(4) Dead or permanent load is one of constant and indeterminate duration, as books on a shelf.  In the case of a bridge the weight of the structure itself is the dead load.  All large beams support a uniform dead load consisting of their own weight.

The effect of dead load on a wooden beam may be two or more times that produced by an immediate load of the same weight.  Loads greater than the elastic limit are unsafe and will generally result in rupture if continued long enough.  A beam may be considered safe under permanent load when the deflections diminish during equal successive periods of time.  A continual increase in deflection indicates an unsafe load which is almost certain to rupture the beam eventually.

Variations in the humidity of the surrounding air influence the deflection of dry wood under dead load, and increased deflections during damp weather are cumulative and not recovered by subsequent drying.  In the case of longleaf pine, dry beams may with safety be loaded permanently to within three-fourths of their elastic limit as determined from ordinary static tests.  Increased moisture content, due to greater humidity of the air, lowers the elastic limit of wood so that what was a safe load for the dry material may become unsafe.

When a dead load not great enough to rupture a beam has been removed, the beam tends gradually to recover its former shape, but the recovery is not always complete.  If specimens from such a beam are tested in the ordinary testing machine it will be found that the application of the dead load did not affect the stiffness, ultimate strength, or elastic limit of the material.  In other words, the deflections and recoveries produced by live loads are the same as would have been produced had not the beam previously been subjected to a dead load.[11]

[Footnote 11:  See Tiemann, Harry D.:  Some results of dead load bending tests of timber by means of a recording deflectometer.  Proc.  Am.  Soc. for Testing Materials.  Phila.  Vol.  IX, 1909, pp. 534-548.]

Maximum load is the greatest load a material will support and is usually greater than the load at rupture.

Safe load is the load considered safe for a material to support in actual practice.  It is always less than the load at elastic limit and is usually taken as a certain proportion of the ultimate or breaking load.

The ratio of the breaking to the safe load is called the factor of safety. (Factor of safety = ultimate strength / safe load) In order to make due allowance for the natural variations and imperfections in wood and in the aggregate structure, as well as for variations in the load, the factor of safety is usually as high as 6 or 10, especially if the safety of human life depends upon the structure.  This means that only from one-sixth to one-tenth of the computed strength values is considered safe to use.  If the depth of timbers exceeds four times their thickness there is a great tendency for the material to twist when loaded.  It is to overcome this tendency that floor joists are braced at frequent intervals.  Short deep pieces shear out or split before their strength in bending can fully come into play.