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ACI Building Code: Operationsand Discussion of Recent &
Coming Changes
James K. Wight
Chair of ACI Committee 318Dept. of Civil & Env. Eng.
University of Michigan
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ACI 318: Philosophy for
Committee Operation
Consensus Document Based on Practical EngineeringPrinciples
Membership: 53% Practicing Engineers, 33%Academics, and 14% Trade or Govt. Associations
Goal is for continuous improvement in safety andusability of the code, not just simplification
New items for the code need a reason statement tojustify the proposed change and must containexample(s) to show the impact on current designpractice
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ACI 318 Code Committee
Code organization: 22 Chapters
4 Appendices
Committee organization: 8 Subcommittees
Technical change proposals are developed by
subcommittees and approved by main committee. Each chapter/appendix is the responsibility of a
different subcommittee.
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Whats new in 318-05
Consistent notation Alternative torsion design procedure
Use of spiral reinforcement with fy of 100 ksi
for confinement Limits on reinforcement spacing for crack
control and skin reinforcement
Need for shear reinforcement at slab-columnconnections of structures in high seismicperformance categories
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ACI 318-02 Notation Issues
Angle of inclination of inclined stirrups(shear - chapter 11)
Modification factor for top bars(anchorage chapter 12)
Ratio of flexural stiffness of beam toflexural stiffness of slab (two-way slabs
chapter 13)
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ACI 318-02 Notation Issues
Reinforcement ratio for vertical web
reinforcement in walls (ordinary
systems) Chapter 14
Reinforcement ratio for horizontal web
reinforcement in walls (specialsystems)- Chapter 21
n
n
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Notation and Terminology Task
Group
Task group formed to reduce the number ofconflicts and duplicate definitions in 318-05.
Task group recommendations were sent
directly to the main committee and were notballoted by any of the eight subcommittees.
Task group generated more than half of thechanges to 318-02 that were released forpublic comment.
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Approach Taken
Each term used in the code should have aunique definition.
Consolidate similar terms where possible.
Add or revise subscripts to distinguish amongterms and give meaning to them.
Existing terms were not modified unlesschanges were required to avoid duplication ofnotation.
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Consolidation of Terms
318-02 fy yield strength (most chapters and appendices)
fyh yield strength (11, circular hoop)
fyh yield strength (21, all transverse reinf.)
fy yield strength (11, longitudinal steel)
fyv yield strength (11, transverse steel)
fyt yield strength (12, transverse steel)
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Consolidation of Terms
318-05 fy specified yield strength of reinforcement
fya specified yield strength of anchor steel
fyt
specified yield strength of transverse reinforcement
=
v y
s
A f d
V s =
v yt
s
A f d
V s
=
g c
s c y
A f
. A f0 45 1
=
g c
s ch yt
A f
. A f0 45 1
ACI 318-02 ACI 318-05
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Development Length
318-02 reinforcement location factor (12)
coating factor (12)
reinforcement size factor (12)
lightweight aggregate concrete factor (12)
A
3
40
yd b
c t r
b
fd
f c K
d
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Development Length
318-05: factor used to modify development lengthbased on
reinforcement location reinforcement coating
reinforcement size
A
3
40
y t e sd b
c t r
b
fd
f c K
d
t e s
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Text Versus Notation
The possibility of using only notation within theCode text was discussed.
11.1.3 Computation of maximum factored
shear forceVu at supports ...
11.1.3 Computation of maximum Vu at
supports ...
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Defining of Terms in Code
A limited number of common terms are not defined eachtime they are used:
Terms are defined the first time that they are used in asection, and only the notation is used subsequently.
yf
cf
d b wb hn nV uVnM uM
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Alternate Design Procedure
in Torsion
Section 10.6.7 permits an alternate torsion designprocedure for deep thin members where the equivalenttube analogy leads to very unconservative designs
h/bt 3
bt
h
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Alternate Design Procedure
in Torsion
Permitted to use method given in PCI Handbookand/or method given in paper by Zia and Hsu,PCI Journal, May-June 2004. Tn is taken equal toTc plus Ts and for the concrete contribution in
torsion they use:
( )20.8 2.5 1.5
101
c c
e
g c
T f x y
P
A f
=
= +
y1x1
x2
y2
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Use of High-Strength Steel for
Spiral Reinforcement
Section 10.9.3 permits the use of steelwith a yield strength, fy, up to 100 ksi (690MPa) for spiral reinforcement.
fy 100 ksi
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Use of High-Strength Steel for
Spiral Reinforcement
In Chapter 21 (earthquake-resistantdesign), the value of fy permitted fordesign is limited to 60 ksi (21.11.5). Note:this limit is to be enforced when checkingthe shear strength of spiral columns, butthe higher yield strength (100 ksi) ispermitted when designing spiral as
confinement reinforcement.
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Limits on Bar Spacing for
Crack Width Control
Section 10.6.4 Applies to reinforcementnearest to the extreme tension fiber.
s
ss
cc
or
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Limits on Bar Spacing for
Crack Width Control Limiting equation was changed to reflect
potentially higher working stresses resultingfrom changes in load factors and phi factors.
40,00015 2.5
40,00012
c
s
s
s cf
sf
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Spacing Limit for Skin
Reinforcement in Deep Beams
In 2002 Code, thelimit was based ond, not h.
Also, the spacinglimit was given asa function of the
size of the bar.
s
s
s
s 36 in.
h/2
cc
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Spacing Limit for Skin
Reinforcement in Deep Beams
Research has shown that the size of the bar isnot important, so the code was simplified andthe spacing limit is the same as that for crack-
control reinforcement.
40,00015 2.5
40,00012
c
s
s
s c
f
s
f
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Need for Shear Reinforcement
at Slab-Column Connections
Although not designed as part of the systemfor resisting lateral loads, transfer momentscan be developed at these connections due
to lateral drift of the building during anearthquake.
If connection area is already subjected to
high shear stresses for resisting gravityloads, displacement-induced transfermoments can cause a punching shear
failure.
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Need for Shear Reinforcement
at Slab-Column Connections
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Need for Shear Reinforcement
at Slab-Column Connections
Section 21.11.5 requires a minimum amount ofshear reinforcement as a function of gravityinduced shear and expected story drift.
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Potential Changes for ACI 318-08
Modification (modernization) of analysisportions of the code.
Include in-plane design of structural walls in
Chapter 14. Use of steel studs as shear reinforcement at
slab-column connections.
Include adhesive and grouted anchors inAppendix D.
Simplify the analysis and design procedures
for slender columns.
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Potential Changes for ACI 318-08
Modification of the stress block for usewith high-strength concrete.
Include the size effect for shear strength
contributed by concrete in deep beams. Include seismic design categories within
Chapter 21.
Seismic design requirements for foundations,including concrete piles.
General reorganization and reduction of
textbook material included in the Code.
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Summary for ACI 318-05
Primary change was to achieve consistentnotation throughout the code.
Limited number of technical changes.
Expect more technical changes in 2008edition of the Code.