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TRANSCRIPT
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ABSTRACT:
This presentation is a chapter by chapter review of ACI 318-19 “Building Code Requirements for Structural Concrete”, released in August 2019 to replace ACI 318-14. Highlighted are the code provisions which the author of this presentation has used most often while engaged in the design of industrial, marine, and commercial reinforced concrete structures. Figures and short example problems illustrating use of the provisions are included. The emphasis is on non-prestressed, non-seismic structures designed by traditional methods.
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CHAPTER 1 – GENERAL
1.1- SCOPE OF
ACI 318
1.3.1 The purpose of this Code is to provide for public health and safety by establishing minimum requirements for strength, stability, durability, and integrity of concrete structures.
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CHAPTER 2 - NOTATION AND TERMINOLOGY
2.2 – NOTATION
a = depth of equivalent rectangular stress block, inchesb = width of compression face of member, inchesc = distance from extreme compression fiber to neutral axis, inchesd = distance from extreme compression fiber to centroid of longitudinal tension reinforcement, inchesh = overall thickness, height, or depth of member, inchesl = span length of beam or one-way slab; clear projection of cantilever, inches.
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2.3-
TERMINOLOGY
• base of structure – level at which horizontal earthquake ground motions are assumed to be imparted to a building. This level does not necessarily coincide with the ground level.
• Design story drift ratio – relative difference of design displacement between the top and bottom of a story, divided by the story height.
• Load, service – all loads, static or transitory, imposed on a structure or element thereof, during the operation of a facility, without load factors
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2.3-
TERMINOLOGY
(CONT’D)
• spiral reinforcement –continuously wound reinforcement in the form of a cylindrical helix.
• steel element, brittle – element with a tensile test elongation of less than 14 percent, or reduction in area of less than 30 percent at failure.
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CHAPTER 3 - REFERENCED
STANDARDS
3.2-REFERENCED
STANDARDS 3.2.3 ASCE/SEI 7-16
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3.2-
REFERENCED
STANDARDS
3.2.4 ASTM A615 and ASTM A706
3.2.5 AWS D1.4
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CHAPTER 4 – STRUCTURAL
SYSTEM REQUIREMENTS
4.4.6-SEISMIC
FORCE-RESISTING
SYSTEM
4.4.6.1 Every structure shall be assigned to a Seismic Design Category in accordance with the general building code.
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4.4.6-SEISMIC
FORCE-RESISTING
SYSTEM (CONT’D)
4.4.6.2 Structural systems designated as part of the seismic-force-resisting system shall be restricted to those systems designated by the general building code…
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4.4.6-SEISMIC
FORCE-RESISTING
SYSTEM (CONT’D)
4.4.6.3 Structural systems
assigned to Seismic Design
Category A shall satisfy the
applicable requirements of
this Code. …not required to
be designed in accordance
with Chapter 18.
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4.4.6-SEISMIC
FORCE-RESISTING
SYSTEM
4.4.6.4 Structural systems
assigned to Seismic Design
Category B, C, D, E, or F shall
satisfy the requirements of
Chapter 18 in addition…
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CHAPTER 5 – LOADS
5.3 – LOAD FACTORS
AND COMBINATIONS
1.4D
1.2D + 1.6L
1.2D +1.0W +1.0L
1.2D + 1.0E +1.0L
0.9D +1.0W
0.9D +1.0E
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5.3 – LOAD FACTORS
AND COMBINATIONS 5.3.5 If W is “service-level”
use 1.6W in place of 1.0W
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CHAPTER 6 – STRUCTURAL
ANALYSIS
6.2.5. - SLENDERNESS
EFFECTS
6.2.5.1 Slenderness effects
permitted to be neglected
for:
a) Columns not braced
against sidesway if kl/r LE
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b) Columns braced against
sidesway if kl/r LE 34 +
12(M1/M2) and LE 40
For sway frames, obtain k
from nomograph
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6.2.5. SLENDERNESS
EFFECTS (CONT’D)
6.2.5.2 r = sqrt( Ig / Ag ) or
0.3 times depth of
rectangular column or 0.25
times the diameter of
circular column
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6.2.5. SLENDERNESS
EFFECTS (CONT’D)
6.2.5.3 Second order
moments must not exceed
1.4 ( First order moments )
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6.3.2-T BEAM
GEOMETRY
6.3.2.1 Effective flange
widths nonprestressed
beams:
T beam: Web width plus
each side: minimum
( 8 times slab thickness,
one half clear distance to
adjacent web, one eighth
clear span of beam )
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6.3.2 T BEAM
GEOMETRY
(CONT’D)
L beam: Web width
plus: minimum( 6 times
slab thickness, one half
clear distance to
adjacent web, one
twelfth clear span of
beam )
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6.4 – ARRANGEMENT
OF LIVE LOAD
6.4.2 For one-way slabs
and beams:
a) Maximum positive
moment near midspan :
L on alternate spans
b) Maximum negative
moment at support: L
on adjacent spans
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6.4 – ARRANGEMENT
OF LIVE LOAD
(CONT’D)
6.4.3 For two-way slab
systems.
Moments at least the
values resulting from L
on all panels.
