【正文】 tudinal pressive stress distribution varying linearly from zero at the top surface to a maximum of concrete stress, = , at the bottom, where is the distance from the concrete centroid to the bottom beam, and is the moment of the inertia of the crosssection, is the depth of the beam. An upward camber is then created. Figure shows the prestressed beams after loads have been applied. The loads cause the beam to deflect down, creating tensile stresses in the bottom of the beam. The tension from the loading is pensated by pression induced by the prestressing. Tension is eliminated under the bination of the two and tension cracks are prevented. Also, construction materials (concrete and steel) are used more efficiently. Circular prestressing , used in liquid containmeng tanks , pipes , and pressure reactor vessels , essentially follows the same basic principles as does linear prestressing . The circumferential hoop . or “hugging” stress on the cylindrical or spherical structure , neutralizes the tensile stresses at the outer fibers of the curvilinear surface caused by the internal contained pressure . From the preceding discussion , it is plain that permanent stresses in the prestressed structural member are created before the full dead and live loads are applied in order to eliminate or considerably reduce the net tensile stresses caused by these loads . With reinforced concrete , it is assumed that the tensile strength of the concrete is negligible and disregarded . This is because the tensile forces resulting from the bending moments are resisted by the bond created in the reinforcement process . Cracking and deflection are therefore essentially irrecoverable in reinforced concrete once the member has reached its limit state at service load .The reinforcement in the reinforced concrete member does not exert any force of its own on the member , contrary to the action of prestressing steel . The steel required to produce the prestressing force in the prestressed member actively preloads the member , permitting a relatively high controlled recovery of cracking and deflection . Once the flexural tensile strength of the concrete is exceeded , the prestressed member starts to act like a reinforced concrete element .Prestressed members are shallower in depth than their reinforced concrete counterparts for the same span and loading conditions . In general , the depth of a prestressed concrete member is usually about 65 to 80 percent of the depth of the equivalent reinforced concrete member . Hence , the prestressed member requires less concrete , and about 20 to 35 percent of the amount of reinforcement. Unfortunately , this saving in material weight is balanced by the higher cost of the higher quality materials needed in prestressing . Also, regardless of the system used , prestressing operations themselves result in an added cost : formwork is more plex ，since the geometry of prestressed sections is usually posed of flanged sections with thin webs .In spite of these additional costs, if a large enough number of precast units are manufactured, the difference between at least the initial costs of prestressed and reinforced concrete systems is usually not very large. And the indirect longterm savings are quite substantial, because less maintenance is needed, a longer working life is possible due to better quality control of the concrete, and lighter foundations are achieved due to the smaller cumulative weight of the superstructure.Once the bean span of reinforced concrete exceeds 70 to 90 feet ( to m), the dead weight of the beam bees excessive, resulting in heavier membersand,consequently,greater longterm deflection and cracking. Thus,for larger spans,prestressed concrete bees mandatory since arches are expensive to construct and do not perform as well due to thesevere longterm shrinkage and creep they large spans such as segmental bridges or cablestayed bridges can only be constructed through the use of prestressing .Prestressed concrete is not a new concept, dating back to 1872,when . Jackson ,an engineer from California, patented a prestressing system that used a tie rod to construct beams or arches from individual block. After a long lapse of time during which little progress was made because of the unavailability of highstrength steel to overe prestress losses, . Dill of Alexandria, Nebraska , recognized the effect of the shrinkage and creep(transverse material flow) of concrete on the loss of prestress. He subsequently developed the idea that successive posttensioning of unbonded rods would pensate for the timedependent loss of stress in the rods due to the decrease in the length of the member because of creep and shrinkage. In the early 1920s, of Minneapolis developed the principles of circular prestressing He hoopstressing horizontal reinforcement around walls of concrete tanks through the use of turnbuckles to prevent cracking due to internal liquid pressure , thereby achieving watertightness . thereafter , prestressing of tanks and pipes developed at an accelerated pace in the United States,with thousands of tanks for water,liquid,and gas storage built and much mileage of prestressed pressure pipe laid in the two to three decades that followed. Linear prestressing continue to develop in Europe and in France,in particular through the ingenuity of Eugene Freyssinet,who proposed in 192328 methods to overe prestress losses through the use of highstrength and highductility ,he introduced the now wellknown and wellaccepted Freyssinet system. . Abeles of England introduced and developed the concept of partial prestressing between the 193