134 ANNUAL REPORT 
the arch, it follows, that a single [-beam gives no reinforcement to the 
top surface of the concrete at the haunch. It was to avoid this defect and 
Fig. 70.—Floor and Post Construction, Hennebique System. 
to secure a simpler, more conveniently obtained form of steel that 
Thacher introduced his system described below. 
Thacher System.—The Thacher system of reinforced concrete is 
particularly designed for bridge construction. It consists of pairs of 
flat iron or steel bars laid near the upper and lower surfaces of the arch. 
spliced and riveted so as to be continuous from one abutment or pier to 
the next. These two members are tied together with vertical iron rods 
spaced two or three feet apart. Large headed rivets are set in the bars 
at short intervals to give a better bond with the concrete. The pairs of 
bars are spaced about three feet apart laterally and not connected. These 
bars act as the flanges of a beam of which the web consists of the body 
of concrete. The advantages of this system are: The simplicity of the 
metal forms, the convenience in obtaining, shipping and handling, and 
the ability to place the metal nearest to the maximum tensile straining 
points. It gives greater moment of inertia and consequently greater 
strength for the same amount of metal than does the original system from 
which it sprang; namely, the Melan I-beam system. The special 