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6.4 – ARRANGEMENT
OF LIVE LOAD
6.4.3.3 Generally also
check 0.75L
checkerboard for
positive moments,
0.75L adjacent panels
only for negative
moments
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6.5 – SIMPLIFIED METHOD
OF ANALYSIS FOR
NONPRESTRESSED
CONTINUOUS BEAMS AND
ONE-WAY SLABS
6.5.1 Restrictions on
use: Prismatic
members, uniform
loads, unfactored live
load LE 3 times
unfactored dead load,
at least two spans,
longer of two adjacent
spans not more than 1.2
times shorter span
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6.5 – SIMPLIFIED METHOD
OF ANALYSIS FOR
NONPRESTRESSED
CONTINUOUS BEAMS AND
ONE-WAY SLABS (CONT’D)
6.5.2 Design factored moments:
Coefficients* factored uniform
load *clear span squared
• Positive end span moment
with discontinuous end
integral with support : 1/14
• Positive end span moment
with discontinuous end
unrestrained: 1/11
• Positive interior span
moments: 1/16
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6.5 – SIMPLIFIED METHOD
OF ANALYSIS FOR
NONPRESTRESSED
CONTINUOUS BEAMS AND
ONE-WAY SLABS (CONT’D)
• Negative moment
interior face of exterior
column: 1/16
• Negative moment
exterior face of first
interior support: 1/9 for
2 spans, 1/10 more than
2 spans
• Negative moment face
of other supports: 1/11
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6.5 – SIMPLIFIED METHOD
OF ANALYSIS FOR
NONPRESTRESSED
CONTINUOUS BEAMS AND
ONE-WAY SLABS (CONT’D)
6.5.3 Moment
redistribution not
allowed for values from
simplified analysis
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6.5 – SIMPLIFIED METHOD
OF ANALYSIS FOR
NONPRESTRESSED
CONTINUOUS BEAMS AND
ONE-WAY SLABS
6.5.4 Gravity load
design shears: simple
span values except 15%
higher at exterior face
of first interior support
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6.6.3 - SECTION
PROPERTIES
A = Gross A
Columns : I = 0.7 * Gross I
Beams: I = 0.35 * Gross I , or 0.7
* Gross I of web for T beams
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6.6.3 - SECTION
PROPERTIES (CONT’D)
6.6.3.1.2 For factored lateral load
analysis, permitted to use I = 0.5 *
Gross I for all members
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6.6.4 – SLENDERNESS
EFFECTS, MOMENT
MAGNIFICATION
METHOD
6.6.4.4.1 Stability index Q for a
building story equals: (total
factored vertical load * first order
story drift due to factored story
shear) / (factored story shear *
centerline story height)
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6.6.4 - SLENDERNESS
EFFECTS, MOMENT
MAGNIFICATION
METHOD
6.6.4.4.2 The critical buckling
load of a member, Euler value :
(9.86 * effective(EI)) / (kL *kL)
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6.6.4 - SLENDERNESS
EFFECTS, MOMENT
MAGNIFICATION
METHOD
6.6.4.4.4 For lateral load analysis
with no sustained lateral loads:
Effective (EI) = 0.4 Gross EI
(E, ksi from 19.2.2:
57 *sqrt (compressive strength
of concrete, psi)
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6.6.4.6 – MOMENT
MAGNIFICATION
METHOD: SWAY
FRAMES
6.6.4.6.1 Column end moment =
end moment due to gravity loads
plus moment magnifier times
moment due to lateral loads
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6.6.4.6 - MOMENT
MAGNIFICATION
METHOD: SWAY
FRAMES (CONT’D)
6.6.4.6.2 Moment magnifier =
1 / (1-Q) GE 1.0
OR:
Moment magnifier = 1 / (1 –
(Sum of all story column vertical
loads / 0.75 * sum of all story
column critical buckling loads)
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CHAPTER 7 – ONE – WAY SLABS
7.2.2 –
MATERIALS
Design properties for
concrete: Chapter 19
Design properties for steel reinforcement: Chapter 20
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7.3 – DESIGN
LIMITS
7.3.1 Minimum
thickness of solid
nonprestressed one-
way slabs, 60ksi yield
steel reinforcing,
(unless deflections
calculated to be
acceptable)
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7.3 – DESIGN
LIMITS (CONT’D)
Simply supported: h, inches
GE span,inches /20 span =
centerline or clear???
One end continuous: h GE
span/24
Both ends continuous: h GE
span/28
Cantilever: h GE span/10
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7.3 – DESIGN
LIMITS (CONT’D)
7.3.3 Reinforcement
strain limit in
nonprestressed slabs:
Must be tension-
controlled, ie tension
strain in extreme
tension steel at failure
must exceed the
tension yield strain plus
0.003.
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7.3 – DESIGN
LIMITS (CONT’D)
(Note: 22.2.2.1
Maximum strain at the
extreme concrete
compression fiber shall
be assumed equal to
0.003. ie “Failure” in
reinforced concrete is
still considered to be
the moment the first
point reaches a
compression strain of
0.003)
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7.3 – DESIGN
LIMITS (CONT’D)
(Note: 20.2.2.2: Modulus of
elasticity for nonprestressed bars
and wires shall be permitted to
be taken as 29,000,000psi)
(Note: 22.2.1.2: Strain in
concrete and nonprestressed
reinforcement shall be assumed
proportional to the distance from
the neutral axis.)
(Note: 22.2.2.2: Tensile strength
of concrete shall be neglected in
flexural and axial strength
calculations,)
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7.3 – DESIGN
LIMITS (CONT’D)
(Note: 21.2.2.1: For deformed
reinforcement the yield strain shall
be the yield stress divided by the
modulus of elasticity. For a yield
strength of 60ksi, it shall be
permitted to take the yield strain as
0.002.)
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7.3. DESIGN
LIMITS (CONT’D)
7.3.4 Stress limits in prestressed
slabs
Prestressed slabs shall be
classified as Class U, T, or C in
accordance with 24.5.2
Stresses in prestressed slabs
immediately after transfer and at
service loads shall not exceed the
permissible stresses in 24.5.3 and
24.5.4.
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7.4. REQUIRED
STRENGTH
7.4.2 Factored moment
For slabs built integrally with
supports, face of support
moment can be used as the
design maximum.
7.4.3 Factored shear
For nonprestressed slabs built
integrally with supports,
generally can use the shear at “d”
from support face as the design
maximum.
