Design of singly reinforced concrete

A Beam is any structural member which resists load
mainly by bending. Therefore it is also called
flexural member. Beam may be singly reinforced or
doubly reinforced. When steel is provided only in
tensile zone (i.e. below neutral axis) is called singly
reinforced beam, but when steel is provided in
tension zone as well as compression zone is called
doubly reinforced beam.

To decide the size (dimensions) of the member and
the amount of reinforcement required.
To check whether the adopted section will perform
safely and satisfactorily during the life time of the
structure.

THE NORMAL DISTANCE FROM THE TOP EDGE
OF THE BEAM TO THE BOTTOM EDGE OF THE
BEAM IS CALLED OVER ALL DEPTH. IT IS
DENOTED BY ‘D’.
THE NORMAL DISTANCE FROM THE TOP EDGE
OF BEAM TO THE CENTRE OF TENSILE
REINFORCEMENT IS CALLED EFFECTIVE
DEPTH. IT IS DENOTED BY ‘d’.

THE DISTANCE BETWEEN THE BOTTOM OF THE BARS
AND BOTTOM MOST THE EDGE OF THE BEAM IS
CALLED CLEAR COVER.
CLEAR COVER = 25mm OR DIA OF MAIN BAR, (WHICH
EVER IS GREATER).
THE DISTANCE BETWEEN CENTRE OF TENSILE
REINFORCEMENT AND THE BOTTOM EDGE OF THE
BEAM IS CALLED EFFECTIVE COVER. EFFECTIVE
COVER = CLEAR COVER + ½ DIA OF BAR.

END COVER = 2XDIA OF BAR OR 25mm (WHICH EVER
IS GREATER)
THE LAYER / LAMINA WHERE NO STRESS EXIST IS
KNOWN AS NEUTRAL AXIS. IT DIVIDES THE BEAM
SECTION INTO TWO ZONES, COMPRESION ZONE
ABOVE THE NETURAL AXIS & TENSION ZONE BELOW
THE NEUTRAL AXIS.

THE NORMAL
DISTANCE BETWEEN THE TOP EDGE OF THE
BEAM & NEUTRAL AXIS IS CALLED DEPTH OF
NETURAL AXIS. IT IS DENOTED BY ‘n’.
THE DISTANCE BETWEEN THE
RESULTANT COMPRESSIVE FORCE (C) AND
TENSILE FORCE (T) IS KNOWN AS LEVER ARM. IT
IS DENOTED BY ‘z’. THE TOTAL COMPRESSIVE
FORCE (C) IN CONCRETE ACT AT THE C.G. OF
COMPRESSIVE STRESS DIAGRAM i.e. n/3 FROM
THE COMPRESSION EDGE. THE TOTAL TENSILE
OF THEFORCE (T) ACTS AT C.G.
REINFORCEMENT.
LEVER ARM = d-n/3

THE REINFORCEMENT PROVIDED TENSILE
ZONE IS CALLED TENSILE REINFORCEMENT. IT
IS DENOTED BY Ast.
THE REINFORCEMENT PROVIDED
COMPRESSION ZONEIS CALLED COMPRESSION
REINFORCEMENT. IT IS DENOTED BY Asc

THE BEAM SECTION CAN BE OF THE FOLLOWING
TYPES:
1. BALANCED SECTION
2. UNBALNCED SECTION
(a) UNDER- REINFORCED SECTION
(b) OVER-REINFORCED SECTION
A SECTION IS KNOWN AS
BALANCED SECTION IN WHICH THE COMPRESSIVE
STREE IN CONCRETE (IN COMPRESSIVE ZONES) AND
TENSILE STRESS IN STEEL WILL BOTH REACH THE
M A X I M U M P E R M I S S I B L E V A L U E S
SIMULTANEOUSLY.

SECTION IS KNOWN AS CRITICAL NEUTRAL AXIS (nc). THE AREA
OF STEEL
PROVIDED AS ECONOMICAL AREA OF STEEL. REINFORCED
CONCRETE SECTIONS ARE DESIGNED AS BALANCED SECTIONS.
2. UNBALNCED SECTION:-THIS IS A SECTION IN WHICH THE
QUANTITY OF STEEL PROVIDED IS DIFFERENT FROM WHAT IS
REQUIRED FOR THE BALANCED SECTION.
UNBALANCED SECTIONS MAY BE OF THE FOLLOWING TWO
TYPES:
(a) UNDER-REINFORCED SECTION
(b) OVER-REINFORCED SECTION

(a) IF THE AREA OF
STEEL PROVIDED IS LESS THAN THAT REQUIRED FOR
BALANCED SECTION, IT IS KNOWN AS UNDER-
DUE
REINFORCEMENT THE POSITION
TO LESS
OF ACTUAL
NEUTRAL AXIS (n) WILL SHIFT ABOVE THE CRITICAL
NEUTRAL AXIS (nc)i.e. n< nc. IN UNDER-REINFORCED
SECTION STEEL IS FULLY STRESSED AND CONCRETE IS
UNDER STRESSED (i.e. SOME CONCRETE REMAINS UN-
UTILISED). STEEL BEING DUCTILE, TAKES SOME TIME TO
BREAK. THIS GIVES SUFFICIENT WARNING BEFORE THE
FINAL COLLAPSE OF THE STRUCTURE. FOR THIS REASON
AND FROM ECONOMY POINT OF VIEW THE UNDER-
REINFORCED SECTIONS ARE DESIGNED.

