where 
24 BULLETIN 1259, U. S. DEPARTMENT OF AGRICULTURE. 
increment is expressed as a fraction of the live load or 
live-load stress and is determined by the following 
formulas. 
For highway loads, /= .q£"^ |q^ 
For electric raihvav loads, /= — ^~^y!i-^ 
12Z + 1200 
/= impact fraction. 
Z= loaded length in feet producing the maximum 
_ static stress in the member considered. 
11. Longitudi- Upou bridges carrying electric railway traffic, the 
nai force. forces due to traction and momentum shall be considered 
as longitudinal forces having a magnitude equal m 
amount to 20 per cent of the moving railway load on one 
track only. Provision shall be made in the design of 
floor members, ghders, trusses, trestle towers, etc., for 
these forces applied at the elevation of the tops of rails. 
12. Lateral The forcc due to wind and lateral vibrations shall be 
taken as horizontal and shall be treated as a uniformly 
moving load actmg only on the lateral and sway bracing. 
This lateral force shall be taken at 30 pounds per square 
foot on one and one-haK times the side area of all trusses 
or girders plus the area of the railings and the vertical 
projection of the floor. In the case of truss spans it 
shall be considered as divided between the loaded and 
the unloaded chords in the proportion of two-thirds to 
the former and one-third to the latter. In adchtion to 
the foregoing a moving load of 150 pounds per linear 
foot shall be considered as acting in the plane of the 
bridge floor on highway bridges and 300 pounds per 
linear foot upon bridges for combined highway and 
electric railway service. However, in the case of struc- 
tures having a reinforced concrete floor slab engaging the 
flanges of the steel floor members this additional loaded 
chord load need not be considered. 
The transverse bracing of trestle towers and bents 
shall be proportioned to resist the lateral forces of the 
superimposed spans acting at the tops of the columns 
together with a pressure of 30 pounds per square foot 
acting against an area equal to one and one-half times the 
horizontally projected side areas of the columns, struts, 
and bracing. 
The longitudinal bracing of trestle towers shall be 
proportioned to resist not less than 0.7 of the transverse 
lateral forces above described in addition to the tractive 
force due to electric railway traffic, when the latter 
exists. 
^ 13. Centrifugal Bridges carrying electric railway traffic on a curve 
shall be designed to resist a lateral force equal to 10 per 
cent of the moving railway load, without impact allow- 
ance. This load shall be assumed to act 4 feet above 
the rail. 
