G.15.5 Prestress and Poststress Member Load
BETA
angle when rotating the member coordinates, if necessary. Two types of prestress load specification are available; PRESTRESS
, where the effects of the load are transmitted to the rest of the structure, and POSTSTRESS
, where the effects of the load are experienced exclusively by the members on which it is applied.-
The cable is assumed to have a generalized parabolic profile. The equation of the parabola is assumed to be
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The angle of inclination of the cable with respect to the local x-axis (a straight line joining the start and end joints of the member) at the start and end points is small which gives rise to the assumption that
sinθ = θ = dy/dxHence, if the axial force in the cable is P, the vertical component of the force at the ends is P(dy/dx) and the horizontal component of the cable force is,
P[1 - (dy/dx)2]1/2Users are advised to ensure that their cable profile meets this requirement. An angle under 5 degrees is recommended.
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The member is analyzed for the prestressing / poststressing effects using the equivalent load method. This method is well documented in most reputed books on Analysis and Design of Prestressed concrete. The magnitude of the uniformly distributed load is calculated as
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The force in the cable is assumed to be same throughout the member length. No reduction is made in the cable forces to account for friction or other losses.
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The term
MEMBER PRESTRESS
as used in STAAD.Pro signifies the following condition. The structure is constructed first. Then, the prestressing force is applied on the relevant members. As a result, the members deform and depending on their end conditions, forces are transmitted to other members in the structure. In other words,PRE
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The term
MEMBER POSTSTRESS
as used in STAAD.Pro signifies the following condition. The members on which such load is applied are first cast in the factory. Following this, the prestressing force is applied on them. Meanwhile, the rest of the structure is constructed at the construction site. Then, the prestressed members are brought and placed in position on the partially built structure. Due to this sequence, the effects of prestressing areexperienced
by only the prestressed members and not transmitted to the rest of the structure. In other words,POST
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As may be evident from Item (6) above, it is not possible to compute the displacements of the ends of the
POSTSTRESSED
members for the effects of poststressing, and hence are assumed to be zero. As a result, displacements of intermediate sections (See SECTION DISPLACEMENT command) are measured relative to the straight line joining the start and end joints of the members as defined by their initialJOINT COORDINATES
.
See TR.32.5 Prestress Load Specification for details.