SPECIFICATIOXS FOE STEEL HIGHWAY BRIDGES. 
21 
The following weights are to be used in computing the 
dead load : 
Weight per cubic foot, 
Substance. pounds. 
Steel 490 
Iron, cast 450 
Bronze 524 
Timber (treated or untreated) 60 
Concrete 144 
Loose sand and earth 100 
Rammed sand or gravel 120 
Macadam or gravel rolled 140 
Cinder filling 60 
Asphalt wearing surface 150 
Granite block paving 160 
Vitrified brick paving 150 
Granolithic pavement 150 
LIVE LOAD. 
Steel bridges shall be designed for either uniform or 2. General. 
concentrated loads as hereinafter specified. 
For the purposes of design, bridges will be classified, on 
the basis of traffic, as follows : 
Class A — Heavy-traffic bridges on primary or secondary roads. 
Class B — Light-traffic bridges. 
Class C — Bridges carrying a highway traflac of exceptionally 
heavy load units. 
Class D — Bridges carrying electric railway lines, in addition to 
highway traffic. 
For purposes of design, loadings will be divided into 
four classes, as follows: 
Class 1-20 'T^®^® loading classifications indicate the number 
Class 2-151 ^^^ weight of trucks in tons, on which the load- 
Class 2-20 J 
3. Traffic 
sification. 
clas- 
4. Loading clas- 
sification. 
ing specification is based. 
Loadings for the different classes of bridares shaU be as , ^\ selection of 
J, ,, » ® loadc. 
loUows : 
Class of bridge. 
Class of loading. " 
Roadway 
width less 
than 18 
feet. 
Roadway- 
width 18 
feet or 
more. 
A 
2-15 
1-15 
2-20 
b:::::::::: ::: 
c 
1-15 
1-20 
Class D bridges shall be designed for the specified high- 
way live loads, of the proper class, on any portion of the 
roadway; or the electric railway live loads on the car 
tracks and the highway live loads on the remaining 
portion of the roadway. 
Concentrated highway loads shaU consist of typical 8. concentrat- 
trucks having total loaded weights of 15 tons or 20 tons, ®^^°^^- 
distributed as shown in Figure 5. The mdth of each 
rear truck tire shall be assumed as 1 inch per ton of gross 
weight of truck and load. 
