Study On Loading Property And Application Of Prestressed Concrete Box Beam With Pretensioned Bent-up Tendons

Prestressed concrete beams with pretensioned bent-up tendons are newly prestressed beams, and have begun used in bridge engineering of China. In comparison with post-tensioning method, pretensioning method can omit making hollow tubes (metal or plastic) placed in the forms, omit anchorage devices or end steel plates and omit grouting the space in the tubes by pumped grout. In addition, since the tendons are well bonded to the concrete in pretensioned concrete beams, the corrosion of prestressing reinforcemens due to the concrete cracking may be avoided, thus the durability of the members is more improved. Because the Prestressed concrete beams with pretensioned bent-up tendons have many advantages, this new type prestressed concrete beams have been widely used in flyovers of high speed highway or high speed railway in recent years.In this thesis, based on the requirements of application for prestressed concrete box beam with pretensioned bent-up strands, the box beam which has a span length of 35m is one of the bridge beams of Huaihe River and is the first application of this new type prestressed beams in high speed highway bridge of China, the prestressing parameters of bent-up strands, the loading properties of 2 prestressed concrete box beams with pretensioned bent-up strands which have span length of 12m and have the span length of 35m respectively, and the local stress at anchorage zone and bent-up zone of strands in box beam' s web plate are tested and analyzed. The design method of the prestressed concrete box beam with pretensioned bent-up strands is suggested, and this may be as a reference for the applications of the newly prestressed box beams in engineering. The research work of this thesis is shown as following:1. Experimental study on prestressing parameters of bent-up strands.The frictional loss and limit tensile strength for the bent-up prestressed strands are tested and analyzed, the main factors affecting the frictional loss and tensile strength of the strands are discussed in detail, and the formulas for calculating the frictional loss and tensile strength of the bent-up strands are suggested. The transmission length of prestress for the bent-up strands in concrete are also tested and analyzed, the results show that the transmission length of prestress can also be calculated by the stipulations in Chinese Codes.2. Study on loading behavior of prestressed concrete model box beam of 12m with pretensioned bent-up strands.The test results of prestressed concrete model box beam which has a span length of 12m with pretensioned bent-up strands under loading are given. The prestress losses of strands, the stress of concrete, the cracking resistance, the deflection and the bearing capacity of the prestressed concrete model box beam are analyzed. The formulas calculating the bearing capacity, the cracking resistance, and the deflection of pre-stressed concrete box beams by pre-tensioned method with bent-up tendons are suggested.3. Experiment study on loading behavior of prestressed concrete box beam of 35m with pretensioned bent-up strands in high speed Highway Bridge across Huaihe River.The test results of a prestressed concrete box beam with pretensioned bent-up tendons under loading are presented, the box beam which has a span length of 35m is one of high speed Highway Bridge across Huaihe River in Henan Province of China. The prestress losses, the stresses of tendons and concrete are measured; the bending behaviors such as the cracking resistance, the deflection and flexural capacity of the prestressed concrete box beam is discussed and analyzed. The results show that the bending behaviors of the prestressed concrete box beam with pretensioned bent-up tendons are better than that of conventional post-tentioned prestressed concrete bridge beam, and this may be as a reference for the applications of this newly pre-stressed beam in bridge.4. Analysis on local stress of strand anchorage zone for prestressed concrete box beams with pre-tensioned bent-up tendonsIn order to prevent the concrete cracking at strands anchorage zone and strands bent-up zone of the prestressed concrete box beams of 35m span length with bent-up tendons in the bridge across Huaihe River, the analysis results on local stress at strand anchorage zone for 4 working states with different web thickness and different strand anchorage loss length for the prestressed concrete box beams are given, the results show that local stress of concrete at strand anchorage zone may decrease obviously applying the measures of working state 4 that the thickness of web at the end is increased from 300mm to 400mm and the anchorage loss length of partial stands should be sited. The measures which are increasing the web thickness and sitting the anchorage loss length of strands at the end of beam to prevent concrete cracking are suggested and verified by the measurement results in practical engineering.5. Technical suggestions for stretching and anchoring strands of prestressed concrete box beam with pretensioned bent-up tendons.Based on research results and monitor results for the prestressed concrete box beam with pretensioned bent-up strands in high speed Highway Bridge across Huaihe River, the technical measures for stretching and anchoring strands of prestressed concrete box beam with pretensioned bent-up tendons are suggested for reference in bridge engineering.