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7.5. DESIGN
STRENGTH
Strength reduction factors: 21.2
Nominal moment capacity: 22.3
Nominal shear capacity: 22.5
T beam effective flange designed
as a cantilever
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7.6
REINFORCEMENT
LIMITS
Tension steel reinforcement area per unit width GE 0.0018h
7.6.3 Minimum shear
reinforcement: Required for slabs
when design shear exceeds
concrete shear capacity
7.6.3.3 If shear reinforcement is
required, minimum values of
9.6.3.4 apply.
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7.6
REINFORCEMENT
LIMITS (CONT’D)
7.6.4 Minimum shrinkage and
temperature reinforcement: 24.4
(Note: 24.4: Minimum ratio of
deformed shrinkage and
temperature reinforcement area to
gross concrete area is 0.0018.
Maximum spacing 5h or 18inches)
(Note: Placement of minimum
shrinkage and temperature
reinforcement all on the “tension”
side is not required.)
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7.7.
REINFORCEMENT
DETAILING
Cover: 20.5.1
Development lengths: 25.4
Splices: 25.5
Bundled bars: 25.6
Minimum spacing: 25.2
Maximum spacing
nonprestressed and Class C
prestressed slabs, bonded
reinforcement closest to the
tension face: 24.3
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7.7
REINFORCEMENT
DETALING
(CONT’D)
Nonprestressed and Class T and C
prestressed slabs with unbonded
tendons, maximum spacing of
deformed longitudinal
reinforcement the lesser of 3h and
18 in.
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7.7. –
REINFORCEMENT
DETAILING (CONT’D)
7.7.3.1 Calculated tensile or compressive force in reinforcement at each section of the slab shall be developed on each side of that section.
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7.7
REINFORCEMENT
DETAILING
(CONT’D)
7.7.3.2 Critical locations for development of reinforcement are points of maximum stress and points along the span where bent or terminated tension reinforcement is no longer required to resist flexure
7.7.3.3 Generally must extend
reinforcement “d” or 12 bar
diameters beyond where
needed.
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7.7
REINFORCEMENT
DETAILING
(CONT’D)
7.7.3.5 Flexural tension reinforcement
shall not be terminated in a tension
zone unless….a) Factored shear ≤ 2/3
shear strength.
7.7.3.8 At least one third of required
maximum positive moment
reinforcement must extend into
“simple” supports. For “other”
supports, one fourth required to extend
6 inches into support. At least one third
of the required negative moment
reinforcement at a support must extend
beyond the point of inflection the
greatest of “d”, 12 bar diameters, or the
clear span/16.73
7.7 -
REINFORCEMENT
DETAILING
(CONT’D)
7.7.7 Structural integrity
reinforcement in cast-in-place
one-way slabs
Minimum ¼ of maximum positive
moment reinforcement shall be
continuous; at non-continuous
supports shall develop the yield
stress at the face of the support.
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Figure - Simple span one-way
slab check
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CHAPTER 8 – TWO – WAY SLABS
8.1- SCOPE
Scope: a) Solid slabs d) Two-way
joist systems in accordance with
8.8.
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8.2 - GENERAL
8.2.1 …permitted to be designed
by any procedure satisfying
equilibrium and geometric
compatibility…The direct design
method or the equivalent frame
method is permitted.
Commentary: The direct design
method and the equivalent
frame method are limited in
application to orthogonal frames
subject to gravity loads only.
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8.2 GENERAL
(CONT’D)
Note: ACI 318-14 Sections 8.10
Direct Design Method and 8.11
Equivalent frame method were
discontinued in ACI 318-19.
8.2.4 A “drop panel” projects
below the slab at least one-
fourth of the adjacent slab
thickness and extends in each
direction from the centerline
of support a distance not less
than one sixth the centerline
span length in that direction.
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8.3 – DESIGN
LIMITS
8.3.1 Minimum slab thickness
8.3.1.1 Nonprestressed slabs
without interior beams on all
sides, deflections not
calculated and shown
adequate: Without drop
panels – 5 inch minimum
thickness; with drop panels –
4 inch minimum thickness;
and satisfy Table 8.3.1.1.
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8.3 – DESIGN
LIMITS (CONT’D)
Table 8.3.1.1 for 60 ksi yield
reinforcing, no drop panels,
with edge beams minimum h:
Exterior panels: Long direction
clear span/33
Interior panels: Long direction
clear span/33
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8.3 – DESIGN
LIMITS (CONT’D)
8.3.3 Reinforcement strain limit
in nonprestressed slabs: Must
be tension-controlled
8.3.4 Stress limits in prestressed
slabs: Design as Class U with
maximum tensile stress 6
times the sqrt (compressive
strength of concrete)
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8.4 – REQUIRED
STRENGTH
8.4.1 General
8.4.1.5 A column strip is a design
strip with a width on each
side of the column centerline
equal to the lesser of
0.25(centerline span in
direction being designed) and
0.25(perpendicular span). A
column strip shall include
beams within the strip, if
present.
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8.4 – REQUIRED
STRENGTH
(CONT’D)
8.4.1.6 A middle strip is a design
strip bounded by two column
strips.
8.4.1.8 For monolithic or fully
composite construction
supporting two-way slabs, a
beam includes that portion of
slab, on each side of the
beam extending a distance
equal to the projection of the
beam above or below the
slab, whichever is greater, but
LE four times the slab
thickness. 86
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8.4.2 – FACTORED
MOMENT
8.4.2.1 For slabs built integrally
with supports, maximum
design moments can be taken
as face of support values.
