 

[hokie66]

You need to go back to management, tell them that you are certain the slab is not adequate to serve as a base for the press, but you are not qualified to design the base. If this is a big industrial company, perhaps it has a central engineering group who can help. If not, you need to commission a structural engineering consultant.

 

[LCruiser]

They must mean to store it temporarily, since you need the concrete to cure anyway. Tough problem, since you need to consider the dynamic response of the underlying strata.

 

[DRC1]

Talk to the machine manufacterer. They can generally provide guidance on what is need for floor support. I assume this is a slab on earth. If this is an elevated slab contact a structural engineer ASAP.
Sidewalks ae generally 4 inches or thicker, For a machine you need a fairly thick slab. The risk is that the slab will crack and cause uneven settlement of the machine, This in turns affects alignmet, which can increase component wear and reduce production.

 

[somguy2u]

i have contacted a local consulting firm, the amount of unknown information about the building's structure is to great. hopefully they can shed some light on the situation. i will let you all know what happens.

 

[concretemasonry]

Your machine frequency is quite low.

I was in charge of a plant where we had a few 40,000# pieces of equipment with a different frequency.

Our most practical method that worked was following the old "rule of thumb" of having the foundation weight 10x times the machinery. There are two reasons why the mas concept works:

1. Your machine is probably quite rigid and the slab and its performance should be as stiff or stiffer than the equipment for minimizing vibration and effects and prolonging the life of the equipment.

2. Once you cut the existing slab for a new foundation the area is fixed and the extra concrete to get the mass is "dirt" cheap. - You did not say what the footprint of the operating area of the machine was.

Make sure your foundation is well isolated fron the floor slab since the vibration could be transmitted to other parts of the structure or, more, importantly, to separate it from any instrumentation and controls that could be affected.

 

[msquared48]

Hokie is correct here.

I would suspect though that as mentioned, you want to totally isolate the slab - probably in the neighborhood of 8 to 12" thick depending on the loads seen, plus consider putting the whole assembly on an augercast piling system. You will definitely need to abandon the existing 3.5" slab. That just will not work.

Mike McCann
MMC Engineering

 

[dochardnose]

I suggest you look at ACI 351 for information.

 

[Ron]

Hokie66 is on target. Get a consultant involved, otherwise you'll have a cracked concrete floor slab, and potentially a damaged press. The press foundation should be isolated from the slab.

 

[IFRs]

Can you spread the load over a larger area?