Prestress reduction

A more important effect of prestressiag is its effect on the mean stress at the bore of the cylinder when an internal pressure is appHed. It may be seen from Figure 6 that when an initially stress-free cylinder is subjected to an internal pressure, the shear stress at the bore of the cylinder increases from O to A. On the other hand, when a prestressed cylinder of the same dimensions is subjected to the same internal pressure, the shear stress at the bore changes from C to E. Although the range of shear stress is the same ia the two cases (distance OA = CE), the mean shear stress ia the prestressed cylinder, represented by point G, is smaller than that for the initially stress-free cylinder represented by point H. This reduction in the mean shear stress increases the fatigue strength of components subjected to repeated internal pressure. 

Reduction in prestress due to stress relaxation under continuously applied load can lead to looseness of joints or leakage of seals. 

Prestressed-brick linings can be used to protect the outer shell. In this case, the bricks are installed with a special thermosetting-resin mortar. After lining, the vessel is subjected to internal pressure and heat. This expands the steel vessel shell, and the mortar expands to take up the space. The pressure and temperature must be at least as high as the maximum that will be encountered in service. After the mortar has set, reduction of pressure and temperature will allow the vessel to contract, putting the brick in compression. The upper tem-... 

In order to obtain high yield strength values, wires can be cold worked, hot rolled, or quenched and tempered (Table 15.2) [3,4]. Cold-worked prestressing steel wires are obtained by drawing wires of steel with a ferritic-perlitic microstructure at room temperature, so that the reduction in the cross section leads to an increase of strength. Prestressing bars with diameter up to 36 mm are manufactured by... 

There are three known side effects of ECE. The first is the acceleration of alkali silica reactivity (ASR), another is reduction in bond at the steel concrete interface. A third issue is hydrogen evolution. Hydrogen evolution is inevitable with ECE and so this process must not be used on steel used for prestressing. 

Whilst the work of Boothby and Williams has shown that meaningful reductions In the trace element content can be made, albeit at a price. It Is unlikely that complete elimination can be achieved. Additionally, even assuming the elimination of reinforcement and prestressing tendons from the active zone. It would appear from discussions with liner designers that they see no alternative to a steel liner securely fixed to the vessel concrete. There will also always be steel penetrations connected to the liner. Thus the emission of gamma radiation from steel In activated concrete will be a persistant problem to some degree. 

Only damage that produce stiffness reductions can be identified. For instance, a loss of prestress will only result in a measurable change in eigenfreqnencies if it is accompanied by initiated cracks. 

The bent cap beams were modeled as linear elastic beam-column elements with geometric properties developed using effective width of the cap beam and reduction of its stiffness due to cracking. The superstructnre consists of prismatic prestressed concrete members and was also modeled as linear elastic beam-[Pg.438]

The purpose of a model test is to verify the methods of analysis and to provide a visible physical demonstration of the adequacy of the design requirements. As described earlier, model tests have been carried out to verify the service and ultimate state behaviour of vessels and to assess prestressing, reinforcement and liner requirements. The scale chosen for a specific model depends on the objective of the test and on the reduction in size of important vessel components. For a complete vessel a suitable scale varies from 1 25 to 1 10, and scales of 1 25 and 1 50 have been used for the investigation of the top slab behaviour. A large number of models for shear modes of failure have been tested with... 

The low compressive stresses obtainable by thermal toughening and the reduction of prestressing in daily use are the reasons why low-expansion glass ceramics are usually not prestressed by thermal toughening. 

Protect against stress corrosion of prestressed reinforcement in concrete by careful reduction of stress, by elimination of corrosion through good concreting practice, and by appropriate protection of embedded steel. Note Protect reinforcement cables awaiting full stressing and grouting. 

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