1.1 New Rebar Material & Revised Limitations
2
1.1.1 Use of High-Strength Reinforcement
2
1.1.2 Minimum Slab Thickness Requirements
3
1.1.3 Upper Limit for fy is introduced for minimum flexural reinforcement
As,min calculations.
3
1.1.4 Upper Limit for fy is introduced for maximum axial compressive strength,
pn,max calculations.
4
1.1.5 Lower Limit of 0.5db is introduced for transverse reinforcement index,
K tr , when fy ≥ 550MPa.
4
1.1.6 Modulus of Rupture Equation is modified for fy exceeding 550MPa in
non-prestressed slab systems.
1.2 Minimum Reinforcement Provisions Are Revised
5
5
1.2.1 Modulus of Rupture Equation is modified for fy exceeding 550MPa in
non-prestressed slab systems.
5
1.2.2 Minimum Shear Reinforcement in Non-Prestressed Beams is revised. 6
1.3 New Reinforcement Strain Limit Is Introduced for Non-Prestressed
Members
6
1.3.1 Adjustment to the Strength Reduction Factor, ∅
6
1.3.2 Reinforcement Strain Limit is revised for slabs and beams. Satisfy
Tension is controlled in Table 11.
1.4 Significant Updates to Shear Provisions
7
7
1.4.1 One-Way Shear Strength Provisions in Relation with Vc for NonPrestressed Members are modified substantially.
7
1.4.2 Two-Way Shear Strength in Vc for Non-Prestressed Members is revised
regarding as size effect factor.
1.4.3 Bidirectional One-Way Shear Effects are to be considered.
1.4.4 Upper Limit for the Nominal Shear Strength of Walls, Vn
9
9
10
1.4.5 The Nominal Shear Strength, Vn , Calculations for Walls are modified. 10
1.4.6 In-Plane Vu Criteria is modified for determining the minimum
reinforcement for walls.
11
1.5 Hanger Reinforcement Provisions Are Introduced
11
1.6 New Equation for Effective Moment of Inertia for Cracked Section
12
1.7 Modification of Development Length Provisions
12
1.7.1 Considering Reinforcement Grade Factor, Ψg for Development Length in
Tension, ld [Simple]
12
1.7.2 Considering Reinforcement Grade Factor, Ψg for Development Length in
Tension, ld [General]
13
1.7.3 Considering Factors, ψr ψo ψc for Development Length of Standard
Hooks, Ldh in Tension
1.8 Modification of Earthquake-Resistant Provision
13
14
1.8.1 Maximum Longitudinal Reinforcement Ratio in Beams of Special
Moment Frames are revised.
14
1.8.2 Spacing of the Hoops, S_0, in Columns of Intermediate Moment Frames
are revised.
14
1.8.3 Spacing of the Hoops in Beams of Special Moment Frames are revised.
15
1.8.4 Spacing of the Hoops in Column of Special Moment Frames are revised.
15
1.8.5. The Calculation of Design Shear Forces, Vu, in Special Structural Walls
is revised.
References
15
17
OVERVIEW
The newly ACI318 Code, ACI318-19 is similar with previous code, ACI318-14 in basic approach. But,
there are a number of significant differences in the current documents.
Through this brief document, you can discover the differences between the two codes, understand
their factors and design concepts.
In this document, Comparison between ACI 318-19 and ACI318-14 Code has 8 classified sections.
1. New rebar material
2. Minimum reinforcement provisions
3. New reinforcement strain limit
4. Significant updates to shear provisions
5. Hanger reinforcement provisions
6. New equation for Ie,
7. Modification of development length provisions
8. Modification of earthquake-resistance provisions.
Also, the specific standards are based on the ACI318M-19 and ACI318M-14 as SI Units for US
Codes.
ACI318-19 Updates for ULS Design of Reinforcement Concrete
1
1.1.
