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
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