44 BULLETIN 1259, U. S. DEPARTMENT OF AGRICULTURE. 
f 
The gusset plates engaging the pedestal pm at the end 
of a truss shall be rigidly connected by a diaphragm which 
shall, in general, take direct bearing on the pin. Simi- 
larly, the pedestal webs shall, where practicable, be con- 
nected by a diaphragm which shall, in general, take bear- 
ing on the pin. 
A diaphragm shall be provided between gusset plates 
engaging main members whenever the end tie plate is 
located at a distance of 4 feet or more from the point of 
intersection of the members. In general, the web of this 
diaphragm shall be located in the plane of the latticed 
flange. 
ml^onr?p1at^e?.^ ^^^® ^^^ masoury platcs supporting trusses and 
colmnns shall each have a thickness of not less than % 
inch. The bottom chords of trusses shall be raised above 
the bridge seat at least 2 inches by the use of metal plates 
or pedestals. 
VIADUCTS. 
88. Type Viaducts shall consist usually of alternate tower spans 
and free spans of plate girders or riveted trusses supported 
on trestle towers. However, in viaducts having a 
colunm height less than 35 feet, trestle bents may alter- 
nate with the towers. 
In viaducts requiring freedom of waterway and in 
structures having a less total column height than 20 
feet, the num.ber of intermediate trestle bents may be 
increased over that specified above, but, in general, shall 
not exceed four in number. Ample rigidity shall be 
secured in the attachment of the superimposed spans 
to the column caps of the bents. 
tower?^^*^ ^°^ Each trestle bent shall be composed preferably of two 
main supporting columns. Towers shall be composed of 
two bents rigidly braced and strutted both longitudinally 
and transversely. 
beSs^ ^ ° ^ ^ ® For viaduct spans supported on single bents, the 
bents, if less than 20 feet in height, shall be pin-connected 
to their base sections or shall be designed to resist bending. 
91. Batter. Columns preferably shall have a transverse batter of 
not less than 1 horizontal to 12 vertical. 
92. Depth f The dcpths of plate girders in viaducts preferably shall 
^^"^ ^^^' be uniform. 
93. Girder con- Girders of towcr spans shall be fastened at each end 
nections. ^^ ^-j^^ io^^ of the columus or to the cross girders. Girdei^ 
between towers shall have one end riveted and shall be 
provided with an effective expansion joint at the other 
end. No bracing or sway frame shall be cormnon to 
abutting spans. 
94. Bracing. Towcrs shall bc thorouglily braced, both transversely 
and longitudinally, with a double system of stiff tension 
diagonals having riveted connections. Longitudinal and 
transverse struts shall be placed at caps and bases and 
at all intermediate panel points. All bracing connections 
shall be made by gusset plates. 
