This section is from the book "Safe Building", by Louis De Coppet Berg. Also available from Amazon: Code Check: An Illustrated Guide to Building a Safe House.
If a beam is supported at two ends, and loads are applied to the beam, it is evident: 1st, that the beam will bend under the load, or deflect.
2d, that if the loading continues, the beam will eventually break, or be ruptured.
The methods of calculating deflection and rupture differ very greatly. In some cases, where deflection in a beam would do no damage - such as cracking plaster, lowering a column, making a floor too uneven for machinery, etc., - or where it would not look unsightly, we can leave deflection out of the question, and calculate for rupture only. "Where, however, it is important to guard against deflection, we must calculate for both.
Deflection when non-important.