ABSTRACT
Prestressing is more a philosophy than a specifi c technique. It means preparing a structure to receive a load by applying a pre-emptive countervailing load. For instance, if it is known that a column will be defl ected 100 mm to the left by applied loads, the designer can arrange to bend it 50 mm to the right; the column then only has to be designed to resist a defl ection of ±50 mm, rather than the full 100 mm. A more substantial example is a pin-ended concrete arch that the designer knows will be stressed by sagging moments at mid-span caused by the shortening due to creep, shrinkage and temperature drop. He can pre-empt this load case by jacking the springings together causing moments of opposite sign, Figure 4.1. Clearly, this has to be done under careful control, as he has to take into account all the other load cases, some of which might combine unfavourably with the loads he has jacked into the arch. For instance, the designer must consider the possibility that the arch may be subjected to a temperature rise immediately after his jacking operation, before it has experienced shortening due to shrinkage and creep.
