Reinforced Concrete Column1

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Reinforced concrete column


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Introduction to column

Columns act as vertical supports to beams andslabs, and to transmit the loads to thefoundations.

Columns are primarily compression members,although they may also have to resist bendingmoment transmitted by beams.

Columns may be classified as short or slender,braced or unbraced depending on variousdimensional and structural factors.


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Column sections

Common column cross sections are: (a)

square, (b) circular and (c) rectangular section.

The greatest dimension should not exceed

four times its smaller dimension. (h4b).

For h>4b, the member should be regarded as

a wall for design purpose.


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Failure modes of columns

Columns may fail in one of three mechanisms:

1. Compression failure of the concrete or steelreinforcement;

2. Buckling3. Combination of buckling and compressionfailure.

Compression failure is likely to occur with

columns which are short and stocky. Buckling is probable with columns which are long

and slender.


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Failure modes of columns

Compression

failureBuckling


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Short and slender columns (Clause

3.8.1.3, BS 8110)

A braced column is classified as being short if :


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Braced and unbraced columns (clause

3.8.1.5, BS 8110)

A column may be considered braced in a given

plane if lateral stability to the structure as a

whole is provided by wall or bracing or

buttressing designed to resist all lateral forces

in that plane. It should otherwise be

considered as unbraced.


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Braced and unbraced columns (clause 3.8.1.5,

BS 8110)


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Braced and unbraced columns (clause 3.8.1.5,

BS 8110)


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Effective height of column (clause

3.8.1.6, BS 8110)

The effective height, le of a column in a givenplane may be obtained from the followingequation:

Where bis a coefficient depending on the fixityat the column ends and lo is the height of the

columns. Effective height for a column in two plane

directions may be different.


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Effective height of column (clause

3.8.1.6, BS 8110)

bfor braced column can be obtained fromTable 3.19.

End condition 1 signifies that the column end is fully

restrained. End condition 2 signifies that the column end is partially

restrained .

End condition 3 signifies that the column is nominallyrestrained.


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End conditions (clause 3.8.1.6.2, BS 8110)

End condition 1the end of the column is

connected monolithically to beams on either sidewhich are at least as deep as the overall dimension

of the column in the plane considered. Where the

column is connected to foundation, it should be

designed to carry moment.


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End condition 2the end of column is connectedmonolithically to beams or slabs on either side which

are shallower than the overall dimension of the

column in the plane considered.



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End condition 3the end of the column is

connected to members which, while not specifically

designed to provide restraint to rotation of the

column will nevertheless, provide some nominal

restraint.



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Example 3.17 classification of column (Arya,

2009)

Determine if the column shown below is short.


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Example 3.17 classification of column (Arya,

2009)


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Short column design

The short column are divided into three

categories:

1. Columns resisting axial load only,

2. Columns supporting an approximately

symmetrical arrangement of beams,

3. Columns resisting axial loads and uniaxial or

biaxial bending


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B2 will resist an axial load only, as it supports beams

equal in length and symmetrically arranged.


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C2 supports a symmetrical arrangement of beams

but which are unequal in length. If (a) the loadings

on the beam are uniformly distributed, (2) the beamspans do not differ by more than 15 percent, the

column C2 belongs to category 2.

If the column does not meet criteria (a) and (b), then

the column belon s to cate or 3.


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Theoretical strength of reinforced concrete

column

The equation is derived on the assumption that the axial load is

applied perfectly at the centre of the column.


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Clause 3.8.4.3 Nominal eccentricity of short columns

resisting moments and axial force

To allow for nominal eccentricity, BS 8110reduce the theoretical axial load capacity by

about 10%.

Design maximum axial load capacity of shortcolumn is:


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Clause 3.8.4.4 Short braced columns supporting an

approximately symmetrical arrangement of beam

The column is subjected to axial and small

moment when it supports approximately

symmetrical arrangement of beams:

The design axial load capacity:


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Column resisting an axial load and

uniaxial bending

For column resisting axial load and bending moment

at one direction, the area of longitudinal

reinforcement is calculated using design charts in

Part 3 BS 8110. The design charts are available for columns having a

rectangular cross section and symmetrical

arrangement of reinforcement.


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uniaxial bending

Design charts are derived based on yield stress of460 N/mm2for reinforcement steel. They areapplicable for reinforcement with yield stress of

500 N/mm

2

, but the area of reinforcementobtained will be approximately 10% greater thanrequired.

Design charts are available for concrete grades

25, 30, 35, 40, 45 and 50. The d/h ratios are in the range of 0.75 to 0.95 in

0.05 increment.


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Design chart for column resisting an axial load and

uniaxial bending moment, (Part 3, BS 8110)


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biaxial bending

The columns are subjected to an

axial and bending moment in both x

and y directions.

The columns with biaxial momentsare simplified into the columns with

uniaxial moment by increasing the

moment about one of the axes thendesign the reinforcement according

the increased moment.


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Column resisting an axial load and biaxial

bending (clause 3.8.4.5, BS 8110)

f d l l d l


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Reinforcement details: longitudinal

reinforcement (clause 3.12.5, BS 8110)

1. Size and minimum number of barsbar size should not be

less than 12 mm in diameter. Rectangular column should

reinforced with minimum 4 bars; circular column should

reinforced with minimum 6 bars.

2. The area of longitudinal reinforcement should lie in the

limits:

3. Spacing of reinforcementthe minimum distance between

adjacent bars should not be less than the diameter of the

bar or hagg + 5 mm.


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Reinforcement detailslinks (clause 3.12.7, BS 8110)

The axial loading on the column may cause bucklingof the longitudinal reinforcement and subsequent

cracking and spalling of concrete cover.

Links are passing round the bars to prevent buckling.


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1. Size and spacing of linksthe diameter ofthe link should be at least one quarter of the

largest longitudinal bar size or minimum 8

mm. The maximum spacing is 12 times of the

smallest longitudinal bar.

2. Arrangement of links

Reinforcement detailslinks (clause 3.12.7, BS 8110)


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Example 3.20 axially loaded column (Arya, 2009)

Design the longitudinal and links for a 350mm square, shortbraced column based on following information.


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Example 3.20 axially loaded column (Arya, 2009)

E l 3 21 C l i i l


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Example 3.21 Column supporting an approximately

symmetrical arrangement of beam ( Arya, 2009)

An internal column in a braced two-storey building supporting

an approximately symmetrical arrangement of beams

(350mm wide x 600 mm deep) results in characteristic dead

and imposed loads each of 1100 kN being applied to the

column. The column is 350 mm square and has a clear height

of 4.5 m. Design the longitudinal reinforcement and links.

E l 3 21 C l ti i t l


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Links

Flink= diameter of largest longitudinal bar/4

= 32/4 = 8 mm (equal to minimum bar size of 8 mm)

The spacing of the links

= the lesser of (12 smallest longitudinal bar or the smallest cross

sectional dimension of column)

= the lesser of (12x25 = 300 mm or 350 mm)

= 300 mm




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Example 3.22 Columns resisting an

axial load and bending moment

Design the longitudinal and shear reinforcement for

a 275 mm square, short braced column which

supports either

(a) An ultimate axial load of 1280 kN and a moment of62.5 kNm about the x-x axis

(b) An ultimate axial load of 1280 kN and bending

moment of 35 kNm about the x-x axis and 25 kNm

about the y-y axis


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Example 3.22 Columns resisting an axial load and bending moment


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