Application of Loads

There are three[12] general methods in which loads may be applied to beams, namely: 

[Footnote 12:  A fourth might be added, namely, vibratory, or harmonic repetition, which is frequently serious in the case of bridges.]

(1) Static loading or the gradual imposition of load so that the moving parts acquire no appreciable momentum.  Loads are so applied in the ordinary testing machine.

(2) Sudden imposition of load without initial velocity. “Thus in the case of placing a load on a beam, if the load be brought into contact with the beam, but its weight sustained by external means, as by a cord, and then this external support be suddenly (instantaneously) removed, as by quickly cutting the cord, then, although the load is already touching the beam (and hence there is no real impact), yet the beam is at first offering no resistance, as it has yet suffered no deformation.  Furthermore, as the beam deflects the resistance increases, but does not come to be equal to the load until it has attained its normal deflection.  In the meantime there has been an unbalanced force of gravity acting, of a constantly diminishing amount, equal at first to the entire load, at the normal deflection.  But at this instant the load and the beam are in motion, the hitherto unbalanced force having produced an accelerated velocity, and this velocity of the weight and beam gives to them an energy, or vis viva, which must now spend itself in overcoming an excess of resistance over and above the imposed load, and the whole mass will not stop until the deflection (as well as the resistance) has come to be equal to twice that corresponding to the static load imposed.  Hence we say the effect of a suddenly imposed load is to produce twice the deflection and stress of the same load statically applied.  It must be evident, however, that this case has nothing in common with either the ordinary ‘static’ tests of structural materials in testing-machines, or with impact tests."[13]

[Footnote 13:  Johnson, J.B.:  The materials of construction, pp. 81-82.]

(3) Impact, shock, or blow.[14] There are various common uses of wood where the material is subjected to sudden shocks and jars or impact.  Such is the action on the felloes and spokes of a wagon wheel passing over a rough road; on a hammer handle when a blow is struck; on a maul when it strikes a wedge.

[Footnote 14:  See Tiemann, Harry D.:  The theory of impact and its application to testing materials.  Jour.  Franklin Inst., Oct., Nov., 1909, pp. 235-259, 336-364.]