40 BULLETIN 1259, U. S. DEPAETMEXT OF AGKICULTURE. 
Kailings of the first class may consist of not less tiian 
two lines of horizontal rails of approved section. 
Railings of the second class shall consist of an upper and 
lower horizontal rail connected by a suitable web. The 
clear distance between the floor and the lower rail shall 
not exceed 6 inches. 
Preferably the unsupported length of any rail section 
shall not exceed 8 feet, and all connections to posts, 
truss nienabers, etc., shall contain not less than two rivets 
each. Ample provision shall be made for movement due 
to temperature. 
PLATE GIRDERS. 
65. Proportion- Plate girdcrs shall be proportioned either by assuming 
^^' the flanges to be concentrated at their centers of gravity 
or by the moment of inertia of the net section. In the 
former case one-eighth of the gross area of the web is 
available as net flange area but the effective depth shall 
not be assumed to be greater than the distance back- to- 
back of flange angles. For girders having unusual cross 
sections the moment-of-inertia method shall be used. 
Flange sec- The gross scctiou of the compression flange shall not 
be less than the gross section of the tension flange. The 
compression flange preferably shall be stayed against 
lateral deflection at intervals not exceeding 12 times its 
width. 
The flange angles shall form as large a portion of the 
gross area of the flange as practicable. 
When flange plates are used, at least one plate on the 
top flange shall extend the full length of the girder. Any 
adjditional flange plates shall be of such length as to allow 
two rows of rivets to be placed at each end of the plate 
beyond its theoretical end, and there shall be a sufficient 
munber of rivets at the ends of each plate to develop its 
full stress value before the theoretical end of the next 
outside plate is reached. 
Flange cover plates shall be equal in thickness, or shall 
diminish in thickness from the flange angles outward. 
No plate shall have a thickness greater than that of the 
flange angles. 
67. Web plates. Wcb platcs shall be proportioned for both the vertical 
and horizontal shearing stresses. Splices in web plates 
shall be avoided as far as possible, but, when used, they 
shall be designed to develop the full value of the web 
plate for both bending and shearing stresses. 
68. Flange riv- The number of rivets connecting the flange angles to 
the web plates shall be sufficient to develop the increment 
of flange stress transmitted to the flange angles, combined 
with any load that is applied directly to the flange. For 
electric railways, one wheel load, when applied directly 
to the flange, shall be assumed to be distributed uniformly 
over a length of 3 feet. 