New Rebar Material
& Revised
Limitations
New provisions are introduced for the use of high-strength reinforcing. ACI 318-19 permits the use
of Grade 690 for SI unit, reinforcement to resist moments and axial forces from gravity and wind
load combinations.
The use of higher-grade reinforcement raised concerns about serviceability like, cracking and
deflections, which were addressed through a series of changes for slab and beam minimum
reinforcement, an effective moment of inertia and requirements for deflection calculations for twoway slabs.
Therefore, the limitations also revised due to the use of high-strength reinforcing bars.
1.1.1 Use of High-Strength Reinforcement
A major thrust of the ACI318-19 Code cycle was to expand permissible applications of highstrength reinforcement.
Current U.S. building codes limit rebar strength based on decades-old research, with most
reinforcement used in concrete construction in the United States being Grade 420. Producers are
now able to produce rebar, however, that is almost twice as strong as it was several decades ago.
Strength and ductility concerns were addressed by introducing new requirements for mechanical
properties of reinforcing bars, adjusting the method for calculating the strength reduction factor
for a moment and combined moment and axial load, revising development length provisions, and
limiting the value of fy that can be used for calculating the maximum axial compressive strength,
pn,max of columns.
Table 1. The Comparison between ACI318-14 and ACI318-19
It is likely that Grade 690 reinforcement will be used mostly for vertical bars of shear walls and
columns, though it might also be used for heavily loaded floor systems.
Substantial new research and others have demonstrated acceptable performance of members of
special seismic systems reinforced with Grade 550 and Grade 690.
Recognizing this, ACI 318-19 now permits special moment frames with Grade550 reinforcement
and special structural walls with Grade 550 and Grade 690.
The provisions allow the use of the higher grades to resist moments, axial forces, and shear. To
accommodate these higher grades, additional restrictions on hoop spacing, beam-column joint
dimensions, and lap splice locations have been added that will contribute to the more reliable
performance of special structural systems.
ACI318-19 Updates for ULS Design of Reinforcement Concrete
2
Table 2. ACI318-14 9 (SI unit) and ACI318-19 (SI unit)
For usage of flexural, axial force and shrinkage and temperature, also shear forces, Grade 550 is
allowed for special moment frames. Also, Grade 690 is allowable for special structural walls.
But, longitudinal reinforcement over 550MPa is not permitted for intermediate moment frames and
ordinary moment frames resisting earthquake demands.
1.1.2 Minimum Slab Thickness Requirements
Table 3. Minimum Thickness of Non-Prestressed Two-Way Slabs without Interior Beams
ACI 318-19, Table 3 for minimum thickness of non-prestressed two-way slabs without interior
beams is revised to include Grade 550.
This change affects the calculation of minimum slab thickness for fy exceeding Grade 420. The
previous code that allowed up to Grade 520 was changed to Grade 550, and related regulations
were modified.
1.1.3 Upper Limit for 𝐟𝐲 is introduced for minimum flexural reinforcement 𝐀𝐬,𝐦𝐢𝐧 calculations.
Table 4. Minimum Flexural Reinforcement in Non-Prestressed Beams
For the calculation of the minimum flexural reinforcement amount for beams with 𝐟𝐲 exceeding
550MPa, ACI 318-19, 9.6.1.2 states that “the value of 𝐟𝐲 shall be limited to a maximum of 550MPa.”
ACI 318-14 did not permit the use of flexural reinforcement exceeding 550MPa for beams. This
change affects the flexural designs of beams with 𝐟𝐲 exceeding 550MPa.
ACI318-19 Updates for ULS Design of Reinforcement Concrete
3
The 𝐀𝐬,𝐦𝐢𝐧 values calculated by equations 9.6.1.2 (a) and (b) (Table 4) will be same as for Grade 550
reinforcement even if the higher strength reinforcement is utilized in the design.
1.1.4 Upper Limit for 𝐟𝐲 is introduced for maximum axial compressive strength, 𝐩𝐧,𝐦𝐚𝐱
calculations.
