TRANSPORTATION ON LAND BY VEHICLES:— THE ROADS 



59 



■M^m^gttjfeSJft' -- — '-'rtftf^ 



Bridge across stream used for log drives. Notice the clever 

 combination in the bridge of bracing and suspending members. 



being filled with rock is framed sideways with splits 

 or boards. Its foundation is formed by layers of 

 branches. 



Abutments built crib-fashion should have a 

 heavy filling of rock, and should be built as full cribs, 

 in case of "wet spans." In case of wet spans, also, 

 the breadth and height of the cribs, and the distance 

 between them must be such as to offer plenty of 

 room for the water to pass during freshets. 



The height (thickness) of the stringers is of 

 more importance than their width; stringers placed 

 on corbels are more strongly supported than string- 

 ers without corbels. By a number of corbels, the 

 upper ones gradually increasing in length, a very 

 strong support can be obtained for long stringers. 



For waggon bridges, at Biltmore, three stringers 

 (white oak) are used, up to 40 feet long without 

 supports. In railroad bridges, stringers are 28 feet long, covering 2 spans of 14 feet each. Red oak 

 and yellow poplar are not better, but are more easily found than white oak and chestnut of good lengths. 



The covering of the bridges for waggon and sled roads consists either of poles or half splits (punching), or of 

 2-inch plank. In- 

 ferior lumber can 

 be used by doub- 

 ling the layer.— 

 The expense of 

 bridge building 

 depends largely 

 on the distance 

 overwhich the 

 bridge timbers 

 must be hauled, 

 and on the pre- 

 sence or absence 

 of a sawmill. 



The expense in the Appalachian forests for dry 

 spans covered with punching is -v 1 per running foot; 

 for wet spans with cribs )( I'SO per running foot ; for 

 a suspension bridge 100 feet long -vSSO; for a bra- 

 ced bridge 100 feet long >■ 250. 



II. Suspension bridges are those the span of which 

 is supported overhead. The oblique members of the 

 bridge yield the maximum of effect if inclined at a grade 

 of 45 degrees. The caps are held to the oblique mem- 

 bers by perpendicular "Kingrods" and "Queenrods," 

 the former attached to the ends and the latter attached 

 to the middle of the oblique members. The rods, 

 made of 1 '2 or 2 - inch iron and threaded at both ends, 

 may be replaced by "Kingposts" and "Queenposts " 



III. Braced bridges are frequently found as " dry 

 spans" over deep gullies. Here, the oblique members Braced bridge across Looking-glass Creek, BiUmore E state. 



Suspended bridge spanning 130 feet, on Davidson's River, 

 Biltmore Estate. 



