Axially Loaded Column

PREPARED BY:
Asst. Prof. GAURANG PRAJAPATI
CIVIL DEPARTMENT
AXIALLY LOADED COLUMN
Mahatma Gandhi Institute Of
Technical Education
& Research Centre, Navsari (396450)
ELEMENTARY STRUCTURAL DESIGN
6TH SEMESTER
CIVIL ENGINEERING

INTRODUCTION
 Column :
• If for a compression member, effective length > 3 x least lateral dimension, it is
called column.
 Pedestal :
• If for a compression member, effective length < 3 x least lateral dimension, it is
called pedestal.
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TYPES OF COLUMN
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A. As per Bracing:
1. Braced column:
2. Unbraced Column:
B. As per Length:
1. Long column:
2. Short column:
C. As per Lateral reinforcement & Longitudinal reinforcement:
1. Tied column:
2. Spiral column:
3. Composite column

TYPES OF COLUMN
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A. As per Bracing:
1. Braced column:
 In braced frames, lateral loads like wind,
earthquake, etc. Are resisted by some special
arrangements like shear wall, bracing, etc.
 Side sway is not possible in such columns.

TYPES OF COLUMN
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A. As per Bracing:
2. Unbraced Column :
 In unbraced frames, No special bracing systems
are provided to resist horizontal forces.
 Side sway occurs in such columns.
 Such columns shall have to be designed to resist
horizontal loads.

TYPES OF COLUMN
1. Long column : (slender column)
 If
𝐿𝑒
𝐷
> 12 &
𝐿𝑒
𝑏
> 12 → long column
 Where D= depth, b= width , Le= eff. Column length
 It is failed by material failure & buckling together.
 Bucking is dominant.
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B. As per Length:

TYPES OF COLUMN
2. Short column :
 If
𝐿𝑒
𝐷
< 12 &
𝐿𝑒
𝑏
< 12 → short column
 Where D= depth, b= width , Le= eff. Column length
 It is failed by material failure only.
 Buckling is negligible.
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B. As per Length:

TYPES OF COLUMN
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 Longitudinal reinforcement is provided in the column to increase its load carrying
capacity.
 When column is subjected to compressive force, length of column decreases and
lateral dimensions increases.
 Thus, the longitudinal bars assume the shape of bow and developing cracks on
the outer face of column.
 It may cause failure of column.

TYPES OF COLUMN
1. Tied column :
 When lateral ties are provided as transverse reinforcement in columns, they are
called tied columns.
 They may be used for square, rectangle, circular, L, T-shape columns.
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TYPES OF COLUMN
2. Spiral Column :
 When longitudinal bars are covered laterally by a wire in the form of helix or
spiral, they are called Spiral columns.
 They are normally provide in circular columns.
 Due to higher ductility, spiral columns are suitable for earthquake resistance.
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TYPES OF COLUMN
3. Composite column :
 When column are provided with structural steel sections like I, H, steel tube, etc.
In place of longitudinal rein, it is called composite column.
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MINIMUM ECCENTRICITY IN COLUMN:
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 All columns shall be designed for minimum eccentricity.
emin =
𝒍
𝟓𝟎𝟎
+
𝑫
𝟑𝟎
IS 456
Clause No.: 25.4
Page No.: 42
where
l = unsupported length of column (eff. Span)
D = lateral dimension (overall depth)
 Minimum eccentricity for column should not be less than 20 mm.
emin > 20 mm IS 456
Clause No.: 25.4
Page No.: 42

FORCE (LOAD) ALONG THE COLUMN (PU):
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 All columns shall be designed for minimum eccentricity.
Pu = 0.4 fck Ac + 0.67 fy Asc
IS 456
Clause No.: 39.3
Page No.: 71
where
fck = Grade of concrete
fy = Grade of steel
Ac = Area of concrete
= Ag- Asc
Asc = Area of longitudinal reinforcement for column
= n ×
𝜋
4
d2
Ag = Gross area of column
= b x D → for Rectangle column
= b2 → for Square column
=
𝜋
4
D2 → For Circular column

REINFORCEMENT REQUIREMENTS OF COLUMNS:
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 Min steel = 0.8 % of Ag IS 456,
Clause : 26.5.3.1 (a),
Page No.: 48 Max steel = 6 % of Ag
 For Rectangle column → Min. No. of bar = 4 IS 456,
Clause : 26.5.3.1 (c),
Page No.: 48 For Circular column → Min. No. of bar = 6 (min)
 Min. diameter of bar = 12mm
IS 456,
Clause : 26.5.3.1 (d),
Page No.: 48
 Spacing of bar ≯300 mm
IS 456,
Clause : 26.5.3.1 (g),
Page No.: 48

LATERAL TIES:
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 Pitch : The vertical distance between two successive (consecutive) ties in column is called
pitch.
 It is taken minimum of
1. Least (min) Lateral dimension (min of b & D)
2. 16 x ∅min
3. 300 mm
IS 456
Clause No.: 26.5.3.2.d (2)
Page No.: 71
 Diameter of Lateral ties :
 It is taken maximum of
1.
𝟏
𝟒
x ∅max
2. 6 mm
IS 456
Clause No.: 26.5.3.2.d (1)
Page No.: 71

LATERAL TIES:
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COLUMNS WITH HELICAL REINFORCEMENT (CIRCULAR COLUMN):
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 Fact load = 1.05 x Design fact load (Pu)
 Strength of column with helical reinforcement = 1.05 x Strength of tied column
Pu =
𝐅𝐚𝐜𝐭 𝐥𝐨𝐚𝐝
𝟏.𝟎𝟓
IS 456
Clause No.: 39.4
Page No.: 71

SELECTION OF SIZE OF COLUMN:
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 Minimum eccentricity for column should not be less than 20 mm.
emin ≤ 0.05 (lateral dimension)
emin ≤ 0.05 (D)
IS 456
Clause No.: 39.3
Page No.: 71
20 ≤ 0.05 D
20
0.05
≤ D
D ≥ 400 mm
 Min. Dimension of column is greater than or equal to 400 mm.

LATERAL TIES:
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 They are used in columns.
 The load is carried by the vertical steel bars, while the ties are provided to keep the
bars in their required position without getting disturbed while compacting the
concrete by the needle vibrators

PROVISION OF LATERAL TIES (OPEN/ CLOSE):
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 If 48 ф > c/c distance between corner bars, Provide open tie.
 If 48 ф < c/c distance between corner bars, Provide close tie
Where, Ф = diamete of lateral tie
 c/c distance between bar
For Square column  c/c distance = D – 2 (c.c) – 2 (
∅
2
)
For Rectangle
column
 c/c distance = b – 2 (c.c) - 2 (
∅
2
) (Along shorter side)
 c/c distance = D – 2 (c.c) – 2 (
∅
2
) ( Along longer side)
Where, c.c. = clear cover for column = 50 mm
Ф = diameter of main steel at corner (Greater diameter)
b = smaller dimension of column
D = greater (Max.) dimension of column

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 Examples of open tie:

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 Examples of close tie:
 If along one side, open tie has to be provided and
Along the other side, close tie has to be provided.
(Normally in Rectangle column)
 If along both side, close ties
are provided

Axially Loaded Column

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