What types of construction will we see?
There are 5 different types of construction used in the bridges we will be visiting.
Howe Truss
Howe truss is a type of bridge design that was introduced by an American architect William Howe. It utilizes similar design such as Pratt truss, but with a strong difference. Here the diagonal structural beams slope toward the bridge center, while Pratt truss utilizes diagonal beams that slope outward from the center of the bridge. This approach makes diagonal members of Howe truss bridge in compression, while vertical web members are in tension.
William Howe was born in Spencer, Massachusetts, on May 12, 1803. After working as an apprentice in his father’s sawmill, he enrolled and graduated as an engineer at Leicester Academy in Leicester, Massachusetts. After several years of buildings homes and churches, in 1840 he managed to finally focus on his dream projects – building bridges. That same year he designed his first railroad bridge over the Connecticut River utilizing never before seen truss design that he devised. This Howe Design was purchased by his employer Amasa Stone for exclusive use in New England, where he created hundreds of bridges with this design approach.
Howe made numerous smaller improvements to his design and patented them under new Howe Truss design in 1846.
William Howe died suffering wounds from carriage accident on September 19, 1852.
Pratt Truss
Since its introduction in 1844, this bridge design became part of hundreds of bridges created up to Second World War. It was designed by the Thomas Willis Pratt (1812 – 1875) and his father Caleb Pratt, a pair of American engineers, just several years after William Howe patented his famous Howe truss design. This bridge design immediately became widely used during the period when many bridges moved from wood components toward all-steel construction designs. Its most compelling feature was the ability was to span great distances using simple construction methods. It was regularly used to span anchor points that are up to 250 feet apart. It was most commonly used in railroad bridge construction, although it was also a preferred choice for creating other types of bridges all around the world until early 20th century.
Thomas Willis Pratt was born in 1812 in from Boston, Massachusetts. He was schooled at college in Troy, NY at the Rensselaer Institute and Rensselaer Polytechnic Institute, but he never graduated. He instead returned home and worked at Boston and Worcester RR and the Providence & Worcester RR engineering companies when the majority of bridges were built using wood and with truss designs made by S. H. Long, Elias Towne, and William Howe.
Seeing how Howe’s designs that were enhanced with vertical metal members started replacing long-used Long and Towne Trusses, Pratt decided to enhance 1840 Howe truss and Long Truss design by making diagonal structures made from steel, verticals from wood switching diagonals to flow into other direction than Howe’s. With proper camber and pre-stress, newly created Pratt Truss became structurally stable, enabling it being used over long spans and fixing several disadvantages present in all then commonly used truss designs. Pratt truss patent was accepted on April 4, 1844, under a category of “TRUSS FRAME OF BRIDGES (Truss Bridge)”. Although his name was listed first, many speculate that Thomas’s father Caleb was listed on the patent application as a form of a tribute to his long career in engineering.
The basic form of Pratt truss includes triangular truss design whose diagonal members slope toward the center of the bridge. When under load, this design makes diagonal members feel tension (the force that expands the object apart), while vertical members feel suspension (the force that pushes objects into one self). If the diagonal members are made from the solid material (such as metal bars), the heavy load of the bridge may cause the need for implementing reinforcements to the center area of the Pratt truss bridge, since that part of the bridge will experience the strongest force loads. Those center areas can be reinforced with stronger materials, or be subdivided into K or Y-shaped patterns. Pratt truss bridges are statically determinate (all of its support reactions and member forces can be calculated using only the equations of static equilibrium), which made them capable for use in scenarios where bridge designers needed to span great distances.
Town Lattice Truss
On January 28, 1820, architect Ithiel Town was granted a patent for a wooden truss bridge, also known as Town’s Lattice Truss. An architect and civil engineer, Town had already built the first covered bridges to span the Connecticut River, one at Hartford and two in Massachusetts at Springfield and Northampton. He based all three on the wooden arch truss patent design of Theodore Burr. Town’s innovative lattice design evolved from this experience; he recognized the need to be able to build bridges quickly, out of readily available materials with local, relatively unskilled workers.
Town’s innovation included a support system based on an uninterrupted series of crisscrossed diagonals that connected the horizontal top and bottom chords to form a series of overlapping triangles. Unlike the use of the arch in Burr’s design, Town’s approach distributed the load equally with no vertical timbers. By fastening each triangle at its points of intersection, the design prevented any one structure from moving independently when subjected to stress. This distributed the load borne by one triangle across all of the others. Not only was Town’s method of load distribution more efficient, it could also be achieved with lighter-weight planks of pine or spruce connected with wooden pins. The resulting structure was much lighter and considerably less expensive to build than an arch truss bridge. The light, almost insubstantial, appearance of Town’s bridges prompted comparisons to the common trellis found in every flower garden.
Town built a small bridge in Whitneyville, Connecticut, to introduce his design. The ease of construction, strength, and ability to build a lattice truss bridge on piers spanning long distances quickly made the design the common choice for covered bridges and early railroad bridges until the post-Civil War era. Town’s bridge design accommodated early trains simply by doubling the quantity of planks and pins. The lattice truss bridge became so widely used across the eastern states in the 19th century that Town, who received royalties of $1 to $2 dollars per foot for use of his patented design, became a wealthy man.
