= 18. CHAPTER 18 — EARTHQUAKE-RESISTANT STRUCTURES
= 18.12 — Diaphragms and trusses
== 18.12.1 Scope
=== 18.12.1.1 This section shall apply to diaphragms and
collectors forming part of the seismic-force-resisting system
in structures assigned to SDC D, E, or F and to SDC C if
18.12.1.2 applies.
=== 18.12.1.2 Section 18.12.11 shall apply to diaphragms
constructed using precast concrete members and forming
part of the seismic-force-resisting system for structures
assigned to SDC C, D, E, or F.
= R18.12 — Diaphragms and trusses
== R18.12.1 Scope
Diaphragms as used in building construction are structural
elements (such as a floor or roof) that provide some or all of
the following functions:
(a) Support for building elements (such as walls, partitions,
and cladding) resisting horizontal forces but not
acting as part of the seismic-force-resisting system
(b) Transfer of lateral forces from the point of application
to the vertical elements of the seismic-force-resisting
(c) Connection of various components of the vertical
seismic-force-resisting system with appropriate strength,
stiffness, and ductility so the building responds as intended
in the design (Wyllie 1987).
American Concrete Institute – Copyrighted © Material – www.concrete.org
336 ACI 318-19: BUILDING CODE REQUIREMENTS FOR STRUCTURAL CONCRETE
No further reproduction or distribution is permitted.
=== 18.12.1.3 Section 18.12.12 shall apply to structural trusses
forming part of the seismic-force-resisting system in structures
assigned to SDC D, E, or F.
== 18.12.2 Design forces
=== 18.12.2.1 The earthquake design forces for diaphragms
shall be obtained from the general building code using the
applicable provisions and load combinations.
== 18.12.3 Seismic load path
=== 18.12.3.1 All diaphragms and their connections shall
be designed and detailed to provide for transfer of forces
to collector elements and to the vertical elements of the
seismic-force-resisting system.
=== 18.12.3.2 Elements of a structural diaphragm system that
are subjected primarily to axial forces and used to transfer
diaphragm shear or flexural forces around openings or other
discontinuities shall satisfy the requirements for collectors
in 18.12.7.6 and 18.12.7.7.
== R18.12.2 Design forces
=== R18.12.2.1 In the general building code, earthquake
design forces for floor and roof diaphragms typically are
not calculated directly during the lateral-force analysis that
provides story forces and story shears. Instead, diaphragm
design forces at each level are calculated by a formula
that amplifies the story forces recognizing dynamic effects
and includes minimum and maximum limits. These forces
are used with the governing load combinations to design
diaphragms for shear and moment.
For collector elements, the general building code in the
United States specifies load combinations that amplify
earthquake forces by a factor Ωo. The forces amplified
by Ωo are also used for the local diaphragm shear forces
resulting from the transfer of collector forces, and for local
diaphragm flexural moments resulting from any eccentricity
of collector forces. The specific requirements for earthquake
design forces for diaphragms and collectors depend
on which edition of the general building code is used. The
requirements may also vary according to the SDC.
For most concrete buildings subjected to inelastic earthquake
demands, it is desirable to limit inelastic behavior of
floor and roof diaphragms under the imposed earthquake
forces and deformations. It is preferable for inelastic behavior
to occur only in the intended locations of the vertical seismic-force-
resisting system that are detailed for ductile response,
such as in beam plastic hinges of special moment frames, or
in flexural plastic hinges at the base of structural walls or in
coupling beams. For buildings without long diaphragm spans
between lateral-force-resisting elements, elastic diaphragm
behavior is typically not difficult to achieve. For buildings
where diaphragms could reach their flexural or shear strength
before yielding occurs in the vertical seismic-force-resisting
system, the licensed design professional should consider
providing increased diaphragm strength.
For reinforced concrete diaphragms, ASCE/SEI 7 Sections
12.10.1 and 12.10.2 provide requirements to determine
design forces for reinforced concrete diaphragms. For precast
concrete diaphragms in buildings assigned to SDC C, D, E, or
F, the provisions of ASCE/SEI 7 Section 12.10.3 apply.
== R18.12.3 Seismic load path
=== R18.12.3.2 This provision applies to strut-like elements
that occur around openings, diaphragm edges, or other
discontinuities in diaphragms. Figure R18.12.3.2 shows
an example. Such elements can be subjected to earthquake
axial forces in combination with bending and shear from
earthquake or gravity loads.
