g, Bollman skew BfUDGi at l.lvsvillc (now Daniels), Maryland, built 



in 1853 1854. '/ i of Maryland Hi • ■ '& 



the smaller the segment of .1 circle represented by 

 the arch (that is. the flatter the arch), the greater the 

 stress in the arch ring and the resulting horizontal 

 thrust on the abutments. 



The piers for the numerous arches necessary to 

 permit an optimum amount of rise relative to the 

 span would have presented .1 dangerous restriction to 

 stream flow in time of flood. Bv the use of timber 

 trusses such crossings could be made in one or two 

 -p. ins with, at the most, one pier in the stream, thus 

 avoiding the problem. 



The principal timber bridges as far west as ( lumber- 

 land wen- of Latrobe's design. These were good, 

 solid structures ol composite construction, in which 

 a certain amount of cast iron was used in joints and 

 wrought iron for certain tension members. I 1 1<\ 

 were, however, more empirical than efficient and. for 

 the most part, not only grossly overdesigned but of 

 decidedly difficult fabrication and construction. 



What is interesting about the Latrobian timber 

 , however, is the effect they appear to have had 

 upon Bollman's subsequent work in the design of his 

 own truss. This effect is evidenced l>\ the marked 

 analog) between the primary structural elements of 

 the two types. I he Latrobe truss at Elysville (fig. 2> 

 was only partially a truss, inasmuch as the great 

 of the load was not carried from panel to panel, 

 finally to appeal at the abutments as .1 pure vertical 

 reaction, but was carried from each panel I except the 



four at the centei 1 directly to the bearing points at 

 the piers In heavy diagonal struts, aftei the fashion oi 

 the famous 18th-century Swiss trusses of the Gruben- 

 manns. It was .1 legitimate structural device, and the 

 simplest means of extending the capacity ol a spanning 

 system. However, it was defective in th.it the struts 

 applied considerable horizontal thrust to the abut- 

 ments, requiiinu heavier masonry than would other- 

 wise ha\ e been necessai y . 



It is quite likely thai Latrobe did not have absolute 

 confidence in the various pure truss systems already 

 patented by I own. Long, and others, and preferred 

 for such strategic service a structure in which the 

 panel members acted more or less independently of 

 one .mother. It will lie seen that, similarly, the 

 individual panel loads in Bollman's truss were • 

 totheendsof theframe by members acting independ- 

 ently ol one another. 



The Bollman Truss 



There had never been any question about the many 

 serious inadequacies of wood as .1 bridge material. 



Dee. iv and lire risL. .ilw.ivs present, were the principal 

 ones, involving continuous expenditure lor repl.ice- 

 oi defective members ami lor lire watches. It 

 was, in hut. understood by the management and 

 engineering staff ol' the B. & O. that their timber 

 bridge superstructures, though considered the finest 

 in the country, were more or less expedient and wire 



l'AIT.R 36: ENGINEERING CONTR Hit I n >\s i >l W1MHI KOI. I.MAN 



