Page 1
Short Notes on Concrete Structures
Working Stress Method
Modular Ratio
S
C
E
m
E
=
o m = Modular ratio
o E
S
= Modulus of elasticity of steel
o E
C
= Modulus of elasticity of concrete
Equivalent Area of Concrete
CS
AmA =
o A
C
= Area of concrete
o AS = Area of steel
Critical Depth of Neutral Axis (X
C
)
C
mc
X d
tmc
æö
÷ ç
= ÷
ç
÷
÷ ç
èø +
Here, D = Overall depth
d = Efffective depth
cbc
s = c = permissible stress in concrete
st
s = t = permissible stress in steel
Actual depth of Neutral axis (X
a
)
2
()
2
a
st a
BX
mA d x =-
Special case :
(i) when
ac
XX = for balanced section
(ii) when
ac
XX > for over reinforced section
(iii) when
ac
XX < for under reinforced section
Doubly Reinforced Rectangular Section
Critical depth of Neutral axis, (X
C
)
C
mc
Xd
tmc
=·
+
Actual depth of Neutral axis, (X
a
)
2
(1.5 1) ( ) ( )
2
a
SC a c st a
bX
mAX d mAdx +- - = -
Page 2
Short Notes on Concrete Structures
Working Stress Method
Modular Ratio
S
C
E
m
E
=
o m = Modular ratio
o E
S
= Modulus of elasticity of steel
o E
C
= Modulus of elasticity of concrete
Equivalent Area of Concrete
CS
AmA =
o A
C
= Area of concrete
o AS = Area of steel
Critical Depth of Neutral Axis (X
C
)
C
mc
X d
tmc
æö
÷ ç
= ÷
ç
÷
÷ ç
èø +
Here, D = Overall depth
d = Efffective depth
cbc
s = c = permissible stress in concrete
st
s = t = permissible stress in steel
Actual depth of Neutral axis (X
a
)
2
()
2
a
st a
BX
mA d x =-
Special case :
(i) when
ac
XX = for balanced section
(ii) when
ac
XX > for over reinforced section
(iii) when
ac
XX < for under reinforced section
Doubly Reinforced Rectangular Section
Critical depth of Neutral axis, (X
C
)
C
mc
Xd
tmc
=·
+
Actual depth of Neutral axis, (X
a
)
2
(1.5 1) ( ) ( )
2
a
SC a c st a
bX
mAX d mAdx +- - = -
Singly Reinforced T-Section
Effective width of flange
? For beam casted monolithic with slab
0
12
6
6
22
wf
f
w
l
bd
B Minimum or
ll
b
ìæö
ï
÷ ï ç
÷ ++
ï ç
÷
ç ï ÷ ç
èø
ï
ï
ï
ï
=
í
ï
ï
ï
ï
++
ï
ï
ï
ï î
? For isolated T-beam
0
0
4
fw
l
Bb
l
B
=+
æö
÷ ç
+ ÷
ç
÷
ç ÷
èø
l
0
= Distance between points of zero moments in the beam
B = Total width of flange
b
w
= Width of web
Critical depth of Neutral axis (X
c
)
C
mc
Xd
tmc
æö
÷ ç
= ÷
ç
÷
÷ ç
èø +
? When Neutral axis is in flange area
o Actual depth of Neutral axis
2
()
2
a
st a
BX
mA d X =-
Here, X
a
= Actual depth of Neutral axis
? Moment of resistance (M
r
)
When Neutral axis is in web area
? For actual depth of neutral axis
Page 3
Short Notes on Concrete Structures
Working Stress Method
Modular Ratio
S
C
E
m
E
=
o m = Modular ratio
o E
S
= Modulus of elasticity of steel
o E
C
= Modulus of elasticity of concrete
Equivalent Area of Concrete
CS
AmA =
o A
C
= Area of concrete
o AS = Area of steel
Critical Depth of Neutral Axis (X
C
)
C
mc
X d
tmc
æö
÷ ç
= ÷
ç
÷
÷ ç
èø +
Here, D = Overall depth
d = Efffective depth
cbc
s = c = permissible stress in concrete
st
s = t = permissible stress in steel
Actual depth of Neutral axis (X
a
)
2
()
2
a
st a
BX
mA d x =-
Special case :
(i) when
ac
XX = for balanced section
(ii) when
ac
XX > for over reinforced section
(iii) when
ac
XX < for under reinforced section
Doubly Reinforced Rectangular Section
Critical depth of Neutral axis, (X
C
)
