RAILWAY CONSTRUCTION. 149 



secure or agreeable foothold as timber. The ascent or descent of 

 the bridge may consist either of steps and landings, or of ramps, 

 according to circumstances or expediency. Sometimes these 

 bridges are made with curved tops, terminating in steps when 

 nearing the steps, or ramps. It is very questionable whether 

 such an arrangement is a good one or a safe one. There is 

 always a feeling of insecurity when walking over a sloping 

 surface broken up by steps, and experience points out that it is 

 better to continue the footway level right across to the place 

 where the passenger must change his direction to go down the 

 stairs or ramp. 



Fig. 157 gives cross-section of a covered lattice-girder foot- 

 bridge, 62 feet 6 inches span, and 10 feet wide, suitable for 

 an important station. The upper boom of girder consists of 

 two angle irons and top plate, and the bottom boom of two 

 channel irons. The cross-girders are rolled joist-irons resting 

 on the top tables of the channel irons. Four of the cross-girders 

 are extended outwards, and carry plate-iron outside vertical 

 brackets to stiffen the main girders. Three-inch longitudinal 

 planking is laid down from end to end of the bridge, and on this 

 is laid 1^-inch transverse flooring, in narrow widths, to form the 

 walking deck. The footbridge is lighted from the sides by 

 continuous glazed sashes fixed in strong wooden framework, as 

 shown. The roof is covered with canvas bedded in white lead, 

 and painted in the same way as an ordinary carriage roof. 



The above examples of under-line and over-line bridges are 

 given more with a view of illustrating some of the many different 

 descriptions of flooring, rather than to point out or suggest the 

 type of main girder to carry the load. The description and size 

 of the main girders can be varied to suit the span of the bridge, 

 the requirements of the traffic, and the opinion of the designer. 

 For spans up to 50 feet it will generally be found that web-plate 

 girders are both simpler and cheaper than lattice or truss girders ; 

 at the same time, there are occasions where plate girders can be 

 advantageously adopted for very much larger spans, as, for 

 instance, in the example given in Fig. 145, where the deep plate 

 girders form a most efficient screen. 



Figs. 160 to 194 give diagram sketches of a few out of the 

 many forms of open, or truss, girders which have been adopted 

 for large spans. There are many types from which to make a 

 selection, each one possessing its own special features and 



