= 4.5 — Structural analysis
== 4.5.1 Analytical procedures shall satisfy compatibility of
deformations and equilibrium of forces.
== 4.5.2 The methods of analysis given in Chapter 6 shall be
permitted.
= R4.5 — Structural analysis
The role of analysis is to estimate the internal forces
and deformations of the structural system and to establish
compliance with the strength, serviceability, and stability
requirements of the Code. The use of computers in structural
engineering has made it feasible to perform analysis
of complex structures. The Code requires that the analytical
procedure used meets the fundamental principles of equilibrium
and compatibility of deformations, permitting a number
of analytical techniques, including the strut-and-tie method
required for discontinuity regions, as provided in Chapter 6.
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= 4.6 — Strength
== 4.6.1 Design strength of a member and its joints and
connections, in terms of moment, shear, torsional, axial, and
bearing strength, shall be taken as the nominal strength Sn
multiplied by the applicable strength reduction factor ϕ.
== 4.6.2 Structures and structural members shall have design
strength at all sections, ϕSn, greater than or equal to the
required strength U calculated for the factored loads and
forces in such combinations as required by this Code or the
general building code.
= R4.6 — Strength
The basic requirement for strength design may be
expressed as follows:
_ design strength ≥ required strength
_ ϕSn ≥ U
In the strength design procedure, the level of safety is
provided by a combination of factors applied to the loads and
strength reduction factors ϕ applied to the nominal strengths.
The strength of a member or cross section, calculated
using standard assumptions and strength equations, along
with nominal values of material strengths and dimensions,
is referred to as nominal strength and is generally designated
Sn. Design strength or usable strength of a member or cross
section is the nominal strength reduced by the applicable
strength reduction factor ϕ. The purpose of the strength
reduction factor is to account for the probability of understrength
due to variations of in-place material strengths and
dimensions, the effect of simplifying assumptions in the
design equations, the degree of ductility, potential failure
mode of the member, the required reliability, and significance
of failure and existence of alternative load paths for
the member in the structure.
This Code, or the general building code, prescribes design
load combinations, also known as factored load combinations,
which define the way different types of loads are
multiplied (factored) by individual load factors and then
combined to obtain a factored load U. The individual load
factors and additive combination reflect the variability in
magnitude of the individual loads, the probability of simultaneous
occurrence of various loads, and the assumptions
and approximations made in the structural analysis when
determining required design strengths.
A typical design approach, where linear analysis is applicable,
is to analyze the structure for individual unfactored
load cases, and then combine the individual unfactored load
cases in a factored load combination to determine the design
load effects. Where effects of loads are nonlinear—for
example, in foundation uplift—the factored loads are applied
simultaneously to determine the nonlinear, factored load
effect. The load effects relevant for strength design include
moments, shears, torsions, axial forces, bearing forces, and
punching shear stresses. Sometimes, design displacements
are determined for factored loads. The load effects relevant
for service design include stresses and deflections.
In the course of applying these principles, the licensed
design professional should be aware that providing more
strength than required does not necessarily lead to a safer
structure because doing so may change the potential failure
mode. For example, increasing longitudinal reinforcement
area beyond that required for moment strength as derived
from analysis without increasing transverse reinforcement
could increase the probability of a shear failure occurring
prior to a flexural failure. Excess strength may be undesirable
for structures expected to behave inelastically during
earthquakes.
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= 4.7 — Serviceability
== 4.7.1 Evaluation of performance at service load conditions
shall consider reactions, moments, shears, torsions,
and axial forces induced by prestressing, creep, shrinkage,
temperature change, axial deformation, restraint of attached
structural members, and foundation settlement.
== 4.7.2 For structures, structural members, and their connections,
the requirements of 4.7.1 shall be deemed to be satisfied
if designed in accordance with the provisions of the
applicable member chapters.
= R4.7 — Serviceability
Serviceability refers to the ability of the structural system
or structural member to provide appropriate behavior and
functionality under the actions affecting the system. Serviceability
requirements address issues such as deflections and
cracking, among others. Serviceability considerations for
vibrations are discussed in R6.6.3.2.2 and R24.1.
Except as stated in Chapter 24, service-level load combinations
are not defined in this Code, but are discussed in
Appendix C of ASCE/SEI 7-16. Appendixes to ASCE/SEI 7
are not considered mandatory parts of the standard.
= 4.8 — Durability
== 4.8.1 Concrete mixtures shall be designed in accordance
with the requirements of 19.3.2 and 26.4, considering applicable
environmental exposure to provide required durability.