Table 5. Maximum Axial Compressive Strength
For the calculation of the maximum axial compressive strength, Pn,max for non-prestressed and
prestressed members with exceeding g Grade 550, ACI 318-19, 22.4.2.1 states that “the value of fy
shall be limited to a maximum of 550MPa.”
ACI 318-14 did not permit the use of reinforcement exceeding Grade 550.
This change affects the designs of structural members exceeding Grade 550. The P0 values
calculated by Eq. 22.4.2.2 and 22.4.2.3 (Table 5) will be the same as for Grade 550 reinforcement
even if the higher strength reinforcement is utilized in the design.
1.1.5 Lower Limit of 𝟎. 𝟓𝐝𝐛 is introduced for transverse reinforcement index, 𝐊 𝐭𝐫 , 𝒘𝒉𝒆𝒏 𝒇𝒚 ≥
𝟓𝟓𝟎𝑴𝑷𝒂.
Table 6. Reinforcement Detailing of Transverse Reinforcement
ACI 318-19, 9.7.1.4, 10.7.1.3, 25.4.2.2 (Table 6) states that “along development and lap splice
lengths of longitudinal bars with Grade 550 or more, 𝐊 𝐭𝐫 , shall not be smaller than 0.5db.”
ACI 318-14 did not permit the use of flexural reinforcement with Grade 550 or more for beams and
columns.
The requirement for a minimum value of 𝐊 𝐭𝐫 along development and splice lengths improves
ductility.
ACI318-19 Updates for ULS Design of Reinforcement Concrete
4
1.1.6 Modulus of Rupture Equation is modified for fy exceeding 550MPa in non-prestressed
slab systems.
Table 7. Modulus of Rupture Equation for fy Exceeding 550 MPa for Two-Way Slab Systems
ACI 318-19, 8.3.1.1 states that “for fy exceeding 550MPa, the calculated deflection limits in 8.3.2
shall be satisfied assuming a reduced modulus of rupture, fr = 0.41√𝑓𝑐 ′ for a two-way slab. This
change leads to higher deflections for exceeding grade 550, in non-prestressed two-way slab
systems.
1.2.
Minimum
Reinforcement
Provisions Are
Revised
ACI 318-14 requirements were based on reinforcement type and grade.
The minimum flexural reinforcement regulations according to the reinforcement type and grade
were integrated into one in ACI318-19
1.2.1 Modulus of Rupture Equation is modified for 𝐟𝐲 exceeding 550MPa in non-prestressed
slab systems.
Table 8. Minimum Flexural Reinforcement in Non-Prestressed 1 and 2-Way Slabs
ACI 318-19, 7.6.1.1 (Table 8) states that “a minimum area of flexural reinforcement, As,min , of
0.0018 Ag shall be provided.”
Deformed bars with the yield strength of reinforcement, fy < 420MPa: The minimum area of
flexural reinforcement, As,min , was equal to 0.0020 Ag.
Therefore, the As,min requirement is reduced by 10% in ACI 318-19. Deformed bars or welded wire
reinforcement with the yield strength of reinforcement, fy ≥ 420MPa: The minimum area of flexural
reinforcement, As,min , was greater of two equations.
Therefore, As,min , requirement stayed unchanged for Grade 420 reinforcement in ACI 318-19.
However, for higher grade reinforcement, there is an increase in As,min , requirement. For example,
for Grade 550, Grade 690 reinforcements, the new, As,min , requirement is 0.0018 Ag, instead of the
ACI 318-14 value of 0.0014 Ag (i.e. 28.6% increase in ACI 318-19).
ACI318-19 Updates for ULS Design of Reinforcement Concrete
5
This change unifies the minimum flexural reinforcement requirement irrespective of reinforcement
grade.
