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By: Susan Bugher, P.E.
One of my favorite books quotes is about the spy who likes to pose as an engineer. His reason: "when I tell people I'm an engineer their eyes glaze over and they don't ask any more questions."
Today's topic is building loads, almost guaranteed to make your eyes glaze over.
however . . .
If you have a lot of books, if you are afraid to sleep at night because you fear you will wake up in the basement, if you have no idea at all if your concern is justified, prop your eyelids open and read on.
Buildings are designed to safely support anticipated dead loads and live loads. Dead loads are permanent loads (the weight of the building). Live loads are all loads that are not permanent. Live load includes your books, the people who walk through, and the refrigerator that stays for 30 years.
Design live loads are specified in building codes. The Uniform Building Code (UBC) is one of the more common codes. There are several others. In the US building codes are adopted by governmental entities. The agency that issues building permits can tell you what building code (if any) is in effect in your area.
Some common design live loads:
homes: 30 to 40 psf (pounds per square foot) (attics: 0 to 20 psf)
retail stores: 75 to 100 psf
warehouses: 125 to 250 psf
library stack rooms: 150 psf or more
Some night when you are up worrying calculate your own uniform live load. That will give you numbers to worry about - which will be a change from your previous unformulated fears.
To obtain the total live load estimate the weight of your books (weigh a representative foot long section of books), add the weight of the maximum number of people in the room at any time (weigh a representative person), estimate and add the weight of shelving and other furnishings.
When you have the total live load weight divide it by the number of square feet in the room. This is YOUR uniform live load. It may help you decide whether or not you have cause for concern.
A few other considerations:
The way the load is distributed makes a difference. Heavier than average loads may be okay if they are placed directly above the supporting walls and columns. Lighter than average loads may be a problem if they are concentrated near the center of a room or along a single joist.
Buildings may be weakened by deterioration, poor construction or other factors. The load classification of a building may not be obvious. A warehouse designed for light loads and one designed for heavy loads may look alike. Their load capacity is not the same.
The above discussion deals only in generalities. Consult a profession engineer or architect for advice about your particular situation.
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