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CASED COLUMNS 
 
Encased I sections or filled hollow sections carries more load. In cased columns, 
the advantages derived from the properties of concrete and steel are used. The 
concrete is strong in stronger in compression and it provides greater rigidity. The 
solid concrete casing assists in carrying the load and the entire load is resisted by 
concrete and steel. The design of the above columns is currently based on IS 
11384-1985. As the above code is on working stress method the guide lines given 
in BS5950, Part I is presented here. The role of concrete is that it acts as a fire 
protection for the encased steel columns and also prevents the column from 
buckling about the weak axis. As per the BS5950, Part I the column must satisfy 
the following specifications. 
 
(i) The steel section is either a single rolled or fabricated I or H section 
with equal flanges, channels and compound sections can also be used. 
(ii) The steel section should not exceed 1000mmx500mm. The dimension 
100mm is in the direction of web. 
(iii) Primary structural connections should be made in the steel section. 
(iv) The steel section is unpainted and free from dirt, grease, rust, scale etc. 
(v) The steel section is encased in concrete of at least Grade 20, to BS 
8110. 
(vi) The cover on the steel is to be not less than 50mm. The corners may be 
chamfered. 
(vii) The concrete extends the full length of the member and is thoroughly 
compacted. 
(viii) The casing is reinforced with bars not less than 5mm diameter at a 
maximum spacing of 200mm to form a cage of closed links and 
longitudinal bars. The reinforcement is to pass through the centre of 
the cover. 
(ix) The effective length is not to exceed 40b
c
, 100b
2
c 
/ d
c
 or 250 r 
whichever is the least, where  
b
c
  = minimum width of solid casing. 
d
c
 = minimum depth of solid casing. 
r = minimum radius of gyration of steel section. 
Page 2


                                              
CASED COLUMNS 
 
Encased I sections or filled hollow sections carries more load. In cased columns, 
the advantages derived from the properties of concrete and steel are used. The 
concrete is strong in stronger in compression and it provides greater rigidity. The 
solid concrete casing assists in carrying the load and the entire load is resisted by 
concrete and steel. The design of the above columns is currently based on IS 
11384-1985. As the above code is on working stress method the guide lines given 
in BS5950, Part I is presented here. The role of concrete is that it acts as a fire 
protection for the encased steel columns and also prevents the column from 
buckling about the weak axis. As per the BS5950, Part I the column must satisfy 
the following specifications. 
 
(i) The steel section is either a single rolled or fabricated I or H section 
with equal flanges, channels and compound sections can also be used. 
(ii) The steel section should not exceed 1000mmx500mm. The dimension 
100mm is in the direction of web. 
(iii) Primary structural connections should be made in the steel section. 
(iv) The steel section is unpainted and free from dirt, grease, rust, scale etc. 
(v) The steel section is encased in concrete of at least Grade 20, to BS 
8110. 
(vi) The cover on the steel is to be not less than 50mm. The corners may be 
chamfered. 
(vii) The concrete extends the full length of the member and is thoroughly 
compacted. 
(viii) The casing is reinforced with bars not less than 5mm diameter at a 
maximum spacing of 200mm to form a cage of closed links and 
longitudinal bars. The reinforcement is to pass through the centre of 
the cover. 
(ix) The effective length is not to exceed 40b
c
, 100b
2
c 
/ d
c
 or 250 r 
whichever is the least, where  
b
c
  = minimum width of solid casing. 
d
c
 = minimum depth of solid casing. 
r = minimum radius of gyration of steel section. 
                                              
BS5950, Part I guidelines for estimating the compressive strength of column. 
 
a) The radius of gyration about yy axis is shown in figure, r
y
 should be taken 
as 0.2b
c
 but not more than 0.2 (B+150) where B = overall width of flange. 
The radius of gyration for the zz axis should be taken as that of the steel 
section. 
b) The compression resistance 
c
P is 
 
cs c
y
cu
g c
P p
p
A f
A P =
?
?
?
?
?
?
?
?
+ = 45 . 0 
 
y c
y
cu
g cs
p A
p
f
A P
?
?
?
?
?
?
?
?
+ = 25 . 0 
 
Where 
c
A = gross sectional area of concrete. Casing in excess of 75mm from the 
steel section is neglected. Finish is neglected. 
 
g
A =gross area of the steel section 
 
cu
f =characteristic strength of the concrete at 28 days. This should not exceed 
40N/mm
2
. 
 
c
p =compressive strength of steel section determined using 
x
r and 
z
r in the 
determination of which 
2
/ 335 mm N p
y
= 
 
y
p = design strength of steel 
 
Cased Column 
 
Page 3


                                              
CASED COLUMNS 
 
Encased I sections or filled hollow sections carries more load. In cased columns, 
the advantages derived from the properties of concrete and steel are used. The 
concrete is strong in stronger in compression and it provides greater rigidity. The 
solid concrete casing assists in carrying the load and the entire load is resisted by 
concrete and steel. The design of the above columns is currently based on IS 
11384-1985. As the above code is on working stress method the guide lines given 
in BS5950, Part I is presented here. The role of concrete is that it acts as a fire 
protection for the encased steel columns and also prevents the column from 
buckling about the weak axis. As per the BS5950, Part I the column must satisfy 
the following specifications. 
 