C
mc
Xd
tmc
=·
+
Actual depth of Neutral axis, (X
a
)
2
(1.5 1) ( ) ( )
2
a
SC a c st a
bX
mAX d mAdx +- - = -
Singly Reinforced T-Section
Effective width of flange
? For beam casted monolithic with slab
0
12
6
6
22
wf
f
w
l
bd
B Minimum or
ll
b
ìæö
ï
÷ ï ç
÷ ++
ï ç
÷
ç ï ÷ ç
èø
ï
ï
ï
ï
=
í
ï
ï
ï
ï
++
ï
ï
ï
ï î
? For isolated T-beam
0
0
4
fw
l
Bb
l
B
=+
æö
÷ ç
+ ÷
ç
÷
ç ÷
èø
l
0
= Distance between points of zero moments in the beam
B = Total width of flange
b
w
= Width of web
Critical depth of Neutral axis (X
c
)
C
mc
Xd
tmc
æö
÷ ç
= ÷
ç
÷
÷ ç
èø +
? When Neutral axis is in flange area
o Actual depth of Neutral axis
2
()
2
a
st a
BX
mA d X =-
Here, X
a
= Actual depth of Neutral axis
? Moment of resistance (M
r
)
When Neutral axis is in web area
? For actual depth of neutral axis
? Moment of resistance (M
r
)
Limit State Method
Design stress strain curve at ultimate state
? Design value of strength
o For concrete
0.67
0.45
1.5
ck
dck
mc
f f
ff
g
== =
mc
g = Partial factor of safety for concrete = 1.5
f
d
= design value of strength
o For steel
0.87
1.15
y
dy
f
ff ==
Singly Reinforced Beam
? Limiting depth of neutral axis (x
u, lim
)
,lim
700
0.87 1100
u
y
x d
f
=´
+
? Actual depth of neutral axis (X
u
)
0.87
0.36
yst
u
ck
fA
CT X
fb
=? =
? Lever arm = d – 0.42 X
u
? Ultimate moment of resistance
0.36 ( 0.42 )
ucku u
MfbXd X =-
0.87 ( 0.42 )
uyst u
MfAd X =-
Special cases
1. Under-reinforced section : X
u
< X
u,lim
0.36 ( 0.42 )
ucku u
MfbXd X =-
0.87 ( 0.42 )
uyst u
MfAd X =-
2. Balanced section: X
u
= X
u,lim
,lim ,lim
0.36 ( 0.42 )
ucku u
MfbXd X =-
,lim
0.87 ( 0.42 )
uyst u
MfAd X =-
3. Over reinforced section : X
u
> X
u,lim
X
u
limited to X
u,lim
Moment of resistance limited to (M
u,lim
)
Doubly Reinforced Section
Page 4
Short Notes on Concrete Structures
Working Stress Method
Modular Ratio
S
C
E
m
E
=
o m = Modular ratio
o E
S
= Modulus of elasticity of steel
o E
C
= Modulus of elasticity of concrete
Equivalent Area of Concrete
CS
AmA =
o A
C
= Area of concrete
o AS = Area of steel
Critical Depth of Neutral Axis (X
C
)
C
mc
X d
tmc
æö
÷ ç
= ÷
ç
÷
÷ ç
èø +
Here, D = Overall depth
d = Efffective depth
cbc
s = c = permissible stress in concrete
st
s = t = permissible stress in steel
Actual depth of Neutral axis (X
a
)
2
()
2
a
st a
BX
mA d x =-
Special case :
(i) when
ac
XX = for balanced section
(ii) when
ac
XX > for over reinforced section
(iii) when
ac
XX < for under reinforced section
Doubly Reinforced Rectangular Section
Critical depth of Neutral axis, (X
C
)
C
mc
Xd
tmc
=·
+
Actual depth of Neutral axis, (X
a
)
2
(1.5 1) ( ) ( )
2
a
SC a c st a
bX
mAX d mAdx +- - = -
Singly Reinforced T-Section
Effective width of flange
? For beam casted monolithic with slab
0
12
6
6
22
wf
f
w
l
bd
B Minimum or
ll
b
ìæö
ï
÷ ï ç
÷ ++
ï ç
÷
ç ï ÷ ç
èø
ï
ï
ï
ï
=
í
ï
ï
ï
ï
++
ï
ï
ï
ï î
? For isolated T-beam
0
0
4
fw
l
Bb
l
B
=+
æö
÷ ç
+ ÷
ç
÷
ç ÷
èø
l
0
= Distance between points of zero moments in the beam
B = Total width of flange
b
w
= Width of web
Critical depth of Neutral axis (X
c
)
C
mc
Xd
tmc
æö
÷ ç
= ÷
ç
÷
÷ ç
èø +
? When Neutral axis is in flange area
o Actual depth of Neutral axis
2
()
2
a
st a
BX
mA d X =-
Here, X
a
= Actual depth of Neutral axis
