BRIDGES — AMMANN 575 



SHOP FABRICATION 



In the fabrication of structural steel members in the shops impor- 

 tant improvements have been made in the past 20 years "which were 

 essential for the building of large bridges. Inaccuracies in the 

 fabrication of steel members were largely responsible for the failure 

 of the Quebec Bridge in 1907. Since then more accurate methods 

 and powerful nuichines have been introduced so that in the present- 

 day large bridges a remarkable degree of accuracy is being obtained. 

 Thus, for instance, the towers of the George Washington Bridge were 

 erected with an accuracy of three-sixteenths inch in a height of 600 

 feet, and the l,6T5-foot arch span of the Bayonne Bridge was closed 

 with a difference of one-half inch from the theoretical length. 



To-day individual members of greater size and weight are being 

 completely assembled in the shops. While 30 years ago members 

 of 25 to 30 tons weight were exceptional, the weight attained to-day 

 is not infrequently 80 to 100 tons and in a few cases 150 tons. The 

 accurate fitting together of connecting members is also being given 

 great care to-day. In some cases whole trusses, or large portions 

 thereof, have been completely assembled at the shops. 



FIELD ERECTION 



When we compare the present day erection of large bridges with 

 that of 20 or 30 years ago, we notice two striking improvements; 

 the speed with which enormous masses of steel for large bridges as 

 well as buildings are being assembled in the field, and the avoidance 

 of cumbersome falsework and erection equipment. The structures 

 often appear during construction as if they were erecting themselves, 

 and this is literally the fact to the extent that frequently members of 

 the final structure proper are being used to lift or temporarily sup- 

 port other members or parts of the structure. Where falsework is 

 unavoidable, it is almost invariably built of steel members which 

 are often members of the permanent structure. 



Erection of bridges by the so-called cantilever method with or 

 without partial use of false work, is very common to-day, not only 

 for cantilever bridges, but for simple and continuous trusses and for 

 arches. The absence of false work of any kind is particularly strik- 

 ing in the erection of the towers of suspension bridges. A simple 

 frame carrying the erection derricks and lifting itself up along the 

 completed portion of the tower is the only temporary structure. 



The erection of the wire cables in America is accomplished by the 

 old and well-tried method of " aerial spinning," in which the indi- 

 vidual wires are pulled from one anchorage to the other over the 

 towers. Then, packed in bundles or strands, the wires are lifted 



