Ultimate Equivalent Load Balance Method For Plastic Design Of PC Continuous Beams (Slabs) With Unbonded Tendons

Recently, the prestressed concrete structures are widely used in the civil engineering field in China. Great developments have been observed for the calculation of bearing capacity of normal section, the rules of plastic internal force redistribution and the design method of moment modification both in China and abroad. But the change of secondary moment and its influence in the ultimate state of bearing capacity are in dispute. The calculation processes of bearing capacity of normal section and plastic design are complicated. What's more, the moment modification coefficients of presstressed concrete and reinforced concrete continuous beams (slabs) are not the same. Therefore, it is necessary to propose a method for the plastic design of prestressed concrete continuous beams (slabs), which is clear in concept, easy for calculating and the same with the plastic design of reinforced concrete structures.The basic principle and profound meaning of load balance method proposed by L.Y.Lin have been excavated, and the concept of "ultimate equivalent load balance" has been presented in the paper. By applying this concept, prestressed concrete continuous beams (slabs) are simplified as reinforced concrete beams with axial prestressing and surplus working load. So the influence of secondary moment can not be considered when calculating the bearing capacity of normal section.Calculation diagram and formula of ultimate equivalent load balance method for ultimate strength are developed. By comparing the traditional design method, unified design method and ultimate equivalent load balance method, it has been found that the influence of stress increment of prestressed tendon on secondary moment had been neglected in traditional design method and unified design method for prestressed concrete continuous beams. Influence of primary moment, secondary moment, axial prestressing and vertical load on moment modification is discussed; the calculation formula of concrete structure and prestressed concrete structure moment modulation coefficient in American code is analyzed, and the results of some model experiments inside china are calculated and analyzed. The function which can be applied to ultimate equivalent load balance method is develped for calculating moment modulation coefficientβin critical section of the support, where the reciprocal of relative depth of compression zone is treated as independent variable. The "ultimate equivalent load balance method" used for plastic design unifies prestressed concrete and reinforced concrete continuous beams (slabs).