8.4.2.2 Factored slab moment
resisted by column (interior
column with no beams or
capital considered here)
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8.4.2 – FACTORED
MOMENT
(CONT’D)
8.4.2.2.2 A fraction (0 to 1.0)of the
moment transferred from slab to
column is considered transferred
by flexure (the remainder by
nonsymmetric punching shear
described in 8.4.4.2). The
fraction assumed transferred by
flexure is: 1 / ( 1 +
0.667(sqrt((column dimension in
direction being analyzed + slab
“d” value) / (column dimension
in perpendicular direction + slab
“d” value)
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8.4.2 – FACTORED
MOMENT
(CONT’D)
8.4.2.2.3 The effective slab width
for resisting this flexure
component is the column
width + 1.5 slab “h” each side
of column
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8.4.3 – FACTORED
ONE WAY SHEAR
8.4.3 Factored one-way shear –
Generally can use value at “d”
from face of support as the
design value
8.4.4 Factored two-way shear
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8.4.4.1 – CRITICAL
SECTION
8.4.4.1.1 Slabs shall be evaluated
for two-way shear in the vicinity
of columns, concentrated loads,
and reaction areas at critical
sections in accordance with
22.6.4.
8.4.4.1.2 Slabs reinforced with
stirrups or headed shear stud
reinforcement shall be
evaluated for two-way shear at
critical sections in accordance
with 22.6.4.2.
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8.4.4.1 – CRITICAL
SECTION (CONT’D)
8.4.4.2 Factored two-way shear
stress due to shear and factored
slab moment resisted by the
column
8.4.4.2.3 The factored shear stress
resulting from the eccentric
shear portion of the moment
transfer from slab to column
shall be assumed to vary linearly
about the centroid of the critical
section.
(Commentary gives help with
critical section geometric
property J )94
8.4.4.1 – CRITICAL
SECTION (CONT’D)
(Note: 22.6.4.1: For two-way
shear, critical sections shall be
located so that the perimeter
is a minimum but need not be
closer than 0.5d to the
column edge)
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8.5 – DESIGN
STRENGTH
8.5.1 General
8.5.1.1 For each applicable
factored load combination,
ensure that nominal
capacities multiplied by
capacity reduction factors
exceed the corresponding
effects of the factored loads.
a) Moment at all sections along
the span in each direction
b) Moment in the slab at the
column connection
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8.5 – DESIGN
STRENGTH
(CONT’D)
c) Shear at all sections along the
span in each direction for
one-way shear
d) Punching shear stress,
including moment transfer
from eccentric shear, at
columns
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8.5 – DESIGN
STRENGTH
(CONT’D)
8.5.1.2 Capacity reduction factors
from 21.2
8.5.2 Moment
8.5.2.1 Nominal moment
capacity from 22.3
8.5.3 Shear
8.5.3.1.1 For one-way shear 22.5
8.5.3.1.2 For two-way shear 22.6
8.6 – Reinforcement limits :
Minimum flexural reinforcement
near tension face .0018h per unit
width.
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8.7 –
REINFORCEMENT
DETAILING
8.7.1 General –Cover 20.5.1;
Development length 25.4;
Splice lengths 25.5
8.7.2 Flexural reinforcement
spacing: minimum spacing
25.2, maximum spacing the
lesser of 2h and 18 inches at
critical sections and the lesser
of 3h and 18 inches at other
sections.
8.7.3 Corner restraint in slabs
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8.7 –
REINFORCEMENT
DETAILING
(CONT’D)
8.7.3.1.2 Reinforcement shall be
provided for a distance in
each direction from the
corner equal to one-fifth the
longer span.
8.7.4 Flexural reinforcement in
nonprestressed slabs
8.7.4.1 Termination of
reinforcement
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8.7 –
REINFORCEMENT
DETAILING
(CONT’D)
8.7.3.1.2 Reinforcement shall be
provided for a distance in
each direction from the
corner equal to one-fifth the
longer span.
8.7.4 Flexural reinforcement in
nonprestressed slabs
8.7.4.1 Termination of
reinforcement
102
8.7 –
REINFORCEMENT
DETAILING
(CONT’D)
8.7.4.1.1 For slab supported on
spandrel beam, column, or
wall, reinforcement
perpendicular to
discontinuous edge: Positive
moment reinforcement shall
extend to the edge of slab and
have embedment, straight or
hooked, at least 6 inches.
Negative moment
reinforcement shall be
developed at the face of
support.
103
8.7 –
REINFORCEMENT
DETAILING
(CONT’D)
Figure 8.7.4.1.3 – Minimum
extensions for deformed
reinforcement in two-way
slabs without beams.
Example, without drop panels,
column strip top steel: At
least 50% of required steel at
exterior supports must extend
at least 0.3 of the clear span
beyond the face of support.
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105
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8.7 –
REINFORCEMENT
DETAILING
(CONT’D)
8.7.5 Flexural reinforcement in
prestressed slabs
8.7.6 Shear reinforcement –
stirrups
Stirrups permitted as shear
reinforcement; anchorage and
geometry to satisfy 25.7.1;
maximum spacing d/2 in
analyzed span direction, 2d in
perpendicular direction
8.7.7 Shear reinforcement –headed stud
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8.8 –
NONPRESTRESSED
TWO-WAY JOIST
SYSTEMS
8.8.1 General
Top slab designed to span in two
directions; rib width at least 4
inches all locations; rib depth
not greater than 3.5 times
minimum rib width; clear
spacing between ribs shall not
exceed 30 inches; can
increase section 22.5 shear
strength by 10%; at least one
bottom bar developed at face
of support.
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CHAPTER 9 - BEAMS
9.0 - BEAMS
From Chapter 2: “beam” = member
subjected primarily to flexure
and shear, with or without axial
force or torsion; beams in a
moment frame that forms part
of the lateral-force-resisting
system are predominantly
horizontal members; a girder is a
beam.
