Stability Analyses for a Multi-Span Tied Steel Arch Bridge: AASHTO Effective Length Method, Eigenvalue Analysis and AISC Direct Analysis Method

The Smith Avenue Bridge (High Bridge), spanning the Mississippi River in St. Paul, Minnesota, consists of 3 continuous tied steel arch spans measuring 282'-3"", 520' and 241'-9"". Originally constructed in 1987, the bridge is currently undergoing comprehensive rehabilitation including the replacement of its original concrete deck. In support of the rehabilitation design, the Minnesota Department of Transportation (MnDOT) tasked AECOM with load rating of the arch spans. Early in the design, AECOM recognized the conservatism inherent in the simplified Effective Length Method utilized for arch rib axial capacity calculations within the AASHTO LRFD Bridge Design Specifications. As permitted by AASHTO LRFD, AECOM employed eigenvalue analysis as a more refined means to determine the critical buckling load for the arch rib. Additionally, MnDOT selected a peer review team (Michael Baker International) to perform an independent analysis for verification of the eigenvalue approach. The peer reviewer utilized the Direct Analysis Method found in Chapter C of the AISC Steel Construction Manual, another refined analysis method, to load rate the arch rib. The increase in axial capacity gained by the eigenvalue analysis conducted by AECOM allowed the arch rib to rate for current AASHTO design loadings as well as MnDOT permit vehicles and was shown to be conservative by the results of the Direct Analysis Method conducted by Michael Baker. A comparison of the results determined using the AASHTO Effective Length Method, AASHTO method employing eigenvalue analyses and the Direct Analysis Method is presented in this paper.

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