workshop floor - strength question

Relatively soft materials aren't a problem provided the weight is spread over a wide enough area. Tanks rarely get stuck in mud because caterpillar tracks massively reduce the pressure on the ground.

Looking up polystyrene insulation I found 3 pressure resistant grades made specifically for insulating floors, able to take 100kPa, 150kPa and 300kPa for 10% compression. Polystyrene made to insulate walls is no good.

300kPa is a bit over 40 pounds per square inch so my flat-bottomed sheet-metal lathe stand with two 13x13" plinths would support 13520 lbs (over 6 tons). As my lathe actually weighs about 600lbs, say 800lbs with the stand, the pressure would be only 2.4 psi, and I'd expect polystyrene under the plinth to deflect by about 0.06mm.

Although concrete screed isn't as good at distributing pressure as chipboard or floorboards, I think it would be safe to support heavy machines provided a stiff board were placed underneath to guarantee load spreading. For example, a length of kitchen worktop 52" x 24" under my 800lb lathe and stand would reduce the floor loading to about 0.7psi, which is trivial.

I'd be nervous of doing anything that increased pressure on a polystyrene insulated floor such as putting a heavy machine on wheels. But this is mainly because I don't know how 50mm of concrete screed behaves laid on top of polystyrene. I suspect the combination is much stronger than I give it credit for because floors are generously specified to take loads. Is there a builder or architect on the forum?

Dave

With my new house build, the concrete floor minimum thickness was increased from 50mm for the rest of the house to 100mm for the workshop. I also increased the concrete to 30mpa instead of the std 25mpa, as the higher cement content makes for a slightly nicer finish on the floor as well. The poly styrene under the floor areas that goes down to the base, so makes a concrete rib that is 200mm deep in the workshop and 150mm deep for the rest of the house. The foam is specific for house construction, but I don't know what compressive strength was used. My driveway is cracking and is placed on sand and they did not reinforce it. Not happy about that. The cracks are now happening 2 years latter. It is also only around 75mm thick. Which I think is too thin for an un reinforced structure.

How about the opposite. Just heat the machine say to 22C with sheet and blanket over it when not in use. Avoids rust. Then stand on a duckboard which has a small heater panel if your feet get cold and a radiant heater aimed at your back if your body gets cold.

Thanks all for your input.

I think its clear now 50mm of concrete on a compliant base is quite risky, and especially so with electrical elements embedded in it. I will have to consider something substantially thicker, probably 100mm min. In a house this wouldn't be helpful as it just slows the warm-up time. But in the workshop it would only be a background heat just to keep the room a little above ambient, so perhaps 100mm is not a problem.

I had planned plenty of insulation and the calculations suggest it wouldn't take much energy to maintain a reasonable differential with the outside ambient. I already have solar panels on the house and they generate far more than I can use, so this would heat the workshop most days, and for free.

The real problem now is that the planning people want to reject the whole concept due to 'green belt' concerns. So all workshop design activities are on hold until this is sorted, ....if it can be sorted.

Gerry

Maybe the planners don't like the word "Workshop" They probably think that you are about to start a major manufacturing enterprise there.

"Shed" or "Hobbyroom" may be acceptable to them

Howard

Have you taken in to account vibration?

My workshop and whole ground floor of my house are 6 repurposed garages. ( I bought some ex council flats & converted them to a house) All the floors were 10" of reinforced concrete on the original build. The floors had a slight slope to them to disuade water from entering under the garage doors. When the conversion was done doors were removed and bricked up to comply with modern insulation standards. The floors were also insulated in such a way as to bring them level over the whole area, a variance of 50mm front to back. So 100mm one end & 150mm the other. The 50mm ish fibre reinforced screed was then laid. This has been more than adequate and has not cracked or shrunk etc over the last 12 years. I have my Boxford lathe on its cabinet with flood coolant tank and many chucks & backplates in them. A 6 legged bench with a 110kg Ajax mill & a 55Kg minimill + all the tooling associated with them. Several very heavy cabinets on wheels + stock standing up + the workbench which is loaded down with "stuff" as is usual. So all in all more than a couple of tons which gets moved around occasionally. Not seen or felt that there was any movement in such a thin screed. I did not specify a deeper screed for the workshop as at the time it was planned to be a normal use room.

regards

It's not that simple I fear, it all depends on the relative stiffness of the chipboard and concrete. If te chipboard really did convert point load into uniformly distributed, you could consider it the other way up, a 52"*24"worktop supported at the corners with 800 lb of sand distributed across the top surface. I wouldn't be surprised if it broke, at the very least it will deflect quite a long way.

True the sum isn't that simple because the board bends! Does it matter? I don't think a kitchen worktop would deflect much when laid flat on a floor because the load is in compression and is supported underneath throughout. It would protect the concrete screed considerably. Be interesting to measure it. Lying on the floor is different from glued or screwed to the floor, but I've no idea how much better a fixed board would be.

A worktop flat on the floor isn't the same case as being supported at the corners and acting as a bridge. The latter imposes a severe tension load and the corners ground the total forces in concentrated form. Nonetheless, not impossible to support 800lb on a kitchen worktop bench provided the structure underneath supports it properly.

