Time-dependent behaviour of composite prestressed concrete beams
Creators: name: Rao, Vadaparty Jagannadha
Publisher: name: University of Calgary
Year: 1973
Subjects: subject: TA 683.5 B3 R35 1973 Microfiche
subject: Concrete beams
subject: Prestressed concrete construction

description: Bibliography: p. 123-126.
description: Key words: composite prestressed concrete beams; long time behaviour, creep; shrinkage; relaxation; untensioned steel, deck reinforcement; super imposed dead load; laboratory; field; prediction; varying stiffness approach, analysis; forces; strains; moments, curvature; camber; loss of prestress; and computer programme. The analysis of .the time-dependent behaviour of composite prestressed composite beams has until now been based upon the solution of a large number of simultaneous equations involving unknown forces. In this present investigation, the author develops, as a main objective, a new rational method called the "varying stiffness approach" for the analysis of the above kind of problem. This is achieved successfully by considering the two basic deformations - normal strain and curvature - as unknowns in each interval of time. All the forces in the elemental areas of the cross section are expressed in terms of these deformations using the elemental stiffnesses and compatibility conditions. Thus the problem is reduced to finding the two unknown deformations from the two equations of equilibrium, which the forces have to satisfy. The forces are determined by using these two deformations and the force-displacement relations of each interval. The parameters considered in this analysis are creep, shrinkage, relaxation of steel, presence of longitudinal web and deck reinforcement, and superimposed dead load. Equations are developed for creep and shrinkage, from tests on prisms; and relaxation charts· are used for the relaxation of steel. A comprehensive computer programme is prepared for the analysis of 6 beams with all the parameters included. A fair agreement is observed between the analytical results and experimental values. A slight discrepancy is observed in the assumption of principle of plane sections, near the interface after composite action and the possible reasons are explained. Trost’s method of approximate analysis has been found to be not sufficiently accurate to be used for this kind of problem. The results of step-by-step analysis using either C.E.B. timedependent parameters, or Branson's standard creep and shrinkage curves, are found to be in good agreement. Both experimental and analytical results show that the role of web or deck reinforcement depends on the superimposed dead load. As a secondary objective, field and laboratory tests were performed on a variety of concrete prisms, to obtain the influence of the environment. Shrinkage and total strains are found to be less in field environment. The influence of the field environment is found to be lesser when the prisms contain reinforcing steel. Finally, it is recommended that time-dependent analysis be made using varying stiffness approach. It is also recommended that minimum deck reinforcement and maximum web reinforcement, be used in the absence of superimposed dead load, and the reverse in the presence of superimposed dead load.