Standard Requirements for Seismic Evaluation and Retrofit of Existing Concrete Buildings Public Discussion Draft

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Standard Requirements for Seismic Evaluation and Retrofit of Existing Concrete Buildings

2 (369.1) and Commentary 3 Reported by Committee 369 4 5

Wassim Ghannoum, Chair

Anna Birely Adolfo Matamoros

Sergio Brena Steven McCabe

Casey Champion Murat Melek

Jeffrey Dragovich Jack Moehle

Kenneth Elwood Arif Ozkan

Una Gilmartin Robert Pekelnicky

Garrett Hagen Arne Halterman

Jose Pincheira

Wael Hassan Mario Rodriguez

Mohammad Iqbal Murat Saatcioglu

Jose Izquierdo-Encarnacion Siamak Sattar

Afshar Jalalian Halil Sezen

Thomas Kang Roberto Stark

Dominic Kelly Andreas Stavridis

Insung Kim John Wallace

Laura Lowes Tom Xia

Kenneth Luttrell

369.1 Public Discussion

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Keywords: ASCE 41; acceptance criteria; anchorage; axial failure; bond-strength; concrete; 2

connection; deformation-controlled; demand-capacity ratio; force-controlled; foundation; 3

dynamic analysis; earthquake; effective flexural strength; stiffness; effective width; linear static 4

analysis; load-deformation relationship; m-factor; modeling parameters; moment-frames; 5

nonlinear analysis; plastic hinge; plastic rotation; probability of failure; posttensioned; prestress; 6

shear strength; slab-column moment frames; seismic rehabilitation; retrofit; retrofit measure; 7

stiffness; structural wall. 8

PREFACE

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In this standard, reference to ASCE 41 implies reference to the ASCE/SEI 41-17 standard. In this 10

standard, reference to ACI 318 implies reference to the ACI 318-14 Building Code. 11

This standard provides retrofit and rehabilitation criteria for reinforced concrete buildings based 12

on results from the most recent research on the seismic performance of existing concrete 13

buildings. The intent of the ACI 369.1 standard is to provide a continuously updated resource 14

document for modifications to Chapter 10 of ASCE 41, similar to how the National Earthquake 15

Hazards Reduction Program (NEHRP) Recommended Seismic Provisions produced by the 16

Federal Emergency Management Agency (FEMA) (FEMA 450) have served as source 17

documents for the International Building Code (IBC) and its predecessor building codes. 18

Specifically, this version of ACI 369.1 serves as the basis for Chapter 10, “Concrete” of ASCE 19

41. 20

This standard should be used in conjunction with Chapters 1 through 7 of ASCE/SEI 41-21

17. Chapter 1 of ASCE 41 provides general requirements for evaluation and retrofit, including 22

the selection of performance objectives and retrofit strategies. Chapter 2 of ASCE 41 defines 23

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performance objectives and seismic hazards. Chapter 3 of ASCE 41 provides the requirements 1

for evaluation and retrofit, including treating as-built information and selecting the appropriate 2

screening procedures. Chapter 4 of ASCE 41 summarizes Tier 1 screening procedures, while 3

Chapters 5 and 6 summarize Tier 2 deficiency-based procedures and Tier 3 systematic 4

procedures for evaluation and retrofit, respectively. Chapter 7 of ASCE 41 details analysis 5

procedures referenced in ACI 369.1, including, linear and nonlinear analysis procedures, 6

acceptance criteria, and alternative methods for determining modeling parameters and 7

acceptance criteria. Chapter 8 of ASCE 41 provides geotechnical engineering provisions for 8

building foundations and assessment of seismic-geologic site hazards. References to these 9

chapters can be found throughout the standard. The design professional is referred to the FEMA 10

report, FEMA 547, for detailed information on seismic rehabilitation measures for concrete 11

buildings. Repair techniques for earthquake-damaged concrete components are not included in 12

ACI 369.1. The design professional is referred to FEMA 306, FEMA 307, and FEMA 308 for 13

information on evaluation and repair of damaged concrete wall components. 14

This standard does not provide modeling procedures, acceptance criteria, and rehabilitation 15

measures for concrete-encased steel composite components. Future versions will provide 16

provision updates for concrete moment frames and will add provisions for concrete components 17

and systems omitted in the present version of the standard. 18 19 20 INTRODUCTION 21 22

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Earthquake reconnaissance has clearly demonstrated that existing concrete buildings designed 1

before the introduction of seismic design codes in the 1980’s are more vulnerable to severe 2

damage or collapse when subjected to strong ground motion than concrete buildings built after 3

that period. Seismic rehabilitation of existing buildings where new components are added or 4

existing components are modified or retrofitted with new materials, or both, can be used to 5

mitigate the risk to damage in future earthquakes. Seismic rehabilitation is encouraged not only 6

to reduce the risk of damage and injury in future earthquakes, but also to extend the life of 7

existing buildings and reduce using new materials in the promotion of sustainability objectives. 8

It is not possible to codify all problems encountered in the process of performing the seismic 9

evaluation and retrofit of reinforced concrete buildings, nor is the intent of the standard to do so. 10

The standard provides a basic framework for modeling and evaluation of structures that reflects 11

the latest information available from researchers and practicing engineers, so that seismic 12

evaluation and retrofit can be performed with a consistent set of criteria. Many provisions in the 13

standard rely on the use of sound engineering judgement for their implementation. The 14

commentary of the standard provides references that describe in detail the implementation of 15

methodologies adopted in the standard. 16

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CHAPTER 1 - GENERAL

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1.1―ScopeThis standard sets forth requirements for the seismic evaluation and retrofit of concrete

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components of the seismic-force-resisting system of an existing building. These building standard 20

requirements apply to existing concrete components, retrofitted concrete components, and new 21

concrete components. Provisions of this standard do not apply to concrete-encased steel composite 22

components. 23

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Chapter 2 specifies data collection procedures for obtaining material properties and performing 1

condition assessments. Chapter 3 provides general analysis and design requirements for concrete 2

components. Chapters 4 through 9 provide modeling procedures, component strengths, acceptance 3

criteria, and retrofit measures for cast-in-place and precast concrete moment frames, concrete 4

frames with masonry infills, cast-in-place and precast concrete shear walls, and concrete braced 5

frames. Chapters 10 through 12 provide modeling procedures, strengths, acceptance criteria, and 6

retrofit measures for concrete diaphragms and concrete foundation systems. 7

C1.1—Scope 

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These  standard  requirements  were  developed  based  on  the  best  knowledge  of  the  seismic  9

performance of existing concrete buildings at the time of publication. These requirements are not  10

intended to restrict the licensed design professional from using new information that becomes  11

available  before  the  issuance  of  the  next  edition  of  this  standard.  Such  new  information  can  12 include tests conducted to address specific building conditions.  13 This standard provides short descriptions of potential seismic retrofit measures for each concrete  14 building system. The licensed design professional, however, is referred to FEMA 547 for detailed  15 information on seismic retrofit measures for concrete buildings. Repair techniques for earthquake‐ 16 damaged concrete components are not included in this standard. The licensed design professional  17 is referred to FEMA 306, FEMA 307, and FEMA 308 for information on evaluation and repair of  18 damaged concrete wall components.  19 Concrete‐encased steel composite components behave differently from concrete sections reinforced  20 with reinforcing steel. Concrete‐encased steel composite components frequently behave as over‐ 21 reinforced sections. This type of component behavior was not represented in the data sets used  22

