Figure 15. — North Street (now Guilford 

 Avmue) bridge, Baltimore. In this transi- 

 tional composite structure cast iron was used 

 only in the relatively short sections of the 

 upper chord. For the long unsupported 

 compression members of the web s\slem. 

 standard wrought-iron angles and channels 

 were built up into a large section. The 

 decorative cast-iron end posts were non- 

 structural. (Photo in the L. N. Edwards 

 Collection, Museum of History and Tech- 

 nology.) 



links was maintained b) .1 vertical post <>r strut. 

 also cast. 



Much of the appeal of this design lay unquestion- 

 ably in the sense of security derived from the fact 



that each of the systems acted independently to 

 carry its load to the abutments. The lower chords, 

 actually nonfunctional in the primary structure, were 

 included merely to preserve the proper longitudinal 

 spacing between the lower ends of the struts. A 

 certain lack of rigidity was inherent in the system due 

 to that very discontinuity which characterized its 

 action; however, this was compensated for by a pair 

 of light diagonal stay rods crossing each panel. 

 These rods served the additional function of distrib- 

 uting concentrated loads to adjacent struts much in 

 the manner of the bridging between floor joists in a 

 building. 



In the Winchester span the floor system was of 

 timber for reasons of economy. This was a very 

 minor weakness inasmuch as any stick could be 

 quickly replaced, and without disturbing the function 

 of the structure. Bollman received a patent for his 

 truss in January 1852, and in the same year published 

 a booklet describing his system in general and the 

 Harpers Ferry span in particular. Here, he first 

 calls it a "suspension and trussed bridge," which is 

 indeed an accurate designation for a system which is 

 not strictly a truss because it has no active lower 

 chord. (The analogy to a suspension bridge is quite 

 clear, each pair of primary rods being comparable 

 to a suspension cable.) Thereafter, Bollman's inven- 

 tion was generally termed a suspension truss. 



INFLUENCE OF THE TRUSS 



Bollman's 1852 publication was widely disseminated 

 here and abroad and studied with respectful interest 

 by the engineering profession. Its drawings of the 

 structure were copied in a number of leading technical 

 journals in England and Germany. Although there 

 is no record that the type was ever reproduced in 

 Europe, there can be little doubt that this successful 

 structural use of iron by the most eminent railroad 

 in the United States and its endorsement by an engi- 

 neer of Latrobe's status gave great impetus to the 

 general adoption of the material. This influence was 

 certainly equal to that of Stephenson's tubular iron 

 bridge of 1850 over the Menai Strait, or Roebling's 

 iron-wire suspension bridge of 1855 over Niagara 

 gorge. The Bollman design had perhaps even greater 

 influence, as the B. & O. immediately launched the 

 system with greal energy and in great numbers to 

 replace its timber spans; on the other hand, Roebling's 

 structure was never duplicated in railroad service, 

 and Stephenson's only once. 



90 



lit I I I.TIN 240: CONTRIBUTIONS FROM THE MUSEUM OF HISTORY AND TECHNOLOGY 



