A replacement bridge was needed across the mississippi river between minneapolis and st. paul, in an area where the river valley is lined with natural limestone bluffs. there was strong support to replace the existing deck arch structure in kind, so design proceeded on a steel and concrete alternate. the concrete arch alternate consists of two arch spans, each 555 ft in length. each span has two ribs;each rib is a twin-cell, conventionally reinforced concrete box. staged construction was necessary to maintain traffic on the existing bridge. this meant one arch rib would be completed and in service for more than a year before the second rib is completed. the first design challenge was analyzing the structure. the analysis included time-dependent loads in order to predict the magnitude of creep stresses and deflections in the arch ribs. an in-house computer program was utilized which indicated that the crown of the arch rib would deflect downward about 12 in. over time. this large deflection induces large moments in the arch at the fixed supports, so aashto's serviceability criteria for flexural members were applied to the section. no. 9 bars at 6 in. at each face satisfied all design criteria. the second design challenge was predicting the magnitude and evaluating the effects of differential creep due to staged construction. the program predicted that the stage one arch will have deflected 2 in. morethan the stage two arch at the time of closure. after closure, the stage one arch restrains deflection in the stage two arch and changes are accommodated by torsional creep. no. 6 stirrups at 12 in. are adequate for these torsional forces. the third design challenge was preparing adequate construction plans. the plans define the shape ofthe arch by giving coordinates for the dead load polygon and valuesfor vertical and horizontal camber to offset deflections. the plansalso provide pedestal elevations for the spandrels which differ foreach stage. the elevation difference accounts for the differential creep due to staged construction. this paper appears in transportation research record no. 1290, third bridge engineering conference, march 10-13, 1991, denver, colorado, volume 1.
Samenvatting