112d STEEL HIGHWAY BRIDGES. CHAP. III. 



The specifications for steel bridges prepared by the U. S. Office of Public Roads, and the 

 specifications for steel bridges of the West Virginia Highway Commission and the Oregon Highway 

 Commission specify the impact factor, / = ioo/(L + 300), where L is the loaded length of the 

 bridge in feet that produces maximum stress in the member. 



The Montana Highway Commission specifies 25 per cent impact. 



The Department of Public Roads of Kentucky requires no impact allowance for bridges with 

 concrete floors, and 25 per cent for bridges with wooden floors. 



The Utah Highway Commission specifies 25 per cent impact for floors, and 15 per cent for 

 trusses. 



For concrete highway bridges the impact allowance varies from no impact allowance, as 

 specified by the highway commissions of Illinois, Iowa, Michigan, Nebraska and Wisconsin; an 

 allowance of 15 per cent of the live load, as specified by the highway commission of West Virginia, 

 to an allowance of 50 per cent of the live load, as specified by the U. S. Office of Public Roads. 

 Watson's "General Specifications for Concrete Bridges," third edition, 1916, uses an impact al- 

 lowance of 7 = i5o/(L + 300), where L is the loaded length of the bridge in feet that produces 

 maximum stress. 



Ketchum's Specifications for Impact. The author has adopted the following impact factors 

 for concrete bridges and steel bridges. 



(a) For concrete arches with spandrel filling on culverts with a minimum filling of one foot, 

 no allowance for impact. 



(b) For concrete slab and girder bridges and trestles, and arches without spandrel filling, 30 

 per cent for impact. 



(c) For steel bridges the following allowance for impact. For the floor and its supports in- 

 cluding floor slabs, floor joist, floorbeams and hangers, 30 per cent. 



For all truss members other than the floor and its supports, the impact increment shall be 

 7 = ioo/(L + 300), where L = length of span for simple highway spans (for trestle bents, towers, 

 movable bridges, arch and cantilever bridges, and for bridges carrying electric trains, L shall be 

 taken as the loaded length of the bridge in feet producing maximum stress in the member). 



CONCENTRATED LIVE LOADS. Traction engines weighing 20 tons are quite common in 

 the west and northwest. The heaviest motor truck in common use has a capacity of 75 tons and 

 a total weight of 13 tons, with nearly 10 tons on the rear axle. With an overload of 50 per cent, 

 which is not unusual, this truck would carry 14 tons on the rear axle. The maximum road roller 

 weighs 20 tons. 



The highway commissions of the different states have adopted concentrated live loads as fol- 

 lows: Illinois specifies a 15-ton traction engine; Iowa specifies a 15-ton traction engine for bridges 

 with reinforced concrete floors; Wisconsin specifies a 15-ton road roller; Michigan specifies an i8-ton 

 road roller; Nebraska specifies a 2o-ton traction engine; Minnesota specifies a 2o-ton traction 

 engine; New York specifies a 1 5-ton road roller; all loadings to be used without impact. 



Utah specifies an i8-ton road roller with 25 per cent impact; Oregon specifies a 15-ton road 

 roller for medium traffic and a 2O-ton road roller for heavy traffic; Ohio specifies a 15-ton concen- 

 trated load with l6f per cent impact; Montana specifies a 2O-ton traction engine with 25 per cent 

 impact; the Massachusetts Railway Commission specifies a 2O-ton motor truck with 14 tons on the 

 rear axle, with an allowance of 50 per cent for impact on the floor and its supports; Mr. J. A. L. 

 Waddell in "Bridge Engineering" specifies for class A bridges an i8-ton motor truck with impact 

 allowance as given above. 



For additional data see article entitled "Concentrated Live Loads for Highway Bridges," 

 by Milo S. Ketchum, printed in University of Colorado Journal of Engineering, October, 1916. 



Ketchum's Specifications for Concentrated Moving Loads. The author has adopted the 

 following specifications for moving concentrated loads. 



(a) That highway bridges on main roads or near towns or cities shall be designed to carry 

 a 2O-ton motor truck with axles spaced 12 ft. and wheels with a 6-ft. gage, with 14 tons on rear axle 

 and 6 tons on front axle. The truck to occupy a space 10 ft. wide and 32 ft. long. The rear wheels 

 to have a width in inches equal to the total load in tons (20 in. for a 2o-ton truck). 



(b) That bridges not on main roads shall be designed for a 1 5-ton motor truck with axles 

 spaced 10 ft. and wheels with a 6-ft. gage, and occupying a space IO ft. wide and 30 ft. long, with 

 10 tons on rear axle and 5 tons on front axle, and with rear wheels 15 in. wide. 



(c) To provide for impact and vibration and unevenness of road surface thirty (30) per cent 

 is to be added to the maximum live load stresses. Only one motor truck is to be assumed to be on 

 a bridge at one time. 