112
9.1 - SCOPE
9.1 - Scope: nonprestressed,
prestressed, one-way joists
9.8, deep beams 9.9
9.2 – General
9.2.1 Materials:
Concrete design properties
Chapter 19; steel
reinforcement design
properties Chapter 20;
embedments 20.6
113
9.2 GENERAL
(CONT’D)
9.2.1 Materials:
Concrete design properties
Chapter 19; steel
reinforcement design
properties Chapter 20;
embedments 20.6
9.2.2 Connection to other members:Cast-in-place beam to column and slab to column joints: Chapter 15
114
9.2 - GENERAL
(CONT’D)
Precast concrete construction
force transfer: 16.2
9.2.3 Stability: Spacing of lateral
bracing shall not exceed 50
times the least width of the
compression flange or face.
9.2.3.2 In prestressed beams,
buckling of thin webs and
flanges shall be considered….
115
9.2 - GENERAL
(CONT’D)
9.2.4 T-beam construction
9.2.4.1 In T-beam construction,
flange and web concrete shall
be place monolithically or
made composite in
accordance with 16.4.
9.2.4.2 Effective flange width
shall be in accordance with
6.3.2.
116
9.2- GENERAL
(CONT’D)
9.2.4.3 For T-beam flanges where
the primary flexural slab
reinforcement is parallel to
the longitudinal axis of the
beam, reinforcement in the
flange perpendicular to the
longitudinal axis of the beam
shall be in accordance with
7.5.2.3.
117
9.2 GENERAL
(CONT’D)
9.2.4.4 For torsional design
according to 22.7, the
overhanging flange width
used to calculate section
properties shall satisfy: Not
greater than beam projection
above or below slab, not
greater than 4 times slab
thickness, flanges to be
neglected if they cause
section properties to
decrease.
118
119
9.3 – DESIGN
LIMITS
9.3.1 Minimum beam depth
9.3.1.1 For nonprestressed
beams not supporting or
attached to partitions or
other construction likely to be
damaged by large deflections,
overall beam depth h shall
satisfy the limits in Table
9.3.3.1, unless the calculated
deflection limits of 9.3.2 are
satisfied.120
9.3 – DESIGN
LIMITS (CONT’D)
Table 9.3.1.1 – Minimum depth of
nonprestressed beams ( for normal
weight concrete and 60ksi yield steel
reinforcement)
Simply supported: L/16
One end continuous: L/18.5
Both ends continuous: L/21
Cantilever: L/8
( L = span = centerline or clear?)
9.3.2 Calculated deflection limits 24.2
121
122
9.3 – DESIGN
LIMITS (CONT’D)
9.3.3 Reinforcement strain limit
in nonprestressed beams
9.3.3.1 Nonprestressed beams
with factored axial
compression stress less than
0.1 of the compressive
strength shall be tension
controlled in accordance with
Table 21.2.2.
(Note: Previous permission to
have extreme tension strain of
0.004 with a capacity
reduction factor reduced from
0.9 has not been retained.)123
9.3 – DESIGN
LIMITS (CONT’D)
9.3.4 Stress limits in prestressed
beams
9.3.4.1 Prestressed beams shall
be classified as Class U, T, or C
in accordance with 24.5.2.
9.3.4.2 Stresses in prestressed
beams immediately after
transfer and at service loads
shall not exceed permissible
stresses in 24.5.3 and 24.5.4.
124
9.4 –REQUIRED
STRENGTH
9.4 – Required strength: Load
combinations from Chapter 5;
required strength from
Chapter 6; reactions induced
by prestressing in accordance
with 5.3.11.
9.4.2 Factored moment: For
beams built integrally with
supports, moments at faces of
supports can be used as
design values.
125
9.4 –REQUIRED
STRENGTH
(CONT’D)
9.4.3 Factored shear: Generally can use shear at “d” from support face as maximum design value.
9.4.4 Factored torsion
126
9.5 –DESIGN
STRENGTH
9.5.4.1 When the torsion due to
factored loads is less than the
“Threshold torsion” given in
22.7 multiplied by the
capacity reduction factor
(0.75), it is permitted to
neglect torsional effects;
minimum reinforcement
requirements of 9.6.4 and
detailing requirements of
9.7.5 and 9.7.6.3 need not be
satisfied.
9.5.4.2 Nominal torsion capacity:
22.7
127
9.5 – DESIGN
STRENGTH
(CONT’D)
9.5.4.3 Longitudinal and
transverse reinforcement
required for torsion shall be
added to that required for
shear, moment, and axial
force.
128
9.6 –
REINFORCEMENT
LIMITS
9.6.1 Minimum flexural
reinforcement in
nonprestressed beams
9.6.1.1 A minimum area of
flexural reinforcement shall
be provided at every section
where tension reinforcement
is required by analysis.
129
9.6 –
REINFORCEMENT
LIMITS (CONT’D)
9.6.1.2 The minimum area of
tension side reinforcement
shall be the larger of
a) 3sqrt(concrete compressive
strength)(beam web
width)(beam “d”) / yield
stress of reinforcing steel
b) 200 (beam web width) (beam
“d”) / yield stress of
reinforcing steel
(Transition point is concrete
compression strength of
4444psi)
130
9.6 –
REINFORCEMENT
LIMITS (CONT’D)
9.6.2 Minimum flexural
reinforcement in prestressed
beams
9.6.2.1 For beams with bonded
prestressed reinforcement,
the areas of nonprestressed
and prestressed tension
reinforcement must be
adequate to develop a
factored moment at least 1.2
times the beam cracking
moment.
131
9.6.3 – MINIMUM SHEAR
REINFORCEMENT
9.6.3.1 For nonprestressed
beams, a minimum area of
shear reinforcement must
generally be provided
wherever the factored beam
shear exceeds
0.75sqrt(concrete
compressive strength)(width
of beam web)(“d”)
132
9.6.3 – MINIMUM SHEAR
REINFORCEMENT
Exceptions: Beam “h” LE 10
inches; T-beam “h” LE 24
inches and LE the greater of
2.5 times flange thickness or
0.5 times beam web width;
one-way joist systems defined
in 9.8
Note: No exception for footings
and pilecaps.
