# Fatigue Testing of Shear Reinforcement in Prestressed Concrete T-Beams of Bridges
## 1 Introduction  
Bridges are an important part of the national and international infrastructure. Due to higher traffic volume, especially concerning heavy load traffic, the requirements on bridge structures have increased significantly in recent years [1; 2; 3; 4]. While prestressed bridges were designed based on approaches using principle stresses as design criterion in the past, verifications based on truss models have been common practice since the 1980s. Numerous German bridges were designed and built in the 1960s and 1970s for lower load classes [5]. Therefore, many existing bridges exhibit a deficit in shear strength, applying the current standards (e.g., Eurocode 2 [6; 7; 8; 9]) because of the lower required shear reinforcement of previous design rules [10; 11]. The verification of shear resistance under cyclic loading has been required for concrete bridges in Germany since 2003 by introducing the DIN Fachbericht 102 [12], which is based on Eurocode 2 [13]. Hence, a guideline to assess the load bearing capacity of existing bridges [14; 15] was introduced and has been continuously refined--e.g., in [16; 17; 18; 19; 20; 21; 22]. The second amendment is already in progress [23]. The key element of this assessment guideline is a stepwise verification procedure, which is similar to the level-of-approximation approach of fib Model Code 2010 [24]. This procedure accounts for the characteristics of older bridges and enables a more refined determination of the structural capacity. Independent of this German guideline and its refinement, research is currently being conducted all over the world on the load-bearing capacity of concrete bridges and individual structural parts, especially under fatigue loading [25; 26; 27; 28; 29; 30; 31; 32; 33; 34; 35]. However, in numerous cases, the resistance under shear and torsion, as well as fatigue loads, cannot be verified [36; 37; 38], although the webs of these prestressed bridges do not reveal any shear cracks. A further refinement of the shear assessment under static and fatigue loading can reduce theneed for strengthening measures or restrictions of usage significantly and will extend the service life of existing bridge structures. To investigate the cyclic shear capacity of prestressed concrete beams with and without shear reinforcement, experimental tests on 43 beams have been carried out until now [39; 40; 41; 42; 43] at the Institute of Structural Concrete of RWTH Aachen University. The main parameters of the investigation were shear reinforcement ratio, degree of prestressing and the cyclic part of the shear force. Beside the number of cycles until failure, the redistribution of shear stresses and forces in the shear reinforcement was the most important aspect of the investigations. The definition of cross-section geometry, shear reinforcement ratio, degree of prestressing and the load levels for the test specimen was based on an evaluation of typical existing federal highway bridges in Germany built between 1960 and 1970. The previous tests on prestressed I-beams without shear reinforcement have indicated that an abrupt shear failure occurs without announcement due to the development of an inclined crack. In contrast, fatigue failure on prestressed T-beams without shear reinforcement is announced by the slow development of flexural shear cracks. Due to the missing shear reinforcement, however, the failure is still abrupt [39; 41]. In the tests on prestressed I-beams with shear reinforcement, it was observed that the fatigue failure occurred progressively. Due to the successive failure of individual stirrups and the redistribution to adjacent stirrups, as well as the gradual increase in crack width and deflection, the shear failure was well announced [40; 41; 42; 43]. All beams, both with and without shear reinforcement, had in common that the resistible number of load cycles until the failure of each beam was clearly underestimated by Eurocode 2 [6; 7; 8; 9].