How much load can a 48" (wood) board support if it is supported in middle, but unsupported for 6 inches at ends? It is for a 600 lb mower that must go over a wooden bridge we will are adding width to. The tires of the mower are 45" wide. Could we use 1 x 6" pressure treated decking? The weight is more on the back 2 tires. The 600 lbs includes the person on the mower. Can we get away with 1" X 6"?
Wood – Adding 48“ planks to widen a 36” bridge with no support at ends
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Plywood will not sag over time if you intend to use it for the door faces. I recommend birch plywood for not only its hardness but looks. However your hinges and what they are attached to are the big concern in this project. It looks like you've used a low grade pine to build the unit. This is a soft wood and a couple of cabinet hinges screwed into this are going to loosen quickly. Your best chance for longevity is to use a piano hinge so that you have a continuous hinge from top to bottom with 15 screws or more on each hinge. I highly recommend ordering a stainless steel or flat nickel piano hinge for this. What they have in stock at the box stores is of the "bright" variety and looks cheap to me.
I'd also recommend that you not attempt to build the door to be perfectly flush with the "lip" at the bottom. Leave yourself 1/4 of an inch gap. Units like these always sag at least to some degree and you wind up with the door scratching the "lip" and its going to look really bad with time.
Also take a look at ball catches to use either at the top or bottom of the doors to hold them in place once closed.
To add stability you could use 2x6 members for the cross pieces along the bottom. Then cover the bottom with a half of a sheet of plywood. Pour concrete into the box thus formed at the base by the 2x6's and plywood.
You could get by without having to go all the way to full angle braces across the sides (which would get in the way of your hanging plants anyway) by installing some gussets made of thin plywood as shown below:
At a suitable size these would probably come right up to where the first hanging pot wires go.
I see that you have each corner made of just one 2x4. I would encourage you to make the corners out of two 2x4's attached in an L shape in each corner. This will go a long ways toward making the unit a good bit sturdier and will also make the cross bars on two of the sides have a whole lot more wood to bolt into.
I would echo that the proper fasteners for this structure are going to be 1/4" or 5/16" diameter carriage bolts as opposed to screws or nails.
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Best Answer
Assuming reasonably equal weight distribution, each wheel of a 600 lb. mower and a 200 lb person would carry 200 lbs. Any 2x4 or wider SPF lumber that's reasonably free from large knots will suffice.
The actual calculations are difficult due to the unknown weight distribution (front/rear, tire width, tire pressure, etc.), but experience tells me that you certainly will not break 2x4 or 2x6 (or larger) lumber in that application, especially if you were to run a sleeper under the ends to tie the boards together and distribute load. I consider that overkill, though, and I'd only bother to do it if you see substantial flex in the individual boards as you roll over them.