Question I am about to become the owner of a Cary safe that is approx 33x22x22. I am wondering if my floors will hold it? My wood floors are supported by huge floor joists, built in 1953.
You have several problems which need to be addressed. While floors are generally built to hold quite a bit of weight, it is described as the load limit and expressed in pounds per square foot. This generally would address items which sit flat on the floor. This is described as the "Load Bearing Capacity" of the floor. Wood is rated by the type and size. These ratings can usually be found at most lumberyards. So you will need to know the size of your joists (2x6 or 2x8), the type of wood, and the length of the span. supporting structures under the joists will also increase the rating.
A general rule is that a 2x6 joist can hold about 10 lbs of dead weight per foot, or 30 lbs of live weight (person walking). Multiply that times length, so a 12 foot joist could support about 120 lbs of dead weight. Multiply that times the number of joists the weight spans to get the weight supported.
For instance a 12 foot square room with 2x6 joists set on 16" centers would support around 1100 to 1200 lbs of dead weight.
Load capacity is generally determined by city and county building codes. For instance if your city requires a load capacity of 40 pounds per square foot (PSF), then the engineers that design your house would build the floor to meet this rating. How much load bearing capacity joists add to a floor depends on the type of material used in the joist, the thickness of the joist, joist construction methods and how builders secure the joists.
Assuming that your floor has the 40 PSF rating, and using the measurements for your safe of 22" deep by 22" wide (we will round it up to 24" for easy math), this would provide your safe with a 4 square foot, foot print, and with a 40psf rating, a 10ft by 10ft room would provide you with a load limit for your room of about 4000lbs total (equivalent to about 15 adults). Your safe is probably in the 400 to 600 lb range.
The problem is that YOUR safe does not sit flat on the floor, it sits on four wheels, which provide pressure points on the floor. This much weight would eventually drive holes in the floor. in order for your safe to sit on the floor without causing problems, you will need to place some type of supports UNDER the wheels to help spread the weight of the safe over a larger area. For instance two - 1" thick piece of plywood, 6" wide and at least 30" long, placed across the joists and under two wheels would provide adequate support for the wheels and ensure that the weight of the safe is supported by at least THREE joists.
NOTE: There are no "general" calculations for determining load limit. I've tried to provide you with some basic examples to give you an idea of what to look for. We use the 40psf number as a basis to start with, if we don't have records or engineers to refer to.
You can check with your local planning office to see if they have the building plans for your house, to find out what standards it was made for, or you can measure all of the joists and supports to have someone do the math for you. Again, the more supports under the floor the stronger it is. If you suspect that your safe weighs more than the floor will support, you can have supporting beams added under the floor, under the safe to increase the amount of weight you can have at that particular point in the floor.
Hope this helps,
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