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)
-

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.

The anchorages and exposed free lengths of cables (left for de-tensioning) shall be properly housed in steel boxes and filled with protective agents such as nuclear grade grease or other suitable material.

View of the Condet Bridge after Strengthening

Kemlaka Gede Bridge after Strengthening

1. Minimal disruption to traffic
2. Low weight of the additional component
3. Speed and short duration of construction
4. Low costs involved
5. Future re-stressing operations could be carried out quickly and conventionally if required.

Some projects are listed here involving application of external pre-stressing in strengthening of projects.
a) Flexural Strengthening of Bridges Girders

b) Shear Strengthening of Headstocks or Pier Caps

Shirarika River Bridge Made of HLA Concrete

Transparent Sheaths used for Inspection of Grouting

The use of pre-stressing concrete box girder bridges with corrugated steel webs is being constructed widely considering the cost effectiveness to construct new bridges. In these structures, the concrete webs are placed with corrugated steel plates to reduce the self weight which also simplifies the construction.

Figure below shows the typical section of a pre-stressed concrete bridge with corrugated webs. Basically, the pre-stress is provided by means of bonded tendons arranged in the upper and lower flanges of concrete slab and unbounded external tendon for webs, since the webs are made of steel plates. It is studied and concluded that a reduction of self weight of about 25% can be achieved in this kind of structures compared to conventional PSC box girder bridges.

Number of bridges with corrugated steel webs has been constructed worldwide. Some examples of such bridges constructed will these techniques are described below:

Typical section of a Pre-stressed concrete Bridge with Corrugated Webs

The Givizon-Miyuki Bridge has five spans with a total length of 210m, which was the first bridge to be constructed in Japan using incremental launching method. Weathering steel plates were used for the web. The feature of this bridge is that stud dowel connectors were adopted for the connection between concrete and steel plate. Further, the connection between steel plates in the direction of the bridge axis was by means of bolt connection of the end plates. This method was adopted considering the aesthetic appearance of the bridge.

The Shivasawa Bridge has a span of 50m was also constructed in Japan using supported formwork. The unique feature of this bridge is that “perforbard” connection was adopted for the first time in Japan to connect the steel web and lower flange of concrete slab.

The Kogawauchi-gawa Bridge with a total length of 160m was constructed by the cantilever launching method. Pointed steel plates were used for the web.

The above examples illustrate by the use of external pre-stressing how composite bridges can be designed to enhance better performance by effective use of materials such as steel and concrete.

Complete View of Shirasawa Bridge

(i) In the first type, the unbounded pre-stressing tendons/cables comprise the total pre-stressing force. Segmentally constructed bridge and cast-in-situ box girder bridge superstructure using external cables are an example of this category.

(ii) In the second type, the unbounded elements act as part of total pre-stressing force, the other part being provided by bounded pre-stressing elements embedded in concrete. External pre-stressing provided for repair/rehabilitation/strengthening of existing PSC bridge is an example of this type. The external pre-stress may or may not contribute to the enhancement of ultimate strength.

(iii) In the third type, the external pre-stressing is used as a means of applying external load acting in the desired direction and quantum and the deformation of the structure due to subsequent live load-both in service and under ultimate condition do not affect the magnitude and direction significantly. The external pre-stressing is taken as equivalent load only without considering its contribution to the strength of the structure. This type of application is used in strengthening of existing RCC structure by using pre-stressing elements to relieve part of the dead load effects.

tendon placement in external post-tensioning

(ii) Additional cost for ducts and anchorages.

(iii) Diffusion of Post-Tensioning forces.

(iv) External tendons are more sensitive to fire and more easily attackable by the vandals.

(v) The pre-stressing force disappears over the overall length of the tendon once a section of tendon is broken.

(vi) External tendons are protected by HDPE ducts which are fully cement grouted or wax filled which result in a higher initial material cost for the pre-stress system.

(vii) Under ultimate bending conditions external tendons require more pre-stressing to generate the same moment of resistance.

(viii) Anchorage points and deviators are subjected to high concentrations of forces which need to be properly tied into the structure.