Fig. R18.12.3.2—Example of diaphragm subject to the
requirements of 18.12.3.2 and showing an element having
confinement as required by 18.12.7.6.
American Concrete Institute – Copyrighted © Material – www.concrete.org
PART 5: EARTHQUAKE RESISTANCE 337
18 Seismic
No further reproduction or distribution is permitted.
== 18.12.4 Cast-in-place composite topping slab diaphragms
=== 18.12.4.1 A cast-in-place composite topping slab on
a precast floor or roof shall be permitted as a structural
diaphragm, provided the cast-in-place topping slab is reinforced
and the surface of the previously hardened concrete
on which the topping slab is placed is clean, free of laitance,
and intentionally roughened.
== 18.12.5 Cast-in-place noncomposite topping slab
=== 18.12.5.1 A cast-in-place noncomposite topping on a precast
floor or roof shall be permitted as a structural diaphragm,
provided the cast-in-place topping slab acting alone is
designed and detailed to resist the design earthquake forces.
== 18.12.6 Minimum thickness of diaphragms
=== 18.12.6.1 Concrete slabs and composite topping slabs
serving as diaphragms used to transmit earthquake forces shall
be at least 50 mm thick. Topping slabs placed over precast
=== 18.12.6.1 Continuation
floor or roof elements, acting as diaphragms and not relying
on composite action with the precast elements to resist the
design earthquake forces, shall be at least 65 mm thick.
== R18.12.4 Cast-in-place composite topping slab diaphragms
=== R18.12.4.1 A bonded topping slab is required so that
the floor or roof system can provide restraint against slab
buckling. Reinforcement is required to ensure the continuity
of the shear transfer across precast joints. The connection
requirements are introduced to promote a complete system
with necessary shear transfers.
== R18.12.5 Cast-in-place noncomposite topping slab
=== R18.12.5.1 Composite action between the topping slab
and the precast floor elements is not required, provided that
the topping slab is designed to resist the design earthquake
forces.
== R18.12.6 Minimum thickness of diaphragms
=== R18.12.6.1 The minimum thickness of concrete
diaphragms reflects current practice in joist and waffle
systems and composite topping slabs on precast floor and
=== R18.12.6.1 Continuation
roof systems. Thicker slabs are required if the topping slab
is not designed to act compositely with the precast system to
resist the design earthquake forces.
American Concrete Institute – Copyrighted © Material – www.concrete.org
338 ACI 318-19: BUILDING CODE REQUIREMENTS FOR STRUCTURAL CONCRETE
No further reproduction or distribution is permitted.
== 18.12.7 Reinforcement
=== 18.12.7.1 The minimum reinforcement ratio for
diaphragms shall be in conformance with 24.4. Except for
post-tensioned slabs, reinforcement spacing each way in
floor or roof systems shall not exceed 450 mm. Where welded
wire reinforcement is used as the distributed reinforcement
to resist shear in topping slabs placed over precast floor and
roof elements, the wires parallel to the joints between the
precast elements shall be spaced not less than 250 mm. on
center. Reinforcement provided for shear strength shall be
continuous and shall be distributed uniformly across the
shear plane.
=== 18.12.7.2 Bonded tendons used as reinforcement to resist
collector forces, diaphragm shear, or flexural tension shall be
designed such that the stress due to design earthquake forces
does not exceed 420 MPa. Precompression from unbonded
tendons shall be permitted to resist diaphragm design forces
if a seismic load path is provided.
=== 18.12.7.3 All reinforcement used to resist collector forces,
diaphragm shear, or flexural tension shall be developed or
spliced for fy in tension.
=== 18.12.7.4 Type 2 splices are required where mechanical
splices on Grade 420 reinforcement are used to transfer
forces between the diaphragm and the vertical elements of
the seismic-force-resisting system. Grade 550 and Grade
690 reinforcement shall not be mechanically spliced for this
application.
=== 18.12.7.5 Longitudinal reinforcement for collectors shall
be proportioned such that the average tensile stress over
length (a) or (b) does not exceed ϕfy where the value of fy is
limited to 420 MPa.
