Research On Designing Methods Of Crack-controlling Of Partially Prestressed Concrete Beams Considering Crack-closing

Partially prestressed concrete beams have a good property of deformation-renew. When load is removed enough, the cracks expanded under the biggest service load can close well. To the partially prestressed concrete beams with larger variable load, they still have good normal service performance and durability, when they are allowed to crack in the allowed range momently under the load effect nominal combination and the cracks can close under the load effect quasi-permanent combination. In structure design, if this property is full used, it can help to decrease the dosage of the prestressing tendon, save anchorage device, reduce cost and minish invert arch.In order to apply the crack-closing property of partially prestressed concrete beams to engineering practice, scholars at home and abroad have done a large amount of research work fruitfully. They have got a deep cognition to the course of crack closing and have put forward the empirical formulas respectively. However, their formulas fit well to the test data own to themselves but perforce bad to the others. So it's essential to bring forward a uniform formulas to calculate the crack closing moment. In addition, the previous research work mostly focus on the crack closing mechanism, but neither the applicable conditions and ranges was considered nor the design methods of crack control that considering the crack-closing property was given. In other words, the previous study didn't discuss when and how to make use of the crack-closing property.Aim at hereinbefore problems, the primary work are done: (1) Based on the previous test data and theory study, further research is done to the crack-closing property of the partially prestressed concrete beams, and the crack closing stress value is studied emphatically through whole courses analysis. (2) The uniform formula to crack closing moment is presented, which is fit well to the existed test data all. (3) The author's research indicates that the calculational crack closing moment accord well with the measure one to disposition and unbounded partially prestressed concrete beams, when crack closing stress respectively equal to 1N/mm2 and 2N/mm2,. So crack closing stress can be take as 1N/mm2 and 2N/mm2 when calculate crack closing moment of partially prestressed concrete beams. (4) Introduce the main influencing factors and the calculating method ofthe crack width, and expatiate the virtue of crack controlling using nominal tensile stress. (5) The effect of the transverse crack width on the durability of partially prestressed concrete beams is studied, through analyzing the mechanic of reinforced corrrosio and previous exposed test. (6) The condition of crack closing is studied and combining with the load condition the concept of the unload coefficient is given, base on which, the application range of crack closing property to the partially prestressed concrete beams is given. (7) In the end, the proposal design formulas of crack controlling in the partially prestressed concrete beams is given, considering the crack-closing property and nominal tensile stress method.The design of a engineering example proved that a good economy benefit can be gained using the crack-closing property to crack controlling design of the partially prestressed concrete beams. And the prepositional formula can make a good use of the crack closing property to crack controlling design of the partially prestressed concrete beams. The prepositional formula has the conveniency of application, and clear concept, can be provided as a reference to the engineering designers and revise the code for design of concrete structures.