Figure 13. — The four Bollman spans at Harpers Ferry that survived the 

 Civil War. The spans were completed in 1862-1863. {Photo courtesy of 

 Baltimore and Ohio Railroad.) 



contracts entered into, for the reconstruction of the large 

 Bridges at Little Patuxent and at Bladensburg which will 

 be executed in a few months .... It is proposed to erect 

 a superstructure of Iron upon stone abutments, at each 

 place — with increased span, for greater security against 

 future floods. 



It is interesting to note that it was indeed Bollman 

 trusses to which the president of the railroad had 

 referred. How much earlier than this date Bollman 

 had evolved his peculiar trussing system is not clear. 

 The certain influence of Latrobe's radiating strut 

 system of trussing has been mentioned. As likely an 

 influence was another basic technique commonly 

 used to increase the capacity of a simple timber 

 beam — that of trussing — i.e., placing beneath the 

 beam a rod of iron that was anchored at the ends of 

 the beam and held a certain distance below it at the 

 center by a vertical strut or post. This combination 

 thus became a truss in that the timber portion was no 

 longer subject to a bending stress but to a simple one 

 of compression, the rod absorbing the tensile stress of 

 the < ombination. The effect was to deepen the beam, 

 increasing the distance between its extreme fibers 

 and — by thus reducing the bending moment — 

 reducing the stress in them (see fig. 3). 



It apparentl) '"(mud to Bollman that by extend- 

 ing the number of rods in a longitudinal direction, 

 this effect could be practical!) amplified to such an 



extent as to be capable of spanning considerable 

 distances. He almost certainly did not at first con- 

 template an all-iron system, but rather a composite 

 one such as described. It is entirely likely that such 

 trussed beams, with multiple systems of tension rods, 

 were used by Bollman as bridging in temporary 

 trestlework along the line as early as 1845 (see fig. 4). 



It is impossible to say whether Bollman himself, or 

 Latrobe, w r as struck with the logic of further elabo- 

 rating upon the system and, simultaneously, trans- 

 lating the timber compression member into one of cast 

 iron. Cast iron would naturally have been selected 

 for a member that resisted a compressive stress, as it 

 was considerably cheaper than wrought iron. But 

 more important, at that time wrought iron was not 

 available in shapes of sufficient sectional area to resist 

 the appreciable buckling stresses induced in long com- 

 pression members. The cost of building up members 

 to sufficient size from the very limited selection of 

 small shapes then rolled would have been prohibitive. 



The trussing rods, subjected to tension, were of 

 wrought iron inasmuch as the sectional area had only 

 to be sufficient to resist the primary axial stress. 



The first all-iron Bollman truss was constructed 

 over the Little Patuxent River at Savage Factory, 

 near Laurel, Maryland, in 1850. In the chief en- 

 gineer's report for the year 1850, Latrobe was able to 

 state that the truss had been completed and w as gi\ Lng 



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l-t III I IN 240: CONTRIBUTIONS FROM THE MUSEUM OF HISTORY AND TECHNOLOGY 