? Moment of resistance (M
r
)
When Neutral axis is in web area
? For actual depth of neutral axis
? Moment of resistance (M
r
)
Limit State Method
Design stress strain curve at ultimate state
? Design value of strength
o For concrete
0.67
0.45
1.5
ck
dck
mc
f f
ff
g
== =
mc
g = Partial factor of safety for concrete = 1.5
f
d
= design value of strength
o For steel
0.87
1.15
y
dy
f
ff ==
Singly Reinforced Beam
? Limiting depth of neutral axis (x
u, lim
)
,lim
700
0.87 1100
u
y
x d
f
=´
+
? Actual depth of neutral axis (X
u
)
0.87
0.36
yst
u
ck
fA
CT X
fb
=? =
? Lever arm = d – 0.42 X
u
? Ultimate moment of resistance
0.36 ( 0.42 )
ucku u
MfbXd X =-
0.87 ( 0.42 )
uyst u
MfAd X =-
Special cases
1. Under-reinforced section : X
u
< X
u,lim
0.36 ( 0.42 )
ucku u
MfbXd X =-
0.87 ( 0.42 )
uyst u
MfAd X =-
2. Balanced section: X
u
= X
u,lim
,lim ,lim
0.36 ( 0.42 )
ucku u
MfbXd X =-
,lim
0.87 ( 0.42 )
uyst u
MfAd X =-
3. Over reinforced section : X
u
> X
u,lim
X
u
limited to X
u,lim
Moment of resistance limited to (M
u,lim
)
Doubly Reinforced Section
? Limiting depth of neutral axis
,lim
700
0.87 1100
u
y
Xd
f
=´
+
? For actual depth of neutral axis (X
u
)
12
0.36 ( 0.45 ) 0.87
ck u sc ck sc y st
CT C C T
fbX f f A fA
=? + =
?
+- =
? Ultimate moment of resistance
0.36 ( 0.42 ) ( 0.45 ) ( )
ucku u sc cksc C
MfbXd X f fAdd =- +- -
f
SC
= stress in compression
T-Beam
? Limiting depth of neutral axis
,lim
700
0.87 1100
u
y
Xd
f
=´
+
? Singly reinforced T-Beam
o When NA is in flange area
X
u
< D
f
o
0.87
0.36
yst
uf
ck f
fA
XD
fb
=<
o Ultimate moment of resistance
0.36 ( 0.42 ) 0.87 ( 0.42 )
uckfu u u yst u
MfbXd XorM fAd X =- = -
o When NA is in web area
X
u
> D
f
? X
u
> D
f
and
3
7
fu
DX <
? For actual depth of neutral axis
0.36 0.45 ( ) 0.87
ck w u ck f w f y st
fbx f b b D fA +- =
? Ultimate moment of resistance
0.36 ( 0.42 ) 0.45 ( )
2
f
uckwu u ckfwf
D
Mfbxd x fbbDd
æö
÷ ç
=- + - - ÷
ç
÷
ç ÷
èø
12
0.87 ( 0.42 ) 0.87
2
f
u y st u y st
D
MfAd x fAd
æö
÷ ç
=- + - ÷
ç
÷
ç ÷
èø
12
0.36 0.45 ( )
,
0.87 0.87
ck w u ck f w f
st st
yy
fbx f b b D
AA
ff
-
==
? When Xu > Df and
3
7
fu
DX >
Page 5
Short Notes on Concrete Structures
Working Stress Method
Modular Ratio
S
C
E
m
E
=
o m = Modular ratio
o E
S
= Modulus of elasticity of steel
o E
C
= Modulus of elasticity of concrete
Equivalent Area of Concrete
CS
AmA =
o A
C
= Area of concrete
o AS = Area of steel
Critical Depth of Neutral Axis (X
C
)
C
mc
X d
tmc
æö
÷ ç
= ÷
ç
÷
÷ ç
èø +
Here, D = Overall depth
d = Efffective depth
cbc
s = c = permissible stress in concrete
st
s = t = permissible stress in steel
Actual depth of Neutral axis (X
a
)
2
()
2
a
st a
BX
mA d x =-
Special case :
(i) when
ac
XX = for balanced section
(ii) when
ac
XX > for over reinforced section
(iii) when
ac
XX < for under reinforced section
Doubly Reinforced Rectangular Section
Critical depth of Neutral axis, (X
C
)
C
mc
Xd
tmc
=·
+
Actual depth of Neutral axis, (X
a
)
2
(1.5 1) ( ) ( )
2
a
SC a c st a
bX
mAX d mAdx +- - = -
Singly Reinforced T-Section
Effective width of flange
? For beam casted monolithic with slab
0
12
6
6
22
wf
f
w
l
bd
B Minimum or
ll
b
ìæö
ï
÷ ï ç
÷ ++
ï ç
÷
ç ï ÷ ç
èø
ï
ï
ï
ï
=
í
ï
ï
ï
ï
++
ï
ï
ï
