Doctor of Philosophy
Department of Civil and Mining Engineering
Wong, Yen Wen, The deflections of reinforced and partially prestressed concrete box beams under repeated loading, Doctor of Philosophy thesis, Department of Civil and Mining Engineering, University of Wollongong, 1984. http://ro.uow.edu.au/theses/1275
Several major types of civil engineering structure including off-shore platforms and road and railway bridges are subjected to repeated loading. Earlier studies have indicated that such loading causes higher deflections in concrete structures than static loading. This thesis is devoted to the study of the deflection behaviour of reinforced and partially prestressed concrete box beams. In addition to presenting details and results of a comprehensive experimental programme, a simple and reliable procedure is proposed for the analysis of concrete beam deflections under repeated loading.
Thirty reinforced and five partially prestressed concrete box beams (of 1/4 scale) were fabricated and each tested up to 105 repetitions of load. For each beam, the repeated loading range was kept constant with the lower and upper limits set at 30 percent and 50 percent of the yield load to simulate the dead load and service load respectively. For some beams 70 percent or 90 percent of the yield load was used as the upper limit, simulating overloading conditions. The deflections at both loading limits were measured immediately after 1, 10, 102, 103, 104 and 105 cycles of load.
It was found that after the initial loading cycle the bending rigidity of the beams (hence the instantaneous deflection) was not affected by further repeated loading. The dead load deflection on the other hand increased with increasing number of loading cycles although the rate of increase reduced rapidly as the number of cycles became larger. This amplification of dead load deflection by repeated load can be seen as a parallel phenomenon to the time creep of concrete under sustained loads. Because large deflection can accumulate in a short time under repeated loads, the accumulated deflection is referred to herein as the 'intensive creep' deflection. For some lightly reinforced beams the intensive creep deflection was as high as eight times the initial dead load deflection.
The steel ratio, degree of prestressing and the maximum loading level are found to be the important factors influencing the deflection behaviour of concrete beams under repeated loading.
A simple procedure is presented for the analysis of the total deflections of reinforced and prestressed concrete box beams under repeated loading. The total deflection is obtained by summing the instantaneous live load deflection and the accumulated dead load deflection. The latter may be computed as the product of the initial dead load deflection and the intensive creep factor. Following a comparative study, the most suitable method is chosen from 9 well-known procedures for the computation of initial dead load deflection. Based on the experimental data of the initial and accumulated dead load deflections an empirical formula for the intensive creep factor is derived using statistical means. Parallel to the logarithmic time-creep equation, the formula is considered to be more realistic than an alternative hyperbolic model. For computing the instantaneous live load deflection of any loading cycle, a new equation for the effective moment of inertia is proposed. This equation is shown to be superior to the existing formulas.
All equations recommended for use are simple and explicit. In light of the experimental data obtained herein and those by other researchers, comparisons are made with two other recently published methods. It is concluded that the proposed procedure is more versatile in that it accounts for more variables and allows for the predictions of instantaneous deflection and permanent set in addition to the total deflection. In most cases it also gives more accurate results.