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20.3.3 — A load test shall not be made until that
portion of the structure to be subjected to load is at
least 56 days old. If the owner of the structure, the
contractor, and all involved parties agree, it shall be
permitted to make the test at an earlier age.
20.4 — Loading criteria
R20.4 — Loading criteria
20.4.1 — The initial value for all applicable response
measurements (such as deflection, rotation, strain,
slip, crack widths) shall be obtained not more than 1 hour
before application of the first load increment. Measurements shall be made at locations where maximum
response is expected. Additional measurements shall
be made if required.
20.4.2 — Test load shall be applied in not less than
four approximately equal increments.
R20.4.2 — Inspecting the structure after each load increment
20.4.3 — Uniform test load shall be applied in a manner
to ensure uniform distribution of the load transmitted to
the structure or portion of the structure being tested.
Arching of the applied load shall be avoided.
R20.4.3 — Arching refers to the tendency for the load to be
transmitted nonuniformly to the flexural element being
tested. For example, if a slab is loaded by a uniform
arrangement of bricks with the bricks in contact, arching
would results in reduction of the load on the slab near the
midspan of the slab.
20.4.4 — A set of response measurements shall be
made after each load increment is applied and after
the total load has been applied on the structure for at
least 24 hours.
20.4.5 — Total test load shall be removed immediately
after all response measurements defined in 20.4.4 are
20.4.6 — A set of final response measurements shall
be made 24 hours after the test load is removed.
20.5 — Acceptance criteria
R20.5 — Acceptance criteria
20.5.1 — The portion of the structure tested shall
show no evidence of failure. Spalling and crushing of
compressed concrete shall be considered an indication
R20.5.1 — A general acceptance criterion for the behavior of
a structure under the test load is that it does not show
evidence of failure. Evidence of failure includes cracking,
spalling, or deflection of such magnitude and extent that the
observed result is obviously excessive and incompatible with
the safety requirements of the structure. No simple rules have
been developed for application to all types of structures and
conditions. If sufficient damage has occurred so that the
structure is considered to have failed that test, retesting is not
permitted because it is considered that damaged members
should not be put into service even at a lower load rating.
Local spalling or flaking of the compressed concrete in flexural
elements related to casting imperfections need not indicate
overall structural distress. Crack widths are good indicators
ACI 318 Building Code and Commentary
of the state of the structure and should be observed to help
determine whether the structure is satisfactory. However,
exact prediction or measurement of crack widths in reinforced
concrete elements is not likely to be achieved under field
conditions. Establish criteria before the test, relative to the
types of cracks anticipated; where the cracks will be
measured; how they will be measured; and approximate
limits or criteria to evaluate new cracks or limits for the
changes in crack width.
20.5.2 — Measured deflections shall satisfy either
Eq. (20-1) or (20-2):
Δ 1 ≤ ----------------------20, 000h
Δ r ≤ -----14
If the measured maximum and residual deflections, Δ1
and Δr , do not satisfy Eq. (20-1) or (20-2), it shall be
permitted to repeat the load test.
R20.5.2 — The deflection limits and the retest option
follow past practice. If the structure shows no evidence of
failure, recovery of deflection after removal of the test load
is used to determine whether the strength of the structure is
satisfactory. In the case of a very stiff structure, however, the
errors in measurements under field conditions may be of the
same order as the actual deflections and recovery. To avoid
penalizing a satisfactory structure in such a case, recovery
measurements are waived if the maximum deflection is less
than lt2/(20,000h). The residual deflection Δr is the difference between the initial and final (after load removal)
deflections for the load test or the repeat load test.
The repeat test shall be conducted not earlier than
72 hours after removal of the first test load. The
portion of the structure tested in the repeat test shall
be considered acceptable if deflection recovery Δr
satisfies the condition:
Δ r ≤ -----25
where Δ2 is the maximum deflection measured during
the second test relative to the position of the structure
at the beginning of the second test.
20.5.3 — Structural members tested shall not have
cracks indicating the imminence of shear failure.
R20.5.3 — Forces are transmitted across a shear crack plane
by a combination of aggregate interlock at the interface of
the crack that is enhanced by clamping action of transverse
stirrup reinforcing and by dowel action of stirrups crossing
the crack. As crack lengths increase to approach a horizontal
projected length equal to the depth of the member and
concurrently widen to the extent that aggregate interlock
cannot occur, and as transverse stirrups if present begin to
yield or display loss of anchorage so as to threaten their
integrity, the member is assumed to be approaching imminent
20.5.4 — In regions of structural members without
transverse reinforcement, appearance of structural
cracks inclined to the longitudinal axis and having a
horizontal projection longer than the depth of the
member at midpoint of the crack shall be evaluated.
R20.5.4 — The intent of 20.5.4 is to make the professionals
in charge of the test pay attention to the structural implication
of observed inclined cracks that may lead to brittle collapse
in members without transverse reinforcement.