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to  develop  the  force–deformation  modeling  relationships  and  acceptance  criteria  in  this  1 standard and is not covered in this standard. Concrete encasement is often provided for fire  2 protection rather than for strength or stiffness and typically lacks transverse reinforcement. In  3 some cases, the transverse reinforcement does not meet detailing requirements in AISC 360.  4 Lack of adequate confinement can result in lateral expansion of the core concrete, which exacerbates  5 bond slip and, undermines the fundamental principle that plane sections remain plane.  6 Testing and analysis used to determine acceptance criteria for concrete‐encased steel composite  7 components should include the effect of bond slip between steel and concrete, confinement ratio,  8 confinement reinforcement detailing, kinematics, and appropriate strain limits.  9 To preserve historic buildings, exercise care in selecting the appropriate retrofit approaches and  10 techniques for application.  11 12 13

CHAPTER 2―MATERIAL PROPERTIES AND CONDITION ASSESSMENT

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2.1―General

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Mechanical properties of materials shall be obtained from available drawings, specifications, and 16

other documents for the existing building in accordance with the requirements of ASCE 41 Section 17

3.2. Where these documents fail to provide adequate information to quantify material properties, 18

such information shall be supplemented by materials testing based on requirements of Chapter 2. 19

The condition of the concrete components of the structure shall be determined using the 20

requirements of Section 2.3. 21

Material properties of existing concrete components shall be determined in accordance with 22

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in accordance with Section 2.2.5. A condition assessment shall be conducted in accordance with 1

Section 2.3. The extent of materials testing and condition assessment performed shall be used to 2

determine the knowledge factor as specified in Section 2.4. 3   4 C2.1General  5 Chapter 2 identifies properties requiring consideration and provides requirements for determining  6 building properties. Also described is the need for a thorough condition assessment and utilization of  7

knowledge  gained in analyzing  component and  system behavior. Personnel involved in  material  8

property  quantification  and  condition  assessment  should  be  experienced  in  the  proper  9

implementation of testing practices and the interpretation of results.  10

When modeling a concrete building, it is important to investigate local practices relative to seismic  11

design.  Specific  benchmark  years  can  be  determined  for  the  implementation  of  earthquake‐ 12

resistant  design  in  most  locations,  but  caution  should  be  exercised  in  assuming  optimistic  13 characteristics for any specific building. Particularly with concrete materials, the  date of original  14 building construction significantly influences seismic performance. Without deleterious conditions  15 or materials, concrete gains compressive strength from the time it is originally cast and in place.  16

Strengths  typically  exceed  specified  design  values  (28‐day  or  similar).  In  older  construction,  17

concrete  strength  was  often  very  low  (less  than  3000  psi)  and  it  was  rarely  specified  in  the  18

drawings.. Early adoptions of concrete in buildings often used reinforcing steel with relatively low  19

strength  and  ductility,  limited  continuity,  and  reduced  bond  development.  Continuity  between  20

specific existing components and elements, such as beams, columns, diaphragms, and shear walls,  21

can be particularly difficult to assess because of concrete cover and other barriers to inspection.  22

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Properties of welded wire reinforcement for various periods  of  construction  can  be  obtained  1

from the Wire Reinforcement Institute (WRI 2009).  2

Documentation  of  the  material  properties  and  grades  used  in  component  and  connection  3

construction is invaluable and can be effectively used to reduce the amount of in‐place testing  4

required.  The  licensed  design  professional  is  encouraged  to  research  and  acquire  all  available  5 records from original construction, including photographs, to confirm reinforcement details shown  6 on the plans.  7 Further guidance on the condition assessment of existing concrete buildings can be found in the  8 following:    9  ACI 201.1R, which provides guidance on conducting a condition survey of existing concrete  10 structures;  11  ACI 364.1R, which describes the general procedures used for the evaluation of concrete  12 structures before retrofit; and  13  ACI 437R, which describes methods for strength evaluation of existing concrete buildings,  14 including analytical and load test methods.  15 16

2.2 Properties of In-Place Materials and Components

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2.2.1 Material Properties

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2.2.1.1 General―The following component and connection material properties shall be obtained

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for the as-built structure: 20

1. Concrete compressive strength; and 21

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2. Yield and ultimate strength of conventional and prestressing reinforcing steel, castin -1

place and post-installed anchors, and metal connection hardware. 2

Where materials testing is required by ASCE 41 Section 6.2, the test methods to quantify material 3

properties shall comply with the requirements of Section 2.2.3. The frequency of sampling, 4

including the minimum number of tests for property determination, shall comply with the 5 requirements of Section 2.2.4. 6   7 C2.2.1.1 General―Other material properties and conditions of interest for concrete components  8 include  9 1.  Tensile strength and modulus of elasticity of concrete;  10 2.  Ductility, toughness, and fatigue properties of concrete;  11 3.  Carbon equivalent present in the reinforcing steel; and  12 4.  Presence of any degradation such as corrosion or deterioration of bond between concrete and  13 reinforcement.  14 The extent of effort made to determine these properties depends on availability of accurate, updated  15

construction  documents  and  drawings;  construction  quality  and  type;  accessibility;  and  material  16

conditions.  The  analysis  method  selected—for  example,  linear  static  procedure  (LSP)  or  nonlinear  17

static  procedure  (NSP)—might  also  influence  the  testing  scope.  Concrete  tensile  strength  and  18 modulus of elasticity can be estimated based on the compressive strength and may not warrant the  19 damage associated with any extra coring required.  20 The sample size and removal practices followed are referenced in FEMA 274, Sections C6.3.2.3  21

and  C6.3.2.4.  ACI  228.1R  provides  guidance  on  methods  to  estimate  the  in‐place  strength  of  22

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concrete  in  existing  structures,  whereas  ACI  214.4R  provides  guidance  on  coring  in  existing  1

structures  and  interpretation  of  core  compressive  strength test  results.  Generally,  mechanical  2 properties for both concrete and reinforcing steel can be established from combined core and  3 specimen sampling at similar locations, followed by laboratory testing. Core drilling should minimize  4 damage to the existing reinforcing steel.  5 6