Burr Arch
Theodore Burr obtained the first U.S. patent issued for a specific timber truss configuration in 1806. The Burr arch is, basically, a combination of a typical multiple kingpost truss with a superimposed arch. The arch was added to allow heavier loads on the bridges and to stretch their span capabilities to greater lengths. Surviving examples of Burr arch bridges have spans of up to 222 ft.
Burr's development was immediately popular with bridge builders and has proven durable. More existing North American covered bridges use the Burr arch than any other type. The classic, or conventional, Burr arch supports the ends of the arch components at the abutment, with no connection between the bottom chord and arch as they pass each other (the chord is supported by the abutment directly separated from the arch end). A variation of the Burr arch (sometimes referred to as a modified Burr arch) terminates (and ties) the arch with a connection directly to the bottom chord, which is supported on the abutments.
The actual arches of most Burr arches are in pairs; these sandwich a single multiple kingpost truss between them. The most common connection uses a single bolt to join the arches through each of the vertical members of the truss. This means that the load sharing between the truss and the arch components is largely dependent on the relative stiffnesses of those bolts. The floor beams carry the live loads to the truss bottom chords, and the roof loads bear on their top chords. For these vertical loads to be distributed into the arch, the bolts must resist significant vertical shear forces. The initial, traditional Burr arches used arch components sawn from large, single timbers that were lap-spliced to each other at the verticals. Later, use of continuous but laminated (multiple-layer) timber arches became popular with some builders.
In addition to the critical areas of interest cited above for the multiple kingpost truss that comprises the central portion of the Burr arch structure, special attention should be paid to the ends of the arches and the interconnections of the arch to the truss.
There are about 224 remaining bridges supported by the Burr arches and its multiple variations (about 25 percent of all covered bridges). The Burr arch has individual spans that range from 33 to 222 ft; this longest span is 10 percent longer than the next rival configuration of truss (the Howe). The extant Burr arches were built between the early 1800s and 1988.
Inverted Haupt Truss
The Sayres Bridge, (45-09-06) in Thetford Center, Vermont, is the only Haupt Truss covered bridge in New England, said Richard Sanders Allen in his Covered Bridges of The Northeast, and one of just two in the United States. At the time of that writing, the other was the Bunker Hill Bridge (33-18-01) in Catawba County, North Carolina.
Now, since 1999 there is a third claimant, the Netcher Covered Bridge, 35-04-63. It crosses Mill Creek in Jefferson Township, Ashtabula County, Ohio. It was designed by John Smolen of Smolen-Gulf Bridge fame (35-04-64), 110 feet long using an "inverted Haupt Truss" with arches.
These three covered bridges may have a common claim in that their builders were inspired by Herman Haupt's truss design described in his book, General Theory of Bridge Construction published in 1851, but each of the three truss systems differ in detail.
According to his patent letter, Haupt designed his truss as an improved lattice truss over that patented by Ithiel Town. Haupt, in his letter to the U. S. Patent Office (.Specification of Letters Patent No. 1,445, dated December 27, 1839.) writes, "What I claim as my invention is—The construction of a lattice bridge without counterbraces, but consisting simply of braces inclined at any proposed angle and ties which are perpendicular to the lower chord, the chords being either straight or curved."
Haupt's truss resembles Town's in that both trusses use planks rather than squared timbers, and the truss members are fastened with treenails rather than mortise and tenon. (Netcher uses bolts.) Haupt's patent drawing shows a lattice design in which the braces (compression members) are sent directly to the lower chord ends, crossing several panels (see figure), the tension members here are solely the verticals (ties).
The truss used in the Netcher Road Bridge inverts Haupt's truss not only in form, but also in function. Upside down, Haupt's lattice of compression members and tension bearing "ties" becomes a lattice of tension members. The figure depicts the center five panels and a massive double laminated arch bolted to the ties. The arches, of course, are compression members.
I contacted Brian McKee in Ohio and asked him what he knew about the Netcher Road Bridge: "Is this a 'show bridge' topping on a stringer bridge, or a real working truss? I don't see how an inverted Haupt Truss could work." Brian's reply: "The bridge is mainly supported by a huge laminated arch, not the truss. To call it a Haupt Truss is not really accurate."
In that Haupt stated in his patent letter ". . . consisting of braces inclined at any angle . . ." the Sayres and Bunker Hill trusses are true variants of the Haupt Truss. In the case of the Netcher Road truss, the lattice is all counterbraces, contrary to the Haupt design.
Herman Haupt graduated from West Point in 1835. He resigned his commission to become district superintendent and chief engineer for the Pennsylvania Railroad. When the Civil War began he was drafted to serve as superintendent of military railroads. He pushed his tracks through Virginia, building trestles out of found materials described by Abraham Lincoln as "bean poles and corn stalks."