H2h STEEL HIGHWAY BRIDGES. CHAP. III. 



The Michigan Highway Commission specifies that the floor and its supports be designed for 

 an i8-ton road roller, or 100 Ib. per sq. ft. No allowance is made for impact. 



The floor systems for Di bridges are to be designed for 125 Ib. per sq. ft. or a 2o-ton auto truck; 

 while D 2 bridges are to be designed for 100 Ib. per sq. ft. or a 1 5-ton auto truck. An impact factor 

 of 30 per cent is to be added both for the uniform loads and for the auto truck. 



WIND LOADS FOR HIGHWAY BRIDGES. The Illinois Highway Commission specifies a 

 wind load of 25 Ib. per sq. ft. on the vertical projection of both trusses and the floor system, but in 

 no case shall the wind be less than 300 Ib. per lineal foot on the loaded chord nor less than 150 Ib. 

 per lineal foot on the unloaded chord. 



The Iowa Highway Commission specifies 150 Ib. per lineal foot on the unloaded chord and 

 300 Ib. per lineal foot on loaded chord, all loads considered as moving loads. 



The Wisconsin Highway Commission specifies 150 Ib. per lineal foot on the unloaded chord 

 and 300 Ib. per lineal foot on the loaded chord; 150 Ibs. of the latter being considered a moving 

 load. 



Cooper's 1909 specifications require that highway bridges be designed for a lateral force of 

 150 Ib. per lineal foot on the unloaded chord and a lateral force of 300 Ib. per lineal foot on the 

 loaded chord, 150 Ib. of the load on the loaded chord being treated as a moving load. For spans 

 exceeding 300 ft. add in each case above 10 Ib. for each additional 30 ft. 



The author's specifications for wind loads are given in " General Specifications for Steel High- 

 way Bridges" given in the latter part of this chapter. 



DESIGN OF HIGHWAY BRIDGE FLOORS. Types of Floors. The choice of floor for a 

 highway bridge depends upon the traffic, the cost, including first cost and cost of maintenance, and 

 the climate. A highway bridge floor consists of a sub-floor which has the necessary strength to 

 carry the loads and a wearing surface. Plank floors and reinforced concrete slabs without wearing 

 surface have the sub-floor and wearing surface combined. A highway bridge floor should have 

 a strength and a weight appropriate to the structure of the bridge, and should be well drained. 

 The wearing surface should be waterproof, capable of resisting wear and should be as smooth as 

 possible without being slippery. For proper drainage the wearing surface should have a longi- 

 tudinal grade of not less than I in 50 or a transverse slope of not less than I in 12. Sub-floors for 

 highway bridges are made (i) of reinforced concrete; (2) of buckle plates or other steel sections, 

 and (3) of timber. The most common wearing surfaces for highway bridge floors are (a) concrete, 

 (b) bituminous concrete, (c) asphalt, (d) creosoted timber blocks, (e) brick, (/) stone block, (g) 

 macadam, (h) gravel or earth. The different types of sub-floors and wearing surfaces for highway 

 bridges will be described in some detail. 



Reinforced Concrete Floor Slabs. Reinforced concrete floor slabs on steel highway bridges 

 may be supported on joists or stringers and floorbeams, or by the floorbeams alone. Stringers 

 are used for beam bridges and are commonly used for truss bridges, while the stringerless floor is 

 commonly used on plate girder bridges. The sub-floor slabs are commonly calculated to carry 

 the dead load due to the weight of the slab and of the wearing surface, and a live load consisting 

 of a uniform load per square foot or a concentrated moving load. The thickness of reinforced 

 concrete slabs in short spans is commonly determined by the concentrated moving load. The 

 stresses in reinforced concrete slabs due to a concentrated load will depend upon the distribution 

 of the load over the slab. The different methods for the distribution of concentrated loads in use 

 in different specifications have been described and the specifications adopted by the author have 

 already been given. 



Design of Reinforced Concrete Floor Slabs. The live loads and the distribution of loads on 

 floor slabs as specified by the author are given on pages H2d and H2f. The concrete should be 

 a 1-2-4 Portland cement concrete that will give a compressive strength of not less than 2,000 Ib. 

 per sq. in. when tested in cylinders 8 in. in diameter and 16 in. long after having been stored for 

 28 days in moist air. Allowable compression in slabs, 650 Ib. per sq. in.; allowable tensile stress 

 in steel, 16,000 Ib. per sq. in., modulus of elasticity of steel to be taken as 15 times the modulus of 

 elasticity of concrete, allowable shear as a measure of diagonal tension 40 Ib. per sq. in.; punching 

 shear 120 Ib. per sq. in., bond stress in slabs 120 Ib. per sq. in. 