== 4.8.2 Reinforcement shall be protected from corrosion in
accordance with 20.5.
= R4.8 — Durability
The environment where the structure will be located will
dictate the exposure category for materials selection, design
details, and construction requirements to minimize potential
for premature deterioration of the structure caused by environmental
effects. Durability of a structure is also impacted
by the level of preventative maintenance, which is not
addressed in the Code.
Chapter 19 provides requirements for protecting concrete
against major environmental causes of deterioration.
= 4.9 — Sustainability
== 4.9.1 The licensed design professional shall be permitted
to specify in the construction documents sustainability
requirements in addition to strength, serviceability, and
durability requirements of this Code.
== 4.9.2 The strength, serviceability, and durability requirements
of this Code shall take precedence over sustainability
considerations.
= R4.9 — Sustainability
The Code provisions for strength, serviceability, and
durability are minimum requirements to achieve a safe and
durable concrete structure. The Code permits the owner
or the licensed design professional to specify requirements
higher than the minimums mandated in the Code.
Such optional requirements can include higher strengths,
more restrictive deflection limits, enhanced durability, and
sustainability provisions.
= 4.10 — Structural integrity
== 4.10.1 General
=== 4.10.1.1 Reinforcement and connections shall be detailed
to tie the structure together effectively and to improve overall
structural integrity.
== 4.10.2 Minimum requirements for structural integrity
=== 4.10.2.1 Structural members and their connections shall
be in accordance with structural integrity requirements
in Table 4.10.2.1 .
= R4.10 — Structural integrity
== R4.10.1 General
=== R4.10.1.1 It is the intent of the structural integrity requirements
to improve redundancy and ductility through detailing
of reinforcement and connections so that, in the event of
damage to a major supporting element or an abnormal loading,
the resulting damage will be localized and the structure will
have a higher probability of maintaining overall stability.
Integrity requirements for selected structural member
types are included in the corresponding member chapter in
the sections noted.
== R4.10.2 Minimum requirements for structural integrity
Structural members and their connections referred to in
this section include only member types that have specific
requirements for structural integrity. Notwithstanding,
detailing requirements for other member types address
structural integrity indirectly.
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Table 4.10.2.1—Minimum requirements for
_structural integrity
= 4.11 — Fire resistance
== 4.11.1 Structural concrete members shall satisfy the fire
protection requirements of the general building code.
== 4.11.2 Where the general building code requires a thickness
of concrete cover for fire protection greater than the
concrete cover specified in 20.5.1, such greater thickness
shall govern.
= R4.11 — Fire resistance
Additional guidance on fire resistance of structural
concrete is provided by ACI 216.1M.
= 4.12 — Requirements for specific types of
== 4.12.1 Precast concrete systems
=== 4.12.1.1 Design of precast concrete members and connections
shall include loading and restraint conditions from
initial fabrication to end use in the structure, including form
removal, storage, transportation, and erection.
=== 4.12.1.2 Design, fabrication, and construction of precast
members and their connections shall include the effects of
tolerances.
= R4.12 — Requirements for specific types of
This section contains requirements that are related to
specific types of construction. Additional requirements that
are specific to member types appear in the corresponding
member chapters.
== R4.12.1 Precast concrete systems
All requirements in the Code apply to precast systems and
members unless specifically excluded. In addition, some
requirements apply specifically to precast concrete. This
section contains specific requirements for precast systems.
Other sections of this Code also provide specific requirements,
such as required concrete cover, for precast systems.
Precast systems differ from monolithic systems in that the
type of restraint at supports, the location of supports, and
the induced stresses in the body of the member vary during
fabrication, storage, transportation, erection, and the final
interconnected configuration. Consequently, the member
design forces to be considered may differ in magnitude and
direction with varying critical sections at various stages of
construction. For example, a precast flexural member may
be simply supported for dead load effects before continuity
at the supporting connections is established and may be a
continuous member for live or environmental load effects
due to the moment continuity created by the connections
after erection.
=== R4.12.1.2 For guidance on including the effects of tolerances,
refer to the PCI Design Handbook (PCI MNL 120).
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=== 4.12.1.3 When precast members are incorporated into a
structural system, the forces and deformations occurring in
and adjacent to connections shall be included in the design.
=== 4.12.1.4 Where system behavior requires in-plane loads
to be transferred between the members of a precast floor or
wall system, (a) and (b) shall be satisfied:
(a) In-plane load paths shall be continuous through both
connections and members.
(b) Where tension loads occur, a load path of steel or steel
reinforcement, with or without splices, shall be provided.
