This section is from the book "Building Construction And Superintendence", by F. E. Kidder. Also available from Amazon: Building Construction And Superintendence.

When the span of a wooden beam exceeds about twelve times its depth the beam will usually deflect so much under its full safe load as to crack plastering if applied on the under side, and as floor and ceiling beams are generally used for a greater span than twelve times their depth, they should be computed by the formulae for stiffness rather than for strength.

In determining the size of floor beams the superimposed load and the span are the two elements which vary the most, the beams themselves being usually sawn to regular sizes while the spacing is generally either 12 or 16 inches. In tabulating the size of beams' for different loads, the author has found that tables giving the maximum safe span are the most convenient for general use, and with this view the following tables have been prepared, which show at a glance the maximum span for which different sizes of floor and ceiling joists should be used for different loads and spacings, and it is believed that they will be found applicable to most buildings in which wooden floor joists are used.

By knowing the size of the room to be covered and the purpose for which it is to be used, the size of joist required can be told at a glance. Incidentally the tables also show which kind of wood will be most economical.

If, owing to the room being irregular in shape, the joists must be of different lengths, the spacing or thickness of the joists may be varied, so that the same depth may be used throughout.

The only precautions to be exercised in using these tables are in regard to the superimposed load and the actual size of the timbers.

The total loads for which the maximum spans have been computed are given at the head of each table. The actual weight of the floor (beams, flooring, plastering and deafening, if any) subtracted from the total load will give the superimposed load, i.e., the load which the floor is expected to carry.

The superimposed load to be assumed for any given building is to a large extent a matter of judgment, as circumstances may demand a higher limit for one building than for another, even of the same general class. In general the tables may be considered safe for the classes of buildings indicated.

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