The impact stresses depend so much on local conditions that it is difficult to fix what allowance should be made.
Straining Actions and Working Stresses.
The stresses in the web are greatest at the ends of the span.
Let t be the statical breaking strength of a bar, loaded once gradually up to fracture (t = breaking load divided by original area of section); u the breaking strength of a bar loaded and unloaded an indefinitely great number of times, the stress varying from u to o alternately (this is termed the primitive strength); and, lastly, let s be the breaking strength of a bar subjected to an indefinitely great number of repetitions of stresses equal and opposite in sign (tension and thrust), so that the stress ranges alternately from s to -s.
Judd that this structure may be due to mechanical stresses.