(i) The steel section is either a single rolled or fabricated I or H section 
with equal flanges, channels and compound sections can also be used. 
(ii) The steel section should not exceed 1000mmx500mm. The dimension 
100mm is in the direction of web. 
(iii) Primary structural connections should be made in the steel section. 
(iv) The steel section is unpainted and free from dirt, grease, rust, scale etc. 
(v) The steel section is encased in concrete of at least Grade 20, to BS 
8110. 
(vi) The cover on the steel is to be not less than 50mm. The corners may be 
chamfered. 
(vii) The concrete extends the full length of the member and is thoroughly 
compacted. 
(viii) The casing is reinforced with bars not less than 5mm diameter at a 
maximum spacing of 200mm to form a cage of closed links and 
longitudinal bars. The reinforcement is to pass through the centre of 
the cover. 
(ix) The effective length is not to exceed 40b
c
, 100b
2
c 
/ d
c
 or 250 r 
whichever is the least, where  
b
c
  = minimum width of solid casing. 
d
c
 = minimum depth of solid casing. 
r = minimum radius of gyration of steel section. 
                                              
BS5950, Part I guidelines for estimating the compressive strength of column. 
 
a) The radius of gyration about yy axis is shown in figure, r
y
 should be taken 
as 0.2b
c
 but not more than 0.2 (B+150) where B = overall width of flange. 
The radius of gyration for the zz axis should be taken as that of the steel 
section. 
b) The compression resistance 
c
P is 
 
cs c
y
cu
g c
P p
p
A f
A P =
?
?
?
?
?
?
?
?
+ = 45 . 0 
 
y c
y
cu
g cs
p A
p
f
A P
?
?
?
?
?
?
?
?
+ = 25 . 0 
 
Where 
c
A = gross sectional area of concrete. Casing in excess of 75mm from the 
steel section is neglected. Finish is neglected. 
 
g
A =gross area of the steel section 
 
cu
f =characteristic strength of the concrete at 28 days. This should not exceed 
40N/mm
2
. 
 
c
p =compressive strength of steel section determined using 
x
r and 
z
r in the 
determination of which 
2
/ 335 mm N p
y
= 
 
y
p = design strength of steel 
 
Cased Column 
 
                                              
CASED COLUMN WITH AXIAL LOAD 
 
Ex.17 An internal column in a building has an actual length of 4.5m centre to 
centre of floor beams. The steel section is ISHB250 @ 51kg/m. Calculate the 
compression resistance of the column if it is cased in accordance with the 
codal provision. M25 concrete grade has been use. The casing has been made 
325mm square. 
 
 
Properties of ISHB 250 
 
A=6496mm
2
 
 
zz
r =10.91cm 
 
yy
r =5.49cm 
 
For the above cased column; 
 
( )
( ) mm
mm r
y
80 150 250 2 . 0
65 325 2 . 0
= + ?
= =
 
 
i) effective length = 0.7 (4500) = 3150mm of cased column 
 
ii) 40
c
b =40(325) = 13000mm 
 
iii) 100
c
c
d
b
2
=100x325=32500mm 
 
iv) 250r =250x54.9=13725mm 
 
slenderness ratio = 46 . 48
65
3150
= =
r
kL
 
 
refer Table 9(c) in P42, IS800:2007. 
Page 4


                                              
CASED COLUMNS 
 
Encased I sections or filled hollow sections carries more load. In cased columns, 
the advantages derived from the properties of concrete and steel are used. The 
concrete is strong in stronger in compression and it provides greater rigidity. The 
solid concrete casing assists in carrying the load and the entire load is resisted by 
concrete and steel. The design of the above columns is currently based on IS 
11384-1985. As the above code is on working stress method the guide lines given 
in BS5950, Part I is presented here. The role of concrete is that it acts as a fire 
protection for the encased steel columns and also prevents the column from 
buckling about the weak axis. As per the BS5950, Part I the column must satisfy 
the following specifications. 
 
