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# Permissible stresses for flexure member

Civil - Prestressed Concrete Structures - Design Concepts

Permissible stresses for flexure member

Steel - Cl 8.5.1

Steel stress for pre-tensioned tendons immediately after transfer or post-tensioned tendons after anchorage is: Where f pi = Maximum initial pre-stress, and f pu = Ultimate tensile stress in tendon.

Concrete in compression - Cl 22.8.2.1, 22.8.1.1

Concrete stress after transfer and before losses in extreme fiber

Compression = 0.54 fck to 0.37 fck (for M30 to M60) for post-tension

= 0.51 fck to 0.44 fck (or M40 to M60) for pre-tension

Concrete stress at service loads after transfer and after losses in extreme fiber

Compression = 0.41 fck to 0.35 fck (for M30 to M60) for post-tension

= 0.34 fck to 0.27 fck (or M40 to M60) for pre-tension

Concrete in tension - Cl 22.7.1

Concrete stress after transfer and before losses in extreme fiber

1. For Type 1 members, Tension = 0.

2. For Type 2 members, Tension = 3.0 MPa to 4.5 MPa

3. For Type 3 members, Tension = 4.1 MPa to 4.8 MPa

Concrete stress at service loads after transfer and after losses in extreme fiber

Tension = same as at transfer before losses

Example

Depth not restricted - beam

Design a post-tensioned beam of le = 12 m to carry a live load of 12 kN/m throughout its length. The  width of beam b = 250 mm. f ct = f cw=  - 17MPa and ftt = f tw = 1.4MPa . h= 0.85.    Depth restricted - slab

A post-tensioned concrete bridge slab of le = 10 m is 380 mm thick. It is stressed with parallel cables stressed to 360 kN each. wL = 25 kN/m2. Losses are 20%. Ftt=ftw= 0.7MPa . Calculate the emax and spacing of cable at mid-span.     Depth not restricted - slab

A post-tensioned concrete one-way bridge slab of le = 10 m is stressed with parallel cables stressed to 500 kN each. wL = 25 kN/m2. Losses are 20%.f ct= f cw =-  15MPa and ftt=  f tw = 0   Depth restricted - beam

A pre-tensioned simply supported beam of size 80 mm x 120 mm and le = 3 m caries two 4 kN loads at third points along the span. Losses are 20%. Ftt = 0, f tw = 1.4MPa . Design the beam with 3mm wires for fi 1400 MPa each.  below bottom in heavy girder - beam

An unsymmetrical I section has the following sectional property: h = 1000 mm, A = 345 000 mm2 , Zt = 95 x106 mm3, Zb = 75 x 106 mm3 , cgc = 440 mm from top, MG = 1012 kN-m, ML = 450 kN-m. Design the section if f ct= f cw =- 15MPa and f tt= f tw = 0. h= 0.85  Study Material, Lecturing Notes, Assignment, Reference, Wiki description explanation, brief detail

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