In Civil Engineering ( Bridge works) Under what situation shall engineers use jacking at one end only and from both ends in pre-stressing work?

. 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. These frictional losses 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 centerline. 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 a even distribution and to provide symmetry of pre-stress force along the structure. 

In fact, stressing at one end only has the potential advantage of lower cost when compared with stressing from both ends. For multiple spans (e.g. two spans) with unequal span length, jacking is usually carried out at the end of the longer span so as to provide a higher pre-stress force at the location of maximum positive moment. On the contrary, 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 sufficiently long, jacking from both ends should be considered.

it depends on the structural part to be prestressed

I support the answers of Mr. Shakeel and Mr. Kingsbergen.

I am with Mr, Shakeel 

Thanks

If you have a blind end you pre stress is from one end. But normally I pre stress only from one end . The only difference is the end head you use

In my mind need both , for prestressed work 

Friction and imperfect alignment need to be take care.

If you make sure that this effect is taken care off there is no difference.

Sometimes space on site do not allow jack at both end plus cost is also considered.