1.2.2 Minimum Shear Reinforcement in Non-Prestressed Beams is revised.
Table 9. Minimum Shear Reinforcement in Non-Prestressed Beams
In ACI318-19, the minimum shear reinforcement in non-prestressed beams is also revised.
In ACI318-14, the formula using the Vc calculation formula was judged, but in ACI318-19, it has
been changed to a fixed formula. But, it seems almost no numerical impact between 14 and 19 as
shown.
1.3.
New Reinforcement
Strain Limit Is
Introduced for NonPrestressed
Members
This is new definition of εt , associated with tension-controlled limit.
1.3.1 Adjustment to the Strength Reduction Factor, ∅
Table 10. Strength Reduction Factor ø
Before ACI 318M-14, the compression-controlled strain limit was defined as 0.002 for Grade 420
reinforcement and all prestressed reinforcement, but it was not explicitly defined for other types of
reinforcement. The compression-controlled strain limit εty is defined in 21.2.2.1 and 21.2.2.2 (Table
10) for deformed and prestressed reinforcement, respectively.
Beams and slabs are usually tension-controlled, whereas columns may be compression-controlled.
Some members, such as those with small axial forces and large bending moments, experience net
tensile strain in the extreme tension reinforcement between the limits of εty and "εty + 0.003".
These sections are in a transition region between compression-controlled and tension-controlled.
In ACI 318-19, Table 10 for strength reduction factor, ∅, for moment, axial force, or combined
moment and axial force, the tension-controlled strain limit is defined as an expression of fy to
explicitly cover non-prestressed reinforcement grades other than Grade 420. Therefore, the
beginning with the 2019 Code, the expression, εty + 0.003, defines the lower limit on εt for tensioncontrolled behavior.
ACI318-19 Updates for ULS Design of Reinforcement Concrete
6
1.3.2 Reinforcement Strain Limit is revised for slabs and beams. Satisfy Tension is controlled
in Table 11.
Table 11. Reinforcement Strain Limit
ACI 318-14, 7.3.3.1, 8.3.3.1 and 9.3.3.1 stated that “for non-prestressed slabs and beams, εt , shall
be at least 0.004.”
ACI 318-19 states that “non-prestressed slabs & beams shall be tension-controlled in accordance
with Table 10.”
This change leads to more economical designs for non-prestressed slabs and beams by
eliminating the need to add more tension-steel just to satisfy a lower steel strain limit without any
gain in the flexural design strength due to reduced strength reduction factor, ∅.
1.4.
Significant Updates
to Shear Provisions
Maybe, this section is large change parts from ACI318-14.
ACI 318-19 sections on one-way shear and two-way shear (i.e., punching shear) consolidate what
was previously a wide range of equations.
1.4.1 One-Way Shear Strength Provisions in Relation with Vc for Non-Prestressed Members are
modified substantially.
Table 12. Vx for One-Way Member without Shear Reinforcement
One-way shear strength provisions in relation with Vc for non-prestressed members are modified
substantially.
ACI 318-19 updates shear capacity to reflect research findings on the effect of flexural
reinforcement, ρw , and section depth, size effect factor, λs , using test data.
ACI318-19 Updates for ULS Design of Reinforcement Concrete
7
ACI 318-19, 22.5.5.1 states that, “for non-prestressed members, Vc shall be calculated by Table
22.5.5.1 and 22.5.5.1.1 through 22.5.5.1.3.”
The new Table 22.5.5.1 is titled Vc for non-prestressed members and aims to unify the ACI 318-14
provisions of :
- 22.5.5: Vc for non-prestressed members without axial force
- 22.5.6: Vc for non-prestressed members with axial compression
- 22.5.7: Vc for non-prestressed members with significant axial tension
- The new Table 22.5.5.1 introduces criteria for Av and Av,min , that results in a set of equations to
calculate Vc .
Table 13. Vc for Non-Prestressed Members
ACI 318-19, 22.5.5.1.1 states that “Vc shall not be taken greater than 0.42λ√𝑓 ′ 𝑐𝑏𝑤 𝑑. ".
