372 REPORTS ON THE STATE OF SCIENCE, ETC. 
in a specification prepared by Mr. Schneider for the Pencoyd Co., and is now well 
known as the Pencoyd formula.! 
The additional allowance termed ‘ impact’ is based on the loaded length of the 
girder when the greatest load or bending moment is brought to bear on the member 
considered. The addition to the load thus made, taken in conjunction with the 
standard stress allowed, covers the necessary provision for fatigue and vibration, etc. 
This formula and slight modifications of it have been very extensively adopted.? 
Recent Investigations. 
The question of bridge renewal in America led to the elaborate investigations 
made by the American Railway Engineering Association, from 1907 to 1909, described 
in their Bulletins of 1910. In addition to the question of loading, the important 
30,000 
30,000 +L? “*s 
subject of impact was investigated and a modified coefficient 
adopted, as preferable to that in the Pencoyd formula. 
The investigations of the Indian Railway Bridge Committee, carried out for the 
Government of India, were started in 1917 and are still in progress.’ 
The work has been directed principally to the consideration of the ‘ impact’ 
question, and the possible substitution of a new formula for the Pencoyd one which 
was adopted for Indian Railway bridges in 1903. 
The experimental work in this country has been done by the Ministry of Transport, 
and investigations made in 1920 are given in the Report of 1921.4 
, ; . 120 
In that report the impact coefficient suggested is +L 
In connection with each of these investigations, a large number of experiments 
have been made in which strains taken on various lengths, and deflections of girders, 
‘have been measured with the object of getting values of fo=fe 
& 
In some cases only the maximum strain during the passage of the load is measured, 
as with the La Touche gauge, and in others a complete photographic record of the 
1 Wxtract from the section book of the Pencoyd Co. : 
‘ Effect of impact.—In proportioning the members of the structures, the effects of 
impact and vibration shall be considered and added to the maximum strains resulting 
from the above-mentioned engine and train loads. The effect of impact is to be 
determined by the following formula : 
300 ) 
Se (; + 300 
where I = impact to be added to the live-load strain, 
S = calculated maximum live-load strain, 
L = length of loaded distance in feet which produces the maximum strain 
in member. 
‘ Permissible Tensile Strains.—All parts of the structure shall be so proportioned 
that the sum of the maximum loads, together with the impact, shall not cause the 
tensile strain to exceed : 
On soft steel, 15,000 pounds per sq. in. 
On medium steel, 17,000 pounds per sq.’in. 
The same limiting unit strains shall also be used for members strained by wind 
pressure, centrifugal force, or momentum of train.’ 
2 An interesting comparative table giving the flange areas of rail bearers, cross 
girders, and main girders required to satisfy five representative specifications applied 
to a particular bridge has been prepared by Mr. Conrad Gribble, Proc. Inst. C.E., 
vol. cc., p. 238. 
3 Indian Railway Bridge Committee : 
lst Report 1917, Technical paper 187. 
2nd ,, 1918 > 198. 
ord ss  eLOLo os 211. 
4th’ 4) "1920" Vol. IT.’ 224. 
4th’''5.’'° 1920: Vol. IT. ';; 225. 
5th "",,*) 1921 “p 228. 
See also Technical papers 194 and 199. 
4 Ministry of Transport, ‘ Testson Railway Bridges inrespect of Impact Effect,’ 1921. 
