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.
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Protection of Pre-stressing Steel/Anchorages/Deviators
Pre-stressing tendons/steel shall be protected during temporarily exposed stages by coating with water salable oils, grease or other suitable means. The permanent protection to the steel shall be provided by the cement grout, nuclear-grade grease or other suitable means. The external exposed steel components of deviations supporting brackets etc shall be protected by clear epoxy paint coating.
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.
Rehabilitation of Bridges with Lightweight Concrete
The deck of the Puttesund Bridge in Norway was rehabilitated by replacing concrete sidewalls with aluminum replacing portion of slab and surfacing with lightweight concrete and installing external pre-stressing. This bridge was original constructed in 1970 and rehabilitated in 2002 to extend its life according to current demands.
Puttesund Bridge, Norway
Shear Strengthening of Headstocks or Pier Cap
The headstocks of the Hawthorne and Morrison Bridges in USA were strengthened by External Pre-stressing. A system of externally mounted post-tensioned bars has been used to increase the compressive strength of the concrete caps thus increasing the load carrying capacity of the head stocks / pier cap. A steel I-beam was used as an anchoring block on each end of the head stocks. Figure below shows the typical application of the external pre-stressing in head stocks / pier cap strengthening.
Morrison Bridge Headstock Strengthening, USA