=== 18.12.7.5 Continuation
(a) Length between the end of a collector and location at
which transfer of load to a vertical element begins
(b) Length between two vertical elements
== R18.12.7 Reinforcement
=== R18.12.7.1 Minimum reinforcement ratios for diaphragms
correspond to the required amount of temperature and
shrinkage reinforcement (refer to 24.4). The maximum
spacing for reinforcement is intended to control the width
of inclined cracks. Minimum average prestress requirements
(refer to 24.4.4.1) are considered to be adequate to limit the
crack widths in post-tensioned floor systems; therefore, the
maximum spacing requirements do not apply to these systems.
The minimum spacing requirement for welded wire reinforcement
in topping slabs on precast floor systems is to avoid
fracture of the distributed reinforcement during an earthquake.
Cracks in the topping slab open immediately above the
boundary between the flanges of adjacent precast members, and
the wires crossing those cracks are restrained by the transverse
wires (Wood et al. 2000). Therefore, all the deformation associated
with cracking should be accommodated in a distance not
greater than the spacing of the transverse wires. A minimum
spacing of 250 mm for the transverse wires is required to reduce
the likelihood of fracture of the wires crossing the critical cracks
during a design earthquake. The minimum spacing requirements
do not apply to diaphragms reinforced with individual
bars, because strains are distributed over a longer length.
=== R18.12.7.3 Bar development and lap splices are designed
according to requirements of Chapter 25 for reinforcement
in tension. Reductions in development or splice length for
calculated stresses less than fy are not permitted, as indicated
in 25.4.10.2.
=== R18.12.7.5 Table 20.2.2.4(a) permits the maximum design
yield strength to be 550 MPa for portions of a collector, for
example, at and near critical sections. The average stress
in the collector is limited to control diaphragm cracking
over the length of the collector. The calculation of average
stress along the length is not necessary if the collector is
=== R18.12.7.5 Continuation
designed for fy of 420 MPa even if Grade 550 reinforcement
is specified.
American Concrete Institute – Copyrighted © Material – www.concrete.org
PART 5: EARTHQUAKE RESISTANCE 339
18 Seismic
No further reproduction or distribution is permitted.
=== 18.12.7.6 Collector elements with compressive stresses
exceeding 0.2fc′ at any section shall have transverse reinforcement
satisfying 18.7.5.2(a) through (e) and 18.7.5.3,
except the spacing limit of 18.7.5.3(a) shall be one-third of
the least dimension of the collector. The amount of transverse
reinforcement shall be in accordance with Table 18.12.7.6.
The specified transverse reinforcement is permitted to be
discontinued at a section where the calculated compressive
stress is less than 0.15fc′.
If design forces have been amplified to account for the
overstrength of the vertical elements of the seismic-forceresisting
system, the limit of 0.2fc′ shall be increased to
0.5fc′, and the limit of 0.15fc′ shall be increased to 0.4fc′.
Table 18.12.7.6—Transverse reinforcement for
=== 18.12.7.7 Longitudinal reinforcement detailing for collector
elements at splices and anchorage zones shall satisfy (a) or (b):
(a) Center-to-center spacing of at least three longitudinal
bar diameters, but not less than 40 mm, and concrete clear
cover of at least two and one-half longitudinal bar diameters,
but not less than 50 mm.
(b) Area of transverse reinforcement, providing Av at least
the greater of 0.062 sqrt(fc')(bw.s/fyt) and 0.35bw.s/fyt, except
as required in 18.12.7.6
== 18.12.8 Flexural strength
=== 18.12.8.1 Diaphragms and portions of diaphragms shall
be designed for flexure in accordance with Chapter 12. The
effects of openings shall be considered.
=== R18.12.7.6 In documents such as the NEHRP Provisions
(FEMA P750), ASCE/SEI 7, the 2018 IBC, and the
Uniform Building Code (ICBO 1997), collector elements
of diaphragms are designed for forces amplified by a factor
Ωo to account for the overstrength in the vertical elements
of the seismic-force-resisting systems. The amplification
factor Ωo ranges between 2 and 3 for most concrete structures,
depending on the document selected and on the type
of seismic-force-resisting system. In some documents, the
factor can be calculated based on the maximum forces that
can be developed by the elements of the vertical seismicforce-
resisting system.
