TWO MARK QUESTIONS

UNIT I METHODS OF DESIGN OF CONCRETE STRUCTURES

1. What are the assumptions made in the working stress method?
The following are the assumptions made in working stress method:
a) At any cross-section, plane sections before bending remain plain after bending
b) All tensile stresses are taken up by reinforcement and none by concrete, except as
otherwise specifically permitted
c) The stress-strain relationship of steel and concrete, under working loads, is a straight
line
d) The modular ratio m has the value -280/3

2. State the advantages of limit state method over others.

The advantages of limit state method over the other methods are the following:
a) In the limit state method of analysis, the principles of both elastic as well as plastic
theories used and hence suitable for concrete structures
b) The structure designed by limit state method is safe and serviceable under design loads
and at the same time it is ensured that the structure does not collapse even under the
worst possible loading conditions
c) The process of stress redistribution, moment redistribution etc., are considered in the
analysis and more realistic factor of safety values are used in the design
d) Hence the design by limit state method is found to be more economical
e) The overall sizes of flexural members (depth requirements) arrived by limit state method
are less and hence they provide better appearance to the structure
f) Because of the modified assumptions regarding the maximum compressive strains in
concrete and steel, the design of compressive reinforcement for double reinforced beams
and eccentrically loaded columns by limit state method gives realistic values which is not
so in other methods

3. Define – Characteristic Strength in Limit State Method

The term characteristic strength means that value of the strength of the material below which
not more than 5% of the test results are expected to fall.

4. What is meant by balanced section?

When the maximum stress in steel and concrete simultaneously reach their allowable values,
the section is said to be balanced section. In this section the actual neutral axis depth is equal
to the critical neutral axis.

5. Define – Limit State

The acceptable limit for safety and serviceability requirements before failure occurs is called a
limit state. The aim of design is to achieve acceptable probabilities that the structure will not
become unfit for the use for which it is intended, that is, that it will not reach a limit state.
 6. What are the expressions recommended by the IS 456-2000 for Modulus of Elasticity
and Flexural Strength?
a) Flexural strength, f= 0.7/ fck N/
Where ‘fck’ is the characteristic cube compressive strength of concrete in N/
b) Ec=5000 fck
Where, Ec is the short term static modulus of elasticity in N/

7. Write the formula for the neutral axis depth factor K in working stress design.
Neutral axis depth factor K= σbc.m/(σbc.m + σst)
where σbc is permissible stress in concrete. σst is permissible stress in steel
m is modular ratio

8. State the working stress method.

Working stress method is based on behaviour of the structure at working load. It ensures
satisfactory behavior under service load and is assumed to possess adequate safety against
collapse. Also stresses in concrete and steel do not exceed the allowable stresses at service
loads.

9. Write the shortcomings of working stress method.

The shortcomings of working stress method are:
a) Though the stress-strain relation is linear for mild steel, it is not so in case of high yield
strength deformed bars, which are more commonly used in practice
b) It does not provide a true factor of safety against failure or objectionable deformation.
c) This method ignores the effect of creep and shrinkage of concrete

10. State the advantages of limit state method.

Limit state design (LSD) refers to a design method used in structural engineering. A limit
state is a condition of a structure beyond which it no longer fulfills the relevant design criteria.
The condition may refer to a degree of loading or other actions on the structure, while the
criteria refer to structural integrity, fitness for use, durability or other design requirements.
A structure designed by LSD is proportioned to sustain all actions likely to occur during its
design life, and to remain fit for use, with an appropriate level of reliability for each limit
state. Building codes based on LSD implicitly define the appropriate levels of reliability by
their prescriptions.

11. State some of the important limit states of failure or collapse.

Some of important limit states of failure or collapse are listed below:
a) Failure of one or more critical sections in flexure,shear,torsion or due to their
combinations
b) Failure due to fatigue under repeated load
c) Failure due to bond and anchorage failure of reinforcement
d) Failure due to elastic instability of structural members
e) Failure due to impact, earthquake, fire or frost
f) Failure due to destructive effects of chemicals, corrosion of reinforcements

12. State the moment of resistance of a beam section.

A balanced section of a reinforced concrete beam is designed such that the steel begins to
yield just as the concrete begins to crush. The bending moment at that point is called the
moment of resistance.

13. Write the type of failure undergone by over reinforced and under reinforced beams?
 Over reinforced beam fails at small deflection, resulting in brittle failure whereas under
reinforced beam fails under large deformation resulting in ductile failure.

