The following figure shows the common types of precast sections. Hence, it is not as common as reinforced concrete. The following classifications are discussed. Source of prestressing force This classification is based on the method by which the prestressing force is generated. There are four sources of prestressing force: Mechanical, hydraulic, electrical and chemical.
Devdas Menon External or internal prestressing This classification is based on the location of the prestressing tendon with respect to the concrete section. Pre-tensioning or post-tensioning This is the most important classification and is based on the sequence of casting the concrete and applying tension to the tendons. Linear or circular prestressing This classification is based on the shape of the member prestressed.
Full, limited or partial prestressing Based on the amount of prestressing force, three types of prestressing are defined. Uniaxial, biaxial or multi-axial prestressing As the names suggest, the classification is based on the directions of prestressing a member.
The individual types of prestressing are explained next. Source of Prestressing Force Hydraulic Prestressing This is the simplest type of prestressing, producing large prestressing forces.
The hydraulic jack used for the tensioning of tendons, comprises of calibrated pressure gauges which directly indicate the magnitude of force developed during the tensioning.
Mechanical Prestressing In this type of prestressing, the devices includes weights with or without lever transmission, geared transmission in conjunction with pulley blocks, screw jacks with or without gear drives and wire-winding machines. This type of prestressing is adopted for mass scale production. Devdas Menon Electrical Prestressing In this type of prestressing, the steel wires are electrically heated and anchored before placing concrete in the moulds.
This type of prestressing is also known as thermo- electric prestressing. External or Internal Prestressing External Prestressing When the prestressing is achieved by elements located outside the concrete, it is called external prestressing. The tendons can lie outside the member for example in I-girders or walls or inside the hollow space of a box girder.
This technique is adopted in bridges and strengthening of buildings. In the following figure, the box girder of a bridge is prestressed with tendons that lie outside the concrete. Figure Hurst Book Free Download. Lin, Ned H. Burns Book Free Download. Please enter your comment! Please enter your name here. You have entered an incorrect email address! Devdas Menon 1 There is no secondary moment in the spans due to the prestress. The M2 diagram coincides with the M1 diagram.
The shift of the pressure line due to external loads can be measured from the profile directly. A concordant profile can be developed from the moment diagram due to external loads for a certain load combination using the following theorem. Theorem Every real moment diagram for a continuous beam on non-settling supports produced by any combination of external loads, whether transverse loads or moments, plotted to any scale, is one location for a concordant tendon in that beam.
The theorem can be proved based on the condition of no deflection at the supports due to external loads. Also, for a concordant profile since there is no reaction at any support, there is no possibility of deflections at the supports.
Thus, it is easy to obtain a concordant profile from the moment diagram of the external loads for a certain load combination, drawn to a certain scale. The following figure shows the steps of the development of concordant profile from the moment diagram. Devdas Menon Discussion The computation of the concordant profile helps in the layout of the tendon profile. The tendon profile need not be designed to be a concordant profile.
It should be such that the stresses in concrete at transfer and at service are within the allowable values. If a concordant profile is selected then the calculations become simpler. The steps are as follows.
For the preliminary design, the type and depth h of the section can be selected based on architectural requirement and deflection criteria. Revise the section if necessary. The zone between emax and emin along the length of the beam is the limiting zone. The equations of emax and emin are same as that for a simply supported beam. The value of P0 can be estimated from Pi as follows. Devdas Menon b. Equal to Pi for post-tensioned members.
The value of Pi can be estimated as follows. If the profile is a concordant profile, the pressure line due to prestress coincides with the profile of the CGS. Some of the key features of the book are: 1. Discussion on limit state design of full and partial prestressed concrete beam. Detailed examples of design of small to long span beams, water tanks, cylindrical shells, folded plates, domes and bridges are illustrated. Latest codes of practice are incorporated and comparison of the designs based on different methods and illustrated.
Economical analysis and the influence of design variable and methods on cost are discussed.
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