Trying to determine support weight

Question

I have 3 2X4 walls 16" high 30" long and 3/4 inch MDF plywood top. How can I figure its load support capabilities? What I'm doing is building a platform to put a washer and dryer on and need this to be able to support a full washer with water. Under the platform I will be putting 2 slide out draws. Can anyone help me with calculating the support weight load.

Answer 1

TL;DR

A 2x4 carcase with a 3/4 in. MDF top is probably not going to work. It's hard to say for sure, but unless that MDF can transfer the loads directly to a component that transfers the load to the floor, this is going to fail eventually. MDF does not have great elastic load handling ability (it doesn't like to bend without catastrophic failure or weakening). Also, unless you are using some sort of marine-grade material, as it gets wet over time it will get weaker and weaker until it fails suddenly.

Essay Answer

Disclaimer: I am not an engineer! Static and dynamic loading of lumber structures is a complicated subject that I have no expertise in. For anything non-trivial we have to consider all sorts of loading and elasticity factors, and how that affects the various shear and compression limits.

That being said, most civil engineering is collected wisdom, usually collected as a result of premature and sudden failures. And since we are talking about white goods here (and, say, not a bridge) we can make some edumacated guesses without raising the ire of any trained engineers who stumble upon this.

There is a lot of published information on the compression or shear limits of lumber, both with and perpendicular to the grain. For instance Cornell has published a wood column calculator. There are other sites that break down the various compression and shear values of typical hard- and soft-woods, so definitely do some web searches.

But, at the end of the day, even softwood has pretty good with-grain compression values - thousands of pounds per square inch - that you should be able to design a reasonably robust carcase.

I would approach this in two ways:

1. Static loading.

   • With the white goods as a dead load, think about transferring the maximum wet mass from the four legs of each directly to components within the carcase directly to the floor.
   • Try not to bridge or cantilever the mass. The idea is that you are sort of extending the feet through the carcase to the floor.
   • Design your drawers, etc., around these extensions.
   • You could even augment or replace these extensions with steel or iron components because those materials have pretty good compressive load strength.
   • Think about how posts, etc. transfer the mass of a deck or floor into the ground, ideally without twisting or bowing. Now imagine a small deck that was a tiny dance floor where two heavy dancers really knew how to do the jitterbug...

2. Dynamic loading.

   • Modern washers really test structures sensitive to vibration and torque!
   • Build your carcase so it has very positive connections to the white goods, and allow yourself the ability to level the appliance easily (this is going to be your largest challenge).
   • The point being that a washerful of cotton linens set to wash with extra water, long wash cycle, and high-speed spin is going to teach anything you build about how centripetal force works.
   • Unless you really have to, don't try to build in leveling to your design; let the washer level itself as part of its mating with the stand.
   • Because your carcase is now the actual feet of the washer, all those things that apply to washer feet (how sticky it is, how it wants to move across the surface) should be part of your design.
   • Most washer feet come to a rather small diameter. Is this important? Would a flat 4x4 foot want to rock, or transfer more unwanted vibration to the floor? I have no idea! But vibration mitigation is its own arcane science, so maybe try to steal some ideas from existing designs.

Ok, I've written a lot of fancy words here. What does this mean? What are some of the back-of-the-envelope things we can come up with?

Well, Something like a 4x4 or even a 2x4, properly braced internally against twisting on at least two planes should be sufficient as a with-grain load transfer component. Attention to how the carcase interfaces with the white goods, and how it interfaces with the floor is an important part of this. Reading the appliance specs and making sure they are proper distances from each other (if side-by-side) and walls is something you want to get right before you start making sawdust.

I'd design this so that any drawer components are "floating", and not part of the mass-transfer carcase. You are building appliance feet extenders that happen to have some hollow spaces for storage space.

Finally, the overall height of your design might be significant. I'm not sure if this is a consideration in this case, but vibration is often a function of overall length of moving components. Are we working with material and forces here that could set up a standing wave? I'm guessing no one in your house wants an otherwise safe washer platform that has the side-effect of vibrating the dishes out of the cupboard.

Answer 2

Are you using MDF of plywood? Use 3/4" plywood rated for sub-flooring, if you make a good structural frame of 2x4's with center support build it like a floor structure make sure that the top "joist" are supported. When in doubt OVER BUILD! Do not use MDF it is not structural material and water will destroy it quickly. Lastly paint it with a good paint to seal it. I am not an engineer or such, but have been involved in the trades for about 30 yrs.

Answer 3

I personally think this is a terrible idea. No way it's going to hold up to a washer's spin cycle unless you make it out of steel and weld your parts together.

Why not just build some drawers to go on top of the washer, or build simple cabinets with drawers which float an inch or so above the washer? Makes it easier to access the washer for future repairs and you don't have to risk the whole thing collapsing during the spin cycle.