| In this paper, significance of tensioning control is discussed. By investigating existing control methods of prestress tensioning, the viewpoint, that the traditional tensioning technology has many disadvantages and is an obstacle of development of prestressed structures, is presented. Therefore, this tensioning technology seems to not be consistent with the rapid advancement in design of prestressed concrete structures. Taking full advantage of leading edge scientific and technical developments to improve traditional prestress tensioning technology is an urgent need in prestressed concrete construction. Just by combining computer technology and civil engineering, this paper originally presents the concept of digital prestress tensioning.According to characteristics of modern information techniques and prestressed structures, following two Solution schemes of digital tensioning are presented. Scheme one: Utilizing a common stressing device assisted by an intelligent control instrument, which is in turn composed of compound sensor, displacement sensor, control and display instrument, achieves dual open-loop control of the prestress tensioning. Scheme two: By altering the common stressing device to make messages exchangeable between hydraulic equipment and an intelligent control device, closed-loop control of the tensioning process is achieved. In essence, a prestress tensioning robot, which can finish the tensioning process by pressing only a button, will bemanufactured.In this paper, the intelligent control instrument is successfully manufactured and a set of corresponding technology procedure is presented. The digital tension system has been appraised by a group of experts, and the result of appraisal is that the digital tension system supplies a gap of prestessed concrete structures and up to international advanced level. Besides, the system has been patented (Patent No. ZL00262315.3). The system's application in a prestressed concrete structure shows that digital tensioning technology can achieve dual synchronous control of stress and elongation, and can control prestress tensioning with high accuracy (within 1 % error) and efficiency (reducing manpower by as much as 60%).Computing method of tensioning elongation with the effects of lateral constrains and axial deformations is studied. Besides, a computer program is presented on calculating instantaneous prestress losses and tensioning elongation, which can be used conveniently in design or in construction and can be solidified in close-loop digital tensioning system.In the experiment in-situ, the instantaneous losses and interrelation between different tendons in tensioning are tested. Especially, the change of effective prestress under periodical environmental temperature conditions is monitored hour by hour. The tested results show that effective prestress have great relativity with environment temperature and concrete's temperature, and so temperature action on prestressed structures should be included in design and construction. On the base of these tested results and other experimental data of environment temperature, main factors which affect the temperature distribution are analyzed, the viewpoint that periodical environment temperature can be simplified as a syntonic curve is presented. A computation model of inner temperature field in prestressed concrete members is built upaccording to heat transmission theory. And the heat transmission model is computed with FEM. Then the paper analyzed rules of difference in temperature. Different in temperature is divided into transverse temperature difference and lengthways difference, and corresponding formulas are respectively presented. According to different transverse difference in temperature and lengthways difference in temperature, the paper gave the methods of computing temperature effect respectively, these methods can be straightly used in computation or design of prestressed concrete structures. |
PDF Full Text Request