Will floor bend/sag over time under the weight of my desk?

Question

For context, I'm a musician. I have a large workstation desk that, when outfitted with all my equipment, clocks in at 219 pounds. (I could probably get that down to maybe 205).

The legs/feet are oddly designed. The two front feet are 16"x5/8", and the two back feet are 2"x1 1/4". Right now the surface area of all four of those feet come in at 25in sq. That would be 8.76lbs per square inch. The desk is currently on carpet on a concrete foundation so no worries at all about the weight.

Now, I'm wanting to move into a shed we have but I'm worried that these odd feet dimensions will have two small of weight distribution for the floor.

The shed floor has joists every 12", 23/32" cdx plywood, then pergo gold underlayment and the top is pergo timbercraft laminate flooring coming in at 12mm (10mm plank and 2mm pad). In order to center the desk, the feet land exactly between the joists. Of course the joists couldn't perfectly be where i need them, right??

What're your thoughts on this? Possible? Or will the floor bend over time in those areas where the feet are in contact with the ground? (This would look clunky, but I've thought about potentially getting oak 2x4s and bolstering along either side of all 4 feet, sistering them together so as to add an additional 108in sq to the desk feet surface area, totaling 133, which would then distribute the weight as 1.65lbs per square inch. Though it still doesn't change the fact that the weight is going directly on board between joists.)

Thanks so much in advance for sticking with this tedious read and providing any thoughts or ideas for me!

Answer 1

I cannot calculate nor have the schedules to refer to for deflection and load capacity. However I can tell you from experience, the construction of your shed floor should be well capable of supporting your desk, no matter where it is situated, without issue for many years.

Answer 2

It's fine, although the 2 mm pad below your laminate might not like the 44 psi maximum pressure under the small legs.

Your 23/32" plywood should have a span rating of 24" for subfloor applications. I've highlighted the testing standard information for your plywood:

Since your 24" rated plywood has been installed to joists on 12" centers, your worst case deflection under the 200# test load should be about (12"/24")³ = 1/8 times the tabulated maximum deflection of 0.25", i.e. (1/8)(1/4") = 1/32". For your 50# concentrated loads, the anticipated deflection is (50#/200#)(1/32") = 1/128". No worries there, then, with an span length over 360 of (12")/360 = 0.033" ≈ 1/32". I might start worrying for a steady 200# concentrated load with its 1/32" deflection matching L/360.

The tabulated 400# minimum static load should double to an 800# minimum capacity for your installation.

The pressure number you generated was a little misguided. Assuming that the desk is rocking across two legs, I get a maximum bearing pressure of (219#/2)/[(2")(1.25")] = 44 psi. The bearing strength of plywood is about 360 psi, where that corresponds to about 0.04" of crushing. Obviously the plywood has no problem, but your laminate's spongy layer might not like the pressure.

Another check for concentrated loading would be punching shear. For your plywood, the 2018 Manual for Engineered Wood Construction provides a through-the-thickness shear strength of 115#/in (Table M9.2-4). For your 2"x1-1/4" leg the punching shear strength is (2"+1.25"+2"+1.25")(115#/in) = 750#, so no problem there.