4 MATERIALS OF CONSTRUCTION. Art. 6. 



AH loads, (dead or live), which are not gradually applied, 

 produce vibrations which gradually disappear if the loads come 

 to rest on the structure. In other words, the distortions of the 

 structure are momentarily greater than for the same static loads 

 and these are evidences of stresses, exceeding the static stresses, 

 which may be called vibratory stresses. 



If the impulses causing vibration are applied at intervals 

 corresponding in a certain way with the time of vibration of the 

 structure, the effect will be cumulative, and the stresses may be 

 very great. Practically, there is more or less interference 1 . 



Vibratory stresses are usually the result of a number of 

 different causes, and it is important that the construction be of 

 such a character as to keep them within proper limits ; allowance 

 is usually made for them in a more or less arbitrary manner 

 under the name of impact stresses. 



IMPACT STRESSES usually include all stresses in excess of 

 those due to the same loads gradually applied. The total equiva- 

 lent static stress is, then, equal to the sum of the dead load, live 

 load, and impact stresses. In getting stresses, all loads are 

 treated as being gradually applied, producing static stresses. 

 To make allowance for impact, a certain percentage of the live 

 load stress is added, depending upon the directness with which 

 the live load stress is imparted. The impact stress is greater for 

 a short span than for a long cne 2 . 



6. Materials of Construction. The ordinary materials of 

 construction are stone, brick, concrete, timber, cast iron, cast 

 steel, wrought iron, and steel. Steel is the material most largely 

 used for structures in which the engineer has most to do with 

 stresses. 



These materials have various properties to a greater or 

 lesser degree, such as density, hardness, brittleness. ductility, 

 elasticity, strength, etc. The most important of these are strength, 

 elasticity and ductility. 



7. Elasticity and Stress. The materials of construction are 

 more or less elastic, that is, they undergo changes of form under 

 the action of forces. Let Fig. 3 represent a bar held rigidly at 



*For a full account of measurements of vibrations of railway 

 bridges and their causes, see a paper by Professor F. E. Turneaure in 

 the Trans. Am. Soc. C. E. Vol. 41, p. 411, and a paper by Professor 

 S. W. Robinson in the Trans. Am. Soc. C. E. Vol. 16, p. 42. 



3 See the bridge specifications of the Am. Ry. Engineering and Main- 

 tenance of Way Assoc. 