133
9.6.3 – MINIMUM
SHEAR
REINFORCEMENT
9.6.3.2 For prestressed beams
the threshold factored shear
is 0.5( 0.75)(Nominal shear
capacity of prestressed
concrete)
9.6.3.4 Minimum area of shear
reinforcement where torsion
can be neglected,
nonprestressed:
0.75sqrt(concrete compressive
strength)(beam web
width)(stirrup spacing)/steel
yield stress
But use a concrete compressive
strength at least 4444psi.134
9.6.4 – MINIMUM
TORSIONAL
REINFORCEMENT
9.6.4.1 A minimum area of
torsional reinforcement shall
be provided in all regions
where the torsion due to
factored loads exceeds 0.75
times the threshold torsion
from 22.7.
9.6.4.2 If torsional reinforcement is required, the minimum stirrup areas are similar to those for shear alone, however, only exterior legs of the stirrups are counted.
135
9.6.4 – MINIMUM
TORSIONAL
REINFORCEMENT
9.6.4.3 If torsional reinforcement is
required, there is a minimum
area of longitudinal
reinforcement required:
5 sqrt(concrete compressive
strength)(Concrete area of
torsion beam)/yield stress
reinforcement
- (required area of transverse
torsion reinforcement per unit
length of beam)(perimeter of
outer torsion stirrups) , where
the required area of torsion
reinforcement per unit length
need not be taken as less than
25(beam web width)/steel
yield stress 136
9.7 –
REINFORCEMENT
DETAILING
9.7.1 General: Concrete cover
20.5.1; Development lengths
25.4; Splices 25.5; bundled
bars 25.6
9.7.2 Reinforcement spacing:
Minimum spacing 25.2
137
9.7 –DETAILING
(CONT’D)
9.7.1 General: Concrete cover
20.5.1; Development lengths
25.4; Splices 25.5; bundled
bars 25.6
9.7.2 Reinforcement spacing:
Minimum spacing 25.2
9.7.2.3 For nonprestressed and
Class C prestressed beams with
“h” greater than 36 inches,
longitudinal skin reinforcement
shall be uniformly distributed
on both sides of the beam for a
distance 0.5h from the tension
face, minimum spacing
according to 24.3.2138
139
9.7.3 –FLEXURAL
REINFORCEMENT IN
NONPRESTRESSED
BEAMS
9.7.3.1 Calculated tensile or
compressive force in
reinforcement at each section
of the beam shall be
developed on each side of
that section.
9.7.3.2 Critical locations for
development of
reinforcement are points of
maximum stress and points
along the span where bent or
terminated tension
reinforcement is no longer
required to resist flexure.
140
9.7.3 –FLEXURAL
REINFORCEMENT IN
NONPRESTRESSED
BEAMS
9.7.3.3 Reinforcement shall
extend beyond the point at
which it is no longer required
to resist flexure for a
distance equal to the greater
of “d” and 12 bar diameters,
except at supports of simply-
supported spans and at free
ends of cantilevers.
141
9.7.3 –FLEXURAL
REINFORCEMENT IN
NONPRESTRESSED
BEAMS
9.7.3.4 Continuing flexural
tension reinforcement shall
extend at least its
development length beyond
the point where bent or
terminated tension
reinforcement is no longer
required to resist flexure.
142
9.7.3 –FLEXURAL
REINFORCEMENT IN
NONPRESTRESSED
BEAMS (CONT’D)
9.7.3.5 Flexural tension
reinforcement shall not be
terminated in a tension zone
unless (a), (b), or (c) is
satisfied:
(a) Shear due to factored loads LE
0.667(0.75)(Nominal shear
capacity) at the cutoff point
(b) (c)
143
144
9.7.3.8 –
TERMINATION OF
REINFORCEMENT
9.7.3.8.1 At simple supports, at
least one-third of the
maximum positive moment
reinforcement shall extend
along the beam bottom into
the support at least 6 inches,
except for precast beams
where such reinforcement
shall extend at least to the
center of the bearing length.
145
9.7.3.8 –
TERMINATION OF
REINFORCEMENT
(CONT’D)
9.7.3.8.2 At other supports, at least one-fourth of the maximum positive moment reinforcement shall extend along the beam bottom into the support at least 6 inches and if the beam is part of the primary later-load-resisting system, shall be anchored to develop the steel yield stress in tension at the face of the support.
146
9.7.3.8 –TERMINATION
OF REINFORCEMENT
(CONT’D)
9.7.3.8.3 At simple supports and
points of inflection, the bar
diameter of positive moment
tension reinforcement shall
be limited such that the
development length of the
bar satisfies (a) or (b), unless
reinforcement terminates
beyond the support
centerline with a standard
hook or equivalent:
147
9.7.3.8 –TERMINATION
OF REINFORCEMENT
(CONT’D)
(a) Bar development length LE 1.3( Nominal moment capacity/ Shear due to factored loads) +la , if end of reinforcement is confined by a compressive reaction, or(b) Bar development length LE ( Nominal moment capacity / Shear due to factored loads) + la, if not.La is the embedment length beyond the center of support or point of inflection, limited to the greater of “d” and 12 bar diameters
148
9.7.3.8 –TERMINATION
OF REINFORCEMENT
(CONT’D)
9.7.3.8.4 At least one-third of the
negative moment
reinforcement at a support
shall have an embedment
length beyond the point of
inflection at least the greatest
of “d”, 12 bar diameters, or
the clear span/16.
149
150
9.7.4 –FLEXURAL
REINFORCEMENT IN
PRESTRESSED
BEAMS
9.7.4.3.1 Post-tensioned
anchorage zones shall be
designed and detailed in
accordance with 25.9.
