72                                           THE   TECHNOGRAPH.

RELATIVE COST OF HEAVY VS. RE-INFORCED BRIDGES.
 

By Albert F. Robinson, '80, Engineer Bridg^es and Building, C. R. I. & P.
R'y., Topeka, Kansas.
 

Frequently an engineer must decide what is the heaviest bridge
he can build with economy. In 1882 and 1883 the writer had charge
of the designing and construction of some heavy iron bridges for one
of our best western railroads. At that time the bridge companies
bidding on these structures considered them unusually heavy; but
during the past season several of these bridges were taken down and
re-inforced because they were already considered too light for the
service required. The writer has frequently found it necessary to
urge the use of heavier bridges, and the data prepared for such dis¬
cussions has suggested the following tables.

Tables I (page 73) shows a comparison between the cost of
building a light bridge and afterwards re-inforcing it, and the cost
of building the heavier bridge orginally. Only single track structues
are considered, though the same relative proportion of cost and weight
doubtless exist in double track bridges.

The bridges built in 1882 were according to the Erie Railroad
specifications of 1879, and the "heavy bridges" are in accordance
with the. Lehigh Valley Railroad specifications of late date. The
actual '^shipping weights" were used in nearly every case. The
prices, 4.7 cents and 5.1 cents per pound for the work erected and
painted, are the lowest of which the writer has any record for the
year 1882. During the year just passed the prices for bridge work
have been about 3.75 cents per pound for griders up to 80 feet and 4
cents per pound for truss spans of ordinary length. These prices in¬
clude erection, painting, and freight to Chicago, 111.

The reasons for taking 15 years as the period for comparison
are: 1st, It is almost 15 years since the first "Erie" specifications
were published (about the limit of our observation for purposes of
comparison). 2d, A large portion of the bridges which were fully
up to the requirements at the time they were built, have had to be
re-inforced within less than 15 years. 3d, It is hardly safe to pre¬
dict the probable direction or rate of increase in the rolling load
for a greater period than 15 years.

In Table I no charge has been made for freight between the
bridge and the shops, for the delay or interruption of traffic on ac¬
count of the work, or for the danger from floods when the track is