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Behavior of end zone of precast/pretensioned concrete bridge girders
A simplified shear design method is proposed for reinforced and prestressed concrete beams, based on analysis of the influencing factors and correlation with test results. The proposed method takes into account significant parameters influencing concrete contribution to shear capacity. Shear reinforcement contribution to shear capacity includes consideration of variable diagonal cracking angle. A non-iterative procedure and a simple formula for diagonal cracking angle, including effect of axial force, are proposed. The beneficial aspects of LRFD longitudinal reinforcement anchorage requirement and the maximum shear limit in LRFD are retained in this proposed method. Design examples are given. The example shows how Av/s is calculated and demonstrates to difference in results of the proposed method with those of the AASHTO LRFD, AASHTO Standard, and ACI-318. ^ In a recently concluded investigation on shear limits in precast prestressed concrete girders for Nebraska Department of Roads (NDOR), it was determined that the AASHTO LRFD limit of 0.25f′cbdv for maximum shear reinforcement is attainable as long as an adequate number of strands is anchored into the abutment diaphragms. In addition, extending strands in prestressed concrete girders beyond member ends and bending them into cast-in-place pier diaphragms can be a cost-effective method of controlling creep and shrinkage effects in bridges designed as simple spans for girder and deck weights and continuous spans for additional loads. In this paper, pullout capacity of 0.5 in. and 0.6 in. (12.7 and 15.2 mm) diameter strands is evaluated. Two numerical design examples are included together with design recommendations for determining the required number and length of strands that need to be bent and embedded into the diaphragms. ^ The current AASHTO LRFD and Standard Specifications require that 4 percent of the total prestressing force be used for the design of end zone reinforcement in pretensioned concrete girders. In addition, AASHTO LRFD requires that the end zone reinforcement be placed within a distance from the end equal to one-fifth of the girder depth, while AASHTO Standard requires that the same amount of steel be placed within one-fourth of the girder depth. This requirement creates excessive reinforcement congestion in the end zone. In this study, a literature review was conducted to document the development of end zone reinforcement specifications. Analytical methods using strut-and-tie and equilibrium analysis models for end zone reinforcement design are discussed. Further, strains and stresses in the end zone reinforcement of various girders designed based on the current specifications were determined experimentally at prestress transfer. The results showed that the stress level in the end zone reinforcement was less than the stress recommended by the specifications. The locations of maximum moment predicted by the analytical model developed by Gergely and Sozen were found consistent with the crack locations observed. As a result of the analytical and experimental investigations, new end zone reinforcement details were developed. The new details installed in inverted tees and NU I-girders have been tested experimentally. The results showed the new details would reduce the amount of current reinforcement by 40 percent and it is effective in controlling crack propagation in the end zone, and eliminating the end zone congestion. ^
Jongpitaksseel, Nipon, "Behavior of end zone of precast/pretensioned concrete bridge girders" (2003). ETD collection for University of Nebraska - Lincoln. AAI3092561.