Sam asked if I took vibration into account? No, and I don't know how to! I guess machine vibration wouldn't break a screed on polystyrene floor, or underground heating elements, but it might break electrical or plumbing connections. No idea how vibration proof underfloor heating is.

Dave

Edited By SillyOldDuffer on 12/02/2022 13:32:58

SOD tried to argue that putting a worktop under a lathe would make the load applied from the floor to the concrete uniform. The lathe is (I assume) applying 4 point loads, so that would be exactly equivalent to having it upside down on 4 points supporting a uniform load as I said. It is irrelevant that you could support 800kg on a worktop by supporting it properly, you could put more legs on the lathe if you were that way inclined.

In reality the board will spread out the load a bit but to nothing like that extent. To work it out properly would not be trivial. According to Google the Young's modulus of 'particle board' is around 0.3 GPa Echip, whereas concrete is between 30 and 50 GPa Econc depending on the mix, say 40, so 133 times greater. Steel is 207 GPa for comparison. Worktop is typically 40mm thick, whereas the OP's concrete was 50mm. Stiffness is proportional to the cube of thickness, so the concrete will be (50/40)^3 * 133 = 260 times stiffer than the worktop. The bending load will be shared in proportion to the stiffness.

In short, the concrete needs to be thicker, even my kitchen floor is 100mm on top of 100mm hardcore, and the heaviest thing on that is the washing machine. Actually it's me, but we needn't get personal!

Not guilty this time, because I don't claim the load under the worktop is uniform!

All I'm saying is putting a worktop on the floor under a lathe will spread the load, and if the worktop was perfectly rigid, my simple calculation would be about right. Duncan says, correctly, that the actual pressure depends on how stiff the worktop is.

It's the difference between these two constructions:

They have the same areas top and bottom, so a load on the red top would put much the same pressure on the lower disc. The cone on the left represents my view, the column on a disk is closer to Duncan's position. The problem with the column bearing on the disc is that it will tend bend the disc so the pressure underneath isn't uniform: it will be higher directly under the column than at the edge.

The risk is putting a worktop flat on on a underheated floor wouldn't spread the weight of a heavy four-footed lathe evenly enough to prevent damage. An experiment is called for! I can't think of an easy way of measuring or calculating a pressure distribution caused by the support flexing. Suggestions anyone?

On the plus side, heavy objects like lathes are often successfully moved across soft ground by laying boards underneath. On the down side, boarding a lathe safely across a lawn doesn't prove an expensive floor would be completely protected.

Dave

Oh yes you did, I quote For example, a length of kitchen worktop 52" x 24" under my 800lb lathe and stand would reduce the floor loading to about 0.7psi, which is trivial. You didn't mention perfectly rigid, although I admit you did say stiff, unfortunately worktop isn't stiff compared with concrete. It would have to be infinitely stiff to make your sum correct. If it were in 2 dimensions, one could use the equations for a beam supported on a flexible foundation, but they are not to be sneezed at, and I deliberately said one, not I. Bit pointless really, as building regs will require at least 100mm

My garage is single skin brick with a concrete base, I don't heat the entire thing while I am not in there. I use several 40W Dimplex heaters to keep the garage above the dew point and stop condensation (and therefore rust) on my machines and use a little 1KW fan heater when I am going to be in there for a while. I put the dimplex heater under the lathe, milling machine, pillar drill and my tool box and it works really well and is cheap, especially with electric prices being what they are at the moment

Hi Gerry,

You might get away with a concrete sectional 'shed' and stay away from talk of insulation and heating.

Ah, but it's what I meant that matters, not what I actually said. And being wrong is irrelevant in a world full of Alternative Facts.

Either I'm innocent or a dog ate my homework.

Time to move on from Duffer-gate: the forum demand I tackle the big issues, like how best to sharpen pencils with a milling machine!

Dave

Anyone remember chisel points in the DO, and several different hardnesses of lead? I had a file to hand to touch up the point. Then they invented the Pentel things with small diameter leads, so all the clutch pencils went in the bin. Pity really, would have been a good basis for a scriber. To sharpen pencils on a milling machine you'd need a universal dividing head, and a lot of patience.. Of course the 4 axis CNC men would do it in an instant

Expanded polystyrene has a compressive strength of 70 to 20 kPa.

At the bottom end 70 kPa is about 7000 kg/m^2, seven tons per square metre.

Which ought to be ample.

polyfoamxps.co.uk/ground-floor-insulation-what-you-need-to-know/

Neil

I've removed a road bridge using a 5 ton demolition machine sitting on 1 metre of polystyrene topped with two layers of plywood, all sitting on railway lines. No kittens were killed.

Timber absolutely does distribute a point load over a wider area, if it didn't they wouldn't use it for crane mats.

Neil, in these situations it isn't only about the ultimate compressive strength, its about the compliance or stiffness of the materials as well.

Your polystyrene will only 'fight back' with any effort when it's considerably crushed. A hard brittle material like concrete, on top of soft material may crack well before the compliant polystyrene has built up any significant resistance to the load.

Imagine putting a large sheet of 0.1mm thick glass over a very large area of layer of sponge rubber. You wouldn't be able to walk on the glass without it breaking. So basically the stiffest material has to have sufficient strength to carry the load before it will be able to spread that load successfully into the softer more compliant material.

Gerry

N

Edited By gerry madden on 14/02/2022 23:22:23