2.2.1.2 Nominal or Specified Properties―Nominal material properties, or properties specified in

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construction documents, shall be taken as lower-bound material properties. Corresponding 8

expected material properties shall be calculated by multiplying lower-bound values by a factor 9

taken from Table 1 to translate from lower-bound to expected values. Alternative factors shall be 10

permitted where justified by test data. 11

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2.2.2 Component Properties―The following component properties and as-built conditions shall

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be established: 14

1. Cross-sectional dimensions of individual components and overall configuration of 15

the structure; 16

2. Configuration of component connections, size, embedment depth, and type of 17

anchors, thickness of connector material, anchorage and interconnection of 18

embedments and the presence of bracing or stiffening components; 19

3. Modifications to components or overall configuration of the structure; 20

4. Most recent physical condition of components and connections, and the extent of 21

any deterioration; 22

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5. Deformations beyond those expected because of gravity loads, such as those caused 1

by settlement or past earthquake events; and 2

6. Presence of other conditions that influence building performance, such as 3

nonstructural components that can interact with structural components during 4 earthquake excitation. 5 C2.2.2 Component PropertiesComponent properties are required to properly characterize  6 building performance in seismic analysis. The starting point for assessing component properties  7 and condition is retrieval of available construction documents. A preliminary review should  8 identify primary gravity‐ and seismic‐force‐resisting elements and systems and their critical  9 components and connections. If there are no drawings of the building, the licensed design  10 professional should perform a thorough investigation of the building to identify these elements,  11 systems, and components as described in Section 2.3.  12 13

2.2.3 Test Methods to Quantify Material Properties

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2.2.3.1 General―Destructive and nondestructive test methods used to obtain in-place mechanical

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properties of materials identified in Section 2.2.1 and component properties identified in Section 16

2.2.2 are specified in this section. Samples of concrete and reinforcing and connector steel shall 17

be examined for physical condition as specified in Section 2.3.2. 18

When determining material properties with the removal and testing of samples for laboratory 19

analysis, sampling shall take place in primary gravity- and seismic-force-resisting components in 20

regions with the least stress. 21

Where Section 2.2.4.2.1 does not apply and the coefficient of variation is greater than 20%, the 22

expected concrete strength shall not exceed the mean less one standard deviation. 23

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2.2.3.2 Sampling―For concrete material testing, the sampling program shall include the removal

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of standard cores. Core drilling shall be preceded by nondestructive location of the reinforcing 2

steel, and core holes shall be located to avoid damage to or drilling through the reinforcing steel. Core 3

holes shall be filled with concrete or grout of comparable strength having nonshrinkage properties. If 4

conventional reinforcing steel is tested, sampling shall include removal of local bar segments and 5

installation of replacement spliced material to maintain continuity of the reinforcing bar for 6

transfer of bar force unless an analysis confirms that replacement of the original components is 7

not required. 8

Removal of core samples and performance of laboratory destructive testing shall be permitted to 9

determine existing concrete strength properties. Removal of core samples shall use the procedures 10

included in ASTM C42. Testing shall follow the procedures contained in ASTM C42, ASTM C39, 11

and ASTM C496. Core strength shall be converted to in-place concrete compressive strength by 12

an approved procedure. 13

Removal of bar or tendon samples and performance of laboratory destructive testing shall be 14

permitted to determine existing reinforcing steel strength properties. The tensile yield and ultimate 15

strengths for reinforcing and prestressing steels shall follow the procedures included in ASTM 16

A370. Reinforcing samples that are slightly damaged during removal are permitted to be machined 17

to a round bar as long as the tested area is at least 70% of the gross area of the original bar. 18

Prestressing materials shall meet the supplemental requirements in ASTM A416, ASTM A421, or 19

ASTM A722, depending on material type. Properties of connector steels shall be permitted to be 20

determined by wet and dry chemical composition tests and direct tensile and compressive strength 21

tests as specified by ASTM A370. Where strength, construction quality or both of anchors or 22

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embedded connectors are required to be determined, in-place testing shall satisfy the provisions of 1 ASTM E488-96. 2   3 C2.2.3.2 Sampling―ACI 214.4R and FEMA 274 provide further guidance on correlating concrete  4 core strength to in‐place strength and provide references for various test methods that can be  5 used to estimate material properties. Chemical composition can be determined from retrieved  6 samples to assess the condition of the concrete. Section C6.3.3.2 of FEMA 274 (1997b) provides  7 references for these tests.  8 When concrete cores are taken, care should be taken when patching the holes. For example, a  9 core through the thickness of a slab should have positive anchorage by roughening the surface  10 and possibly dowels for anchorage. For that case, the holes should be filled with concrete or grout  11 and the engineer should provide direction for filling the hole so that the added concrete or grout  12 bonds to the substrate.  13 The reinforcing steel system used in the construction of a specific building is usually of uniform  14 grade and similar strength. One grade of reinforcement is occasionally used for small‐diameter  15 bars, like those used for stirrups and hoops, and another grade for large‐diameter bars, like those  16 used for longitudinal reinforcement. In some cases, different concrete design strengths or classes  17

are  used.  Historical  research  and  industry  documents  contain  insight  on  material  mechanical  18

properties used in different construction eras (Section 2.2.5). This information can be used with  19

laboratory  and  field  test  data  to  gain  confidence  in  in‐place  strength  properties.  Undamaged  20

reinforcing steel can be reduced to a smooth bar, as long as the samples meet the requirements  21

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of  ASTM  A370,  excluding  the  limitations  of  Annex  9.  This  type  of  reinforcing  would  occur  in  a  1

situation where only a limited length of bar can be removed for testing.  2

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2.2.4 Minimum Number of Tests―Materials testing is not required if material properties are

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available from original construction documents that include material test records or reports. 5

Material test records or reports shall be representative of all critical components of the building 6

structure. 7

Based on Section 6.2 of ASCE 41, data collection from material tests is classified as either 8

comprehensive or usual. The minimum number of tests for usual data collection is specified in 9

Section 2.2.4.1. The minimum number of tests necessary to quantify properties by in-place testing 10

for comprehensive data collection is specified in Section 2.2.4.2. If the existing gravity-load-11

resisting-system or seismic-force-resisting system is replaced during the retrofit process, material 12

testing is only required to quantify properties of existing materials at new connection points. 13 14 C2.2.4 Minimum Number of Tests―To quantify in‐place properties accurately, it is essential that  15 a minimum number of tests be conducted on primary components of the seismic‐force‐resisting  16 system. The minimum number of tests is dictated by the availability of original construction data,  17 structural system type used, desired accuracy, quality and condition of in‐place materials, level of  18 seismicity, and target performance level. Accessibility to the structural system can influence the  19

testing  program  scope.  The  focus  of  testing  should  be  on  primary  seismic‐force‐resisting  20

components  and  specific  properties  for  analysis.  Test  quantities  provided  in  this  section  are  21

minimal; the licensed design professional should determine whether further testing is needed to  22

