Prestressing Ratio Research Articles

Optimization of Prestressed Concrete Flat Plates

Mathematical programming techniques are used for the solution of a comprehensive optimal design problem formulation. The formulation, based on American codes of practice, includes constraints related to service, ultimate, and collapse limit states, as well as relevant technological constraints. Solutions to the nonlinear programming problem are obtained with an appropriate computer program, OPTIMAL. This is used for solving a wide range of typical flat plate designs with varying span‐to‐depth ratios, live loads, cable layouts, limit states, and allowable tension stresses. A related sensitivity study enables the comparison of optimal and standard solutions based on the following parameters: (1) Rate of equivalent prestressing and total external load; (2) average concrete stress in compression; (3) active design constraints; (4) material costs; (5) dead to total external load ratio; (6) reinforcement index; and (7) partial prestressing ratio. Some areas of possible improvement of current standard procedures are identified.

Static and Fatigue Tests on Partially Prestressed Beams

The fatigue behavior of 12 different sets of partially prestressed concrete beams was experimentally investigated. All beams were rectangular, 9×4.5 in. (229×114mm) in cross section, simply supported on a 9 ft (2.74 m) span and loaded in 4 point bending. Each set consisted of 2 identical specimens designed with the same input parameters. One control beam was tested under static load up to ultimate. The second beam was tested in cyclic fatigue at a constant load range varying between 40% and 60% of the ultimate load capacity of the static specimen. The main input variables were the partial prestressing ratio PPR and the reinforcing index ω¯. Four different levels of PPR and three different levels of ω¯ covering both fully prestressed and fully reinforced were explored. Typical results and observed trends are described. Throughout the tests, measurements of strains in the reinforcement, deflections, crack widths, curvatures and their variation under static and cyclic fatigue loading were systematically recorded. The six partially prestressed and the three fully reinforced beams survived 5,000,000 cycles without suffering fatigue failure. The three fully prestressed beams which were loaded beyond cracking failed, respectively, at 1.21×106,2.17×106 and 1.94×106 cycles.

Compactive Prestress in Shales

The term shale encompasses materials displaying different responses during excavation, compaction and service life. A soft, non-durable shale is easily broken down and may be Compacted as a soil fill. A hard, durable shale may perform satisfactorily when placed as a rock fill. An intermediate shale may be mechanically hard during excavation and compaction but may prove to be non-durable over the service life of a compacted embankment. The New Providence shale used in this study is classified as a medium hard, non-durable shale. Shale samples were compacted from shale aggregate by a California kneading compactor. Conventional oedometer tests were performed on a group of as-compated samples to investigate the compressibility behavior and the compactive prestress induced in the samples. At pressures above the prestress the compressibility of the compacted soil markedly increases. The effect of the compaction variables on the observed behavior of the samples was of interest. These variables were: compacted dry density, molding moisture content, initial void ratio, initial degree of saturation, and nominal compactive pressure. It was found that the prestress was equal to the compaction pressure for the low and intermediate compactive efforts at the dry-of-optimum and near-optimum moisture conditions. Increasing the compaction pressure or the moisture condition resulted in a decrease in the ratio of prestress to compaction pressure to less than unity. Increasing moisture reduces the air permeability and permits the buildup of pore air and pore water pressures during compaction which results in less of the compaction pressure being applied to the soil skeleton. Increasing compaction pressure causes greater aggregate degradation during compaction. Some of the compaction effort is spent breaking aggregates which results in the reduction in the number of large pores. Statistical analysis of the accumulated test data permitted a descriptive model to be developed for the estimation of the compactive prestress for samples of compacted New Providence shale. The most influential variables affecting the level of compactive prestress appear to be the nominal compaction pressure and the molding moisture content.