=== 4.12.1.5 Distribution of forces that act perpendicular
to the plane of precast members shall be established by
analysis or test.
=== R4.12.1.5 Concentrated and line loads can be distributed
among members provided the members have sufficient
torsional stiffness and shear can be transferred across
joints. Torsionally stiff members such as hollow-core or
solid slabs will provide better load distribution than torsionally
flexible members such as double tees with thin flanges.
The actual distribution of the load depends on many factors
discussed in detail in LaGue (1971), Johnson and Ghadiali
(1972), Pfeifer and Nelson (1983), Stanton (1987, 1992),
PCI Manual for the Design of Hollow Core Slabs and Walls
(PCI MNL 126), Aswad and Jacques (1992), and the PCI
Design Handbook (PCI MNL 120). Large openings can
cause significant changes in distribution of forces.
== 4.12.2 Prestressed concrete systems
=== 4.12.2.1 Design of prestressed members and systems shall
be based on strength and on behavior at service conditions
at all critical stages during the life of the structure from the
time prestress is first applied.
=== 4.12.2.2 Provisions shall be made for effects on adjoining
construction of elastic and plastic deformations, deflections,
changes in length, and rotations due to prestressing. Effects
of temperature change, restraint of attached structural
members, foundation settlement, creep, and shrinkage shall
also be considered.
=== 4.12.2.3 Stress concentrations due to prestressing shall be
considered in design.
=== 4.12.2.4 Effect of loss of area due to open ducts shall be
considered in computing section properties before grout in
post-tensioning ducts has attained design strength.
=== 4.12.2.5 Post-tensioning tendons shall be permitted to
be external to any concrete section of a member. Strength
and serviceability design requirements of this Code shall be
used to evaluate the effects of external tendon forces on the
concrete structure.
== R4.12.2 Prestressed concrete systems
Prestressing, as used in the Code, may apply to pretensioning,
bonded post-tensioning, or unbonded posttensioning.
All requirements in the Code apply to prestressed
systems and members, unless specifically excluded. This
section contains specific requirements for prestressed
concrete systems. Other sections of this Code also provide
specific requirements, such as required concrete cover for
prestressed systems.
Creep and shrinkage effects may be greater in prestressed
than in nonprestressed concrete structures because of the
prestressing forces and because prestressed structures typically
have less bonded reinforcement. Effects of movements
due to creep and shrinkage may require more attention than
is normally required for nonprestressed concrete. These
movements may increase prestress losses.
Design of externally post-tensioned construction should
consider aspects of corrosion protection and fire resistance
that are applicable to this structural system.
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== R4.12.3 Composite concrete flexural members
This section addresses structural concrete members, either
precast or cast-in-place, prestressed or nonprestressed,
consisting of concrete cast at different times intended to act
as a composite member when loaded after concrete of the
last stage of casting has set. All requirements in the Code
apply to these members unless specifically excluded. In
addition, some requirements apply specifically to composite
concrete flexural members. This section contains requirements
that are specific to these elements and are not covered
in the applicable member chapters.
== 4.12.3 Composite concrete flexural members
=== 4.12.3.1 This Code shall apply to composite concrete flexural
members as defined in Chapter 2.
=== 4.12.3.2 Individual members shall be designed for all critical
stages of loading.
=== 4.12.3.3 Members shall be designed to support all loads
introduced prior to full development of design strength of
composite members.
=== 4.12.3.4 Reinforcement shall be detailed to minimize
cracking and to prevent separation of individual components
of composite members.
== 4.12.4 Structural plain concrete systems
=== 4.12.4.1 The design of structural plain concrete members,
both cast-in-place and precast, shall be in accordance with
Chapter 14.
= 4.13 — Construction and inspection
== 4.13.1 Specifications for construction execution shall be
in accordance with Chapter 26.
== 4.13.2 Inspection during construction shall be in accordance
with Chapter 26 and the general building code.
= R4.13 — Construction and inspection
Chapter 26 has been organized to collect into one location
the design information, compliance requirements, and
inspection provisions from the Code that should be included
in construction documents There may be other information
that should be included in construction documents that is not
covered in Chapter 26.
= 4.14 — Strength evaluation of existing structures
== 4.14.1 Strength evaluation of existing structures shall be
in accordance with Chapter 27.
= R4.14 — Strength evaluation of existing structures
Requirements in Chapter 27 for strength evaluation of
existing structures by physical load test address the evaluation
of structures subjected to gravity loads only.
Chapter 27 also covers strength evaluation of existing structures by
analytical evaluation, which may be used for gravity as well
as other loadings such as earthquake or wind.
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