ï î
? For isolated T-beam
0
0
4
fw
l
Bb
l
B
=+
æö
÷ ç
+ ÷
ç
÷
ç ÷
èø
l
0
= Distance between points of zero moments in the beam
B = Total width of flange
b
w
= Width of web
Critical depth of Neutral axis (X
c
)
C
mc
Xd
tmc
æö
÷ ç
= ÷
ç
÷
÷ ç
èø +
? When Neutral axis is in flange area
o Actual depth of Neutral axis
2
()
2
a
st a
BX
mA d X =-
Here, X
a
= Actual depth of Neutral axis
? Moment of resistance (M
r
)
When Neutral axis is in web area
? For actual depth of neutral axis
? Moment of resistance (M
r
)
Limit State Method
Design stress strain curve at ultimate state
? Design value of strength
o For concrete
0.67
0.45
1.5
ck
dck
mc
f f
ff
g
== =
mc
g = Partial factor of safety for concrete = 1.5
f
d
= design value of strength
o For steel
0.87
1.15
y
dy
f
ff ==
Singly Reinforced Beam
? Limiting depth of neutral axis (x
u, lim
)
,lim
700
0.87 1100
u
y
x d
f
=´
+
? Actual depth of neutral axis (X
u
)
0.87
0.36
yst
u
ck
fA
CT X
fb
=? =
? Lever arm = d – 0.42 X
u
? Ultimate moment of resistance
0.36 ( 0.42 )
ucku u
MfbXd X =-
0.87 ( 0.42 )
uyst u
MfAd X =-
Special cases
1. Under-reinforced section : X
u
< X
u,lim
0.36 ( 0.42 )
ucku u
MfbXd X =-
0.87 ( 0.42 )
uyst u
MfAd X =-
2. Balanced section: X
u
= X
u,lim
,lim ,lim
0.36 ( 0.42 )
ucku u
MfbXd X =-
,lim
0.87 ( 0.42 )
uyst u
MfAd X =-
3. Over reinforced section : X
u
> X
u,lim
X
u
limited to X
u,lim
Moment of resistance limited to (M
u,lim
)
Doubly Reinforced Section
? Limiting depth of neutral axis
,lim
700
0.87 1100
u
y
Xd
f
=´
+
? For actual depth of neutral axis (X
u
)
12
0.36 ( 0.45 ) 0.87
ck u sc ck sc y st
CT C C T
fbX f f A fA
=? + =
?
+- =
? Ultimate moment of resistance
0.36 ( 0.42 ) ( 0.45 ) ( )
ucku u sc cksc C
MfbXd X f fAdd =- +- -
f
SC
= stress in compression
T-Beam
? Limiting depth of neutral axis
,lim
700
0.87 1100
u
y
Xd
f
=´
+
? Singly reinforced T-Beam
o When NA is in flange area
X
u
< D
f
o
0.87
0.36
yst
uf
ck f
fA
XD
fb
=<
o Ultimate moment of resistance
0.36 ( 0.42 ) 0.87 ( 0.42 )
uckfu u u yst u
MfbXd XorM fAd X =- = -
o When NA is in web area
X
u
> D
f
? X
u
> D
f
and
3
7
fu
DX <
? For actual depth of neutral axis
0.36 0.45 ( ) 0.87
ck w u ck f w f y st
fbx f b b D fA +- =
? Ultimate moment of resistance
0.36 ( 0.42 ) 0.45 ( )
2
f
uckwu u ckfwf
D
Mfbxd x fbbDd
æö
÷ ç
=- + - - ÷
ç
÷
ç ÷
èø
12
0.87 ( 0.42 ) 0.87
2
f
u y st u y st
D
MfAd x fAd
æö
÷ ç
=- + - ÷
ç
÷
ç ÷
èø
12
0.36 0.45 ( )
,
0.87 0.87
ck w u ck f w f
st st
yy
fbx f b b D
AA
ff
-
==
? When Xu > Df and
3
7
fu
DX >
0.15 0.65
fu ff
yX DD =+ <
? For actual depth of neutral axis
12
0.36 0.45 ( ) 0.87 0.87
ck w u ck f w f y st y st
fbX f b b y fA fA +- = +
0.36 0.45 ( ) 0.87
ck w u ck f w f y st
fbX f b b y fA +- =
Design Beams and Slabs and Columns
Effective span
Simply supported beams and slabs ( l
eff
)
?
0
0
minimum
eff
lw
l
ld
ì
+ ï
ï
=
í
ï +
ï î
Here, l
0
= clear span
w = width of support
d = depth of beam or slab
For continuous beam
? If width of support
1
12
< of clear span
0
0
minimum
eff
lw
l
ld
ì
+ ï
ï
=
í
ï +
ï î
? If width of support
1
12
> of clear span
o When one end fixed other end continuous or both end continuous.
l
eff
= l
0
o When one end continuous and other end simply supported
Read More