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Testing is generally not required on components other than those of the seismic‐force‐resisting  1 system.  2 The licensed design professional and subcontracted testing agency should carefully examine test  3

results  to  verify  that  suitable  sampling  and  testing  procedures  were  followed  and  appropriate  4

values for the analysis were selected from the data.  5

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2.2.4.1 Usual Data Collection―The minimum number of tests to determine concrete and

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reinforcing steel material properties for usual data collection shall be based on the following 8

criteria: 9

1. If the specified design strength of the concrete is known, at least one core shall be 10

taken from samples of each different concrete strength used in the construction of the 11

building, with a minimum of three cores taken for the entire building; 12

2. If the specified design strength of the concrete is not known, at least one core shall be 13

taken from each type of seismic-force-resisting component, with a minimum of six 14

cores taken for the entire building; 15

3. If the specified design strength of the reinforcing steel is known, nominal or specified 16

material properties shall be permitted without additional testing; and 17

4. If the specified design strength of the reinforcing steel is not known, at least two 18

strength test coupons of reinforcing steel shall be removed from the building for 19

testing. 20

5. Cast-in-place or post-installed anchors shall be classified in groups of similar type, 21

size, geometry and structural use. In groups of anchors used for out-of-plane wall 22

anchorage and in groups of anchors whose failure in tension or shear would cause 23

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the structure not to meet the selected Performance Objective, 5% of the anchors with 1

a minimum of three anchors of each anchor group shall be tested in-place in tension 2

to establish an available strength, construction quality or both. The test load shall be 3

specified by the licensed design professional and shall be based on the anticipated 4

demand or strength in accordance with available construction information. If the test 5

load is used as the basis for anchor strength calculation, the available anchor strength 6

shall not be taken greater than 2/3 of the test load. Testing of the anchors to failure is 7

not required and a test load lower than the expected failure load shall be permitted. 8

If the test load is not achieved in one or more anchors tested in a group, anchors in 9

that group shall be tested under a tensile load smaller than that specified for the 10

preceding tests. Otherwise, the strength of the tested anchor group shall be ignored. 11

Testing in accordance with 2.2.4.2.5 shall be permitted to determine the available 12

strength based on a statistical distribution of the test results. 13

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2.2.4.2 Comprehensive Data Collection

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2.2.4.2.1 Coefficient of Variation―Unless specified otherwise, a minimum of three tests shall be

2

conducted to determine any property. If the coefficient of variation exceeds 20%, additional tests 3

shall be performed until the coefficient of variation is equal to or less than 20%. If additional 4

testing does not reduce the coefficient of variation below 20%, a knowledge factor reduction per 5

Section 4.4 shall be used. In determining coefficient of variation, cores shall be grouped by grades 6

of concrete and element type. The number of tests in a single component shall be limited so as not 7

to compromise the integrity of the component. 8

2.2.4.2.2 Concrete Materials―For each concrete element type of the seismic-force-resisting

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system, as well as secondary systems for which failure could result in a collapse hazard, a 10

minimum of three core samples shall be taken and subjected to compression tests. A minimum of 11

six total tests shall be performed on a building for concrete strength determination, subject to the 12

limitations of this section. If varying concrete classes or grades were used in the building 13

construction, a minimum of three samples and tests shall be performed for each class and grade. 14

The modulus of elasticity and tensile strength shall be permitted to be estimated from the 15

compressive strength testing data. Samples shall be taken from components, distributed throughout 16

the building, that are critical to the structural behavior of the building. 17

Tests shall be performed on samples from components that are identified as damaged or degraded 18

to quantify their condition. Test results from areas of degradation shall be compared with strength 19

values specified in the construction documents. If test values less than the specified strength in the 20

construction documents are found, further strength testing shall be performed to determine the cause 21

or identify the degree of damage or degradation. 22

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The minimum number of tests to determine compressive strength of each concrete element type

1

shall conform to one of the following criteria: 2

1. For concrete elements for which the specified design strength is known and test results 3

are not available, a minimum of three core tests shall be conducted for each floor level, 4

400 yd3 (306 m3) of concrete, or 10,000 ft2 (930 m2) of surface area, whichever requires

5

the most frequent testing; or 6

2. For concrete elements for which the specified design strength is unknown and test 7

results are not available, a minimum of six core tests shall be conducted for each floor 8

level, 400 yd3 (306 m3) of concrete, or 10,000 ft2 (930 m2) of surface area, whichever

9

requires the most frequent testing. Where the results indicate that different classes of 10

concrete were used, the degree of testing shall be increased to confirm class use. 11

3. Alternately, for concrete elements for which the design strength is known or unknown, 12

and test results are not available, it is permitted to determine the lower bound 13

compressive strength based on core sample testing and applying the provisions in 14

Section 6.4.3 of ACI 562-16. If the lower bound compressive strength is determined in 15

this manner, the expected compressive strength shall be determined as the lower bound 16

compressive strength value obtained from ACI 562-16 Equation 6.4.3 plus one standard 17

deviation of the strength of the core samples. When following the provisions in Section 18

6.4.3 of ACI 562-16, the minimum number of samples per element type shall be four. 19

The sample locations shall be: 20

a. Distributed to quantify element material properties throughout the height of 21

the building 22

(19)

structural system being investigated. 1

2

Quantification of concrete strength via ultrasonics or other nondestructive test methods shall not 3

be substituted for core sampling and laboratory testing. 4   5 C2.2.4.2.2 Concrete Materials―ACI 214.4R provides guidance on coring in existing structures and  6 interpretation of core compressive strength test results.  7 If a structure was constructed in phases or if construction documents for different parts of the  8

structure  were  issued  at  separate  times,  the  licensed  design  professional,  for  the  purpose  of  9 determining sampling size, should consider the concrete in each construction phase or in each set  10 of construction documents as of different type. Section 6.4.3 of ACI 562‐16 provides a method to  11 calculate an equivalent specified concrete strength f’c based on statistical analysis of compression  12

strength  test  results  from  core  samples.    ASTM  E178  provides  guidance  on  consideration  of  13

outliers  in  a  set  of  core  samples.  Equation  6.4.3  in  Section  6.4.3  of  ACI  562‐16  defines  the  14

equivalent specified compressive strength of concrete as a function of the number of tests, the  15

coefficient of variation of the samples, and a factor to account for the number of samples. Section  16