14. Define – Uncracked Phase

The central segment of a beam is subjected to pure flexure. In earlier stages of loading, the
applied moment is less than cracking moment Mcr and the maximum tensile stress fct in the
concrete is less than the flexural tensile strength fcr. This phase is termed as uncracked phase.

15. What are the considerations required for wall thickness of liquid retaining structures?
The considerations required for wall thickness of liquid retaining structures are:
a) The thickness of wall may be atleast 150 mm
b) The thickness of wall may be about 30 mm per m depth+50 mm
c) The thickness of wall may be such that tensile stresses on concrete section may not
exceed the stress in tension

UNIT II LIMIT STATE DESIGN FOR FLEXURE

1. State the check for deflection control in the design of slabs.

The deflection of a structure or part there of shall not adversely affect the appearance or
efficiency of the structure or finishes or partitions. The deflection shall generally be limited to
the following:
a) The final deflection due to all loads including the effects of temperature, creep and
shrinkage and measured from the as-cast level of the, supports of floors, roofs and all
other horizontal members, should not normally exceed span/250
b) The deflection including the effects of temperature, creep and shrinkage occurring after
erection of partitions and the application of finishes should not normally exceed span/350
or 20 mm, whichever is less

2. What is meant by doubly reinforced beams?

The section reinforced in both tension and compression is known as doubly reinforced beams.
The doubly reinforced beams are adopted when the balanced moment is smaller than the
actual moment.

3. What type of slabs are usually used in practice, under reinforced or over reinforced?
The depth of slab chosen from deflection requirements will be usually greater than the depth
required for balanced design. Hence, the area of steel required will be less than the balanced
amount. So, the slab is designed as under reinforced section.

4. Why is it necessary to provide transverse reinforcement in one way slab?

Since the one way slab bends in one direction and also in shorter direction, it is necessary to
provide transverse reinforcement in one way slabs. These slabs adopted when availability of
two supports in one direction

5. Distinguish between under reinforced and over reinforced sections?

A beam reaches its permissible stress in steel under the working moment before concrete
reaches its stress is called as Under reinforced section. A beam reaches its permissible stress
in concrete under the working moment before steel reaches its stress is called over reinforced
section.
6. Sketch the edge and middle strips of a two way slab.

7. State the situations in which doubly reinforced sections are preferred.

Situations in which doubly reinforced sections preferred are:
a) when the members are subjected to alternate external loads and the bending moment in the
section reverses
b) when the members are subjected to loading eccentric on either side of axis
c) when overall size of beam section is limited
d) when beam section is continuous over several supports

8. State the merits and demerits of a balanced section.

Merits: There will be a clear warning of failure of the member in the form of large deflections
associated with well distributed cracks along length of member, before final failure.
Demerits: Balanced sections are more economical but it may not be practicable in all cases.

9. List out the different limit state to be considered in reinforced concrete design.
Different limit state to be considered in reinforced concrete design are:
a) limit state of collapse
b) limit states of serviceability
c) limit state of collapse: flexure, compression, shear and torsion
d) limit state of serviceability: deflection and cracking

10. What is meant by one way slab and two way slab?
One way slab is supported by beams on only 2 sides.
The ratio of longer span panel (L) to shorter span panel (B) is equal to or greater than 2. Thus,
L/B ≥ 2
Main reinforcement is provided in only one direction for one way slabs.
Two way slab is supported by beams in all four sides.
The ratio of longer span panel (L) to shorter span panel (B) is less than 2.
Thus, L/B < 2.
Main reinforcement is provided in both the directions for two way slabs.

11. What will be the minimum reinforcement used for flexure member?
The mild steel reinforcement in either direction in slabs shall not be less than 0.15 of the total
cross-sectional area. However, this value can be reduced to 0.12 when high strength deformed
bars or welded wire fabric are used.
12. State the factors influencing moment developed in two way rectangular slabs.
Moment developed in slab are influenced by following factors:
a) Short and long span length LX & LY
b) Type of supporting edges such as free, fixed and continous
c) Magnitude and type of load on slab such as concentrated, uniformly distributed, etc.,

13. Why is secondary reinforcement provided in one way RC slab?

Secondary reinforcements are provided running perpendicular to the main reinforcements, in
order to take the temperature and shrinkage stresses. It is otherwise called as distribution or
temperature reinforcements

14. What is meant by the terms balanced, under reinforced and over reinforced in bending?
Balanced: Quantity of steel provided is such that, when most distant concrete fibre in
compression zone reaches the allowable stress in compression, the tensile stress in
reinforcement reaches its allowable stress.
Under reinforced: Stress in steel reaches its permissible value, the corresponding extreme
compressive stress reached in concrete will be less than its permissible values.
Over reinforced: When extreme compressive stress in concrete reaches its permissible value,
the corresponding tensile stress in steel will be less than its permissible value.

