Long truss

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 bridges and with efforts to improve the stiffness of timber trusses under moving live loads.

The Long truss combined a distinctive system of adjustable timber joints with an early attempt to apply analytical methods to the design of timber truss bridges. A defining feature was the use of adjustable timber wedges at the interfaces between diagonal braces, vertical posts, and chords, allowing joints to be tightened and maintaining compression in the bracing system during service.^{[1]: 206–209}

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 his use of contemporary statics (including the parallelogram of forces) and simple-beam theory in proportioning elements of the structure.^{[1]: 149, 159}

In this account, Long applied theoretical mechanics to size at least a primary member based on assumed loading, thereby shifting toward theory-informed proportioning rather than reliance on uniform empirical dimensions.^[1]: 164

Long’s 1830 Jackson Bridge pamphlet provides builders with tabulated guidance on how to relate span length and loading assumptions to member dimensions (including chords and braces). It includes tables prescribing chord areas with corresponding load capacities for simple spans.^[5]: 132 This material indicates an attempt to translate analytical ideas into practical proportioning instructions. However, while capacities are stated, the underlying representative live loads are not explicitly specified in the manner later seen in Whipple (1847).^{[5]: 58–60, 140–141}

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

Historians have further interpreted Long’s analytical approach as implying consideration of representative live loads and of load transfer in continuous-span construction. However, complete mathematical treatments of such problems were not published until the late nineteenth century.^{[2]: 62 : 140–141}

Edwards argues that the use of analytical procedures by early designers such as Long and Benjamin Henry Latrobe implies consideration of representative live loads for transportation service, even though those loads were not explicitly stated in published form.^{[2]: 140–141}

He further notes that the first printed record of assumed bridge live loads he identified appears in Squire Whipple’s Essay No. II (1847).^{[2]: 140–141}

Edwards also distinguishes Long’s treatment of continuous spans from earlier practice. While Town’s lattice truss was sometimes constructed continuously over piers, its published specifications did not clearly address load transfer at supports. By contrast, Long’s pamphlets provided explicit directions for the construction of continuous spans and for the transfer of loads to substructure elements, suggesting an early recognition of the load-reaction problem in American bridge engineering.^[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

In this form, the system relied primarily on timber members acting in compression, with force transfer governed by bearing at shouldered joints rather than by metal fasteners. Bolts were used chiefly to secure alignment and resist separation rather than to define a pinned load path.^{[4]: 52–54}

A distinguishing feature was the use of timber wedges inserted at the interfaces between diagonal braces, vertical posts, and chords. Tightening the wedges increased bearing and maintained compression in the bracing system, allowing both diagonals to contribute to stiffness under moving loads.^{[1]: 206–209} In Long’s interpretation, this reduced trembling, springing, and oscillation in service.^{[1]: 155–156} The ability to re-drive the wedges—thereby retightening the truss—also allowed the use of unseasoned ("green") timber in construction, in contrast to contemporary practice that relied on seasoned members to control shrinkage and joint loosening.^[5]: 131

While Long’s design reflected an advanced understanding of load behavior and structural action, it remained constrained by the material and jointing limitations of timber construction, particularly the relative weakness of wood in tension and the shear vulnerabilities of shouldered connections in vertical members.^{[5]: 133–134}

Long subsequently modified the basic panelized, parallel-chord truss in later patent work. In his 1836 patent work and pamphlet, he emphasized systems to improve lateral stiffness and restrain out-of-plane chord movement rather than replacing the basic truss configuration. In his 1839 bridge patents, he introduced several distinct structural concepts. These included alternative parallel-chord truss arrangements in which the stress orientation of the diagonals could be altered relative to the 1830 design, as well as other bridge systems not limited to the original truss configuration. These developments are interpreted as extensions of Long’s effort to control stiffness, load transfer, and truss action more deliberately than in earlier timber systems.^{[5]: 133–135}

A limitation of the original design lay in its vertical timber tension members. Loads from floor beams were transferred into these vertical posts through shouldered connections, producing shear stresses in the wood that could lead to deterioration and eventual structural weakness over time. This vulnerability was identified as one of the system's weaker aspects and later addressed in iron-reinforced designs, notably in William Howe’s 1840 truss patent.^{[5]: 133–134}

The wedge system has sometimes been described as a 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.

Long’s truss design was first embodied in the Jackson Bridge (1829), a Baltimore bridge that carried the Washington Turnpike over the Baltimore and Ohio Railroad. The configuration presented in his 1830 patent described a parallel-chord timber truss divided into panels by vertical posts and reinforced by diagonal braces tightened into compression by adjustable wedges.{^{[1]: 148–150, 206–209}

A subsequent patent granted in 1836 addressed lateral stiffness, introducing a system intended to restrain out-of-plane movement of the top chord. This has been interpreted as an auxiliary structural system rather than a new primary truss form.^{[5]: 58–60}

In 1839, Long obtained multiple additional bridge-related patents, as summarized in contemporary patent listings. These patents covered distinct structural concepts rather than a single revision of the original truss. They included:

• revised parallel-chord truss arrangements with altered diagonal orientation and panel geometry;
• alternative structural configurations intended to modify load distribution and stiffness behavior; and
• a separate suspension-type bridge system.

Taken together, the 1839 patents demonstrate that Long’s work extended beyond a single fixed truss form to a broader experimental program addressing stiffness, load transfer, and structural configuration in timber bridge design, rather than defining only one or two canonical variants.^{[5]: 58–60}

Patent drawings for the 1830 system included upper arch-like braces; however, these elements were apparently not employed in the Jackson Bridge itself and saw little subsequent use in later examples.^{[5]: 58–60}

Long promoted his bridge system through printed pamphlets, 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

Long also employed such demonstration models to communicate load paths and stiffness behavior to prospective clients and builders, complementing his printed pamphlets in marketing the system.^{[5]: 58–60}

The Long truss was initially applied in the Jackson Bridge in Baltimore, which carried the Washington Turnpike over the Baltimore and Ohio Railroad and has been described as the first separate grade crossing of a railroad in the United States.^[2]: 61

Following this initial application, the system was adopted for covered highway bridges in New England and the Mid-Atlantic states during the 1830s and 1840s.^{[1]: 235–236} Long actively promoted the design through a network of agents; by 1836, twenty-six agents operating in eleven states were engaged in marketing and constructing bridges of his patented type.^[1]: 189

The Long truss also saw limited early use in railroads. However, increasing axle loads and the growing demands of railroad service exposed limitations in timber-only systems—particularly the weakness of wood in tension—contributing to the subsequent adoption of hybrid timber–iron designs such as the Howe truss.^: 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}