6.4.3  of  ACI  562‐16  permits  the  engineer  to  select  the  number  of  samples  used  to  evaluate  17 concrete compressive strength but imposes a penalty to the results to account for the uncertainty  18 associated with the number of samples.      19 Equation 6.4.3 of ACI 562‐16 was derived with the objective of calculating the 13% fractile of the  20

in‐place  concrete  compressive  strength,  which  some  studies  have  shown  to  be  approximately  21

equal to the specified compressive strength of concrete f’c (Bartlett and MacGregor, 1996). The 

(20)

first term in Equation 6.4.3 of ACI 562‐16 represents the effect of sample size on the uncertainty  1 of the mean in‐place strength, where the coefficient kc is obtained from a Student’s t distribution  2 with n‐1 degrees of freedom and a 90% confidence level. The second term in Equation 6.4.3 of ACI  3 562‐16 represents the uncertainty attributable to correction factors relating cylinder strength to  4

specified  compressive  strength,  which  were  assumed  to  have  a  normal  distribution,  also  5 estimated with a 90% confidence level. The study by Bartlett and MacGregor (1996) showed that  6 the specified compressive strength f’c corresponds approximately to the 13% fractile of the 28‐ 7 day in‐place strength in walls and columns, and approximately the 23% fractile of the 28‐day in‐ 8

place  compressive  strength  in  beams  and  slabs.  The  former  was  considered  to  be  a  more  9

appropriate measure of specified compressive strength f’c than the latter because the nominal 

10

strength  of  columns  is  more  sensitive  to  concrete  compressive  strength  than  the  strength  of  11

beams and slabs (ACI 214.4).    12

In  Section  2.2.1.2  of  this  standard  it  is  stated  that  nominal  material  properties  or  properties  13

specified  in  construction  documents  shall  be  taken  as  lower‐bound  material  properties  unless  14 otherwise specified. The method to estimate of the specified concrete compressive strength f’c in  15 Section 6.4.3 of ACI 562‐16 was adopted in this standard to obtain the lower bound compressive  16 strength consistent with the provisions in Section 2.2.1.2.  17

ACI  214.4R  provides  guidance  on  coring  in  existing  structures  and  interpretation  of  core  18

compressive  strength  test  results.  The  minimum  of  4  samples  was  adopted  based  on  the  19

recommendations in ACI 214.4. The following equation is provided in ACI 214.4  20

  21

(21)

     (C1) 

1

  2

where nsamples represents the minimum number of samples, COVpopulation represents the estimated 

3

coefficient of variation of the population, and epopulation represents the predetermined maximum 

4 error expressed as a percentage of the population average. For a total of 4 samples the previous  5 equation dictates that the maximum error is equal to the estimate of the coefficient of variation  6 of the population. Bartlett and MacGregor (1995) report that for many batches of cast‐in‐place  7

concrete,  and  samples  obtained  from  many  members,  the  coefficient  of  variation  was  8 approximately 13%. If the maximum error is equal to the coefficient of variation, a maximum error  9 of 13% corresponds to approximately 1.13 standard deviations, which is considered adequate for  10 an estimate of lower bound material properties.    11 Users of the document are cautioned that for coefficients of variation between 13 and 20%, the  12 minimum number of samples needed to limit the error below one standard deviation according  13 to the recommendations in ACI 214.4 is higher than 4. For example, for a coefficient of variation  14 of 20% a minimum of 7 samples is recommended to limit the error to one standard deviation. If  15

the  maximum  error  is  reduced  to  10%  the  minimum  number  of  samples  recommended  is  16 significantly higher. For a coefficient of variation of 15.87% (one standard deviation away from  17 the mean) and a maximum error of 10%, the minimum number of samples recommended is 11,  18 and for a coefficient of variation of 20% and a maximum error of 10%, the minimum number of  19

samples  recommended  is  16.  If  the  coefficient  of  variation  exceeds  20%,  the  requirements  in  20

Section 2.2.4.2.1 shall be satisfied.  21

(22)

  1 2

Ultrasonics  and  nondestructive  test  methods  should  not  be  substituted  for  core  sampling  and  3

laboratory testing as they do not yield accurate strength values directly. These methods should  4

only  be  used  for  confirmation  and  comparison.  Guidance  for  nondestructive  test  methods  is  5

provided in ACI 228.2R.    6

7

2.2.4.2.3 Conventional Reinforcing and Connector Steels―Tests shall be conducted to determine

8

both yield and ultimate strengths of reinforcing and connector steel. Connector steel is defined as 9

additional structural steel or miscellaneous metal used to secure precast and other concrete shapes 10

to the building structure. A minimum of three tensile tests shall be conducted on conventional 11

reinforcing steel samples from a building for strength determination, subject to the following 12

supplemental conditions: 13

1. If original construction documents defining properties exist, then at least three strength 14

coupons shall be removed from random locations from each element or component 15

type and tested; or 16

2. If original construction documents defining properties are unavailable, but the 17

approximate date of construction is known and a common material grade is confirmed, 18

at least three strength coupons shall be removed from random locations from each 19

element or component type for every three floors of the building; and 20

3. If the construction date is unknown, at least six strength coupons for every three floors 21

shall be performed. 22

Refer to Section 2.2.3.2 for replacement of sampled material. 23

(23)

2.2.4.2.4 Prestressing Steel―Sampling prestressing steel tendons for laboratory testing shall only

1

be performed on prestressed components that are part of the seismic-force-resisting system. 2

Prestressed components in diaphragms shall be permitted to be excluded. 3

Tendon or prestress removal shall be avoided if possible. Any sampling of prestressing steel 4

tendons for laboratory testing shall be done with extreme care. It shall be permitted to determine 5

material properties without tendon or prestress removal by careful sampling of either the tendon 6

grip or the extension beyond the anchorage, if sufficient length is available. 7

All sampled prestressed steel shall be replaced with new, fully connected, and stressed material 8

and anchorage hardware, unless an analysis confirms that replacement of original components is 9

not required. 10

2.2.4.2.5 Cast-in-place or post-installed anchors― Cast-in-place or post-installed anchors shall

11

be classified in groups in accordance with 2.2.4.1. In groups of anchors used for out-of-plane 12

wall anchorage and in groups of anchors whose failure in tension or shear would cause the 13

structure not to meet the selected Performance Objective, 10% of the anchors with a minimum of 14

six anchors of each anchor group shall be tested in-place to in tension to establish an available 15

strength, construction quality or both. Testing of the anchors to failure is not required. The test 16

load shall be specified by the licensed design professional and shall be based on the anticipated 17

demand or strength in accordance with available construction information. If the test load is 18

used as the basis for anchor strength calculation, the available anchor strength shall not be taken 19

greater than 2/3 of the test load. Testing of the anchors to failure is not required and a test load 20

lower than the expected failure load shall be permitted. 21

(24)