ACI 318-19, 22.5.5.1.2 states that “In Table 22.5.5.1, that value of shall not be taken greater than
0.05√𝑓 ′ 𝑐.”
ACI 318-19, 22.5.5.1.3 states that “The size effect modification factor, λs , shall be determined by
equation 22.5.5.1.3”.
The changes account for the size effect and low reinforcement levels for members without shear
reinforcement.
It also simplifies and reduces the equations for non-prestressed concrete members with and
without axial load effects.
The impact of the change will vary and can only be determined after implementation for various
conditions.
Figure 1. Size Effect Graph
The size effect factor, λs , is the factor used to modify shear strength based on the effects of
member depth. As shown in the left graph, it becomes less than 1.0 when the effective depth is
250mm as the starting point.
Also, the right graph is a graph comparing the longitudinal reinforcement ratio with the variable
1
factor, 0.66λ(ρw )2 , listed in Table 22.5.5.1.
ACI318-19 Updates for ULS Design of Reinforcement Concrete
8
ρw included in Table 22.5.5.1 Vc for non-prestressed members, the longitudinal flexural
reinforcement affects the shear design.
As the longitudinal flexural reinforcement ratio increases, the shear capacity of concrete increases.
Compared with the constant 0.16λ in ACI318-14, if the longitudinal reinforcement ratio is about
1.42% or more, the shear capacity by the formula of ACI318-19 is greater than that of ACI318-14.
1.4.2 Two-Way Shear Strength in Vc for Non-Prestressed Members is revised regarding as size
effect factor.
Table 14. Vc for Two-Way Members without Shear Reinforcement
ACI 318-19, Table 22.6.5.2 now incorporates the size effect factor, λs , in equations (a), (b), and (c).
Likewise, this change accounts for the depth effect of slabs without shear reinforcement.
For two-way slabs without shear reinforcement and if the effective depth is more than 250mm, the
size effect factor will result in reduced two-way shear strength values as compared to ACI 318-14.
1.4.3 Bidirectional One-Way Shear Effects are to be considered.
Table 15. Interaction of Shear Forces Acting Along Orthogonal Axes
The interaction of one-way shear forces acting along orthogonal axes shall need to be considered
per ACI 318-19 as provision 22.5.1.11 states that “if the shear ratio is more than 0.5 by each
direction, Equation 22.5.1.11 shall be satisfied.”
ACI318-19 Updates for ULS Design of Reinforcement Concrete
9
1.4.4 Upper Limit for the Nominal Shear Strength of Walls, Vn
Table 16. Upper Limit for the Nominal Shear Strength of Walls
ACI 318-19, 11.5.4.2 states that “Vn at any horizontal section shall not exceed 0.66√𝑓 ′ 𝑐𝐴𝑐𝑣 .
In ACI 318-14, the limit for in-plane shear, the nominal shear strength, Vn, was 0.66√𝑓′𝑐 ℎ𝑑.
This limit is intended to guard against diagonal compression failure in structural walls. The change
creates consistency in the calculation of the in-plane shear strength of structural walls between
Code chapters 18 (18.10.4) and 11 (11.5.4).
However, it has no numerical impact as the d is already taken as 0.8lw and Acv equals hlw.
1.4.5 The Nominal Shear Strength, Vn, Calculations for Walls are modified.
Table 17. New Vn Calculations for Walls
ACI 318-19, 11.5.4.3 states that to improve consistency in the Code, the nominal in-plane shear
strength equation in section 11.5.4.3 now has the same form as the shear strength equation used
in section 18.10.4.1.
Accordingly, the sections 11.5.4.6 and 11.5.4.7 of ACI 318-14 for Vc calculations, and section
11.5.4.8 of ACI 318-14 for Vs calculation are completely removed from ACI 318-19. The detailed
calculation option allowed within Wall is also removed by the removal of the ACI 318-14, Table
11.5.4.6. The impact of this change can only be determined after implementation for various
conditions.
