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I am thinking of putting a mezzanine in my garage to support 20,000 lbs of load, not including the weight of the floor. On the floor the area is 20' deep X 32' wide; I was looking at 4" X 12" timbers for floor joists and 3" X 12" timber boards on top for floor.

  1. What is the maximum distance I can run 4" X 12" fir timber joists without beam support?
  2. What is the maximum distance between the 4" X 12" floor joists if I have a 3" X 12" plank timber floor on top?
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Are you going to park a truck on this? 20,000 pounds is a lot. – shirlock homes Apr 9 '13 at 8:59
You should be talking to a structural engineer, this is likely not a DIY project. – Tester101 Apr 9 '13 at 11:09

There's a really simple answer to this question.

You find a structural engineer, and you pay them to answer the question. It seems like you probably have a special situation, and none of the standard tables out there are going to work.

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Typically the load on a floor is measured in pounds per square feet (psf). Where did you get the "20,000 lbs" figure you quote? Do you have a specific load in mind? Is this for general storage? Residential living space? The use of the space would dictate how much load the floor should be designed for. Typical values would be from 40 psf to 100 psf or more, depending on the use, local code requirements, etc. An engineer or other qualified building professional can help you determine the appropriate design loads, and then you can decide how to support it.

If you have a single point load (e.g. a winch suspended from the ceiling for moving around equipment or something), that would be a totally different design requirement.

Side note: where are you planning on finding 4"x12" joists and 3"x12" floor boards?? Those are not standards sizes and would be extremely difficult & expensive to obtain unless you have a known source of reclaimed lumber or something.

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