An lnstrumentatlon and load-testing analysis of the 536-ft, two-span, open spandrel arch-rib Hennepln Avenue Bridge over the Mississippi River In Mlnneapolls, Mlnnesota, was conducted to measure the transverse load distribution among the six arches of the bridge and to determine whether the buckled webs of the arch ribs carry any load. When the structure was rated for Hennepin County in 1983, the load distribution and the ability of the buckled web plates to carry axial stress were questioned. Thus, an Instrumentation analysis under static load was performed. The bridge was Instrumented with 18 strain gauges and was loaded with three 27. S-ton tandem dump trucks positionj! d in nine different loading arrangements. Strain readings were averaged for each loading to determine the magnitude of load carried by each arch rib and then compared with a computer-modeled (linear) transverse distribution. It was found that the arch ribs carried not only axial stress, but also stress due to bending moments. It was concluded from the study that the floor beams and di· aphragms do not transfer loads from one side of the bridge to the other. The results of the web testing demonstrated that a nominal axial load was being carried by the web, and that, although buckled, It was working effectively through a tension-field mechanism similar to that of a simple truss.

The Hennepin Avenue Bridge (SB 90589) over the Mississippi River in Minneapolis, Minnesota, is a historically significant, unique structure functioning as the second-longest solid arch-rib bridge span known to be in use today. Although this is the third bridge on this site, its earliest predecessor was the first recorded bridge across the Mississippi River. Constructed directly north of the Falls of St. Anthony and situated between the Burlington-Northern Railroad fiats area and Nicollet Island, the steel arch bridge is now part of the St. Anthony Fnlls Historic District, birtbplnce of the City of Minneapolis. The bridge continues to function as a major transportation artery linking downtown Minneapolis with the northern and eastern metropolitan areas. Of historic and technical significance, the steel bridge was constructed in longitudinal halves, with two 258-ft spans and supports, a 56-ft roadway, and two 12-ft sidewalks. The north half of the bridge was constructed in 1888 of 3 three-hinged arch ribs. In an attempt to reduce deflections and vibration, the design of the south half of the bridge was revised from a threehinged to a two-hinged arch. The combination of three-and two-hinged solid arch ribs is the most unusual structural feature of the historic bridge.