Precast Bridge Girder Details for Improved Performance

A research study was conducted at South Dakota State University (SDSU) to develop and verify the performance of a modified precast/prestressed bridge girder detail to be used by the South Dakota Department of Transportation (SDDOT). The study was primarily focused on improving the strength and performance characteristics of the longitudinal joints used to join adjacent double tee girders. The SDDOT currently uses a double tee precast/prestressed bridge girder with a longitudinal joint that consists of discrete welded steel connections. The design service life of these bridges was expected to range from 50 to 70 years, but bridges built less than 40 years ago are in need of replacement due to premature deterioration caused by inadequate longitudinal joints. The desired rate of bridge replacement has created a backlog of local bridges that are in need of replacement. The study included an extensive literature review, survey of surrounding departments of transportation (DOT), experimental evaluation, and analytical modeling. The experimental evaluation included laboratory testing of a current or “Conventional” specimen and a modified or “Proposed” specimen in the J. Lohr Structures Laboratory at SDSU. The “Proposed” specimen consisted of the current double tee bridge girder detail with a modified longitudinal joint. The proposed longitudinal joint consisted of a 4 in. wide grouted keyway with overlapping reinforcing steel extending out of the girder deck. Both specimens were tested under cyclic and monotonic loading to assess the performance and strength characteristics. The cyclic loading represented repeated truck traffic based on American Association of State and Highway Traffic Officials (AASHTO) Bridge Design Specifications 2012. An analytical model was created to compare the experimental results and investigate the reaction distribution when the loading position was altered. The experimental results indicate that the current longitudinal joint detail is highly inadequate while the proposed joint detail provides superb performance under cyclic loading. Under the AASHTO prescribed fatigue load, the joint of the “Conventional” specimen began to fail at 31,500 load cycles, the equivalent of only 5.8 years in service, whereas the “Proposed” specimen underwent 800,000 load cycles without any noticeable changes in performance. Contrary to the current joint detail, the proposed joint detail was highly effective in transferring shear between the girders; the “Proposed” specimen was capable of reaching its flexural capacity without any joint degradation. The load carrying capacity at flexural failure of the “Proposed” specimen was more than 1.5 times that of the “Conventional” specimen