DEPARTMENT OF CIVIL ENGINEERING
SNS COLLEGE OF ENGINEERING
COIMBATORE
DESIGN OF RC ELEMENTS
2 MARKS WITH ANSWERS
UNIT-I - LIMIT STATE METHOD CONCEPT AND DESIGN OF BEAMS
1. Reinforced concrete
Concrete in which steel bars are introduced in casing stage to resist the
stresses developed due to external loads.
Concrete- good in compression but weak in tension
Steel- strong in tension but weak in compression
2. Philosophy of limit state design
Structures designed should satisfy the criteria of desirable ultimate strength in
flexures shear, compression, tension and torsion developed under a system of loads.
Serviceability condition-deflection and cracking
Stability against buckling.
3. Methods of design
Working stress method or elastic method or modular ratio method
Load factor method or ultimate load method
Limit state method
4. Modular ratio
The ratio of young’s modulus of steel and young’s modulus of concrete.
Modular ratio = Es/Ec
5. Load factor
The ratio of ultimate load and working load.
Load factor = ultimate load/working load
6. Limit state
A structure is said to have reached it limit state when it become unfit for use during
its expected life.
7. Objective of code
 To provide a safe structure by ensuring strength and serviceability.
 Design procedures, design tables and formula for easy computation.
 Protects structural engineer-protects from failures (improper materials, lack of
supervision)
 Provides guide lines for structural engineers
8. Some codes related to concrete
IS-456, ACI-380, BS-8110, SP-16-1980SP-24-1983
9. Types of loads
Dead load, live load, wind load, snow load, earthquake load.
10. Ultimate load method
In this method, working loads are increased by suitable factors, known as load
factors, to obtain ultimate loads and the structure is designed to resist these loads.
11. Assumptions-ultimate load method
 A section which is plane before bending remains plane after bending.
 Tensile strength of concrete is ignored.
12. Assumptions- Working stress method
 A section which is plane before bending remains plane after bending.
 Bond between steel and concrete is perfect within the elastic limit of steel.
 Tensile strength of concrete can be ignored.
13. Over reinforced section
This is a section in which the quantity of steel provided is more than the quantity
required for a balanced section.
14. Under reinforced section
This is a section in which the quantity of steel provided is less than what is required
for a balanced section.
15. Balanced or critical section
This is section in which the quantity of steel provided is such that, when the most
distant concrete fiber in the compression zone reaches the allowable stress in compression,
the tensile stress in the reinforcement reaches its allowable stress.
16. Lever arm
Distance between the line of action of the resultant compression and the line of
action of the resultant tension is called lever arm.
Lever arm = a = (d-n/3)
17. Moment of resistance
The resisting offered by a beam section to resist the bending moment at the section
is called moment of resistance.
18. Neutral axis
The neutral axis for a beam section is the line of intersection of the neutral layer
with the beam section.
19. Double reinforced beams
Beams with reinforcement in compression and tension zones are called doubly
reinforced beams.
20. Singly reinforced beams
Beams with reinforcement provided in tension zone is called singly reinforced
beams.
21. Steel beam theory- assumptions
Compression is resisted only in compression steel
Tension is resisted only by tension steel
Stress in compression steel=stress in tension steel.
22. Principles of limit states
The limit state of collapse
The limit state of serviceability involving excessive cracking and deflection at
service or working loads.
23. Steel beam theory
If the amount of compression reinforcement required equals or exceeds the amount
of tension reinforcement obtained by using equation Ast= M/(σst.j.d) and
Asc = (mAst(d-xc)/((1.5m-1).(xc-d)),the beam section may be designed by the steel beam
theory.
UNIT-II – LIMIT STATE DESIGN FOR SLABS
1. One way slab
Reinforced concrete slabs supported on two opposite sides or an all four sides with
the ratio of long to short span exceeding 2 are referred to as one way slabs.
Ly/lx >2
2. Two way slab
Reinforced concrete slabs supported on two opposite sides or an all four sides with
the ratio of long to short span not exceeding 2 are referred to as two way slabs.