Compressive stress calculated for the factored forces on a
linearly elastic model based on gross section of the structural
diaphragm is used as an index value to determine whether
confining reinforcement is required. A calculated compressive
stress of 0.2fc′, or 0.5fc′ for forces amplified by Ωo,
is assumed to indicate that integrity of the entire structure
depends on the ability of that member to resist substantial
compressive force under severe cyclic loading. Transverse
reinforcement is required at such locations to provide
confinement for the concrete and the reinforcement.
=== R18.12.7.7 This section is intended to reduce the possibility
of bar buckling and provide adequate bar development
conditions in the vicinity of splices and anchorage zones.
== R18.12.8 Flexural strength
=== R18.12.8.1 Flexural strength for diaphragms is calculated
using the same assumptions as for walls, columns, or beams.
The design of diaphragms for flexure and other actions uses
the applicable load combinations of 5.3.1 to consider earthquake
forces acting concurrently with gravity or other loads.
The influence of slab openings on flexural and shear strength
is to be considered, including evaluating the potential critical
sections created by the openings. The strut-and-tie method is
potentially useful for designing diaphragms with openings.
=== R18.12.8.1 Continuation
Earlier design practice assumed design moments for
diaphragms were resisted entirely by chord forces acting
at opposite edges of the diaphragm. This idealization was
implicit in earlier versions of the Code, but has been replaced
by an approach in which all longitudinal reinforcement,
within the limits of 18.12.7, is assumed to contribute to the
flexural strength of the diaphragm. This change reduces the
required area of longitudinal reinforcement concentrated
near the edge of the diaphragm, but should not be interpreted
as a requirement to eliminate all boundary reinforcement.
American Concrete Institute – Copyrighted © Material – www.concrete.org
340 ACI 318-19: BUILDING CODE REQUIREMENTS FOR STRUCTURAL CONCRETE
No further reproduction or distribution is permitted.
== 18.12.9 Shear strength
=== 18.12.9.1 Vn of diaphragms shall not exceed:
Vn = Acv ( 0.17.λ.sqrt(fc')+rho_t.fy ) ... (18.12.9.1)
For cast-in-place topping slab diaphragms on precast
floor or roof members, Acv shall be calculated using only
the thickness of topping slab for noncomposite topping slab
diaphragms and the combined thickness of cast-in-place and
precast elements for composite topping slab diaphragms. For
composite topping slab diaphragms, the value of fc′ used to
calculate Vn shall not exceed the lesser of fc′ for the precast
members and fc′ for the topping slab.
18.12.9.2 Vn of diaphragms shall not exceed 0.66 sqrt(fc') Acv.
=== 18.12.9.3 Above joints between precast elements in
noncomposite and composite cast-in-place topping slab
diaphragms, Vn shall not exceed:
Vn = Avf.fy.μ ... (18.12.9.3)
where Avf is the total area of shear friction reinforcement
within the topping slab, including both distributed and
boundary reinforcement, that is oriented perpendicular to
joints in the precast system and coefficient of friction, μ,
is 1.0λ, where λ is given in 19.2.4. At least one-half of Avf
shall be uniformly distributed along the length of the potential
shear plane. The area of distributed reinforcement in the
topping slab shall satisfy 24.4.3.2 in each direction.
=== 18.12.9.4 Above joints between precast elements in
noncomposite and composite cast-in-place topping slab
diaphragms, Vn shall not exceed the limits in 22.9.4.4, where
Ac is calculated using only the thickness of the topping slab.
== R18.12.9 Shear strength
The shear strength requirements for diaphragms are
similar to those for slender structural walls and are based
on the shear provisions for beams. The term Acv refers to the
gross area of the diaphragm, but may not exceed the thickness
times the width of the diaphragm. This corresponds
to the gross area of the effective deep beam that forms
the diaphragm. Distributed slab reinforcement ρt used to
calculate shear strength of a diaphragm in Eq. (18.12.9.1)
is positioned perpendicular to the diaphragm flexural reinforcement.
Provision 18.12.9.2 limits the maximum shear
strength of the diaphragm.