9.7.4.3.2 Post-tensioning
anchorages and couplers shall
be designed and detailed in
accordance with 25.8.
151
9.7.5 – LONGITUDINAL
TORSIONAL
REINFORCEMENT
9.7.5.1 If torsional reinforcement is
required, longitudinal torsional
reinforcement shall be
distributed around the
perimeter of closed stirrups that
satisfy 25.7.1.6 or hoops with a
spacing not greater than 12
inches. The longitudinal
reinforcement shall be inside
the stirrup or hoop, and at least
one longitudinal bar or tendon
shall be placed in each corner.
152
9.7.5 – LONGITUDINAL
TORSIONAL
REINFORCEMENT
(CONT’D)
9.7.5.2 Longitudinal torsional
reinforcement shall have a
diameter at least 0.042 times the
transverse reinforcement spacing,
but not less than 3/8 inch.
9.7.5.3 Longitudinal torsion
reinforcement shall extend for a
distance of at least (torsion beam
width + “d”) beyond the point
required by analysis.
9.7.5.4 Longitudinal torsional
reinforcement shall be developed
at the face of the support at both
ends of the beam.
153
9.7.6 –
TRANSVERSE
REINFORCEMENT
9.7.6.1 General: Details in
accordance with 25.7.
9.7.6.2 Shear
9.7.6.2.1 If required, shear
reinforcement shall be
provided using stirrups,
hoops, or longitudinal bent
bars.
154
9.7.6 –
TRANSVERSE
REINFORCEMENT
Table 9.7.6.2.2 – Revised from
ACI 318-14 to include “Across
width” limits
For locations along beam where
required nominal shear
capacity of shear
reinforcement LE
4sqrt(concrete compressive
strength)(beam web
width)(“d”)
155
9.7.6 – TRANSVERSE
REINFORCEMENT
(CONT’D)
Leg spacing “s” LE 24 inches
along length or across width
and:
Nonprestressed: Along length s
LE 0.5d; across width s LE d
Prestressed: Along length s LE
.75h; across width s LE 1.5h
156
9.7.6 – TRANSVERSE
REINFORCEMENT
(CONT’D)
For locations along beam where
required nominal shear
capacity of shear reinforcement
GT 4sqrt(concrete compressive
strength)(beam web
width)(“d”) :
Leg spacing “s” LE 12inches along
length or across width and:
Nonprestressed: Along length s
LE 0.25d; across width s LE
0.5d
Prestressed: Along length s LE
.375h; across width s LE 0.75h
157
158
9.7.6.3 – TORSION
9.7.6.3.1 If required, transverse
torsional reinforcement shall
be closed stirrups satisfying
25.7.1.6 or hoops.
9.7.6.3.2 Transverse torsional
reinforcement shall extend a
distance of at least ( torsion
beam width + d) beyond the
point required by analysis.
159
9.7.6.3 – TORSION
(CONT’D)
9.7.6.3.3 Spacing of transverse
torsional reinforcement shall
not exceed the lesser of
0.125 times the perimeter of
the torsion stirrup or 12
inches.
9.7.6.4 Lateral support of
compression reinforcement
(Note: This is the “Beam”
Chapter.)
160
9.7.6.4 –
LATERAL SUPPORT
OF COMPRESSION
REINFORCEMENT
9.7.6.4.1 Transverse
reinforcement shall be
provided throughout the
distance where longitudinal
compression reinforcement is
required…
161
9.7.6.4 –
LATERAL SUPPORT OF
COMPRESSION
REINFORCEMENT
(CONT’D)
9.7.6.4.2,3,4 Same rules as for
column ties given in 25.7.2:
#3 for #10 or smaller, # 4 for
#11 or larger; spacing along
beam LE 16 longitudinal bar
diameters and LE 48 tie bar
diameters; every corner and
every alternate compression
bar enclosed by tie angle LE
135 degrees and every
unsupported bar not more
than 6 inches clear each side
to a supported bar.162
9.7.7 – STRUCTURAL
INTEGRITY
REINFORCEMENT IN
CAST-IN-PLACE BEAMS
9.7.7.1 For perimeter beams: (a)
At least one-quarter
maximum positive moment
reinforcement and at least
two bars continuous, (b) At
least one-sixth negative
moment reinforcement at
support and at least two bars
continuous; (c) At least
minimum closed stirrups in
accordance with 25.7.1.6
along the entire clear span of
the beam.
163
9.7.7 – STRUCTURAL
INTEGRITY
REINFORCEMENT IN
CAST-IN-PLACE BEAMS
(CONT’D)
9.7.7.2 For other than perimeter
beams, structural integrity
reinforcement shall be in
accordance with (a) OR (b): (a) At
least one-quarter and two bars of
maximum positive moment
reinforcement continuous, (b)
Minimum stirrups entire clear
span.
164
9.7.7 – STRUCTURAL
INTEGRITY
REINFORCEMENT IN
CAST-IN-PLACE BEAMS
(CONT’D)
9.7.7.3 Longitudinal integrity
reinforcement shall pass
through the region bounded
by the longitudinal
reinforcement of the column.
165
9.7.7 – STRUCTURAL
INTEGRITY
REINFORCEMENT IN
CAST-IN-PLACE BEAMS
(CONT’D)
9.7.7.4 At noncontinuous
supports, develop the
tension yield stress of
integrity reinforcement at
face of support.
9.7.7.5 Splice bottom steel
near support and top steel
near midspan.