  1 C2.2.4.2.5 Cast‐in‐place or post‐installed anchors― To estimate ultimate strength of the  2 anchors in accordance with Section 3.6, the frequency of the test should be increased to at least  3 25% of the anchors and the test load should be at least the nominal design strength in  4 accordance with Chapter 17 of ACI 318. In‐place anchor testing performed in accordance with  5 2.2.4.2.5 provides the minimum available tensile strength of a single anchor, which is likely  6 governed by pullout or bond strength in tension. Other failure modes and parameters that affect  7 the strength of the anchors, such as proximity to edges, group effect, presence of cracks, or  8 eccentricity of applied loads, should be considered in accordance with Chapter 17 of ACI 318.  9 10

2.2.5 Default Properties―Default material properties to determine component strengths shall be

11

permitted to be used in conjunction with the linear analysis procedures of ASCE 41 Chapter 7. 12

Default lower-bound concrete compressive strengths are specified in Table 2. Default expected 13

concrete compressive strengths shall be determined by multiplying lower-bound values by an 14

appropriate factor selected from Table 1, unless another factor is justified by test data. The 15

appropriate default compressive strength, lower-bound strength, or expected strength as specified 16

in ASCE 41 Section 7.5.1.3, shall be used to establish other strength and performance 17

characteristics for the concrete as needed in the structural analysis. 18

Default lower-bound values for reinforcing steel are specified for various ASTM specifications 19

and periods in Tables 3 or 4. Default expected strength values for reinforcing steel shall be 20

determined by multiplying lower-bound values by an appropriate factor selected from Table 1, 21

unless another factor is justified by test data. Where default values are assumed for existing 22

(25)

reinforcing steel, welding or mechanical coupling of new reinforcement to the existing reinforcing 1

steel shall not be permitted. 2

The default lower-bound yield strength for steel connector material shall be taken as 27,000 lb/in.2

3

(186 MPa). The default expected yield strength for steel connector material shall be determined 4

by multiplying lower-bound values by an appropriate factor selected from Table 1, unless another 5

value is justified by test data. 6

The default lower-bound yield strength for cast-in-place or post-installed anchor material shall be 7

taken as 27,000 lb/in.2 (186 MPa) unless another value is justified by test data. Component actions

8

on the connections shall be considered as force-controlled actions and default expected yield 9

strength shall not be used. 10

The use of default values for prestressing steel in prestressed concrete construction shall not be 11 permitted. 12 13 C2.2.5 Default Properties―Default values provided in this standard are generally conservative.  14

Whereas  the  strength  of  reinforcing  steel  can  be  fairly  consistent  throughout  a  building,  the  15

strength of concrete in a building could be highly variable, given variability in concrete mixtures  16

and  sensitivity  to  water–cement  ratio  and  curing  practices.  A  conservative  assumption  based  17

upon  the  field  observation  of  the  concrete  compressive  strength  in  the  given  range  is  18

recommended, unless a higher strength is substantiated by construction documents, test reports,  19

or material testing. For the capacity of an element in question, the lower value within the range  20

can be conservative. It  can be appropriate to use the maximum value  in a given range where  21

determining the force‐controlled actions on other components.  22

(26)

Until about 1920, a variety of proprietary reinforcing steels was used. Yield strengths are likely to  1 be in the range of 33,000 to 55,000 lb/in.2 (230 to 380 MPa), but higher values are possible and  2 actual yield and tensile strengths can exceed minimum values. Once commonly used to designate  3 reinforcing steel grade, the terms “structural,” “intermediate,” and “hard” became obsolete in  4 1968. Plain and twisted square bars were occasionally used between 1900 and 1949.  5 Factors to convert default reinforcing steel strength to expected strength include consideration of  6 material overstrength and strain hardening.  7 8 2.3 Condition Assessment 9

2.3.1 General―A condition assessment of the existing building and site conditions shall be

10

performed as specified in this section. 11

The condition assessment shall include the following: 12

1. Examination of the physical condition of primary and secondary components, and the 13

presence of any degradation shall be noted; 14

2. Verification of the presence and configuration of components and their connections, 15

and the continuity of load paths between components, elements, and systems; 16

3. A review and documentation of other conditions, including neighboring party walls 17

and buildings, presence of nonstructural components and mass, and prior remodeling; 18

4. Collection of information needed to select a knowledge factor in accordance with 19

Section 4.4; and 20

5. Confirmation of component orientation, plumbness, and physical dimensions. 21

(27)

  1

C2.3.1  General―The  condition  assessment  also  affords  an  opportunity  to  review  other 

2

conditions that can influence concrete elements and systems and overall building performance.  3

Of  particular  importance  is  the  identification  of  other  elements  and  components  that  can  4

contribute  to  or  impair  the  performance  of  the  concrete  system  in  question,  including  infills,  5 neighboring buildings, and equipment attachments. Limitations posed by existing coverings, wall  6 and ceiling space, infills, and other conditions shall also be defined such that prudent retrofit  7 measures can be planned.  8   9

2.3.2 Scope and Procedures―The scope of the condition assessment shall include critical

10

structural components as described in the following subsections. 11

2.3.2.1 Visual Condition Assessment―Direct visual inspection of accessible and representative

12

primary components and connections shall be performed to 13

 Identify configuration issues; 14

 Determine if degradation is present; 15

 Establish continuity of load paths; 16

 Establish the need for other test methods to quantify the presence and degree of 17

degradation; and 18

 Measure dimensions of existing construction to compare with available design information 19

and reveal any permanent deformations. 20

A visual building inspection shall include visible portions of foundations, seismic-force-resisting 21

members, diaphragms (slabs), and connections. As a minimum, a representative sampling of at 22

(28)

least 20% of the components and connections shall be visually inspected at each floor level. If 1

significant damage or degradation is found, the assessment sample of all similar-type critical 2

components in the building shall be increased to 40% or more, as necessary, to accurately assess the 3

performance of components and connections with degradation. 4

If coverings or other obstructions exist, partial visual inspection through the obstruction shall be 5

permitted to be performed using drilled holes and a fiberscope. 6

  7

C2.3.2.1  Visual  Condition  Assessment―Further  guidance  can  be  found  in  ACI  201.1R,  which 