ACI318-19 Updates for ULS Design of Reinforcement Concrete
10
1.4.6 In-Plane Vu Criteria is modified for determining the minimum reinforcement for walls.
Table 18. Minimum Reinforcement Limits for Walls
This change results in a similar threshold value for Vu criteria where minimum reinforcement shall
be acceptable.
In ACI318-19, depending on the height, hw, by the length of the entire walls, lw, the limits values
may change.
1.5.
Hanger
Reinforcement
Provisions Are
Introduced
Table 19. Minimum Spacing of Shear Reinforcement for Hanger Reinforcement for Shear Transfer
ACI 318-19, Table 9.7.6.2.2 introduces new maximum spacing of legs of shear reinforcement
criteria across the width of beams. Also, a new figure is added to the commentary titled Table.
9.7.6.2.1- Hanger reinforcement for shear transfer.
This change improves the shear behavior of wide beams that require stirrups by providing multiple
stirrup legs. This new criterion affects the determination of the number of legs across the width of
a wide beams in design.
ACI318-19 Updates for ULS Design of Reinforcement Concrete
11
1.6.
New Equation for
Effective Moment
of Inertia for
Cracked Section
Table 20. Effective Moment of Inertia, Ie
In-plane Vu criteria Effective moment of inertia, Ie equations are revised to Bischoff and Scanlon
equations. It has been changed to an equation similar to the Eurocode.
ACI 318-19, 24.2.3.5 states that “for non-prestressed members, unless obtained by more
comprehensive analysis, an effective moment of inertia, Ie , shall be calculated in accordance with
Table 24.2.3.5
As stated in ACI 318-19, 24.2.3.5, the effective moment of inertia approximation developed by
Bischoff and Scanlon is adopted and Branson equation (ACI 318-14, Eq. 24.2.3.5a) is removed from
ACI 318-19.
The commentary further states that the ACI 318-14, Eq.24.2.3.5a has been shown to
underestimate deflections for members with low reinforcement ratios, which often occur in slabs,
and did not consider the effects of restraint. For members with greater than 1% reinforcement and
a service moment at least twice the cracking moment, there is little difference between deflections
calculated using the ACI 318-14 provisions and the ACI 318-19 provisions.
1.7.
Modification of
Development
Length Provisions
1.7.1 Considering Reinforcement Grade Factor, 𝚿𝐠 for Development Length in Tension, 𝐥𝐝
[Simple]
Table 21. Development Length for Deformed Bars and Deformed Wires in Tension - 1
In ACI 318-19, 25.4, the development length equations are revised to cover high-strength
reinforcement and concrete, to account for spacing between bars. New modification factors such
as ψg, reinforcement grade factor, to modify development length based on grade of reinforcement
are introduced.
ACI318-19 Updates for ULS Design of Reinforcement Concrete
12
1.7.2 Considering Reinforcement Grade Factor, 𝚿𝐠 for Development Length in Tension, 𝐥𝐝
[General]
Table 22. Development Length for Deformed Bars and Deformed Wires in Tension - 2
In ACI 318-19, it is also considered reinforcement grade factor, ψg for the general method for
calculating development length in tension, 𝑙𝑑 .
1.7.3 Considering Factors, 𝛙𝐫 𝛙𝐨 𝛙𝐜 for Development Length of Standard Hooks, Ldh in
Tension
Table 23. Development Length of Standard Hooks
For calculating the development length of standard hooks in tension, concrete strength factor, ψc
is considered according to concrete compressive strength, fc.
And, ψo , location factor, and ψr , confining reinforcement factor are redefined in ACI318-19.
ACI318-19 Updates for ULS Design of Reinforcement Concrete
13
1.8.
