Shear and Torsion Design of Dade County Rapid Transit Aerial Guideways


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Title: Shear and Torsion Design of Dade County Rapid Transit Aerial Guideways

Author(s): T. T. C. Hsu and C. S. Hwang

Publication: Special Publication

Volume: 93


Appears on pages(s): 433-466

Keywords: angle of twist; beams (supports); cracking (fracturing); double-tee beams; girders; guideways; prestressed concrete; rapid transit railways; reinforced concrete; rigidity; shear; structural analysis; torsion; warping torsion

Date: 9/1/1986

In 1985 Metropolitan Dade County, Florida, completed its first stage of Metrorail. This 22.5 miles of rapid transit system includes 21.5 miles of aerial guideways . The standard aerial structures used for the guideways are prestressed concrete doubletee girders, each 80 feet long, 12 feet wide, 5 feet deep, and weighing 94 tons. The double-tee design concept was the first of such a structure to be used in a U.S. transit system. It was recommended for reasons of aesthetics as well as economy. As compared to the traditional box sections, the double-tee girders were estimated to have resulted in savings of up to $10 million. Extensive technical studies were conducted to examine the various aspects of design and performance of thee structure. including the testing of two 2/3-scale models and three full scale cale girders. . Special attention has been given to shear and torsional design, since large torsional moments were produced by wind load on vehicles and structures , b y the horizontal nosing /lurching action of the vehicles, and by the centrifugal forces on curved tracks. Vlosov's elastic torsion analysis has been made which considers both the St. Venant tosrsion and the warping torsion. This is the theory that accurately predicted the behavior of the girders before cracking. Based on this theory, torsional rotatoin at service load stage was controlled to provide riding comfort for the passengers. Vlasov's theory based on uncracked section was found to be very uneconomical for the design of shear and torsional reinforcement. Considerable savings were achieved by extending Vlasov's theory to the post-cracking ultimate loads stage using a reduced torsional characteristic number. Recommended design value fork is 30% of the torsional characteristic number for an uncracked beam.