At several locations in the span (i.e. third or quarter points) the tendons are deviated to the correct tendon profile by concrete deviators in external prestressing. The advantages of external prestressing are listed below:
(i) Owing the absence of bond between the tendon and structure, external prestressing allows the removal and replacement of one or two tendon at one time so that the bridge could be retrofitted in the event of deterioration and their capacity could be increased easily. This is essential for bridges in urban areas where traffic disruption is undesirable.
(ii) It usually allows easy access to anchorages and provides the ease of inspection.
(iii) It allows the adjustment and control of tendon forces.
(iv) It permits the designer more freedom in selecting the shape of cross section of bridges.
(v) Webs could be made thinner so that there is a reduction of dead load.
(vi) It enhances a reduction of friction loss because the unintentional angular change like wobble is eliminated. Moreover, the use of polyethylene sheathing with external prestressing has lower friction coefficient than corrugated metal ducts in internal prestressing.
(vii) Improvement of concrete placing in bridge webs owing to the absence of ducts.
The major distinction between internal prestressing and external prestressing lies in the variation in cable eccentricity. The deflected shape of external tendons is not exactly the same as beams because the
displacement of external tendons is controlled by deviators. This is a second order effect at working load and it is very important at ultimate load.
Based on past research, for small span with shallow cross section (i.e. less than 3m deep), the use of internal prestressing requires less steel reinforcement. However, for deeper bridge cross section, the employment of external prestressing results in smaller amount of steel reinforcement.
This question is taken from book named – A Closer Look at Prevailing Civil Engineering Practice – What, Why and How by Vincent T. H. CHU.
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