Research On Ultimate Stress Increment Of Unbonded Prestressed Steel

Unbonded prestressed concrete structure is an important component part of prestressed concrete structures, which has been widely applied in civil engineering. How to calculate the stress in unbonded prestressing tendons at ultimate strength limit state is one of pivotal technical problems during the course of popularizing the unbonded prestressed concrete for long time.Firstly, the paper evaluate unbonded prestressed steel calculational formulae of some country's criterion, and compared with the testing result,then probe into the applicability for those calculational formulae to unbonded prestressed concrete beam under different prestressed steel form.It is dificult to analyze unbonded prestressed beams by commercial finite element programs with conventional methods. To solve this problem, a commercial finite element program, ABAQUS, was used to develop an analysis model for concrete beams prestressed with internal unbonded tendon. The finite element model included the entity element for concrete and truss element for internal unbonded tendon. The coupling of node between the concrete beam and prestressed steel depend on many spring elements with very large stiffness. The modified Risks method is utilized to entire nonlinear analyse of unbonded prestressed concrete beam. The reliability of the analysis model is verified by analytical results of typical test beams incomparison with experimental ones.The influence of various parameters to unbonded prestressed steel ultimate stress increment, including ratio of nonprestressed steel, ratio of unbonded prestressed steel, span-depth ratio, cylindrically compressive strength of concrete, yield strength of nonprestressed steel. Five series 45 broken-line and curvilinear tendon model beams are designed respectively to conduct entire nonlinear analyse, making use of the paper's finite element model. According to the close relationship of those beams' unbonded prestressed steel ultimate stress increment to complex steel index and span-depth ratio, the paper put forward respectively experiential regressive calculational formulae of unbonded prestressed steel ultimate stress increment under broken-line and curvilinear tendon prestressed concrete girder. The reliability of the calculational formulae is verified by analytical results of typical test beams incomparison with experimental ones. Therefore, a sound basis is offered for optimal design of unbonded prestressed beams and for future modification in new code equationofΔσ_pu.