Modification of
EarthquakeResistant Provision
1.8.1 Maximum Longitudinal Reinforcement Ratio in Beams of Special Moment Frames are
Revised.
Table 24. Minimum Longitudinal Reinforcement Ratio, 𝜌𝑚𝑎𝑥
In ACI 318-14, the longitudinal reinforcement in beams of special moment frames, ρ, longitudinal
reinforcement ratio, shall not exceed 0.025 regardless of the rebar strength.
In ACI 318-19, for grade 550, ρ shall not exceed 0.02.
1.8.2 Spacing of the Hoops, 𝐒𝟎 , in Columns of Intermediate Moment Frames are revised.
Table 25. Spacing of the Hoops, 𝐒𝟎 , in Columns of Intermediate Moment Frames
In ACI 318-14, hoop spacing, 𝐒𝟎 , over a length ℓo in columns of intermediate moment frames shall
not exceed the smallest of 4 cases.
In ACI 318-19, according to the rebar grade, the limitation is different and smaller spacing should
be satisfied than ACI318-14.
ACI318-19 Updates for ULS Design of Reinforcement Concrete
14
1.8.3 Spacing of the Hoops in Beams of Special Moment Frames are revised.
Table 26. Spacing of the Hoops in Beams of Special Moment Frames
Hoop spacing in beams of special moment frames shall not exceed the smallest of 3 cases in ACI
318-14.
In ACI 318-19, however, the limitation is different, 5 times of diameters, not 6 times of the smallest
primary flexural reinforcing bar for grade 550.
1.8.4 Spacing of the Hoops in Column of Special Moment Frames are revised.
Table 27. Spacing of Transverse Reinforcement
In ACI 318-14, spacing of transverse reinforcement of special moment frames shall not exceed the
smallest (a) through (c):
In ACI 318-19, for grade 550, the limitation is different, 5db not 6db of the smallest primary flexural
reinforcing bar.
1.8.5. The Calculation of Design Shear Forces, Vu, in Special Structural Walls is revised.
Table 28. Design Shear Forces, Vu in Special Structural Walls
ACI318-19 Updates for ULS Design of Reinforcement Concrete
15
Design forces, Vu in special structural walls in ACI318-14 shall be obtained from the lateral load
analysis in accordance with the factored load combinations.
In ACI318-19, the design shear force, Ve shall be calculated by equation 18.10.3.1(Table 28), by
additionally multiplying Ωv , overstrength factor, and ωv factor to account for dynamic shear
amplification.
Design shears for structural walls are obtained from lateral load analysis with appropriate load
factors increased to account for flexural overstrength at critical sections where yielding of
longitudinal reinforcement is expected, and dynamic amplification due to higher mode effects, as
illustrated in Table 28.
Because Mn and Mpr depend on axial force, which varies for different load combinations, and
loading direction for flanged and coupled walls, the condition producing the largest value of omega,
v should be used. Although the value of 1.5 in 18.10.3.1.2 (Table 28) is greater than the minimum
value obtained for the governing load combination with a phi factor of 0.9 and tensile stress of at
least 1.25 fy in the longitudinal reinforcement, a value greater than 1.5 may be appropriate if
provided longitudinal reinforcement exceeds that required.
Dynamic amplification is not significant in walls with hw/ℓw less than 2. A limit of 0.007hwcs is
imposed on ns to account for buildings with large story heights.
The application of Ωv to Vu does not preclude the application of a redundancy factor if required by
the general building code.
ACI318-19 Updates for ULS Design of Reinforcement Concrete
16
References
AS 318M-19: Building Code Requirements for Structural Concrete, American Concrete Institute
ACI 318M-14: Building Code Requirements for Structural Concrete, American Concrete Institute
“ACI 318-19 Code Revisions Impact Assessment” by StructurePoint
“What’s New in ACI 318-19 ” by Jack P. Moehle, PhD, PE
ACI318-19 Updates for ULS Design of Reinforcement Concrete
17