Ly/lx < 2
3. Limiting conditions to check for deflection control
(L/d) actual < (L/d) max
4. Limiting conditions to check for shear stress
‫ح‬c < ‫ح‬
5. Span/depth ratio in two way slab
Simply supported slabs = 28
Continuous slabs = 32
UNIT-III - LIMIT STATE DESIGN FOR BOND,ANCHORAGE,SHEAR&
TORSION
1. Torsion
In reinforced concrete member torsion occurs in combination with flexure and
shear.
Primary or equilibrium torsion
Secondary or compatibility torsion.
2. Primary torsion
Primary torsion is generally induced by eccentric loading and equilibrium
conditions are sufficient to evaluate the torsion moments acting at critical sections.
3. Primary torsion
This type of torsion is induced by the application of an angle of twist such as
rotation of the member.
3. Bond
When the concrete sets, it adheres to the surface of reinforcement and tightly grips
it. This perfect adhesion between concrete and steel is known as bond.
4. Bond Stress
Bond resists any force that rise to pull out or push the rod. The intensity of the
adhesive force is called bond stress.
5. Anchorage
Whenever some reinforcing bar is to be anchored or two bars have to be given an
overlap, it is essential that they must get sufficient length of embedded length or overlap
length so that no slip takes place.
6. Tensional shear stress
The effect of torsion to induced shear stresses is called tensional shear stress.
7. Bond mechanism
Chemical adhesion
Frictional resistance
Shearing resistance or dilatancy.
8. Chemical adhesion
The creep of cement in concrete
9. Frictional resistance
Movement concrete and steel bars in any structure
10. Frictional resistance
The interlocking of steel bars.
11. Flexural bond stress
This develops due to variation of bending moment or shear force at a section.
12. Anchorage bond stress
Stress developed at the extreme end of bars in tension or compression.
13. Reinforcement splicing
Splicing of reinforcement is required when the bars are to be extended beyond they
are available length.
14. Types of splicing
Lapping of bars
Lap welding of bars
Butt welding of bars
Mechanical connections
UNIT-IV - LIMIT STATE DESIGN OF COLUMNS
1. Column
Vertical compression member that transfers the load safely from beams to the
footing.
2. Pedestal
This is also vertical compression member in which effective length is denoted by
le.le is less than 3times the least lateral dimension.
3. Strut
It is the vertical member in a truss
4. Axially loaded columns may fail in any one of the 3 modes
Pure compression failure
Combined compression and bending failure
Failure due to elastic instability
5. Types of columns
Based on type of reinforcement
Tied columns
Spiral column
Composite column
6. Based on type of loading
Axially loaded column
Column with uniaxial eccentric loading
Column with biaxial eccentric loading
7. Based on slenderness ratio
Short column (Lex/D or Ley/b <12)
Long column (Lex/D or Ley/b >12)
8. Short column
Lex/D or Ley/b <12
The ratio of effective length and least lateral dimension is less than 12.
9. Long column
(Lex/D or Ley/b >12)
Lex/D or Ley/b >12
The ratio of effective length and least lateral dimension is greater than 12.
10. Minimum Eccentricity
emin = (L/500+D/300)
L=Unsupported length
D= Lateral dimension
UNIT-V - LIMIT STATE DESIGN OF FOOTING AND DETAILING
1. Footing
Footing is located below the ground level. It effectively supported super structure
like columns by transmitting the applied loads, moments and other forces to the soil
without exceeding safe bearing capacity.
2. Types of footing
Isolated column footing
 Flat
 Stepped
 Sloped
Combined footing
Raft footing
Strap footing
Pile footing
3. Strap footing
A strap footing consists of spread footing of two columns connected by a strap
beam. This is the economical construction over combined footing when columns are far
off. The strap beam does not rest over soil and no load is transmitted by it, directly on soil.
4. Raft Foundation
When the area required for spread footings without more than the half the area of
building, due to poor safe bearing capacity of soil or heavy load on the building, foundation
is taken to entire building area and a thick reinforced concrete slab is provided to cover the
whole area connecting all the columns with the beams in both directions.