Lesson 1: Introduction to Prestressed Concrete Design

Question 1

true or false:

a pre-stressed concrete structure is different from a conventional reinforced concrete structure due to the application of an initial load on the structure prior to its use

Answer 1

true

Question 2

cracking in tensile region as concrete is _____ in tension

Answer 2

weak

Question 3

excessive deflection as stiffness _____ due to
cracking

Answer 3

decreases

Question 4

concrete is weak in _____

Answer 4

tension

Question 5

after cracking, considerable loss in _____

Answer 5

stiffness

Question 6

it substantially increases the external load required to crack the concrete resulting in a member that is strong, tough and stiff

Answer 6

pre-compression

Question 7

3 objectives of prestressing

Answer 7

• control or eliminate tensile stresses in the concrete (cracking) at least up to service load levels
• control or eliminate deflection at some specific load level
• allow the use of high strength steel and concrete

Question 8

true or false:

the ordinary reinforced concrete also have higher shear capacity compared to pre-stressed concrete

Answer 8

false

pre-stressed concrete also have higher shear capacity compared to the ordinary reinforced concrete

Question 9

5 advantages of prestressed concrete

Answer 9

• flexural cracking is delayed; in the case of fully pre-stressed members, which are free from tensile stresses under working loads, the cross-section is more efficiently utilized
• a pre-stressed concrete flexural members are stiffer underworking loads compared to RC members
• camber due to pre-stressing reduces deflection
• pre-stressed members posses improved resistance to shear, as principal tensile stresses are reduced due to prestressing
• the use of high strength concrete and steel in pre-stressed members results in lighter and slender members

Question 10

4 limitations of prestressed concrete

Answer 10

• cost is relatively higher for normal structures as it requires special site operation
• loss of pre-stress due to shrinkage and creep of concrete and relaxation of steel stresses can reduce useable pre-stressing force
• loss also result from anchorage slip
• design and construction is more difficult

Question 11

5 forms of pre-stressing steel

Answer 11

• tendon
• wires
• strands
• cable
• bars

Question 12

a form of pre-stressing steel described as a stretched element used in a concrete member to impart pre-stress to the concrete

Answer 12

tendon

Question 13

this form of pre-stressing steel is a single unit made of steel

Answer 13

wires

Question 14

two, three or seven wires are wound to form this prestressing steel

Answer 14

strands

Question 15

a group of strands are formed in to make this pre-stressing steel

Answer 15

cable

Question 16

a form of pre-stressing steel in which a tendon can be made up of a single steel bar

Answer 16

bars

Question 17

7 applications of prestressed concrete

Answer 17

• buildings
• underground structures
• towers
• water storage
• offshore structures
• nuclear reaction vessels
• numerous types of bridge system including cable stayed and segmental bridges

Question 18

the tension is applied to the tendons before casting of the concrete; the pre-compression is transmitted from steel to concrete through bond over the transmission length near the ends

Answer 18

pre-tensioning

Question 19

3 limitations of pre-tensioning

Answer 19

• heavy casting beds are required but no permanent end anchorages are necessary
• tendon diameter should be small enough and the surface is rough enough for a good bond; normally multiple-wire stranded tendons are used
• tendons cannot be easily curved to vary the eccentricity, but this can be overcome by curving the beam itself or by harping (deflecting tendons after tensioning)

Question 20

this kind of tendons is normally used in pre-tensioning

Answer 20

multiple-wire stranded tendons

Question 21

these are placed in the concrete at the required center of gravity of steel during post-tensioning

Answer 21

ducts

Question 22

it is required through which prestressing force is transferred during post-tensioning

Answer 22

permanent end anchorage

Question 23

during external post-tensioning, tendons can be placed _____ the concrete section

Answer 23

outside

Question 24

during external post-tensioning, pre-stressing force is transferred by _____ and _____

Answer 24

end anchorage; deviators

Question 25

some segmental bridge applications

Answer 25

• urban bridges
• long span bridges
• environmentally sensitive
• long bridges over water
• arches
• cable stayed bridges
• rail bridges

Question 26

2 nature of concrete-steel interface

Answer 26

• bonded tendon
• unbonded tendon