The Long truss is a patented timber truss bridge system developed by Stephen Harriman Long in 1830. In bridge-history scholarship, it is associated with adjustable bracing details intended to maintain stiffness, and with published builder guidance describing its construction. Historians have also discussed the Long truss as an early attempt to apply calculation-based member sizing and to address practical issues, such as bridges that are continuous over intermediate supports.^{[1]: 149, 159, 232–236 [2]: 62  [1]: 149, 159, 232–236  [2] [3] [4] [5] [6] [7]}

The Long truss emerged during a transitional period in American bridge engineering, following the dominance of arch-dependent systems such as the Burr arch truss and the distributive web of the Town lattice truss, and preceding the analytically proportioned timber-iron trusses that appeared in the 1840s (notably the Howe truss).^{[1]: 19–20, 235–236 [3]: 31–33} The system is frequently discussed in connection with early railroad service requirements for stiffness and controllable deformation in timber superstructures.

Danko identifies Long as “the first bridge designer to make a substantial attempt at applying scientific principles to the design of the simple truss bridge,” noting Long’s use of contemporary statics (including the parallelogram of forces) and simple-beam theory in proportioning elements of the design.^{[1]: 149, 159} In Danko’s account, Long applied theoretical mechanics to size at least a primary member, using a formula to resist stress from a concentrated midspan load, and framed this as a move toward theory-informed proportioning.^[1]: 164 Long’s 1830 Jackson Bridge pamphlet also presents span-based sizing guidance and distributed-load tables for builders, indicating an intent to relate member proportions to assumed loading rather than relying only on uniform scantlings.^[8]

Unlike earlier American timber trusses that relied primarily on empirically increasing member sizes, Long coordinated truss geometry and member sizing with calculated structural action.^{[1]: 232–236} Griggs similarly treats the Long truss as an early example of stress-conscious member proportioning in timber bridge design.^[7]: 260

Edwards, discussing the probability that Long and Benjamin Henry Latrobe applied mathematical theory to bridge design, argues that granting an analytic procedure implies that designers must also have considered and selected representative live loads for transportation service.^{[2]: 140–141} Edwards adds that the first printed record of assumed bridge live loads he identifies appears in Squire Whipple’s Essay No. II (1847).^{[2]: 140–141}

Edwards notes that while Town’s patent specifications and pamphlets proposed building lattice trusses continuous over substructure piers, they did not specify the use of vertical columns at abutments and piers to transfer superstructure loads to the substructure. By contrast, Edwards states that Long’s pamphlets gave full directions for the construction of continuous spans and infers that Long was presumably the first American bridge builder to give special consideration to this load-reaction problem.^[2]: 62 In Edwards’s account, the contribution lies in the specificity of builder-oriented directions on continuity and load transfer, rather than in a complete published mathematical treatment of continuous-beam reactions.^[2]: 62

The Long truss is commonly described as a panelized, parallel-chord timber truss consisting of:

• parallel timber upper and lower chords
• vertical timber posts dividing the span into panels
• diagonal timber braces within each panel
• adjustable wedge mechanisms at brace–post–chord interfaces

A distinguishing feature was the use of timber wedges inserted at interfaces between diagonal braces, vertical posts, and chords. Tightening the wedges increased bearing and could introduce compression in the diagonals prior to full live loading.^{[1]: 206–209} In Danko’s interpretation of Long’s counterbracing concept, the wedges were intended to produce a state in which service loads would not reverse the intended compressive action in the diagonals and would reduce trembling, springing, and oscillation in service.^{[1]: 155–156}

Force transfer in the Long truss depended primarily on timber-to-timber bearing at shouldered joints rather than on metal fasteners; bolts were used chiefly to secure alignment and resist separation rather than to define a pinned load path.^{[4]: 52–54}

The wedge system has sometimes been described as an early form of prestressing. Pierce cautions that such an interpretation is valid only under simplified assumptions; in multi-panel trusses, wedges often function primarily to increase effective bearing area and reduce high side-grain compression stresses at post–chord interfaces rather than to impose a calibrated global prestress state.^{[3]: 47–48} Pierce also notes variation in wedge placement (e.g., wedges only at the lower chord versus at both chords), indicating that wedge use was not uniformly applied as a standardized prestressing mechanism.^[3]: 47

Danko characterizes the Long truss as structurally indeterminate in practice, with force distribution dependent on relative member stiffness, joint conditions, and the maintenance of diagonal compression.^{[1]: 206–209} Because the system did not behave as an idealized pinned, statically determinate truss, force paths could shift with timber shrinkage, moisture cycling, or wedge adjustment.

