Mill Grove Bridge

Information and Findings From DHPA Historic Bridge Survey

Bridge History and Significance

By 1876, the road which angled northwest from Gosport to Cloverdale in Putnam County was a well-used one. It serviced at least four churches and two schools, including the Mill Grove church and school house just north of the Mill Creek crossing. After a visit to the crossing in March 1898, the Board of Commissioners decided to built a "steel highway bridge" here. It approved specifications for a 125-ft. (extreme) span with a 14-ft clear roadway, and set a letting for 2 May. Sixteen firms submitted proposals. At $2,450 for "a steel and wrought iron bridge," the Rochester Bridge Company of Rochester, Indiana, was "the lowest responsible bidder" and secured the contract. Whether Mill Creek was spanned on the Gosport-Cloverdale road before the 1898 through-trusses were erected there remains unknown. The builder of the stone abutments for the steel and wrought iron span also remains unknown. A pony-truss span was added to the Mill Grove Bridge, probably in 1919 by the Vincennes Bridge Company. To accommodate the additional span, the commissioners had a concrete abutment built and likely adjusted and encased the original northern stone abutment as a pier.

Through-truss span (1898)
The 122 ft (c>c) Pratt through-trusses are subdivided into seven 17.85-ft. panels. The end-posts and top-chord members consist of a pair of channels riveted together with a cover plate above and lacing bars below. A pair of eyebars supply each lower-chord member. An eyebar provides each hip vertical and a pair of laced channels serve as each intermediate one. The diagonals consist of a pair of forge-welded eyebars; a pair of forge-welded cylindrical rods with turnbuckles double as the center panel's diagonals counter-braces. Single, adjustable rods counter the diagonals in the third and fifth panels. 15-in. deep rolled-I floor-beams are U-bolted to the lower pins. The support four runs of I-beam stringers plus an outer run of channels on each side. The stringers, in turn, carry the timber deck with its 14-ft. roadway with 16 feet of vertical clearance. The portal struts are latticed and knee-braced. The interior struts consist of two pairs of laced angles fit within the top chords inner channel and attached to upper pins. The interior struts are also reinforced with knee-braces. Adjustable round rods provided both the upper and the lower lateral bracing. Latticed hub-guard rails line the trusses.

Pony-truss span (c.1919)
The full-hip, Pratt pony extends 70 ft. (extreme) in five panels of 13.83 ft. Its fabrication underlines the shift away from eyebars and specialty parts to members made from rolled sections. While, for example, the end-posts and top chords are conventionally made from a set of channels riveted together with a cover plate above and lacing bars below, the lower-chord members consist of a pair of angles riveted together with angle battens. The truss web shows the same shift to off-the-shelf sections. Integrated with an external sway brace, each of the verticals is fabricated from two pairs of angles-one set for the post; another for the brace--riveted together with common batten plates. A pair of angles and battens supply the diagonals and center-panel counter. 18-in. rolled-I floor-beams are bolted to the verticals. They support five runs of I-beam stringers with an outer run of channels on each side. The stringers, in turn, carry the 14-ft wide concrete deck. The external sway braces help to stabilize the trusses as do the adjustable round rods which supply the lower laterals. The pony-trusses are lines with two runs of angles attached to the verticals across the span and with angle posts at the ends.

Statement of Significance
While the Gosport to Cloverdale road may not have been among that handful of the most major arterial highways in the county, it was close to being one until along in the twentieth century when the state highway system was established and bypassed the route. For a time, the Mill Grove Church and school house were important community institutions in and around Taylor township, and so getting across Mill Creek to them was a matter of consequence. Whether Mill Creek was forded or spanned before 1898 remains unclear. In that year, however, the county had a quite substantial wrought iron and steel span erected there by a notable Indiana builder, the Rochester Bridge Company. This may be the only Pratt through-truss example of Rochester fabrication still extant in Indiana. In apparently using both wrought iron and steel in the same structure, the Rochester company's materials use was clearly in transition. The structural design was otherwise unexceptional. The span does retain its original members including its lattices hub-guard rails and portal struts. The pony-trusses' bolting, full-hip design, and external sway braces are noteworthy features of a different company in a different era. The Vincennes Bridge Company became one of the major fabricators of highway bridges in Indiana. Indeed, the firm is still engaged in bridge-building. In the twentieth century, fabricators moved away from pin connections-here to bolting-and from wrought iron to steel; turned from specialty forms like eyebars to standard rolled sections-here evidenced in the generous reliance on angles; adjusted design for heavier and faster-moving loads-see the heavier floor-beams and external sway bracing; shifted from concentration on wagons to automotive traffic-note the use of a concrete roadway. Also more evident in the age of increasingly standardized design, the pony span is altogether undecorated. Even for railing, the Vincennes Bridge Company fell back on simple, unadorned rolled angles.

References
Beam, Longest & Neff, Inc., Owen County Bridge Inventory Rating & Safety Inspection (Indianapolis, 1973). Associated Engineering Consultants, Inc., Bridge Reinspection Study & Report: Owen County (Nashville, 1978). bridge nameplate. Baskin, Forster & Co., Illustrated Historical Atlas of the State of Indiana (Chicago, 1968 reprint of 1876). Owen County, "Commissioners Record," 16:409, 414, 435-438; 26: 294-295, 346, 445, 490.

Bridge Considered Historic By Survey: Yes (Select)