166
9.7.7 – STRUCTURAL
INTEGRITY
REINFORCEMENT IN
CAST-IN- PLACE BEAMS
(CONT’D)
9.7.7.6 Use mechanical or
welded splices in accordance
with 25.5.7 or Class B tension
lap splices in accordance with
25.5.2 (Class B is the higher
strength, 1.3 times
development length)
167
9.8 – NONPRESTRESSED
ONE-WAY JOIST SYSTEMS
9.8.1 General: Regularly spaced
(LE 30inches) ribs at least 4
inches wide with rib depth
not greater than 3.5 times the
minimum width, top slab
“one-way”, can increase
concrete shear strength 10%,
at least one bottom bar
tension developed into
supports.
168
9.9 – DEEP BEAMS
9.9.1 General: A “deep beam” is
loaded on one face and
supported on the other and
has a clear span less than 4h.
9.9.1.2 ….nonlinear distribution
of longitudinal strain over the
depth of the beam.
9.9.1.3 The strut-and-tie
method in accordance with
Chapter 23 is deemed to
satisfy 9.9.1.2
169
9.9.2 – DIMENSIONAL
LIMITS
7.5sqrt(concrete compressive
strength)(beam web width)(d)
GE [ maximum shear
due to factored loads] ?
( Shear at support face?)
170
9.9.3 – REINFORCEMENT
LIMITS
9.9.3.1 Distributed reinforcement
along the side faces of deep
beams shall be at least that
required in (a) and (b):
(a) The area of distributed
reinforcement perpendicular
to the longitudinal axis of the
beam shall be at least
0.0025bws, where s is the
spacing of the distributed
transverse reinforcement.
171
9.9.3 – REINFORCEMENT
LIMITS (CONT’D)
(b) The area of distributed reinforcement
parallel to the longitudinal axis of the
beam shall be at least 0.0025bws2,
where s2 is the spacing of the
distributed longitudinal reinforcement.
9.9.4 Reinforcement detailing: Concrete
cover 20.5.1; minimum spacing
longitudinal reinforcement 25.2;
spacing of (a) above LE 0.2d and LE 12
inches; at simple supports bottom
steel to develop the tension yield
stress; at interior supports top bars
must be continuous- no splices, and
bottom bars can be continuous or
splices with reinforcement from the
adjacent span.172
173
174
CHAPTER 10 - COLUMNS
10.2 – GENERAL
From Chapter 2, “column” = member,
usually vertical or predominantly
vertical, used primarily to support
axial compression load, but can
also resist moment, shear, or
torsion. Columns used as part of a
lateral-force-resisting system resist
combined axial load, moment, and
shear.
10.2 - General: Concrete design
properties Chapter 19; Steel
reinforcement Chapter 20;
Embedments 20.6; Joints for cast-
in-place concrete Chapter 15;
Joints for precast concrete 16.2;
Column connections to
foundations 16.3.175
10.3 – DESIGN LIMITS
10.3.1 Dimensional limits
10.3.1.1 For columns with a
square, octagonal, or other
shaped cross section, it shall
be permitted to base gross
area considered, required
reinforcement, and design
strength on a circular section
with a diameter equal to the
least lateral dimension of the
actual shape.
176
10.3 – DESIGN LIMITS
10.3.1.2 For columns with cross
sections larger than required
by consideration of loading, it
shall be permitted to base
gross area considered,
required reinforcement, and
design strength on a reduced
effective area, not less than
one-half the total area. This
provision shall not apply to
columns in special moment
frames or…
177
10.4 – REQUIRED
STRENGTH
Factored load combinations from Chapter 5; analysis procedures Chapter 6
178
10.5 – DESIGN STRENGTH
10.5.1 General: Nominal
strengths times appropriate
capacity reduction factors
from 21.2 must exceed effects
of factored loads; check axial
force, moment, shear, and
torsion and interaction.
179
10.5 – DESIGN STRENGTH
10.5.2 Axial force and moment:
Nominal capacities in
accordance with 22.4.
10.5.3 Shear: Nominal capacity in
accordance with22.5.
10.5.4 Torsion: If torsion due to
factored loads exceeds the
threshold torsion of 22.7
multiplied by the capacity
reduction factor (0.75 for
torsion), torsion shall be
considered in accordance
with Chapter 9.
180
181
182
183
10.6 – REINFORCEMENT
LIMITS
10.6.1 Minimum and maximum
longitudinal reinforcement: For
nonprestressed columns and for
prestressed columns with after
loss prestress less than 225psi,
area of longitudinal
reinforcement shall be between
one and eight percent of the
gross column area.
10.6.2 Minimum shear reinforcement: Required where shear due to factored loads exceeds half the design shear strength of the concrete; A v GE 0.75sqrt(f’c)(bw)(s)/ fyt , with f’c GE 4444psi
184
10.7 – REINFORCEMENT
DETAILING
10.7.1 General: Cover 20.5.1;
Development 25.4; Bundled
bars 25.6.
10.7.2 Reinforcement spacing:
Minimum spacing 25.2.
10.7.3 Longitudinal
reinforcement: Minimum 4
bars enclosed within
rectangular or circular ties;
minimum 6 bars enclosed by
spirals or in special moment
frames.
185
10.7 – REINFORCEMENT
DETAILING (CONT’D)
10.7.4 Offset bent longitudinal
reinforcement: Transition
maximum slope 1:6; use
dowels if column face offset
exceeds 3 inches.
10.7.5 Splices of longitudinal
reinforcement: Lap,
mechanical, butt-welded, or
end bearing; if tension splice
required, generally Class B.
186
10.7.6 – TRANSVERSE
REINFORCEMENT
10.7.6.1 General: Ties 25.7.2;
Spirals 25.7.3; Hoops 25.7.4;
longitudinal reinforcement
laterally supported in
accordance with 10.7.6.2,
10.7.6.3.
10.7.6.5 Shear: If required, shear
reinforcement shall be
provided using ties, hoops, or
spirals; maximum spacing by
Table 10.7.6.5.2 – similar to
beams for along length limits,
no across width limits.
187
188
189
190