After Town’s lattice truss, Long developed a patented design in 1829 intended for use by the Baltimore and Ohio Railroad, the Washington boulevard crossing, or the Jackson bridge.^{[1]: 148–150} The 1830 patent described a parallel-chord timber truss divided into panels by vertical posts and strengthened by diagonal braces tightened into compression by adjustable wedges.^{[1]: 206–209}

Edwards reports that Long’s initial bridge patent was secured on March 6, 1830, and that subsequent patents recognizing improvements were granted in 1836, 1839, 1847, and 1858.^[2]: 62

Edwards also reports that Long distributed printed pamphlets to promote his bridge system, including an 1830 Baltimore printing titled Description of Jackson Bridge, together with Directions to Builders of Wooden or Frame Bridges and an 1836 Concord, New Hampshire printing titled Description of Col. Long's Bridges, Together with a Series of Directions to Bridge Builders.^{[2]: 62  [9] [10]}

The Long truss bridge in Baltimore, which allowed the Washington Turnpike to cross the railroad, was the first separate railroad crossing in the United States.^[2]: 61

The Long truss was used in covered highway bridges in New England and the Mid-Atlantic states during the 1830s and 1840s.^{[1]: 235–236} By 1836, Long had 26 agents operating in 11 states promoting and constructing bridges of his patented type.^[1]: 189 It also saw early railroad application before increasing axle loads prompted adoption of iron-reinforced systems.^[7]: 260

Pierce estimates that approximately twenty-five covered bridges supported by some variation of the Long system remain extant, though classification varies due to later alterations and hybridization.^[3]: 31

Numerous covered bridges in the northeastern United States are classified as Long truss or Long-type truss structures. Many surviving examples have been documented by the Historic American Engineering Record (HAER) as part of the National Historic Covered Bridge Preservation Program; reported survival counts vary due to modifications and restorations.^[3]: 31

Representative examples often cited in the literature include:

• Bement Covered Bridge (New Hampshire)
• Hamden Covered Bridge (New York)
• Blair Bridge (New Hampshire)

Precise structural classification of individual bridges typically requires inspection of joint details, shoulder geometry, and wedge mechanisms.^{[3]: 31–33}

The Long truss differed from:

• Burr arch truss – which combines arch action with truss reinforcement.
• Town lattice truss – which distributes forces through a lattice web rather than discrete, stress-identified members.
• Howe truss – which uses iron vertical tension rods and more mechanically defined joints, reducing reliance on timber bearing surfaces.
• Pratt truss – which reverses diagonal force orientation and later became dominant in iron construction.

In later bridge-history accounts, the transition from the Long truss to the Howe truss is treated as part of a broader shift from timber compression systems dependent on bearing/friction interfaces to hybrid systems with iron tension members and clearer, more readily adjustable force paths.^[7]: 260

Historians of engineering have identified the Long truss as an early American example of deliberate analytical design applied to timber bridge construction. Danko describes Long as “the first bridge designer to make a substantial attempt at applying scientific principles to the design of the simple truss bridge,” emphasizing the use of contemporary statics and beam theory in proportioning members according to anticipated stresses rather than employing uniform empirical dimensions.^{[1]: 149, 232–236} While the Long truss’s behavior remained dependent on timber bearing and wedge adjustment, its coordinated geometry and member sizing are often presented as a transitional step between empirical carpentry traditions and the analytically proportioned hybrid timber–iron trusses that emerged in the 1840s.^{[1]: 232–236 [7]: 260}

In preservation scholarship, the Long truss is treated as one of the formative patented American truss types of the early nineteenth century, significant both for its mechanical conception and for its role in the professionalization of American bridge engineering.^{[5]: 58–60}