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8/2/2019 20451953 Reinforced Concrete Design Lecture 10 Introduction to Columns
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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
8/2/2019 20451953 Reinforced Concrete Design Lecture 10 Introduction to Columns
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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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