Year constructed: 1918
Bridge type: Concrete Marsh Arch
National Register of Historic Places status: Listed
Length: 88 feet
Width: 17 feet
Spans: 1
FHWA: 078120
Jurisdiction: Boone County
Location: 110th Street and V Avenue over Squaw Creek, 7.5 miles northeast of Ridgeport, Section 4, T85N-R25W (Harrison township)
Details
Crossing Squaw Creek some seven miles northeast of Ridgeport in the northeastern corner of Boone County, this concrete fixed Marsh arch dates to 1918. The Squaw Creek bridge features slotted concrete guardrails with paneled concrete bulkheads and is supported by concrete abutments. The county supervisors awarded a construction contract to the Marsh Engineering Company to erect the structure for an undocumented amount of money. The Des Moines-based firm used a design created by James Marsh, a civil engineer and rainbow arch patent holder. Since its construction in 1918, the Squaw Creek Bridge continues to carry vehicular traffic in essentially unaltered condition.
This medium-scale arch marks a noteworthy innovation in bridge design, an achievement engineered and patented by James Marsh in 1912. Marsh's design represented the hybridization of continuous concrete and segmental steel-arch designs, a radical departure from standard engineering practice. Concrete can withstand a nominal amount of tension. For this reason, most previous concrete arches--both reinforced and mass arches in filled and open spandrel configurations--were built with the arch below the deck, where the downward force of the deck could be carried in compression by the arch ribs and spandrel wall or columns. Marsh's suspended arch reversed this.
His arches, of course, act in compression; but the hangers and floor beams carry the deck in tension. Furthermore, the novel treatment of the deck over sliding steel plates on the floor beams and the use of pin-connected, articulated steel hangers for the end panel points were devices more suited to steel construction than concrete. To make the concrete thus act against its nature, Marsh inserted large amounts of structural steel. His bridges may look like concrete spans, but the arch ribs and hangers carry such heavy and complicated reinforcing that they are, in reality, steel structures encased in concrete. Marsh designed his bridges with either tied (with the arches attached to the abutments at the floor beam level) or fixed (arches extending below the floor beams to the abutments) configurations. Aside from this, all of his rainbow arches were similar, varying only in span length, arch rise and number of hangers.
Marsh's invention did not foretell a new direction in reinforced concrete design. The industry would later turn to other, simpler slab and beam configurations as it developed more sophisticated reinforcing techniques in the 1930s and 1940s. The rainbow arch did, however, denote one of the more interesting early experiments in concrete engineering and represented the proliferation of concrete for road and bridge construction. It is not known how many Marsh arches were built in Iowa in the 1910s and 1920s: judging from county records perhaps not more than one hundred. The large amount of reinforcing steel sheathed within a relatively thin skin of concrete has made them particularly vulnerable to rusting and spalling. As a result, only a few are known to remain. The Squaw Creek Bridge is distinguished as a well-preserved example of an indigenous structural type [adapted from Fraser 1992].