8

provides a system for reporting the condition of concrete in service.  9

10

2.3.2.2 Comprehensive Condition Assessment―Exposure is defined as local minimized removal

11

of cover concrete and other materials to inspect reinforcing system details. All damaged concrete 12

cover shall be replaced after inspection. The following criteria shall be used for assessing primary 13

connections in the building for comprehensive data collection: 14

1. If detailed design drawings exist, exposure of at least three different primary 15

connections shall occur, with the connection sample including different types of 16

connections (for example, beam–column, column–foundation, beam–diaphragm, and 17

diaphragm-wall). If no deviations from the drawings exist or if the deviations from the 18

drawings are consistently similar, it shall be permitted to consider the sample as being 19

representative of installed conditions. If inconsistent deviations are noted, then at least 20

25% of the specific connection type shall be inspected to identify the extent of 21

deviation; or 22

(29)

2. In the absence of detailed design drawings, at least three connections of each primary 1

connection type shall be exposed for inspection. If common detailing among the three 2

connections is observed, it shall be permitted to consider this condition as representative 3

of installed conditions. If variations are observed among like connections, additional 4

connections shall be inspected until an accurate understanding of building construction 5

is gained. 6

2.3.2.3 Additional Testing―If additional destructive and nondestructive testing is required to 7

determine the degree of damage or presence of deterioration, or to understand the internal 8

condition and quality of concrete, test methods approved by the licensed design professional shall 9 be used. 10   11 C2.3.2.3 Additional Testing―The physical condition of components and connectors affects their  12

performance.  The  need  to  accurately  identify  the  physical  condition  can  dictate  the  need  for  13

certain  additional  destructive  and  nondestructive  test  methods.  Such  methods  can  be  used  to  14 determine the degree of damage or presence of deterioration and to improve understanding of  15 the internal condition and concrete quality. Further guidelines and procedures for destructive and  16 nondestructive tests that can be used in the condition assessment are provided in ACI 228.1R, ACI  17 228.2R, FEMA 274 (Section C6.3.3.2), and FEMA 306(Section 3.8).  18

The  nondestructive  examination  (NDE)  methods  having  the  greatest  use  and  applicability  to  19

condition assessment are listed below:  20

 Surface  NDE  methods  include  infrared  thermography,  delamination  sounding,  surface  21

hardness measurement, and crack mapping. These methods can be used to find surface  22

(30)

degradation in components such as service‐induced cracks, corrosion, and construction  1 defects;  2  Volumetric NDE methods, including radiography and ultrasonics, can be used to identify  3 the presence of internal discontinuities and loss of section. Impact‐echo ultrasonics is often  4 used and is a well‐understood technology;  5  On‐line monitoring using acoustic emissions, strain gauges, in‐place static or dynamic load  6

tests,  and  ambient  vibration  tests  can  be  used  to  assess  structural  condition  and  7

performance. Monitoring is used to determine if active degradation or deformations are  8

occurring,  whereas  nondestructive  load  testing  provides  direct  insight  on  load‐carrying  9

capacity;  10

 Electromagnetic methods using a pachometer or radiography can be used to locate, size,  11

or  perform  an  initial  assessment  of  reinforcing  steel.  Further  assessment  of  suspected  12

corrosion  activity  should  use  electrical  half‐cell  potential  and  resistivity  measurements;  13 and  14  Lift‐off testing (assuming original design and installation data are available), or another  15 nondestructive method such as the “coring stress relief” specified in SEI/ASCE 11, can  16

be  used  where  absolutely  essential to  determine  the  level of  prestress  remaining  in  an  17

unbonded prestressed system.  18

19

2.3.3 Basis for the Mathematical Building Model―Results of the condition assessment shall be

20

used to quantify the following items needed to create the mathematical building model: 21

(31)

2. Component configuration and the presence of any eccentricities or permanent 1

deformation; 2

3. Connection configuration and the presence of any eccentricities; 3

4. Presence and effect of alterations to the structural system since original 4

construction; and 5

5. Interaction of nonstructural components and their involvement in seismic force 6

resistance. 7

All deviations between available construction records and as-built conditions obtained from visual 8

inspection shall be accounted for in the structural analysis. 9

Unless concrete cracking, reinforcement corrosion, or other mechanisms of degradation are 10

observed in the condition assessment as the cause for damage or reduced capacity, the cross-11

sectional area and other sectional properties shall be assumed to be those from the design drawings 12

after adjustment for as-built conditions. If some sectional material loss has occurred, the loss shall 13

be quantified by direct measurement and sectional properties reduced accordingly using the 14

principles of structural mechanics. 15

2.4 Knowledge Factor―A knowledge factor () for computation of concrete component 16

acceptance criteria shall be selected in accordance with ASCE 41 Section 6.2.4 with additional 17

requirements specific to concrete components. A knowledge factor, equal to 0.75 shall be used if 18

any of the following criteria are met: 19

1. Components are found to be damaged or deteriorated during assessment, and 20

further testing is not performed to quantify their condition or justify the use of 21

higher values of ; 22

2. Mechanical properties have a coefficient of variation exceeding 20%; and 23

(32)

3. Components contain archaic or proprietary material and the condition is uncertain. 1

2

CHAPTER 3 – GENERAL ASSUMPTIONS AND REQUIREMENTS

3

3.1―Modeling and Design

4

3.1.1 General―Seismic retrofit of a concrete building involves the design of new components

5

connected to the existing structure, seismic upgrading of existing components, or both. New 6

components shall comply with ACI 318, except as otherwise indicated in this standard. 7

Original and retrofitted components of an existing building are not expected to satisfy provisions 8

of ACI 318 but shall be assessed using the provisions of this standard. Brittle or low-ductility 9

failure modes shall be identified as a part of the seismic evaluation. 10

Evaluation of demands and capacities of reinforced concrete components shall include 11

consideration of locations along the length where seismic force and gravity loads produce 12

maximum effects; where changes in cross section or reinforcement result in reduced strength; and 13

where abrupt changes in cross section or reinforcement, including splices, can produce stress 14

concentrations that result in premature failure.  15

C3.1.1 General―Brittle or low‐ductility failure modes typically include behavior in direct or nearly 

16

direct  compression;  shear  in  slender  components  and  in‐component  connections;  torsion  in  17

slender  components;  and  reinforcement  development,  splicing,  and  anchorage.  The  stresses,  18 forces, and moments acting to cause these failure modes should be determined from a limit‐state  19 analysis, considering probable resistances at locations of nonlinear action.  20   21

3.1.2 Stiffness―Component stiffnesses shall be calculated considering shear, flexure, axial

(33)

caused by volumetric changes from temperature and shrinkage, deformation levels under gravity 1

loads and seismic forces shall be considered. Gravity load effects considered for effective 2

stiffnesses of components shall be determined using ASCE/SEI 41 Equation 7-3. 3

4

C3.1.2  Stiffness―For  columns  with low axial loads (below  approximately  0.1Agfc),  deformations 

