Jacking at one end only and from both ends in pre-stressing

Question

When should engineers use jacking at one end only and from both ends in pre-stressing work?

Answer 1

During pre-stressing operation at one end, frictional losses will occur and the pre-stressing force decreases along the length of tendon until reaching the other end. The frictional losses occurred include the friction induced due to a change of curvature of tendon duct and also the wobble effect due to deviation of duct alignment from the centre line. Therefore, the pre-stress force in the mid-span or at the other end will be greatly reduced in case the frictional loss is high. Consequently, pre-stressing, from both ends for a single span i.e. pre-stressing one-half of total tendons at one end and the remaining half at the other end is carried out to enable an even distribution and to provide symmetry of pre-stress force along the structure.

Stressing at one end only has the benefit of lower cost when evaluated against the stressing from both ends. For multiple spans with span length that is not equal, jacking is usually done at the end of the longer span so as to provide a higher pre-stress force at the location of maximum positive moment. While jacking from the end of the shorter span would be conducted if the negative moment at the intermediate support controls the pre-stress force. However, if the total span length is adequately long, jacking from both ends should be taken into consideration.