Q. One-way pre stressing vs two-way pre stressing?

At the time of pre-stressing operation at one end, frictional losses will take place and the prestressing force decreases along the length of tendon till reaching the other end. These frictional losses involve the friction induced because of a change of curvature of tendon duct and also the wobble effect because of deviation of duct alignment from the centreline. Thus the prestress force in the mid-span or at the other end would be greatly decreased in case the frictional loss is high. Thus prestressing, from both ends for a single span which implies prestressing one-half of total tendons at one end and remaining half at the other end is carried out to enable an even distribution and to provide symmetry of prestress force along the structure. 

As a matter of fact stressing at one end only has the potential advantage of lower cost when compared with stressing from both ends. For multiple spans (for example two spans) with unequal span length, jacking is generally carried out at the end of the longer span so as to provide a higher prestress force at the location of maximum positive moment. On contrary jacking from the end of shorter span will be conducted if negative moment at the intermediate support controls the prestress force. Though if the total span length is sufficiently long, jackingfrom both ends must be considered.