15. What is meant by singly and doubly reinforced beams?

Singly reinforced: A simple beam of rectangular cross section, reinforced in tension zone is
known as singly reinforced beam.
Doubly reinforced: When the bending moment reverses, the beam is reinforced both in
compression and tension zones known as doubly reinforced beams.

UNIT III LIMIT STATE DESIGN FOR BOND, ANCHORAGE SHEAR AND
TORSION

1. What is meant by development length of bar?

The reinforcement bar must extend in the concrete sufficiently so that it can develop the
required stress. The extended length of bar for development of required stress is known as
development length.

2. How columns are classified based on type of reinforcement?

a) Tied columns: Longitudinal bars are confined within closely spaced lateral ties
b) Spiral columns: Longitudinal reinforcements are enclosed within closely spaced and
continuously wound spiral reinforcement.
c) Composite columns: Longitudinal reinforcements are in form of structural steel sections
with or without longitudinal bars.

3. State the type of failures, which are undergone by axially loaded columns.
Axially loaded columns may fail in any of the following three modes:
a) Pure compression failure
b) Combined compression and bending failure
c) Failure by elastic instability

4. Define – Bond Stress

 Shear stress developed at the interface of bar and concrete is known as the bond stress. It is
the force per unit of nominal surface area of a reinforcing bar acting on the interface between
bar and surrounding concrete.

5. How can the bond stress be reduced?

The bond stress is higher at locations of high shear force but it can be reduced by providing an
increased number of bars of smaller diameter yielding the same equivalent are of
reinforcement.

6. Why are plain bars not used for main reinforcements?

Plain bars cannot develop the bond resistance due to mechanical interlock and the
development and widespread use of deformed bars is attributed to their superior bond
resistance. The present trend is to prefer deformed bars for main reinforcement and plain bars
for ties and stirrups.

7. When does shear reinforcement necessary in a beam?

Shear reinforcements are provided to prevent the failure of concrete by excessive diagonal
tension in beams.

8. What are the types of reinforcement used to resist shear force?

The types of reinforcement used to shear force are the following:
a) Vertical stirrups
b) Inclined or diagonal reinforcement
c) Lattice girder effect
(i) Single system
(ii) Double system

9. State flexure bond and development bond.

Flexure bond: It arises in flexural members on account of shear or a variation in bending
moment, which in turn causes a variation in axial tension along length of reinforcing bar.
Development bond: It arises over the length of anchorage provided for a bar or near the end
of reinforcing bar. This bond resists the pulling out of bar if it is in tension or conversely, the
pushing in of the bar if it is in compression.

10. What will be the minimum and maximum area of tension reinforcement in a beam?
The minimum area of tension reinforcement shall not be less than that given by the relation,
As = (0.85bd/fy)
where As = Minimum area of tension reinforcement
B = breadth of beam
D = effective depth
Fy = characteristic strength of beam in N/
D = overall depth of member

11. What is meant by face reinforcement?

When the depth of web or rib exceeds 750 mm, side face reinforcement of cross sectional area
not less than 0.1% of the web area is to be provided and distributed equally on two faces and
the spacing of bars not to exceed 300 mm or web thickness whichever is smaller.

12. State the difference between primary and secondary torsion.

Primary torsion: Torsion is induced by an eccentric loading and equilibrium conditions alone
suffice in determining the twisting moments.
 Secondary torsion: Torsion is induced by need for the member to undergo an angle of twist
to maintain deformation compatibility, and the resulting twisting moment depends on the
torsional stiffness of the member.

13. Under what circumstances doubly reinforced bars are used?

Reinforced concrete beams with compression reinforcement will be required in cases where:
(i) The depth of beam is restricted
(ii) Singly reinforced section is insufficient to resist the moment of section

14. What is the need for reinforcement in beams?

Generally beams are provided with main reinforcement on the tension side for flexure and
transverse reinforcement for shear and torsion.

