Prestressed Concrete Structures Question Bank on “Stresses at Serviceability Limit”.

1. The maximum permissible stresses are mainly governed by ___________

a) Tensile stresses

b) Compressive stresses

c) Ultimate stress

d) Principle stresses

Answer: b

Clarification: The maximum permissible stresses in the precast prestressed concrete and the insitu cast concrete are mainly governed by the compressive strength of concrete in the respective elements and in general the permissible stresses in a precast prestressed concrete are governed by the normal rules for prestressed concrete.

2. Certain expectations in serviceability limits of British code are made regarding ___________

a) High stresses

b) Compressive stresses

c) Limited stresses

d) Elongation stresses

Answer: a

Clarification: Certain expectations are made regarding high stresses developed at the interface of the precast and insitu cast elements and in British code BS: 8110 provides for a higher value of compressive stress equal to 0.5f_{cu} which is 50 percent higher than the normally allowable value in prestressed element.

3. The higher value of compressive stress is permissible only in ___________

a) I sections

b) Composite sections

c) L sections

d) Prestressed sections

Answer: b

Clarification: The higher value of compressive stress is permissible only in composite sections with the stipulation that the failure of the section is due to excessive elongation of steel and this requirement is to safe guard against the explosive compressive failure of the concrete at the limit state of collapse.

4. The permissible flexural tensile stress in the insitu concrete at the ___________

a) Contact surface

b) Longitudinal surface

c) Zig zag surface

d) Dump surface

Answer: a

Clarification: The permissible flexural tensile stress in the insitu concrete at the contact surface with the prestressed element, as prescribed in the British code BS: 8110 varies from 3.2 to 5.0n/mm^{2} corresponding to various grades of concrete.

5. The stresses may be increased up to 50 percent for prestressed unit by the use of ___________

a) Allowable tensile stresses

b) Permissible tensile stresses

c) Permitted tensile stresses

d) Constant tensile stresses

Answer: a

Clarification: Theses stresses may be increased by upto 50 percent, provided that the allowable tensile stresses for the prestressed unit is reduced by the same amount the higher values of flexural tensile stresses are permitted since it has been proved by experiments that the development of cracking which are visible is prevented by the uncracked prestressed concrete which is bonded to the insitu concrete.

6. The horizontal shear is generally governed by ___________

a) Shear limit state

b) Ultimate limit state

c) Principle limit state

d) Tensile limit state

Answer: b

Clarification: Horizontal shear is generally governed by the ultimate limit states the British code (BS:8110-1985) identifies three types of surfaces in the design provisions for the maximum permissible design ultimate horizontal shear stresses at the interface.

7. The horizontal shear force, at the interface of the precast and insitu components, are due to ___________

a) Design ultimate loads

b) Design shear loads

c) Design flexural loads

d) Design tensile loads

Answer: a

Clarification: The horizontal shear force at the interface of the precast and insitu components due to design ultimate loads is the total compression from that part of the compression zone above the interface calculated from the ultimate bending moment.

8. The average horizontal design shear stress is computed by dividing ___________

a) Horizontal principle force/Area

b) Precast force/Area

c) Horizontal shear force/Area

d) Load/Area

Answer: c

Clarification: The average horizontal design shear stress is computed by dividing the horizontal shear force by the area which is obtained as the product of the contact width of the bear and its length between the points of zero moments.

9. To obtain the horizontal shear stress the design shear stress are ___________

a) Calculated

b) Analysed

c) Distributed

d) Divided

Answer: c

Clarification: The average design shear stress is then distributed in proportion to the vertical design shear force diagram to obtain the horizontal shear stress at any point along the length of the members and the design shear stresses should not exceed the values compiled for different types of surfaces.

10. When the links are provided, their cross sectional area should be at least?

a) 0.20

b) 0.15

c) 0.20

d) 0.18

Answer: b

Clarification: When links are provided their cross sectional area should be at least 0.15 percent of the contact area and the spacing of the links in T beam ribs with composite flanges should not exceed neither four times the minimum thickness of the insitu concrete nor 600mm.

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