(b) IF THE AREA OF STEEL
PROVIDED IS MORE THAN THAT REQUIRED FOR A
BALANCED SECTION, IT IS KNOWN AS OVER-REINFORCED
SECTION. AS THE AREA OF STEEL PROVIDED IS MORE, THE
POSITION OF N.A. WILL SHIFT TOWARDS STEEL,
THEREFORE ACTUAL AXIS (n) IS BELOW THE CRITICAL
NEUTRAL AXIS (nc)i.e. n > nc. IN THIS
SECTION CONCRETE IS FULLY
STEEL IS UNDER STRESSED.
STRESSED AND
UNDER SUCH
CONDITIONS, THE BEAM WILL FAIL INITIALLY DUE TO
OVER STRESS IN THE CONCRETE. CONCRETE BEING
BRITTLE, THIS HAPPENS SUDDENLY AND EXPLOSIVELY
WITHOUT ANY WARNING.

1.Effective span:- In the case of simply supported beam the
effective length,
l = i. Distance between the centre of support
ii. Clear span + eff. Depth eff. Span = least of i. & ii.
2.Effective depth:- The normal distance from the top edge of
beam to the centre of tensile reinforcement is called effective
depth. It is denoted by ‘d’.
d= D- effect. Cover where D= over all
depth

3. Bearing :- Bearings of beams on brick walls may be taken
as follow:
Up to 3.5 m span, bearing = 200mm
Up to 5.5 m span, bearing =300mm
Up to 7.0 m span, bearing =400mm
4. Deflection control:- The vertical deflection limits assumed
to be satisfied if (a) For span up to 10m
Span / eff. Depth = 20
(For simply supported beam)
Span / eff. Depth = 7 (For cantilever beam)

(b)For span above 10m, the value in (a) should be
multiplied by 10/span (m), except for cantilever for which
the deflection calculations should be made.
(c)Depending upon the area and type of steel the value of
(a&b) modified as per modification factor.
5. Reinforcement :-
(a) Minimum reinforcement:- The minimum area of tensile
reinforcement shall not be less than that given by the
following:
Ast = 0.85 bd / fy

(b)Maximum reinforcement:- The maximum area of
tensile reinforcement shall not be more than 0.4bD
(c)Spacing of reinforcement bars:-
i. The horizontal distance between to parallel main bars
shall not be less than the greatest of the following:
 Diameter of the bar if the bars are of same diameter.
 Diameter of the larger bar if the diameter are unequal.
 5mm more than the nominal maximum size of coarse
aggregate.

ii. When the bars are in vertical lines and the minimum vertical
distance between the bars shall be greater of the following:
15mm.
 2/3rd of nominal maximum size of aggregate.
 Maximum diameter of the bar.
6. Nominal cover to reinforcement :- The Nominal
cover is provided in R.C.C. design:
 To protect the reinforcement against corrosion.
 To provide cover against fire.
 To develop the sufficient bond strength along the
surface area of the steel bar.

As per IS 456-2000, the value of nominal cover to meet
durability requirements as follow:-
Exposure
conditions
Nominal
cover(mm)
Not less than
Mild
Moderate
Severe
Very severe
Extreme
20
30
45
50
75

• Given :
(i) Span of the beam (l)
(ii) Loads on the beam
(iii) Materials-Grade of Concrete and type of steel.
• 1. Calculate design constants for the given materials (k, j and R)
• k = m σcbc m σcbc + σst
• where k is coefficient of depth of Neutral Axis

j = 1- k/3
where j is coefficient of lever arm.
R= 1/2 σcbc kj
where R is the resisting moment factor.
2.Assume dimension of beam: d = Span/10 to Span/8
Effective cover = 40mm to 50mm b = D/2 to 2/3D
3. Calculate the effective span (l) of the beam.
4. Calculate the self weight (dead load) of the beam.
Self weight = D x b x 25000 N/m

5.Calculate the total Load & maximum bending
moment for the beam.
Total load (w) = live load + dead load
Maximum bending moment, M = wl2 / 8 at the centre of beam
for simply supported beam.
M = wl2 / 2 at the support
of beam for cantilever beam.
6. Find the minimum effective depth
M = Mr
= Rbd2
dreqd. = √ M / R.b

7. Compare dreqd. With assumed depth value.
(i)If it is less than the assumed d, then assumption is correct.
(ii)Ifdreqd. is more than assumed d, then revise the
depth value and repeat steps 4, 5 & 6.
8. Calculate the area of steel required (Ast).
Ast = M / σst jd
Selecting the suitable diameter of bar calculate the
number of bars required
Area of one bar = π/4 x φ2 = Aφ No. of bars
required = Ast /Aφ

9. Calculate minimum area of steel (AS) required by the
relation:
AS = 0.85 bd / fy
Calculate maximum area of steel by the area
relation:
Maximum area of steel = 0.04bD
Check that the actual ASt provided is more than
minimum and less than maximum requirements.

10. Check for shear and design shear reinforcement.
11. Check for development length.
12. Check for depth of beam from deflection.
13. Write summary of design and draw a neat sketch.

Er. VIKAS MEHTA
School of Mechanical and civil engineering
Shoolini University
Village Bajhol, Solan (H.P)
vikasmehta@shooliniuniversity.com
+91 9459268898

Design of singly reinforced concrete

More Related Content

What's hot (20)

Similar to Design of singly reinforced concrete (20)

More from Vikas Mehta (20)

Recently uploaded (20)

Design of singly reinforced concrete