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Bridging Memories

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The Buffalo River Bridge at Pruitt, built in 1931, will soon be only a memory because it is being removed since a new span has been opened to take its place. So, for memory's sake, here are some excerpts about the 90-year-old bridge from the Historic American Engineering Record. The full report can be found at ardot.gov

(Pruitt Bridge)
HAER No. AR-23

In February 1931 Arkansas State Highway Department began to consider replacing the bridge over the Buffalo River on the scenic Highway 7 near Jasper, county seat of Newton County. Officials were said to be "not seriously alarmed by the condition of this bridge," indeed, an anonymous representative of the Bridge Department was recorded as saying that the old bridge "was almost as good today as when it was built." However, while the old bridge was not in a condition that required immediate replacement, its older design could withstand only limited loading. In a study of the older bridge by the Highway Department, the bridge engineers suggested that "a three ton load limit sign should be placed on the old Buffalo River bridge."
DESIGN AND CONTRACT
The design of the new bridge for the Buffalo River was not yet underway by April 8, 1931.
The State Highway engineer W.M. Mitchell, reported in a letter of that date that the bridge designers had not "been able to get the plans started due to other work that was ordered ahead of this."
On May 6, Mitchell requested the bridge engineers of the State Highway Department to commence and complete the plans "as early as possible." He specifically requested the urgent attention of the bridge engineers to this project as "considerable pressure is being brought to bear to get this bridge in the next letting. " It cannot be ascertained precisely where that "pressure" was coming from, but the dire need for bridge projects to aid employment in the years of the Depression would suggest that pressure originated at county level.
The plans were commenced on May 11, 1931, and, remarkably, completed the next day.
The contract was advertised on May 13, with an estimated cost of $65,461.43. The contractor, Fred Luttjohann, of Topeka, Kansas, received the contract for the lowest bid of $55,226.09. Work began on the bridge on July 18, 1931, with a contracted building period of 210 days.
Fred Luttjohann was, as with many of the bridge contractors of the 1920s and 1930s, a largely unknown figure. He was involved with a number of Arkansan bridges of the period, but as a contractor he was primarily engaged in subcontracting work, consequently leaving his work as regards the contracts largely supervisory and anonymous. His work, however, was regularly advertised in the State Highway Department magazine of the period. Advertisements there declare that his bridges are "built for the ages."
ENGINEERING DESCRIPTION
The Pruitt Bridge is a steel truss of total length 375 feet, comprised of a center span of 160 feet, two end spans of 80 feet each, and a 55-foot girder approach span on the south end. The Warren end spans and the Pennsylvania main span all have eight panels, where a panel is defined by the space spanned by a main diagonal. The horizontal, vertical, and diagonal sub-struts of the main span radiate from the main diagonal at mid-panel width. The end spans have verticals at every other panel point, meeting the top chord where the diagonals are riveted to it.
The inclined top chord of the end trusses slopes five degrees upward to the polygonal top chord of the main span. The top chord for the entire bridge consists of two 10-inch-deep channels, increasing in weight toward the center of the span, joined by a continuous top plate and lacing bars. The chord is riveted along its length except for a pin connection between the main and secondary span at U7 (see Highway Drawing No. 3223). During construction this joint was riveted to support the center span, which was cantilevered from the end spans. Once the center span was complete, the bridge carried forces as three trusses, and all weight was transferred to the piers and not from one truss to another. So member U7-U8 did not carry any axial load.
The bottom chord, two 12-inch-deep channels with lacing and batten plates, is also pin connected at L8 to a fixed hinge on a concrete pier. The similarly positioned panel point on the other end of the bridge is pinned to an expansion rocker, as are the extreme ends of the end spans.
All verticals, sub-struts, and diagonals are riveted to the chords. Throughout the bridge the web members are 10-inch-deepI sections, oriented with the web transverse to the direction of the bridge.
All lateral, sway, and portal bracing is formed with angles. Top and bottom lateral bracing span one panel diagonally. Those on top are angles with lacing, but the bottom braces are single angles. Sway bracing at each panel point is a three-panel double-intersection Warren truss. Portal braces are trapezoidal with braces reaching from the center of the top strut to the inclined impost. A portal brace on the diagonal at the first vertical of the main span replaces the sway bracing at that point.
The 27-inch-deep I-beam floor girders are supported on the bottom chord and are riveted to the vertical web members. The girders support the 20 foot wide concrete slab deck with curbs without the aid of stringers.

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