ABSTRACT
In theory, this method works very well. In practice, it was later determined that the stress in each wire might vary due to the cooling of the wire after the friction induced by the die, the speed at which the wire was pulled through the die, and by using worn dies. Die drawing is still used today as a method of prestressing, however, the temperature and force in the wire should be constantly monitored to ensure quality control. Besides die drawing, there are also mechanical methods of prestressing cylinders. Mechanical methods generally employ a means of differential gearing to elongate wire or strand in order to achieve the desired force. The force in these systems can be monitored by special load cells and a feedback loop can send the differential gears continuous instructions in
Beginning in 1935, the East Bay Municipal Utilities District (EBMUD) in California, USA began constructing water storage tanks which combined the emerging technologies of prestressed concrete and thin shells. This combination of technologies allowed water-containing structures to have the advantages of concrete, without problems associated with cracking and leaking. Since that time, hundreds of similar structures have been built in the San Francisco Bay Area, the majority of which have been in continuous service for decades. In the aftermath of the 1989 Loma Prieta Earthquake, a study of existing water tanks was made in order to assure uninterrupted supply in the event of another large earthquake or other emergency. Older prestressed concrete water tanks, designed and constructed before modern seismic design, were designed for the hydrostatic load only. Many of these older tanks were determined to be in a condition which would not be able to provide water immediately after an emergency. EBMUD decided to rehabilitate and retrofit thirty-two existing water storage tanks to modern seismic design standards and to repair defects. To the great benefit of EBMUD, the water storage tanks were successfully retrofitted and rehabilitated in much less time and for much less cost when compared to demolition and new construction. Learning from the process which EBMUD has now completed can be used as a model for other water agencies with similar structures.