In addition to satisfying 18.12.9.1 and 18.12.9.2, cast-inplace
topping slab diaphragms must also satisfy 18.12.9.3
and 18.12.9.4. Cast-in-place topping slabs on a precast floor
or roof system tend to have shrinkage cracks that are aligned
with the joints between adjacent precast members. Therefore,
the additional shear strength requirements for topping
slab diaphragms in 18.12.9.3 are based on a shear friction
model (Wood et al. 2000), and the assumed crack plane
corresponds to joints in the precast system along the direction
of the applied shear, as shown in Fig. R22.9.4.3a. The
coefficient of friction, μ, in the shear friction model is taken
equal to 1.0 for normalweight concrete due to the presence
of these shrinkage cracks.
Both distributed and boundary reinforcement in the topping
slab may be considered as shear friction reinforcement Avf.
Boundary reinforcement within the diaphragm was called
chord reinforcement in ACI 318 before 2008. Although the
boundary reinforcement also resists forces due to moment
and axial force in the diaphragm, the reduction in the shear
friction resistance in the tension zone is offset by the increase
in shear friction resistance in the compression zone. Therefore,
the area of boundary reinforcement used to resist shear
friction need not be added to the area of boundary reinforcement
used to resist moment and axial force. The distributed
topping slab reinforcement must contribute at least one-half
of the nominal shear strength. It is assumed that connections
between the precast elements do not contribute to the shear
strength of the topping slab diaphragm.
Provision 18.12.9.4 limits the maximum shear that may be
transmitted by shear friction within a topping slab diaphragm.
American Concrete Institute – Copyrighted © Material – www.concrete.org
PART 5: EARTHQUAKE RESISTANCE 341
18 Seismic
No further reproduction or distribution is permitted.
== 18.12.10 Construction joints
=== 18.12.10.1 Construction joints in diaphragms shall be
specified according to 26.5.6, and contact surfaces shall be
roughened consistent with condition (b) of Table 22.9.4.2.
== 18.12.11 Precast concrete diaphragms
=== 18.12.11.1 Diaphragms and collectors constructed using
precast concrete members with composite topping slab
and not satisfying 18.12.4, and untopped precast concrete
diaphragms, are permitted provided they satisfy the requirements
of ACI 550.5M. Cast-in-place noncomposite topping
slab diaphragms shall satisfy 18.12.5 and 18.12.6.
=== 18.12.11.2 Connections and reinforcement at joints used
in the construction of precast concrete diaphragms satisfying
18.12.11.1 shall have been tested in accordance with ACI
550.4M.
=== 18.12.11.3 Extrapolation of data on connections and
reinforcement at joints to project details that result in larger
construction tolerances than those used to qualify connections
in accordance with ACI 550.4M shall not be permitted.
== R18.12.11 Precast concrete diaphragms
=== R18.12.11.1 ACI 550.5M provides requirements for the
design of precast concrete diaphragms with connections
whose performance has been validated by ACI 550.4M
testing. ACI 550.5M permits a maximum tolerance for positioning
and completion of connections of 13 mm, which can
be difficult to achieve with normal construction practices.
Section 26.13.1.3 requires continuous inspection of precast
concrete diaphragm connections to verify that construction
is performed properly and tolerances not greater than 13 mm
for all connections are achieved. Results from ACI 550.4M
testing are not to be extrapolated to allow greater tolerances.
Topped precast concrete floors designed in accordance
with Chapter 18 need careful consideration of support conditions
to verify precast concrete members have sufficient
seating for anticipated displacements and ability to accommodate
relative rotations between beam supports and the
member (Henry et al. 2017).
== 18.12.12 Structural trusses
=== 18.12.12.1 Structural truss elements with compressive
stresses exceeding 0.2fc′ at any section shall have transverse
reinforcement, in accordance with 18.7.5.2, 18.7.5.3, 18.7.5.7,
and Table 18.12.12.1, over the length of the element.
== R18.12.12 Structural trusses
=== R18.12.12.1 The expressions for transverse reinforcement
Ash are based on ensuring compression capacity of an equivalent
column section is maintained after spalling of cover
concrete.
American Concrete Institute – Copyrighted © Material – www.concrete.org
342 ACI 318-19: BUILDING CODE REQUIREMENTS FOR STRUCTURAL CONCRETE
No further reproduction or distribution is permitted.
Table 18.12.12.1—Transverse reinforcement for
structural trusses_
=== 18.12.12.2 All continuous reinforcement in structural
truss elements shall be developed or spliced for fy in tension.
[ Lanjut Ke 18.13—Foundations ... ]

Back To Contents