Design Of Partially Prestressed Concrete Beam-Column Connection With Singly PrestressedSections Under Cyclic Loadings

Prestressed Concrete framed structure are commonly designed to provide presstressing steel to resist gravity loads. Therefore most of the prestressing tendons are located at the top of the beam at beam-column connections. Experimental and numerical non-linear finite element studies developed by the author show that these types of structures are capable to resist severe cyclic loading provided sufficient care is given. Special attention must be considered in the limitation of ductility demand, partial prestressing ratio and detailings in the potential plastic hinge regions. In this paper a simple and semi-rational design method for partially-prestressed interior beam-column connection in proposed. As the behavior of connections are non-flexural, the design method developed is based on major and minor strut modeling using plasticity theory.

THE STRENGTH OF SLENDER PRESTRESSED CONCRETE COLUMNS

Concrete columns have been the subject of research for many years. However, little is known about the behavior of slender columns and, in particular, of slender prestressed concrete columns. The increased use of such columns has encouraged research, both theoretical and experimental, in order to determine the effects of many variables on their behavior and strength. Some aspects of such an investigation(') are described in this paper. Theoretical failure loads are presented, obtained from the analysis of some 300 columns. These were slender, axially pretensioned, eccentrically loaded columns, with equal end eccentricities, and with a possible initial deflected shape (Fig. 1). The variables considered were prestress, eccentricity, slenderness, concrete compressive strength, concrete tensile strength, initial curvature, and area of steel. The details of the cross-section used in the analysis, are shown in Fig. 1. They also represent the typical dimensions of 36 columns, tested to substantiate the theory. The good agreement obtained between experimental and theoretical results(') provides confidence in the theoretical analysis considering additional variables. The variables investigated experimentally !were eccentricity (e/d = 1/8, 3/4 and 2), slenderness (Lid = 20, 30 and 40), and prestress (f ,y/ f ' = 0.1, 0.3 and 0.5). Since the prestress losses were somewhat underestimated, the above prestress ratios are nominal values. The average values of the actual prestress ratios were 0.09, 0.25 and 0.41, and these were used in the theoretical analysis. The theoretical analysis is based on a finite element approach and uses iteration and suitable numerical methods. A computer program was written which gave, for each particular column, the buckling load based on the cotangency criterion, and the full load-deflection and Ioadstrain curves. The mode of failure, i.e. instability or material failure, was also indicated. The deflected

The influence of transverse prestress on the strength of prestressed concrete bridge slabs

Summary The paper describes the results of tests on eight square prestressed concrete slabs having unbonded wires. The ratio of transverse prestress to longitudinal prestress (γi,) was varied hetween 0 and 100%. A system of wheel loads based on AASHO specifications was used and it was found that, for the upper range of values of γi ultimate loads were considerably in excess of those predicted by the use of yield-line theory. It is suggested that at least part of this increased load capacity could be utilized if a suitable design criterion were adopted.

Stress systems in the vicinity of lunar craters

Observations of the distribution of ridge-systems around certain lunar craters are discussed in terms of a simple theory of rock-failure. It is shown that, at the time of formation of these craters, the surface layers of the Moon were stressed in tension. Solutions are found for the propagation index, a property of the lunar rocks; and for the ratio of the horizontal tensile pre-stress in the Moon to that required to fracture the rocks. A measure of the relative force associated with the origin of two craters is obtained. Consideration of several craters suggests that the horizontal tensile stress does not vary greatly from point to point. Of the two principal theories of crater formation, a quiescent process is favoured because it explains certain observations that the explosive theory does not.

12.プレストレストコンクリートの緊張力減退に関する理論的研究

For the design of prestressed concrete sections, it is important to estimate the loss in initial prestress due to creep and shrinkage of concrete as well as creep (or relaxation) of prestressing wires. Many theoretical solutions of this problem have been derived on the basis of the theory of creep of concrete. But, they can be applied only to the special cases, for example, to a case where wires are arranged practically in one layer, and they are not perfectly valid for the cases where the section has wires arranged in many layers. The author derived the general solution which could be applied to any type of section with or without non-prestressing steels, described how the loss in prestress was distributed on the section and showed that the effective ratio of prestress was not always equal to the one of prestress moment. Moreover, the conventional solution was derived by modifying the theoretical solution described above.