Model Question Paper-I

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M.TECH. DEGREE EXAMINATION

Model Question Paper
-
I

First Semester

Branch: Civil Engineering

Specializ
ation


Computer Aided Structural Engineering

MC
ES
E 106.1 PRESTRESSED CONCRETE (Elective


II)

(Regular


20
11

Admissions)

Time: Three Hours










Maximum: 100 marks

Note: i) Answer
all

questions.



i
i) Reference to IS:
1343
and IS:456
code
s

is permitted





i
ii
) Assume suitable data wherever necessary



I



a)

Briefly explain basic concept of prestressing.

b)

Explain pre
-
cracking and post
-
c
racking.

c)

what is " Pressure line or Thrust line" ? explain its significance.

d)

A prestressed concrete beam 500 mm wide and 750 mm deep has a simply supported
span of 7m. It is prestressed with a linearly bent tendon with zero eccentricity attends
and an ecce
ntricity of 150 mm below the beam axis at mid span. The beam carries a
concentrated load of 250 kN at centre besides its self
-
weight. Compute the stresses at
mid span.

OR

II

a)

Differentiate between post
-
tensioning system and pre
-
tensioning system

of
prestressing.

b)

Distinguish clearly between short
-
term and long
-
term deflections of prestressed
concrete beams.

c)

A prestressed
concrete beam , 200mm wide and 300mm deep is prestressed with
wires (area=320mm
2

) located at a constant eccentricity of 50mm a
nd carrying an
in
i
tial stress of 1000N/mm
2
. The span of beam is 10 m. Calculate the percentage loss
of stress in wires if (a) the beam is pre
-
tensioned and

(b) the beam is post
-
tensioned
,
using the following data:

E
s
=210kN/mm
2

and E
c
=35kN/mm
2

Relation of
steel stress = 5

per cent of the initial stress

Shrinkage of concrete = 300
×10
-
6

for pre
-
tensioning and 200×10
-
6

for post
-
tensioning

Creep coefficient=1.6

Slip at anchorage=1

mm

Frictional coefficient for wave effect =0.0015

per m



III

a)

what are the
different types of flexural failure modes observed in prestressed
concrete beams ?

b)

Explain the term (a) minimum section modulus (b) Maximum eccentricity

c)

Explain the importance of anchorage zone in prestressed members.

d)

The end block of a prestressed concret
e beam is 200 mm wide by 400 mm deep.
Two cables carrying an effective force of 1000kN, each are anchored by Freyssinet
anchorages of 150 mm diameter and 150 mm long with their centers @ 100 mm
the axis of the beam, using steel of Fe 415 grade, design suit
able anchorage zone
reinforcement using IS: 1343 provisions and sketch the details of the reinforcement
in the end block.


OR

IV

a)

Explain the tensile stress distribution in an end block of a post
-
tensioned beam with
sketch.

b)

B
riefly explain the limi
ting zone for prestressing force
.

c)

A post
-
tensioned bonded prestressed concrete beam of rectangular cross
-
section,
400mm wide by 500mm deep, is subjected to a service
-
load bending moment of
166.6 kN m, torsional moment of 46.6 kN m and shear force of 66.6 k
N.
The section
has an effective prestressing force , determined from service load requirements, of
magnitude 500 kN at an eccentricity of 150 mm, provide 5 numbers of 12.5mm
stress
-
relieved strands of cross
-
sectional area 506 mm
2

with an ultimate tensile
s
trength of 1820 N/mm
2
. If the cube strength of concrete is 40 N/mm
2
, design suitable
longitudinal and transverse reinforcement in the beam using IS:1343
-
1980.




V

a)

Design of pre
-
tensioned roof purlin to suit the data below:

Effective span=6 m

live

load=5

kN/m

Load factors:



For dead load =1.4



For live load =1.6

Concrete cube strength,
f
cu
= 50

N/mm
2


Cube strength at transfer,
f
ci
=30

N/mm
2

Tensile strength of concrete
,
f
t
= 1.7

N/mm
2


Modulus of elasticity of concrete,
E
c
=34

N/mm
2


Loss ratio , µ=0.8


Permissible stresses



At transfer:


C
ompressive stress,
f
ct

=15

N/mm
2







Tensile stress,

f
tt

=
1 N/mm
2



At working load :

C
ompressive stress
,
f
cw

= 17 N/mm
2




Tensile stress,
f
tw

=

0 N/mm
2

7mm high tensile steel wires having an ultimate tens
ile strength ,
f
pu

= 1600 N/mm
2


OR

VI

a)

What are the advantages of continuous members in prestressed concrete structures?

b)

What are cap cables? Where are they used?

c)

A continuous beam ABC
(AB=BC=10m) is prestressed by a parabolic cable profile
carrying an eff
ective force of 200kN. The cable profile shown in the fig below. The
beam supports a dead and live load of 0.24 kN/m and 2.36 kN/m respectively.
Calculate the resultant moments developed in the beam and locate the pressure line



d)

Outline the design princ
iples of prestressed compression members and its application
in the design of flag mast.


VII

a)

What are the advantages of using composite construction with prestressed and
in
-
situ

concrete in structural members?

b)

Distinguish betwe
en propped and un
propped con
struction methods in composite
construction using stress diagrams at various stages of construction.

c)

What is differential shrinkage?
Explain

its importance in composite construction.

d)

Design a composite slab for the bridge deck using
standard inverted T
-
sec
tion. The
top flange is 250mm wide and 100mm thick. The bottom flange is 500mm wide and
250mm thick. The web thickness is 100mm and the overall depth of the inverted T
-
section is 655mm. the bridge deck has to support a
characteristic

imposed load of 50
kN/
m
2

over an effective span of 12 m. Grade
-
40 concrete is specified for the precast
pre
-
tensioned T
-
with a compressive strength at transfer of 36 N/mm
2

. Concrete of
grade
-
30 is used for the
in situ

part. Determine the minimum prestress necessary and
check
for safety under serviceability limit state.



OR

VIII

a)

What are the advantages of prestressing in the design

of concrete members subjected
to axial tension? What are the load factors generally specified against cracking and
collapse in such members?

b)

Desig
n a suitable section for the tie member of a truss to supporta maximum design
tensile force of 500 kN. The permissible compressive stress in concrete at transfer is
15 N/mm
2

and no tension is permitted under working loads. The loss ratio is 0.8 7mm
diamete
r wires of ultimate tensile strength of 1700 N/mm
2

with an intial stress of 950
N/mm
2

may be used. The direct tensile strength of concrete is 3 N/mm
2

.
A load
factor of 2 at the limit state of collapse and 1.25 against cracking is required.

c)

Explain the des
ign procedure of prestressed folded plates.

d)

How do you compute the shrinkage and resultant stresses in composite members?