15. List out the IS recommendations regarding longitudinal reinforcements.

The following are the IS recommendations regarding longitudinal reinforcements:
a) The minimum number of longitudinal bars provided in a column shall be four in
rectangular columns and six in circular column
b) The bars shall be not be less than 12 mm in diameter
c) Spacing of longitudinal bars measured along the periphery of column shall not exceed
300 mm

QUESTIONS & ANSWERS

PART – A
1. What is Ultimate Limit State (ULS)
ULS in concerned with the maximum load – carrying capacity of the structure
within the limits of strength of the materials used.

2. What is characteristic load?

Cl 36.1,36.2- pg 67

3. State the 3 types of load.

a) Dead load b) Imposed loadc) Wind load

4. State four objectives of the design of reinforced concrete structure.

Properly designed reinforced concrete structures should:
 Have acceptable probability of performing satisfactorily during their intended life,
 Sustain all loads with limited deformations during construction and use,
 Be durable,
 Adequately resist the effects of misuse and fire.

5. How to fulfill the three objectives of the design of reinforced concrete

 structures?
The three objectives can be fulfilled by:
(i) Understanding the strength and deformation characteristics of concrete and steel,
(ii) Following the clearly defined standards for materials, production,
workmanship and maintenance, and use of structures in service,
(iii) Adopting measures needed for durability.

6. What are the three methods of design of reinforced concrete structural

elements?
The three methods are:
 Limit state method,
 Working stress method,
 Method based on experimental approach

7. How to estimate the design loads in (i) limit state method, and (ii)
working stress method?
(i) In limit state method,
Design loads = Characteristic loads multiplied by the partial safety factor for loads
(ii) In working stress
method, Design loads =
Characteristic loads.

1. Explain the limit state philosophy as detailed in the current IS

code.

✓ The Answer is in Page No.67 of IS 456:2000.

✓ In the method of design based on limit state concept, the structure
shall be designed withstand safely all loads liable to act on it
throughout its life;
✓ it shall also satisfy the serviceability requirements, such as
limitations on deflection and cracking.

✓ The acceptable limit for the safety and serviceability requirements

before failure occurs is called a ‘limit state’.

✓ The aim of design is to achieve acceptable probabilities that the

structure will not become unfit for the use for which it is intended,
that is, that it will not reach a limit state.

✓ 351.1 All relevant limit states shall be considered in design to

ensure an adequate degree of safety and serviceability. In general,
the structure shall be designed on the basis of the most critical
 limit state and shall be checked for other limit states.
✓ 35.1.2 For ensuring the above objective, the design should be
based on characteristic values for material strengths and applied
loads, which take into account the variations in material
strengths and in the loads to be
supported.

✓ The characteristic values should be based on statistical data if

available; where such data are not available they should be based
on experience. The ‘design values’ are derived from the
characteristic values through the use of partial safety factors, one
for material strengths and the other for loads.

✓ In the absence of special considerations these factors should have

the values given in 36 according to the material, the type of
loading and the limit state being considered.

2. Differentiate between working stress method and limit state method.

❖ In the limit state method of analysis, the principles of both elastic as well as
plastic theories used and hence suitable for concrete structures.

❖ The structure designed by limit state method is safe and serviceable under
design loads and at the same time it is ensured that the structure does not
collapse even under the worst possible loading conditions.

❖ The process of stress redistribution, moment redistribution etc., are

considered in the analysis and more realistic factor of safety values are used
in the design.

❖ Hence the design by limit state method is found to be more economical.

❖ The overall sizes of flexural members (depth requirements) arrived by limit
state method are less and hence they provide better appearance to the
structure.

❖ Because of the modified assumptions regarding the maximum compressive

strains in concrete and steel, the design of compressive reinforcement for
double reinforced beams and eccentrically loaded columns by limit state
method gives realistic valued which is not so in other methods.
3. Explain the following terms:
a. Characteristic strength and characteristic
loads.
b. Partial safety factors.
c. Balanced section and under reinforced section.

a.)The Answer is in Page No.67 of IS

456:2000. b.)The Answer is in Page
No.68 of IS 456:2000.
c.) When the maximum stress in steel and concrete simultaneously reach their
allowable values, the section is said to be balanced section. in this section the actual
neutral axis depth is equal to the critical neutral axis. When the percentage of steel in the
section is less than that required for a balanced section. In this section the actual neutral
axis depth is equal to the critical neutral axis.