Being a civil engineer by training, I am still amazed at how special and unique were the Beale Wagon Road bridges built to serve the Star Route for the U.S. Mail out of Ft. Smith, in particular the historically important one across Little River, Creek Nation, Indian Territory, on February 16, 1864. I provide the following final bridge details for future guidance to those who might want to rebuild the 1859 Whipple Bowstring Truss (Arch) Iron Bridge across Little River!
A bowstring truss has a continuously curved top chord and a straight line bottom chord making it look like an archers bow laying on the bowstring. The top arch and bottom chord are connected with vertical and diagonal members. The diagonals are load-bearing members which makes this a true truss.
Often a bowstring truss is confused with a tied arch. However, a tied arch has only vertical, or near-vertical members between the arch and the roadway’s deck. These are tension-only members while the verticals on a bowstring are compression struts and the diagonals are tension.
Through Truss versus Pony Truss
A Through Truss bridge span is where the horizontal roadway deck lies between its supporting opposite truss webs, and the top chords of the two supporting “arches” are connected with overhead lateral bracing to provide added structural stability where needed. A pony truss has no overhead lateral bracing, usually due to its shorter span and smaller truss structures.
TWO EXISTING WHIPPLE BOWSTRING BRIDGES
Normanskill Farm Bridge, Albany County, New York, Whipple Cast & Wrought Iron Bowstring Truss Bridge, clear span ~ 108 feet, built 1869 from Whipple’s design
An original Cast & Wrought Iron Whipple bowstring through truss bridge that now provides a crossing of Normanskill Ravine for an exercise path for the Albany New York State Police K-9 Training Center. Figure 4 shows the bridge without more modern safety rails added as seen in later pictures. See at:
GPS Location: 42°38'09" N, 73°48'02" W
Significance: One of only two known surviving "Whipple" bowstring through truss bridges, and one of the few remaining composite cast- and wrought-iron bridges. This span was built according to the patented design of Squire Whipple, which was used widely during the second half of the nineteenth century mainly in New York State. When Whipple's patent of 1841 expired in 1869, the design was copied down to the last detail by a number of bridge builders, including the builder of this bridge.
Pedestrians are allowed on the bridge, according to signs posted at bridge. Its arched span is about 108 feet, having 9 segments, which would be about the same as the Beale Wagon Road Bridge. It is unclear whether the Beale Wagon Road bridge across Little River, believed to have had a clear span of about 100 feet, used the pony truss arch design or the through truss design, the latter providing added overhead horizontal stability of the main arches. Whipple’s patented design, does not show the extra overhead lateral bracing in Design Plans submitted to New York State in 1859 for a 100-foot Whipple Bowstring Arch design (with or without sidewalks) at:
It is known that Whipple bowstring arch bridges were built with either pony- or through-truss designs for clear spans of around 100 feet during the period from 1856-1889. See these 1889 survey data at:
Annual Report of the New York State Engineer & Surveyor for the Fiscal Year Ending September 30th 1889 - See pages 229-234 for survey of important design parameters on over 200 Whipple bridges built in New York by 1889. (The Ehrmentraut bridge below is part of bridge #107, page 231 in the 1889 survey.)
Ehrmentraut Farm Bridge across Black Creek, Monroe County, New York, span 84.8 feet.
This Whipple bowstring pony truss bridge was originally built in 1859 as part of a multi-lane bridge across the Erie Canal in Brockport, NY. Relocated east of Brockport on the Erie Canal in 1880. It was later dismantled, sold to a local farmer, and then one span was moved south several miles in 1910 to provide farm access across Black Creek near Churchville, NY.
GPS location is: 43°05'17" N, 77°49'50" W
or located on the south side of a local road at 2675 Chili Ave. Ext., Churchville, NY.
The bridge is clearly visible from Chili Ave. Extension, using Street View in Google Earth.