(i) The steel section is either a single rolled or fabricated I or H section 
with equal flanges, channels and compound sections can also be used. 
(ii) The steel section should not exceed 1000mmx500mm. The dimension 
100mm is in the direction of web. 
(iii) Primary structural connections should be made in the steel section. 
(iv) The steel section is unpainted and free from dirt, grease, rust, scale etc. 
(v) The steel section is encased in concrete of at least Grade 20, to BS 
8110. 
(vi) The cover on the steel is to be not less than 50mm. The corners may be 
chamfered. 
(vii) The concrete extends the full length of the member and is thoroughly 
compacted. 
(viii) The casing is reinforced with bars not less than 5mm diameter at a 
maximum spacing of 200mm to form a cage of closed links and 
longitudinal bars. The reinforcement is to pass through the centre of 
the cover. 
(ix) The effective length is not to exceed 40b
c
, 100b
2
c 
/ d
c
 or 250 r 
whichever is the least, where  
b
c
  = minimum width of solid casing. 
d
c
 = minimum depth of solid casing. 
r = minimum radius of gyration of steel section. 
                                              
BS5950, Part I guidelines for estimating the compressive strength of column. 
 
a) The radius of gyration about yy axis is shown in figure, r
y
 should be taken 
as 0.2b
c
 but not more than 0.2 (B+150) where B = overall width of flange. 
The radius of gyration for the zz axis should be taken as that of the steel 
section. 
b) The compression resistance 
c
P is 
 
cs c
y
cu
g c
P p
p
A f
A P =
?
?
?
?
?
?
?
?
+ = 45 . 0 
 
y c
y
cu
g cs
p A
p
f
A P
?
?
?
?
?
?
?
?
+ = 25 . 0 
 
Where 
c
A = gross sectional area of concrete. Casing in excess of 75mm from the 
steel section is neglected. Finish is neglected. 
 
g
A =gross area of the steel section 
 
cu
f =characteristic strength of the concrete at 28 days. This should not exceed 
40N/mm
2
. 
 
c
p =compressive strength of steel section determined using 
x
r and 
z
r in the 
determination of which 
2
/ 335 mm N p
y
= 
 
y
p = design strength of steel 
 
Cased Column 
 
                                              
CASED COLUMN WITH AXIAL LOAD 
 
Ex.17 An internal column in a building has an actual length of 4.5m centre to 
centre of floor beams. The steel section is ISHB250 @ 51kg/m. Calculate the 
compression resistance of the column if it is cased in accordance with the 
codal provision. M25 concrete grade has been use. The casing has been made 
325mm square. 
 
 
Properties of ISHB 250 
 
A=6496mm
2
 
 
zz
r =10.91cm 
 
yy
r =5.49cm 
 
For the above cased column; 
 
( )
( ) mm
mm r
y
80 150 250 2 . 0
65 325 2 . 0
= + ?
= =
 
 
i) effective length = 0.7 (4500) = 3150mm of cased column 
 
ii) 40
c
b =40(325) = 13000mm 
 
iii) 100
c
c
d
b
2
=100x325=32500mm 
 
iv) 250r =250x54.9=13725mm 
 
slenderness ratio = 46 . 48
65
3150
= =
r
kL
 
 
refer Table 9(c) in P42, IS800:2007. 
                                              
2
/ 3 . 185 15
10
46 . 8
198 mm N x f
cd
= - = 
 
 
The gross sectional area of concrete  
 
2
105625 325 325 mm x A
c
= = 
 
Compressive strength of concrete 
 
kN x x P
c
5 . 2084
1000
3 . 185
105625
250
25
45 . 0 6496 = ?
?
?
?
?
?
+ = 
 
Short column strength  
 
kN
x
x P
cs
2284
1000
250
250
105625 25
25 . 0 6496 = ?
?
?
?
?
?
+ = 
 
Compressive strength of column = 2084.5kN 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Page 5


                                              
CASED COLUMNS 
 
Encased I sections or filled hollow sections carries more load. In cased columns, 
the advantages derived from the properties of concrete and steel are used. The 
concrete is strong in stronger in compression and it provides greater rigidity. The 
solid concrete casing assists in carrying the load and the entire load is resisted by 
concrete and steel. The design of the above columns is currently based on IS 
11384-1985. As the above code is on working stress method the guide lines given 
in BS5950, Part I is presented here. The role of concrete is that it acts as a fire 
protection for the encased steel columns and also prevents the column from 
buckling about the weak axis. As per the BS5950, Part I the column must satisfy 
the following specifications. 
 