5

caused by bar slip can account for as much as 50% of the total deformations at yield. Further  6

guidance  regarding  calculation  of  the  effective  stiffness  of  reinforced  concrete  columns  that  7

include the effects of flexure, shear, and bar slip can be found in Elwood and Eberhard (2009).  8

Flexure‐controlled wall stiffness can vary from approximately 0.15EcEIg to 0.5EcEIg, depending on 

9

wall  reinforcement  and  axial  load.  A  method  for  calculating  wall  stiffness  which  provides  10

compatibility with fiber section analysis is offered in C7.2.2.  11

12

3.1.2.1 Linear Procedures―Where design actions are determined using the linear procedures of

13

ASCE 41 Chapter 7, component effective stiffnesses shall correspond to the secant value to the 14

yield point of the component. Alternate stiffnesses shall be permitted where it is demonstrated by 15

analysis to be appropriate for the design loading. Alternatively, effective stiffness values in Table 16

5 shall be permitted. 17

18

C3.1.2.1  Linear  Procedures―The  effective  flexural  rigidity  values  in  Table  5  for  beams  and  19 columns account for the additional flexibility from reinforcement slip within the beam–column  20 joint or foundation before yielding. The values specified for columns were determined based on a  21 database of 221 rectangular reinforced concrete column tests with axial loads less than 0.67Agfc  22 and shear span–depth ratios greater than 1.4. Measured effective stiffnesses from the laboratory  23

(34)

test data suggest that the effective flexural rigidity for low axial loads could be approximated as  1

0.2EIg;  however,  considering  the  scatter  in  the  effective  flexural  rigidity  and  to  avoid 

2 underestimating the shear demand on columns with low axial loads, 0.3EIg is recommended in  3 Table 5 (Elwood et al. 2007). In addition to axial load, the shear span–depth ratio of the column  4 influences the effective flexural rigidity. A more refined estimate of the effective flexural rigidity  5 can be determined by calculating the displacement at yield caused by flexure, slip, and shear  6 (Elwood and Eberhard 2009).    7

The  modeling  recommendations  for  beam–column  joints  (Section  6.2.2.1)  do  not  include  the  8

influence of reinforcement slip. When the effective stiffness values for beams and columns from  9

Table 5 are used in combination with the modeling recommendations for beam–column joints,  10

the  overall  stiffness  is  in  close  agreement  with  results  from  beam–column  subassembly  tests  11 (Elwood et al. 2007).  12 The effect of reinforcement slip can be accounted for by including rotational springs at the ends  13 of the beam or column elements (Saatcioglu et al. 1992). If this modeling option is selected, the  14 effective flexural rigidity of the column element should reflect only the flexibility from flexural  15 deformations. In this case, for axial loads less than 0.3Agfc, the effective flexural rigidity can be  16 estimated as 0.5EIg, with linear interpolation to the value given in Table 5 for axial loads greater  17 than 0.5Agfc.  18

Because  of  low  bond  stress  between  concrete  and  plain  reinforcement  without  deformations,  19

components with plain longitudinal reinforcement and axial loads less than 0.5Agfc can have lower 

20

effective flexural rigidity values than in Table 5.  21

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1

3.1.2.2 Nonlinear Procedures―Where design actions are determined using the nonlinear

2

procedures of ASCE 41 Chapter 7, component load-deformation response shall be represented by 3

nonlinear load-deformation relations. Linear relations shall be permitted where nonlinear response 4

does not occur in the component. The nonlinear load-deformation relation shall be based on 5

experimental evidence or taken from quantities specified in Chapters 4 through 12. For the nonlinear 6

static procedure (NSP), the generalized load-deformation relation shown in Fig. 1 or other curves 7

defining behavior under monotonically increasing deformation shall be permitted. For the nonlinear 8

dynamic procedure (NDP), load-deformation relations shall define behavior under monotonically 9

increasing lateral deformation and under multiple reversed deformation cycles as specified in Section 10

3.2.1. 11

The generalized load-deformation relation shown in Fig. 1 shall be described by linear response from 12

A (unloaded component) to an effective yield B, then a linear response at reduced stiffness from 13

point B to C, then sudden reduction in seismic force resistance to point D, then response at reduced 14

resistance to E, and final loss of resistance thereafter. The slope from point A to B shall be 15

determined according to Section 3.1.2.1. The slope from point B to C, ignoring effects of gravity 16

loads acting through lateral displacements, shall be taken between zero and 10% of the initial slope, 17

unless an alternate slope is justified by experiment or analysis. Point C shall have an ordinate equal 18

to the strength of the component and an abscissa equal to the deformation at which significant 19

strength degradation begins. Representation of the load-deformation relation by points A, B, and 20

C only (rather than all points A–E) shall be permitted if the calculated response does not exceed 21

point C. Numerical values for the points identified in Fig. 1 shall be as specified in Sections 3.2.2.2 22

for beams, columns, and joints, 3.3.2.2 for post-tensioned beams, 3.4.2.2 for slab–column 23

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connections, and 7.2.2 for shear walls, wall segments, and coupling beams. Other load-1

deformation relations shall be permitted if justified by experimental evidence or analysis. 2

3

C3.1.2.2 Nonlinear Procedures―Typically, the response shown in Fig. 1 is associated with flexural  4

response  or  tension  response.  In  this  case,  the  resistance  at  Q/Qy  =  1.0  is  the  yield  value,  and 

5

subsequent strain hardening is accommodated by hardening in the load‐deformation relation as  6

the  member  is  deformed  toward  the  expected  strength.  Where  the response shown in Fig. 1 is  7

associated with compression,  the  resistance  at  Q/Qy  =  1.0  typically  is  the value where concrete 

8

begins  to  spall,  and  strain  hardening  in  well‐confined  sections  can  be  associated  with  strain  9 hardening of the longitudinal reinforcement and an increase in strength from the confinement of  10 concrete. Where the response shown in Fig. 1 is associated with shear, the resistance at Q/Qy =  11 1.0 typically is the value at which the design shear strength is reached and, typically, no strain  12 hardening follows.  13 The deformations used for the load‐deformation relation of Fig. 1 shall be defined in one of two  14 ways, as follows:  15 Deformation, or Type I: In this curve, deformations are expressed directly using terms such  as  16

strain,  curvature,  rotation,  or  elongation.  The  parameters  anl  and  bnl  refer  to  deformation 

17

portions that occur after yield, or plastic deformation. The parameter cnl is the reduced resistance 

18

after the sudden reduction from C to D. Parameters anl, bnl, and cnl are defined numerically in 

19

various  tables  in  this  standard.  Alternatively,  parameters  anl,  bnl,  and  cnl  can  be  determined 

20

directly by analytical procedures justified by experimental evidence.  21

Figure

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