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Introduction to Columns

CE 4108 – Concrete Structures Design 

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Three categories

1. Short compression blocks or pedestals2. Short reinforced concrete columns

 .  columns

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Short compression blocks or pedestals

• Height is less than three times it leastlateral dimension.

•  ACI 2.2 and 10.14: Ma be desi ned

with plain concrete with a designcompressive strength if 0.85φf’c whereφ = 0.65.

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Short reinforced concrete columns

•  A stocky member with little flexibility.Would fail first due to material failure.

• The load it su orts is controlled b the

size of the cross section and by thestrength of the materials.

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Long or slender reinforced concrete columns

• Have large bending deformations thatresult in secondary moments thatreduce the axial load capacity of the

columns.

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Types of columns

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 Axial Load Capacity of Columns

[ ]st  yst gcn A f  A A f P +−=

=

)(85.085.0:)75.0(columnsspiralFor

'φ φ φ 

 ACI Eq. 10-1

[ ]st  yst gcn A f  A A f P +−=

=

)(85.080.0

:)65.0(columnsFor tied

'

φ φ 

φ 

 ACI Eq. 10-2

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• The previous expressions are only to be

used when there is no moment or whenit is very small. Moment is very smallwhen e < 0.10h for tied columns and

when e < 0.05h for spiral columns.

u

u

P

 M e =

h = outside diameter for round column= total depth of square or rectangular column

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Code requirements longitudinal bars

•  ACI 7.6.3: Clear distance between longitudinal

bars shall not be less than 1.5db nor less than1.5”.

01.0=

•  ACI 10.9.1:

•  ACI 10.9.2: Minimum number of longitudinal bars:4 for bars within rectangular or circular ties, 3 barswithin triangular shapes, 6 for bars enclosed

within spirals.

08.0max = ρ 

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Code requirements for ties

•  ACI 7.10.5.1: Minimum tie size: #3 forlongitudinal bars #10 or smaller; #4 forlarger longitudinal bars or for bundled

bars.•  ACI 7.10.5.2: Maximum center-to-center

spacing shall not be more than 16 timesthe diameter of longitudinal bars , 48times the diameter of the ties, or the

least dimension of the column.

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•  ACI 7.10.5.3: The ties must be arranged so that everycorner and alternate longitudinal bar will have supportprovided by the corner of a tie having an includedangle not greater than 135o.

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Code requirements for spirals•  ACI 10.9.3: Minimum spiral

percentage:

  y

c

c

g

s f 

 f 

 A

 A '

145.0  

  

 −= ρ 

 determined with thisexpression:

•  ACI 7.10.4: Spacing maynot be less than 1” andmay not be larger than 3”.

2

)(4

c

bcss

sD

d  Da −= ρ 

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Cover

•  ACI 7.7.1(c): Minimum cover = 1.5” 

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