(i) The steel section is either a single rolled or fabricated I or H section 
with equal flanges, channels and compound sections can also be used. 
(ii) The steel section should not exceed 1000mmx500mm. The dimension 
100mm is in the direction of web. 
(iii) Primary structural connections should be made in the steel section. 
(iv) The steel section is unpainted and free from dirt, grease, rust, scale etc. 
(v) The steel section is encased in concrete of at least Grade 20, to BS 
8110. 
(vi) The cover on the steel is to be not less than 50mm. The corners may be 
chamfered. 
(vii) The concrete extends the full length of the member and is thoroughly 
compacted. 
(viii) The casing is reinforced with bars not less than 5mm diameter at a 
maximum spacing of 200mm to form a cage of closed links and 
longitudinal bars. The reinforcement is to pass through the centre of 
the cover. 
(ix) The effective length is not to exceed 40b
c
, 100b
2
c 
/ d
c
 or 250 r 
whichever is the least, where  
b
c
  = minimum width of solid casing. 
d
c
 = minimum depth of solid casing. 
r = minimum radius of gyration of steel section. 
                                              
BS5950, Part I guidelines for estimating the compressive strength of column. 
 
a) The radius of gyration about yy axis is shown in figure, r
y
 should be taken 
as 0.2b
c
 but not more than 0.2 (B+150) where B = overall width of flange. 
The radius of gyration for the zz axis should be taken as that of the steel 
section. 
b) The compression resistance 
c
P is 
 
cs c
y
cu
g c
P p
p
A f
A P =
?
?
?
?
?
?
?
?
+ = 45 . 0 
 
y c
y
cu
g cs
p A
p
f
A P
?
?
?
?
?
?
?
?
+ = 25 . 0 
 
Where 
c
A = gross sectional area of concrete. Casing in excess of 75mm from the 
steel section is neglected. Finish is neglected. 
 
g
A =gross area of the steel section 
 
cu
f =characteristic strength of the concrete at 28 days. This should not exceed 
40N/mm
2
. 
 
c
p =compressive strength of steel section determined using 
x
r and 
z
r in the 
determination of which 
2
/ 335 mm N p
y
= 
 
y
p = design strength of steel 
 
Cased Column 
 
                                              
CASED COLUMN WITH AXIAL LOAD 
 
Ex.17 An internal column in a building has an actual length of 4.5m centre to 
centre of floor beams. The steel section is ISHB250 @ 51kg/m. Calculate the 
compression resistance of the column if it is cased in accordance with the 
codal provision. M25 concrete grade has been use. The casing has been made 
325mm square. 
 
 
Properties of ISHB 250 
 
A=6496mm
2
 
 
zz
r =10.91cm 
 
yy
r =5.49cm 
 
For the above cased column; 
 
( )
( ) mm
mm r
y
80 150 250 2 . 0
65 325 2 . 0
= + ?
= =
 
 
i) effective length = 0.7 (4500) = 3150mm of cased column 
 
ii) 40
c
b =40(325) = 13000mm 
 
iii) 100
c
c
d
b
2
=100x325=32500mm 
 
iv) 250r =250x54.9=13725mm 
 
slenderness ratio = 46 . 48
65
3150
= =
r
kL
 
 
refer Table 9(c) in P42, IS800:2007. 
                                              
2
/ 3 . 185 15
10
46 . 8
198 mm N x f
cd
= - = 
 
 
The gross sectional area of concrete  
 
2
105625 325 325 mm x A
c
= = 
 
Compressive strength of concrete 
 
kN x x P
c
5 . 2084
1000
3 . 185
105625
250
25
45 . 0 6496 = ?
?
?
?
?
?
+ = 
 
Short column strength  
 
kN
x
x P
cs
2284
1000
250
250
105625 25
25 . 0 6496 = ?
?
?
?
?
?
+ = 
 
Compressive strength of column = 2084.5kN 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
                                              
Column with axial load and moment 
 
Ex.18 A stanchion carries an factorial axial load 500kN and a factored 
bending moment of 250kNm. Design the section if the length is 6m and one 
end of the column is restrained in position and direction whereas, other end is 
restrained only in position but not in direction. 
 
 
Try section ISWB 600@ 133.7kg/m 
 
43 . 91
5 . 52
6000 8 . 0
= =
x
r
kL
 
 
Yura suggested 
 
b
M
d
M
P p
y
z
eff
5 . 7 2 + + = for initializing the size of the column. If the BM is 
predominant then the equivalent BM can be found out from  
 
2
d
P M M
u z eq
+ = 
 
In this case; 
 
kN x
d
M
P P
z
eff
33 . 1333
6 . 0
250
2 500 2 = + = + = 
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