Historic Structures

Bridge Description Bells Ford Covered Bridge, Seymour Indiana

As originally built, the Bells Ford Bridge was a 325 foot two-span timber and iron covered bridge on cut stone piers with a long timber pile approach at the easterly end. The eastern approach was removed from the east end in the early 2000's, and in 1999 the western span blew down in a windstorm, leaving only a single span hovering precariously above the water. The bridge is 14'-9" high between the centers of the chords, 17'-7" wide between the centers of the trusses, and has a 16'- 6" wide roadway. The surviving span has fifteen panels spaced at approximately 10', plus 5' end panels, for a clear span length of approximately 160'.

The trusses have parallel upper and lower chords connected by a web of timber compression members and wrought iron tension members. Three parallel sticks (one 9"xl0 3/4" timber flanked by two 5"xl0 3/4" timbers) bolted together comprise the upper chord. The lower chord is composed of a series of parallel wrought iron bars, increasing in number toward the center of the span, and bolted together at each panel point. The ends of the lower chord are slotted castings that frame into shoes for the endposts. The chords are connected by paired 6%"xl2" vertical endposts, single timber compression members (varying from 6 3/4" x 8 3/4" at the center to 9 5/8"x8 3/4" at the ends) inclined at 20 degrees up towards the center of the bridge, single diagonal rods (1-1/8" to l 1/4" diameter) inclined up towards the center within each panel, and paired diagonal iron bars (increasing in size from 3/4"x3/4" at the center to l 1/4"xl 1/4" at the ends) inclined in the opposite direction across two panels. The tops of the posts and braces are joined to the upper chord and lateral bracing with a pin and contained within a joint box. The end post has a cast joint box to receive the end post and lower chord.

Transverse steel I-beams are suspended below the lower at every panel point. There are eight lines of stringers placed longitudinally on top of the floor beams. The decking is two layers of narrow boards, one laid transversely and one laid longitudinally on top of the stringers. Upper lateral bracing consists of tie beams notched into the upper chord and cross bracing between the tie beams.

The roof is made up of rafters whose lower ends rest on the upper chord and whose upper ends meet at the ridge to form a gable. The roof is covered with wood shingles nailed to purlins on top of the rafters. Vertical plank siding fastened to longitudinal nailers on the outer faces of the trusses covers the exterior of the bridge. The siding reaches to just below the eaves, leaving a narrow opening at the top for light and ventilation. The portals are straight and square with no overhang.

Design

A house joiner from Lebanon, New Hampshire, Simeon S. Post (1805-1872) began his engineering career building bridges for the New York & Erie Railroad. In 1853, Post went to Indiana to supervise construction of the Ohio & Mississippi Railroad. While there, he presumably met Meedy Shields, Robert Pattison and others involved in railroad construction. In 1855, Post returned to the New York & Erie Railroad as consulting engineer and subsequently consulted on numerous railroad and tunnel projects around the country.

In 1863, Post received a patent for improvements in iron bridges, consisting of joint boxes and pins that connected the chords and web members. Though not part of his claim, the patent drawing showed the distinctive truss configuration now known as the Post truss. The Post truss featured parallel inclined compression and tension members. The size of the chords increase toward midspan, where maximum bending moments occur, while the size of the members increase toward the ends of the span, where maximum shear occurs. Because engineering analysis for train loadings was still in its infancy, the Post truss, like the Whipple, Bollman, Haupt and Fink trusses, relied on redundant members to prevent catastrophic failure.

The Post truss design was first used on the Erie Railroad at Washingtonville, New York in 1865. The type's redundant members allowed it to remain rigid under live loads, and it enjoyed a brief but vigorous period of popularity in the 1860s and 70s. In 1868, the Post truss received national recognition when the Union Pacific Railroad used it for the largest bridge on its line, spanning the Missouri River between Council Bluffs, Iowa and Omaha, Nebraska. Use of the Post truss even ventured outside the United States, when in 1876, the Watson Manufacturing Company built three Post truss bridges in Brazil. Railroads reportedly built hundreds of combination Post trusses, but more efficient and economical structures replaced these bridges before the turn of the century. The type was also used for highway bridges, though to a lesser extent. The four surviving examples of this type are highway bridges. The popularity of the Post truss ended around 1880. The last two decades of the nineteenth century saw increasing uniformity and standardization of bridge trusses.