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  • Prestresed Concrete
    • Introduction to prestressed concrete
    • Methods of prestressing
    • Design of PSC members
    • Prestressing force and Ecentricity
    • Ecentricity limits and tendon profile
    • Losses in prestressed concrete structures

  • tanks
    • Introduction to water tanks
    • Load conditions and flexure details for water tanks
    • Minimum and Maximum reinforcement for tanks
    • Slab design for Tanks
    • Design Requirement of Concrete in tanks
    • General design requirements for members of a tank
    • Circular tank Analysis
    • Numerical on the analysis of Circular water tank
    • Design of Rectangular tanks
    • Numerical on Rectangular water tank
    • Underground Tanks
    • Overhead tanks

  • Retaining walls
    • Retaining walls and their types
    • Earth Pressure and Stablity requirements
    • Proportioning and design of cantilever and counterfort walls
    • Numerical on design of a Retaining wall

  • footings
    • Introduction to footings
    • Soil Pressure under Isolated footings
    • General Design Considerations for Footings
    • Special case of Footing
    • Numerical on footings
    • Design of Combined Footing

  • Flat slabs
    • Introduction to flat slabs
    • Proportioning of flat slabs
    • Determination of bending moment and shear force
    • Slab reinforcement
    • Numericals of flat slab

Branch : Civil Engineering
Subject : Design of Concrete Structures-II
Unit : Prestresed Concrete

Design of PSC members


Design of PSC Members
Basis

Sign Convention

In order to derive equations that enable design, we maintain a rigid sign convention:

• Moment:: Positive sag;

• Eccentricity of prestress: Positive above centroid;

• Section Modulus: Negative for the bottom of the member;

• Stress: Positive compression.

With this sign convention, we now have:

Thus the final stresses are numerically given by:

 

Note that the sign convention means that:

• the P/ A terms is always positive;
• the MC /Z term is positive or negative depending on whether it is Zt or  Zb , and;
• the Pe /Z term is negative for Zt since Zt is positive and e is negative and the term is positive for  Zb since now both Zb and e are negative.

These signs of course match the above diagrams, as they should.

Governing Inequalities

Given the rigid sign convention and the allowable stresses in the concrete, and noting that the losses are to be taken into account, the stresses are limited as:

Transfer

Top fibre – stress must be bigger than the minimum allowable tensile stress:

Bottom fibre – stress must be less than the maximum allowable compressive stress:

Service

Top fibre – stress must be less than the maximum allowable compressive stress:

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