Full-scale diagnostic load tests were performed on a typical single-span structure as part of a pilot test program for the engineering condition assessment of the chicago rapid transit system. the purpose of the load testing was to evaluate the actual behavior of a 100-year-old elevated steel structure under train loading. measured static and dynamic responses to train loads were used to evaluate impact loading, and to establish methodology for fatigue life assessment. tests were also performed to evaluate longitudinal braking forces at the support columns, and recent modifications made to the stringer bracing system. the chicago transit system includes approximately 40 miles of elevated steel structures and bridges. the results of the pilot test program were used to develop an expanded test program for an overall condition assessment of the entire elevated transit support system. the static load tests were performed by stopping a test train at known positions along the track structure. strains and deflections were measured at critical locations in the stringers, columns and cross-bracing members. the dynamic tests were performed by moving a test train across a test span at crawl speed and at various operating speeds. strains were measured in the stringers during movements of the test train and in-service trains. dynamic tests were also performed by braking the moving test train to evaluate the effects of both normal and emergency braking. the test data showed the average measured impact loading to be 5 to 10%, as compared with the code design impact of 55%. the braking and stopping of the test train caused bending stresses in the columns approximately equal to the column axial stress. the new stringer cross-bracing was found to be effective in distributing train loads from one track to all four stringers supporting both tracks, causing significant uneven distributionto the two stringers supporting the loaded track. this paper appears in transportation research record no. 1290, third bridge engineering conference, march 10-13, 1991, denver, colorado, volume 1.
Samenvatting