The horizontal stresses in the flanges are greatest at the centre of a span.
The impact stresses depend so much on local conditions that it is difficult to fix what allowance should be made.
In most mechanical systems the working stresses acting between the parts can be determined when the relative positions of all the parts are known; and the energy which a system possesses in virtue of the relative positions of its parts, or its configuration, is classified as "potential energy," to distinguish it from energy of motion which we shall presently consider.
But there are stresses which depend on the relative motion of the visible bodies between which they appear to act.
As subscribers' lines are invariably short, the smallest gauge of wire possessing the mechanical strength necessary to withstand the stresses to which it may be subjected can be employed, and bronze wire weighing 40 lb per mile is commonly used.