Evaluation of improved shear key design for multi-beam prestressed concrete box girder bridges

Field tests and laboratory tests were conducted to evaluate the problem of shear key failure in prestressed box girder bridge beam and to propose a new shear key design. A total of six field tests were conducted on five different multibeam prestressed box girder bridges under service loads. The objective of these tests was to investigate the in-situ performance of the grouted shear keys, located at the longitudinal joints between adjacent girders. Failure of the shear key will typically not only compromise the load-sharing mechanism between adjacent girders, but also lead to the failure of the deck waterproofing system, with attendant corrosion problems.;The tests consisted of monitoring relative displacements occurring across the intergirder joints, as well as flexural strains in the girders themselves during passes of a preweighed tandem-axle dump truck, with axle loads typically about 19 kips (84.7 kN). Relative displacements were measured with a specially designed transducer, having a resolution on the order of 0.1 mil (0.00254 mm), which was bonded to the underside of the bridge across the intergirder joints. Flexural strains in the girders were measured with conventional foil strain gages.;All bridges tested exhibited relative displacements across some of the joints, which indicated a fractured shear key. A finite element analysis of a three dimension bridge model showed that tensile stresses from transverse negative moments in the top flange of the bridge generated from the continuity provided by the shear key, in the current location, may be responsible for the shear key failure.;A proposed shear key solution is presented in a new location at the neutral axis of the box girder. Finite element analysis for the new shear key indicated elimination of the stresses suspected of causing the failure problem. Two series of lab tests were conducted to test a two dimensional representation of the box girder bridge both under static and fatigue load. A two dimensional representative "slice" was examined by finite element analysis to define the boundary conditions required to approximate the three dimensional behavior.;A total of three lab tests were conducted for the current shear key design as well as the proposed new shear key under static loading until failure of the shear key. All tests were repeated for three different grouting materials; non-shrink grout, magphosphate grout and epoxy grout, except that some of the test results could be anticipated from previously completed tests.;A similar lab test program was repeated for fatigue life testing laboratory experiments. Two reaction frames were utilized, each equipped with a 50 kip (223 kN) capacity actuator and a signal generator and controller system. A computer-based data acquisition system monitored a set of DCDT's and foil strain gages during each test. Grouting materials from individual test specimens were tested for tensile strength. Load, deflection, flexural strain and fatigue life were monitored and recorded.;The proposed shear key, in the new neutral axis location, greatly improved the load-carrying capacity of the tested specimens when compared to the current shear key design.