ACI 318 Building Code and Commentary
20.5.5 — In regions of anchorage and lap splices, the
appearance along the line of reinforcement of a series
of short inclined cracks or horizontal cracks shall be
R20.5.5 — Cracking along the axis of the reinforcement in
anchorage zones may be related to high stresses associated
with the transfer of forces between the reinforcement and
the concrete. These cracks may be indicators of pending
brittle failure of the element if they are associated with the
main reinforcement. It is important that their causes and
consequences be evaluated.
20.6 — Provision for lower load rating
R20.6 — Provision for lower load rating
If the structure under investigation does not satisfy
conditions or criteria of 20.1.2, 20.5.2, or 20.5.3, the
structure shall be permitted for use at a lower load
rating based on the results of the load test or analysis,
if approved by the building official.
Except for load tested members that have failed under a test
(see 20.5), the building official may permit the use of a
structure or member at a lower load rating that is judged to
be safe and appropriate on the basis of the test results.
20.7 — Safety
20.7.1 — Load tests shall be conducted in such a
manner as to provide for safety of life and structure
during the test.
20.7.2 — Safety measures shall not interfere with load
test procedures or affect results.
ACI 318 Building Code and Commentary
CHAPTER 21 — EARTHQUAKE-RESISTANT STRUCTURES
In 2008, the provisions of Chapter 21 were revised and renumbered to present seismic requirements in order of increasing SDC; therefore,
change bars are not shown.
21.1 — General requirements
R21.1 — General requirements
21.1.1 — Scope
R21.1.1 — Scope
18.104.22.168 — Chapter 21 contains requirements for
design and construction of reinforced concrete
members of a structure for which the design forces,
related to earthquake motions, have been determined
on the basis of energy dissipation in the nonlinear
range of response.
Chapter 21 contains provisions considered to be the
minimum requirements for a cast-in-place or precast
concrete structure capable of sustaining a series of oscillations
into the inelastic range of response without critical deterioration in strength. The integrity of the structure in the inelastic
range of response should be maintained because the design
earthquake forces defined in documents such as ASCE/
SEI 7,21.1 the IBC,21.2 the UBC,21.3 and the NEHRP21.4
provisions are considered less than those corresponding to
linear response at the anticipated earthquake intensity.21.4-21.7
22.214.171.124 — All structures shall be assigned to a
seismic design category (SDC) in accordance with
126.96.36.199 — All members shall satisfy requirements
of Chapters 1 to 19 and 22. Structures assigned to
SDC B, C, D, E, or F also shall satisfy 188.8.131.52 through
184.108.40.206, as applicable.
220.127.116.11 — Structures assigned to SDC B shall
18.104.22.168 — Structures assigned to SDC C shall
satisfy 21.1.2 and 21.1.8.
22.214.171.124 — Structures assigned to SDC D, E, or F
shall satisfy 21.1.2 through 21.1.8, and 21.11 through
126.96.36.199 — Structural systems designated as part of
the seismic-force-resisting system shall be restricted
to those designated by the legally adopted general
building code of which this Code forms a part, or
determined by other authority having jurisdiction in
areas without a legally adopted building code. Except
for SDC A, for which Chapter 21 does not apply, the
following provisions shall be satisfied for each structural
system designated as part of the seismic-forceresisting system, regardless of the SDC:
(a) Ordinary moment frames shall satisfy 21.2.
(b) Ordinary reinforced concrete structural walls
need not satisfy any provisions in Chapter 21.
(c) Intermediate moment frames shall satisfy 21.3.
(d) Intermediate precast walls shall satisfy 21.4.
(e) Special moment frames shall satisfy 21.5
As a properly detailed cast-in-place or precast concrete
structure responds to strong ground motion, its effective
stiffness decreases and its energy dissipation increases.
These changes tend to reduce the response accelerations and
lateral inertia forces relative to values that would occur were
the structure to remain linearly elastic and lightly
damped.21.7 Thus, the use of design forces representing
earthquake effects such as those in ASCE/SEI 7 requires
that the seismic-force-resisting system retain a substantial
portion of its strength into the inelastic range under
The provisions of Chapter 21 relate detailing requirements
to type of structural framing and seismic design category
(SDC). SDCs are adopted directly from ASCE/SEI 7, and
relate to considerations of seismic hazard level, soil type,
occupancy, and use. Before the 2008 Code, low, intermediate,
and high seismic risk designations were used to delineate
detailing requirements. For a qualitative comparison of
SDCs and seismic risk designations, see Table R188.8.131.52.
The assignment of a structure to a SDC is regulated by the
legally adopted general building code of which this Code
forms a part (see 1.1.9).
The design and detailing requirements should be compatible
with the level of energy dissipation (or toughness) assumed
in the computation of the design earthquake forces. The
terms “ordinary,” “intermediate,” and “special” are specifically
used to facilitate this compatibility. The degree of required
toughness and, therefore, the level of required detailing,
increases for structures progressing from ordinary through
intermediate to special categories. It is essential that structures
assigned to higher SDCs possess a higher degree of toughness.
It is permitted, however, to design for higher toughness in
the lower SDCs and take advantage of the lower design
(f) Special structural walls shall satisfy 21.9.
ACI 318 Building Code and Commentary