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By

Dr. Attaullah Shah

Swedish College of Engineering and Technology Wah Cantt.

CE-407

Lec-02

Structural Engineering

The net stress in the prestressed section

The resultant stress due to the combined effect of applied loads and pre-stress force is given as:

At the ends of the beam where there is no moment the MC/I =0 , the stress becomes

Example

Solution

The kern of a section is the region in which a compressive point load may be applied without producing any tensile stress on the cross section.

Shapes of pre-stressed sections:

− Rectangular shape: Simple and easy to pour

− I-Section and T-section: Economical

− Channels and Boxes

Prestress losses

− The initial prestress becomes smaller with time due to following reasons:

A lump sum value has been recommended by ACI Code

Elastic Shortening

Losses due to shrinkage

Relaxation of creep in the tendons

Slippage in post tension and anchorage system

Friction along the duct

Ultimate Stress of pre-stressed section

Assignment No.1