LIVE LOADS. 112c 



The weight of deck plate girder highway bridges with sidewalks is 



w - 2.5 + Z./4-4 (28) 



The weight of through plate girder highway bridges without sidewalks is 



w = 3 + L/4.25 (29) 



The weight of through plate girder highway bridges with sidewalks is 



v> = 3-3 + L/5.6 (30) 



Weight of Electric Railway Bridges. The Boston Bridge Works gives the following formula 

 for the weight of electric railway bridges, where W = total weight of steel in Ib. per lineal foot of 

 bridge and L is the span of the bridge in feet. 

 Beam bridges 



W = 50 + 5L (31) 



Light truss bridges 



W = 200 + o.8L (32) 



Heavy truss bridges 



W = 250 + i. 5 L (33) 



The beam bridges were designed for 3O-ton cars; the light truss bridges were designed for 

 -ton cars or 1,500 Ib. per lineal foot of bridge, and the heavy truss bridges were designed for 

 ton cars, or 2,000 Ib. per lineal foot of bridge. 



LIVE LOADS. The live loads for highway bridges are usually assumed to consist of a uni- 

 live load for the trusses and a uniform live load or a concentrated moving load for the floor 

 its supports. A few highway bridge specifications require that trusses be designed for a con- 

 itrated moving load as well as for a uniform live load, and also that the floor and its supports be 

 jsigned for a concentrated moving load and that the portion of the floor of the bridge not covered 

 by the concentrated load be covered with a uniform live load. In calculating the stresses in the 

 truss members the uniform live load is commonly assumed as applied in full joint loads at joints 

 on the loaded chord. Moving loads and loads suddenly applied produce stresses that are greater 

 than the static stresses due to stationary loads or to loads gradually applied. This increase in 

 stress due to moving loads or due to loads suddenly applied is called impact stress. 



IMPACT. The effect of impact or increase in live load stresses over the stresses due to the 

 same loads gradually applied, is very much less for highway bridges than for railway bridges. 

 Experiments made by Professor F. O. Dufour and recorded in Journal of Western Society of Engi- 

 neers, June, 1913, show that the effect of impact on steel truss highway bridges with concrete floors 

 is very small. The effect of impact on steel truss bridges with plank floors is considerably larger 

 than for bridges with concrete floors. The maximum impact percentages do not occur with maxi- 

 mum static stresses. Experiments made at the University of Colorado under the author's direction 

 show that the effect of impact on highway bridges is very much less than for railway bridges. 



The specifications of the highway commissions of Illinois, Iowa, Michigan, Nebraska and 

 Wisconsin do not add impact for highway bridges. 



The allowance for impact of the Massachusetts Railway Commission is as follows: For 

 stringers, floorbeams and hangers, when loaded with a 2O-ton auto truck, 50 per cent; for all other 

 loads, floorbeams and stringers, 25 per cent; floorbeam hangers, 40 per cent; counters, 40 per cent; 

 for all other members in trusses, and for main girders the percentage shall be 26$ minus one- 

 twelfth the loaded length in feet, with a maximum of 25 and a minimum of 10 per cent. 



Mr. J. A. L. Waddell in "Bridge Engineering" specifies that highway bridges shall be designed 

 for the impact allowance, / = ioo/(Z, + 200), where L is the loaded length of the bridge in feet 

 that produces maximum stress and n is the total clear width of the roadway and footwalks divided 

 by twenty. The above impact allowance is made for motor-truck loadings but not for road-roller 

 loadings. 



