Comparison of External and Internal Prestressing System
External pre-stressing offers designers the potential to create structures that have a better durability and that can be easily be retrofitted in the case of deterioration or to increase their capacity. This is a very important characteristic for urban bridges for which traffic interruption pose a real problem.
It is generally thought that a change from internal to external prestressing results in a significant increase in the amount of reinforcement required. A comparative study was performed on a realistic model of a five span continuous single cell box girder by Mr. Oliver Burdet and Mr. Marc Badoux (paper published in 16th Congress of IABSE, 2000). The one design is being done with internal prestressing and the other with external prestressing. The parameter investigated are the bridge span and depth of the girder. They performed preliminary design only to determine the amount of pre-stressing of flexural reinforcement and of shear reinforcement.
An extensive study is carried out on a realistic model of a typical highway bridge. The structure is five span continuous box girder bridge with a constant depth. The side span length is kept as 85% of inner span length. The tendons are laid out in a classic parabolic shape for the internal prestressing and in a trapezoidal layout with two deviators in the span for external prestressing as shown in figure given below. The cross section (shown below) has a constant depth, with a thicker bottom flange on intermediate supports. The webs of the bridge with external prestressing are thinner then the webs of the bridge with internal prestressing (0.5m) to account for easier concreting conditions without web tendons. The bottom flange is 0.2m thick in the span and 0.4m over intermediate support.
The pre-stressing was determined on the basis of serviceability criteria in both cases to reach a level of deflection allowable. This level represents the amount used in many structures and typically assures a satisfactory behavior in service. The amount of prestressing required in external prestressing system depends upon the location of deviators in the span. The optimal location for these deviators is at 3/8th and 5/8th of the span but in this study, the span deviators are kept at 1/3 of the span which is close to the optimum. The amount of prestressing required to achieve the same level of deflection with a parabolic cable would be larger.
Parametric study
The comparative parametric study was performed using a simple analytical model of each structure. The two main parameters of the investigation are:-
- Girder span ( 30 to 80m)
- Girder depth (girder span to depth ration as15 to 25)
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The figure given below shows the required around of prestressing force as a function of main bridge span for bridges with internal and external prestressing. It is clear from the figure that external prestressing requires large amount of pre-stressing force as compare to internal pre-stressing while in larger span external prestressing system require typically less pre-stress for span exceeding 35m to 60m.
Figure below indicates that required amount of prestresing is with internal prestressing system seems lower for girder depth smaller than 3m and larger above that value. In most cases, these differences are small and it can be concluded that from a serviceability point of view, the two prestressing types are very close to one another.
The design of the bridge was then completed to satisfy, ultimate limit state safety requirements. At this limit state, external tendons have a disadvantage because they cannot reach their ultimate capacity. Taking into account the amount of severability prestressing determined earlier and the minimal reinforcement required, it has been found that bridges with internal prestressing required no additional ULS reinforcement while bridges with external tendon require this type of reinforcement. The amount of additional ULS reinforcement increases with the span of the bridge. It must be noted that requirement of ULS reinforcement is different frame, code to code.
The amount of shear reinforcement is not very different for both types of prestressing as figure above shows. Bridges with external prestresing have thinner webs which decreases dead load but also decrease the web shear resistance. These two effects partially compensate one another leaving a relatively small advantage to the solution with internal prestressing.
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