Computation Method Of Time-dependent Structure Behavior In Tri-axial Condition

Structure time-dependent behavior concerns alteration of structure inner forces and displacements with time. Its main effects manifest in redundant forces of statically indeterminate structures. There is close relationship between these effects and material properties. On one hand, concrete creep and shrinkage could initial inner forces redistribution in structures. On the other hand, relaxation of pre-stressed cable made by high strength steels in steel-concrete structures can produce inner force loss.Long term researches on concrete creep and shrinkage have been carried out by foreign scholars. Chief mathematic models of concrete creep and shrinkage include the EM model, RC model, RF model and TB model. There is also pseudo TB model for steel relaxation. The combination of TB model and plane finite element theory has gained wide application in industrial projects. But these models are based on uni-axial material time-dependent effects and experiment data, differences exist between theories and practice.A mathematic model for structure time-dependent behavior is proposed. Based on elastic superposition principle and space finite element theory, time-dependent equilibrium equation in tri-axial condition is deduced firstly. Then, elastic stiffness matrix, additional creep stiffness matrix and time-dependent load distribution are introduced with the application of tetrahedron element.Finally, MATLAB programs of two examples are formulated for verification. The first example concerns time-dependent behavior of a steel-concrete structure with the same age. The second example deals with a continuous bridge, which includes construction processes and completion stage. The computation results indicate a good agreement of the tri-axial time-dependent model and former results.