1.
A
pretensioned beam, 200 mm wide and 300 mm deep, is prestressed by 10 wires of
7mm diameter initially stressed to 1200 N/mm2, with their centroids
located 100 mm from the soffit. Find the maximum stress in concrete immediately
after transfer, allowing only for elastic shortening of concrete.
If the concrete
undergoes a further shortening due to creep and shrinkage while there is a
relaxation of 5 percent of steel stress, estimate the final percentage loss of stress
in the wires using the Indian standard code regulations, and the following
data:
Es = 210 kN/mm2.
Modulus of elasticity of concrete is considered from the Indian Standard code regulations,
for a compressive strength of 42 N/mm2. Creep coefficient is
considered for the age of loading at 28 days. Assume any other missing
2. Explain
the general principles of prestressing and advantages of Prestressed concrete
3. In
a post-tensioned beam of length 12 m, a cable is laid symmetrically with its
central 6 m length horizontal and the two straight end portions sloping up at
an angle with the horizontal whose tangent is equal to 0.075. The cable is
tensioned by jacking at one end and is anchored at the remote end of the beam.
At the jacking end the measured stress is 1040N/mm2. The wobble
coefficient K may be assumed as 0.004/m. Calculate the stress in the cable at
the remote end and the two points where the alignment changes. Assume the
coefficient of friction between cable and duct of 0.40. What is the percentage
loss of prestress between the jacking end and the anchored end
4. Explain
the different systems of Prestressing with neat sketches
5. A
prestressed concrete beam of section 200 mm wide by 300 mm deep is used over an
effective span of 6 m to support an imposed load of 4 kN/m. The density of
concrete is 24 kN/m3. (Assume any other missing data). At the centre
of span section of the beam, find the magnitude of:
a)
The concentric prestressing force necessary for zero fibre - stress at the
soffit when the beam is fully loaded; and
b)
The eccentric prestressing force located 100 mm from the bottom of the beam
which would nullify the bottom fibre stresses due to loading.
6. An
unsymmetrical I-section is used to support an imposed load of 2kN/m over a span
of 8m. The sectional details are top flange 300 mm wide and 60 mm thick, bottom
flange 100 mm wide and 60 mm thick, bottom flange 100 mm wide and 60 mm thick;
thickness of the web = 80 mm; overall depth of the beam = 400 mm. At the centre
of the span, the effective prestressing force of 100 kN is located at 50 mm
from the soffit of the beam. Estimate the stresses at the centre of span
section of the beam for the following load conditions:
(a)
Prestress
+ Self-weight
(b)
Prestress
+ Self-weight + Live load
