Load distribution

[MCOGGINS]

If I place 4'x8'sheet of plywood on a flat concrete surface then place a 4x8 steel plate on top of the plywood, then place a 72"L x36"W load in center; will the load distribute out to the total sqft to the concrete surface? How much will it distribute?

 

[JAE]

It will not spread out to the full surface.

The actual distribution is somewhere between full distribution and zero distribution.

The actual amount depends on the relative stiffnesses of the concrete, the plywood and the steel plate.

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[dhengr]

MCOGGINS:
To add to JAE’s first sentence, the load will certainly not distribute uniformly as (Load)/(4'x8'), a uniform load in lbs./sq.ft. over 32 sq.ft.

In fact, if the base of the load is very stiff (engineering judgement, experience and some calcs. come into play here) the load will distribute fairly well over the base area. Thus, you have (Load)/(6'x3'), a uniform load under the base. So, given the bending of the plywd. and stl. pl. you really haven’t gained much by adding them. You would likely do much better by putting a layer or two of 6"x6" wooden timbers/beams, 3 or 4' wide by 8 or 10' long under the load. Because of their greater stiffness the timbers would distribute the load over a greater area at less cost than the plywd. and stl. pl.

 

[MCOGGINS]

Thanks for reply! Plywood is not a good source for equal load distribution on a flat solid surface with plate over the top given the information provided? Understand stiffness and strength of plywood is not that great. Just to provide evenly distribution of the plate itself or get max plate distribution usage of the plate is good?

 

[SlideRuleEra]

MCOGGINS - For a heavy load, plywood between the steel plate and concrete is a help. But not for sharing the load with the steel plate.

A typical concrete surface is "rough" compared to a typical steel plate. What the plywood will do is provide a somewhat compressible cushion that deforms (a little) to match the concrete's surface contours. Then the steel plate more uniformly distributes load, without overloading "high spots" on the concrete surface.

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[MCOGGINS]

no sharing of the load..ok thanks.. but good to get max sqft load distribution of the plate only, 32 sqft.

 

[oldestguy]

Not indicating steel plate thickness or if the load is at all flexible leaves a lot of questions as to what will happen.