CACULATION ON LONG-TERM DEFLECTIONS OF FULLY PRESTRESSED CONCRETE BEAMS

Compared with partially prestressed concrete beams, fully prestressed concrete beams stays compressed status of whole section with higher stresses, which results in excessive long-term deflections due to creep and shrinkage of concrete. Based on the Age-adjusted Effective Modulus Method (AEMM), the beam was divided into several elements along the span and the length of the element was one to two times the height of section. The changes of strain and curvature due to creep and shrinkage of concrete and relaxation of prestressed tendon were first artificially restrained by application of an axial force and a bending moment on the nodes of the elements. Subsequently, these restraining forces were removed by the application of equal and opposite forces on the nodes of the elements, resulting in changes in strain and in curvature. A theoretical formula for long-term deflection of fully prestressed concrete beam at the mid-span was derived. Base on the test results of our group and other researchers, the calculated values using the suggested formula were compared well with the test results. However, the calculated errors using design codes were larger, which resulted in unsafe results.