So now let's go through how to design and engineer a floor system for a shed.
This is true for any wood framed floor system, be it in a shed or a pole barn mezzanine or a second floor of a building.
First you need to determine the uses. You must decide on the loading you will design it to accommodate.
For your example, let's assume you are going to use it like a typical residential floor. That would be like furniture and people walking about on it. Note that if you are going to load your floor with piles of heavy stored things, or create point loads with heavy machines, that you will either pick higher loading factors, or design the structure for the point loads that are going to be created by specific machines.
This residential loading is a uniformly distributed loading of 40 pounds of live load per square foot, with an associated point loading of 250 pounds. The point load is like a person standing there. Because of this acceptable point load, I like to consider machines of that weight to be OK without additional design considerations. But think about a couple of people standing at a machine, using it. You now have a larger point load, and should consider locating the machine over a pier, so loads transfer more easily to the ground, rather in the middle of joist or beam spans.
So to design your space with the 40#/SF LL, you need to add 10#/SF for dead load. This is the weight of the structure itself.
Start by assuming that a 3/4" plywood floor sheathing will be used and the joist spacing will not exceed 16" on center. This will give a stable and rigid floor sheathing.
So first you design the joists. Using a joist calculator, we choose a typically available wood such as hem-fir in a #2 and better grade, and a 6" deep joist, to keep depth of the structure to a minimum. We use a deflection of L/240, which means it can deflect 1/240th of it's span, which for your 6' span, is 1/3".
http://www.awc.org/codes-standards/calculators-software/spancalc
The calculator shows it will span up to 9'10", so we're way over designed here. Great, that will be a nice stiff floor.
Next is designing the beams.
The contributing load for the middle beam is 6 square feet of floor area per lineal foot of beam. 6x50 pounds of total load equals 300 pounds of load per lineal foot of beam.
Next we decide on the spacing of the piers under this beam, so we can size the beam based on it's span.
I would place 4 piers under this beam to cut the spans to 5'.
Using this beam calculator with the same material grade gives us a 2x8 beam.
https://courses.cit.cornell.edu/arch264/calculators/example8.1/
There is a difference between designing the floor of a house and this type of floor for a shed. On a house, you typically have a continuous foundation wall supporting the house walls. Here you have beams supported by piers under the perimeter walls. These beams support the walls and roof, and must be sized for these loads.
I haven't calculated these loads and beams because I don't know your roof structure, wall height or loads. And remember that the 2 joists under the end walls have to be designed as beams to support the walls and roof, and their spans are 6' rather than 5'.
Do you understand all this?
As always, I disclaim that this is adequate for your use. I'm could be just some guy on the internet, probably a socially inept, pimply teenager, living in his mom's basement, and playing on the internet.
Bill
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