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3 — Load test procedure

3 — Load test procedure

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320



CHAPTER 20



CODE



COMMENTARY



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

is advisable.



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

made.



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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

of failure.



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



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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):

2



lt

Δ 1 ≤ ----------------------20, 000h



(20-1)



Δ

Δ r ≤ -----14



(20-2)



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

Δ r ≤ -----25



(20-3)



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.



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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

shear failure.



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.



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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

evaluated.



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.



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ACI 318 Building Code and Commentary



CHAPTER 21



323



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.



CODE



COMMENTARY



21.1 — General requirements



R21.1 — General requirements



21.1.1 — Scope



R21.1.1 — Scope



21.1.1.1 — 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



21.1.1.2 — All structures shall be assigned to a

seismic design category (SDC) in accordance with

1.1.9.1.

21.1.1.3 — 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 21.1.1.4 through

21.1.1.8, as applicable.

21.1.1.4 — Structures assigned to SDC B shall

satisfy 21.1.2.

21.1.1.5 — Structures assigned to SDC C shall

satisfy 21.1.2 and 21.1.8.

21.1.1.6 — Structures assigned to SDC D, E, or F

shall satisfy 21.1.2 through 21.1.8, and 21.11 through

21.13.

21.1.1.7 — 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

through 21.8.



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

displacement reversals.

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 R1.1.9.1.

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

force levels.



(f) Special structural walls shall satisfy 21.9.

ACI 318 Building Code and Commentary



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