CHAPTER XIII. 
THEORY AND PRACTICE OF PIN PROPORTIONING. 
The subject of li bridge pins ’’ is one deserving of more consideration than lias 
been accorded it by engineers, and authors of teclmical works. Until 1878, when 
Hr. Charles Bender, C.E., presented his paper on ^Proportions of Pins used in 
fridges ” t 0 the American Society of Civil Engineers, very little was known concern- 
lu o it ； the usual custom among eugineers when proportioning pins having been to 
allow one square inch of pin area for every eight or ton thousand pounds of shear in 
也〇 section most subject to shearing- stress. As Mr. Bender states generally, and as 
”11 be shown farther on to be true for iron bridges, it is not tlie shear, but the bend- 
lri g- moment, wliicli oausos tlio greatest tendency to rupture ; so that in tiny iron 
structuro it will Lo sufficient, in finding tlie sizes of pins, to calculate the greatest 
foment induced in them by tlio various members coupled tliereon, and to proportion 
，〇 rdingly, due regard being paid to tlia stresses in the eye -bar heads. Before mak- 
lu g an y investigations, it will be well to review and summarize the most important 
results of the investigations of others iu this subject. 
. Tho principal conclusions arrived at by Mr. Bender are, that, for a well-fitting 
of large diamoter, a pressure on the bearing- surface of six tons per square iucli 
not too large ； that for simplicity it is well to assume that this pressure is uni- 
ブラ 1 ァ distributed over the diameter of the pin ； that wrought- iron, after millions 
0 impacts, may break on the side where the stress is tensile, but never on the 
こ 1 e w here ifc is compressive, the ultimate resistance to crushing being about thirty 
on s per square inch; that tlie shearing- stress afc the centre of ft pin is one and 
uee-eightli3 times the average shear on the whole section ; that in iron and 
8 eel the ratio between the greatest allowable tensile and the greatest allowable 
S eö,r ^g-sfci*esses should be as 5 fco 4, which would make the uniformly distributed 
ear 2.91 tons per square inch, to correspond with a tensile stress of 5 tons per 
square inch; and that, owing to various considerations, iron in pins may be strained 
mUc ^ more than similar iron in tension members. 
