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 f_{ck} to 0.44 f_{ck}
(or M40 to M60) for pre-tension

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

Compression = 0.41 f_{ck} to
0.35 f_{ck} (for M30 to M60) for post-tension

= 0.34 f_{ck} to 0.27 f_{ck}
(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 l_{e}
= 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 f_{tt }=
f _{tw} = 1.4MPa . h= 0.85.

**Depth restricted - slab **

A post-tensioned concrete bridge slab of l_{e}
= 10 m is 380 mm thick. It is stressed with parallel cables stressed to 360 kN
each. wL = 25 kN/m2. Losses are 20%. F_{tt}=f_{tw}= 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 f_{tt}= 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 f_{i} 1400 MPa each.

**below bottom in heavy girder - beam**

An unsymmetrical I section has the
following sectional property: h = 1000 mm, A = 345 000 mm^{2} , Zt = 95
x10^{6} mm^{3}, Zb = 75 x 10^{6} mm^{3} , cgc =
440 mm from top, MG = 1012 kN-m, M_{L} = 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

**Related Topics **

Copyright © 2018-2020 BrainKart.com; All Rights Reserved. Developed by Therithal info, Chennai.