Wassim M. Ghannoum University of Texas at Austin

Seismic Evaluation and Retrofit of Existing

Reinforced Concrete Structures

ACTS Conference, June 2014

Dbayyeh, Lebanon

Outline

1. Seismic Evaluation and Retrofit

Hazard

Evolution of standards

Process in the U.S.

2. The Future of Seismic Evaluations and Retrofit in

the U.S.

Memorandum of understanding between ASCE and

ACI

Updates to concrete provisions

Hazard

1. Seismic design codes introduced in the 80’s

2. Most construction prior to that period did not have

adequate strength or detailing

3. Just after that period, design forces were relatively

low and system-level vulnerability not adequately

addressed

4. A large body of nonductile building stock is

present around the world

Hazard

1. Nonductile concrete buildings are the single

largest threat to life in urban areas

2. In California alone nonductile concrete

buildings number over 20,000

Hazard

2011 Christchurch, NZ – Mw = 7.1

90% of fatalities caused by collapse of concrete

buildings

80% of the downtown had to be demolished and

rebuilt

Hazard

Wenchuan, China (2008) – Mw = 7.9

70,000 fatalities

Main source of deaths: concrete and masonry

construction

Hazard

Kocaeli, Turkey (1999) – Mw = 7.4

Deaths: 17,225, Injuries: 44,000

77,300 homes and 245,000 businesses destroyed

Seismic Evaluation and Retrofit in the

U.S.

Tools for Existing

Buildings • 0.75*1976 UBC

• ATC 3-06 (1978)

• ATC 14 (1987)

• FEMA 178 310 ASCE

31-03

Rapid evaluation of existing

buildings

• FEMA 273 356 ASCE

41-06

Advanced evaluation and

rehabilitation of existing

buildings

Seismic Evaluation and Retrofit in the

U.S.

ASCE 31 is a Standard for rapid seismic

evaluation of buildings

ASCE 41 is a Standard for detailed seismic

evaluation and retrofit of existing buildings

Widely adopted in the U.S. and worldwide

Seismic Evaluation and Retrofit in the

U.S.

ASCE 41-13

ASCE 31-

03

ASCE 41-

06

Seismic Evaluation and Retrofit of Existing Buildings

ASCE 41-13 Chapter 1 General Requirements

Chapter 2 Seismic Performance Objectives and Ground

Motions

Chapter 3 Evaluation and Retrofit Requirements

Chapter 4 Tier 1 Screening

Chapter 5 Tier 2 Deficiency-Based Evaluation and Retrofit

Chapter 6 Tier 3 Systematic Evaluation and Retrofit

Chapter 7 Analysis Procedures and Acceptance Criteria

Chapter 8 Foundations and Geologic Site Hazards

Chapter 9 Steel

Chapter 10 Concrete

Chapter 11 Masonry

Chapter 12 Wood and Cold-Formed Steel

Chapter 13 Architectural, Mechanical, and Electrical

Components

Chapter 14 Seismic Isolation and Energy Dissipation

Chapter 15 System-Specific Performance Procedures

Chapter 16 Tier 1 Checklists

Appendix A Guidelines for Deficiency-Based Procedures

Appendix B Use of ASCE 41-13 within Mitigation Programs

ASCE 31

ASCE 31

ASCE/SEI 41-13 - Tier 3 Process

1. Evaluate seismic hazard

A small break is given for existing buildings

2. Determine desired performance objectives

3. Analyze structure under hazard using Modeling

Parameters

linear/nonlinear, dynamic/static

4. Ensure member and structural demands do not

exceed Acceptance Criteria

Operational

Immediate

Occupancy

Life

Safety

Collapse

Prevention

50% / 50 yr

(72 year)

20% / 50 yr

(225 year)

BSE-1

(~475 year)

BSE-2

(~2475 yr)

Building Performance Level E

art

hq

uak

e H

aza

rd L

evel

Seismic Evaluation and Retrofit in the

U.S.

Q/Qy

1.0

A

B C

D E (Peak deformation)

θy θ or D

a

b a, b & c: Modeling Parameters

c

IO, LS & CP: Acceptance Criteria IO: Immediate Occupancy

LS: Life Safety

CP: Collapse Prevention

IO

LS CP

ASCE/SEI 41-13 Generic Backbone Curve

MP or AC

Future of Seismic Evaluations and Retrofit in the U.S.

ASCE 41 is in need of updating

Document is over 1000 pages long

Covers most material types

Committee has fewer than 30 voting members

Agreement between ACI and ASCE signed in 2014

Future of Seismic Evaluations and Retrofit in the U.S.

ACI committee 369, “Seismic Repair and

Rehabilitation” starting a new Standard

ACI 369 Standard will be the source

document for concrete provisions of ASCE

41

Agreement is a foundation for material

specialty organizations and ASCE to work

together

ACI 369 Standard Seismic Evaluation and Retrofit of Existing Concrete Buildings Chapter 1 Scope

Chapter 2 Material Properties and Condition

Assessment

Chapter 3 General Assumptions and

Requirements

Chapter 4 Concrete Moment Frames

Chapter 5 Precast Concrete Frames

Chapter 6 Concrete Frames with Masonry Infills

Chapter 7 Concrete Shear Walls

Chapter 8 Precast Concrete Shear Walls

Chapter 9 Concrete Braced Frames

Chapter 10 Cast-in-Place Concrete Diaphragms

Chapter 11 Precast Concrete Diaphragms

Chapter 12 Concrete Foundations

1. New MP for concrete columns

Current MP defined inconsistently across document

2. Use new extended database with close to 500

column tests

3. Treat circular columns

4. Adjust AC based on new MP

Conservatism should be incorporated in AC not MP

Select AC at percentiles of MP to achieve consistent

probabilities of exceedance across all members

ACI 369 Standard MP and AC for RC columns

MP for RC columns - Dataset Extended database with close to 500 column tests

Ghannoum, W. M., and Sivaramakrishnan, B. (2012). "ACI 369 Rectangular Column Database.“ DOI: 10.4231/D36688J50

Ghannoum, W. M., and Sivaramakrishnan, B. (2012). "ACI 369 Circular Column Database.“ DOI:10.4231/D39Z90B9T

https://nees.org/resources/3658 ; https://nees.org/resources/3659

MP for RC columns - Dataset Extended database with close to 500 column tests

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10

10

20

30

40

50

60

70

80

90

Axial Load Ratio

Nu

mb

er

of C

olu

mn

s

Rectangular

Circular

0 0.25 0.5 0.75 1 1.25 1.5 1.750

20

40

60

80

100

120

140

s/d

Nu

mb

er

of C

olu

mn

s

Rectangular

Circular

0 0.0025 0.005 0.0075 0.01 0.0125 0.015 0.0175 0.02 0.0225 0.025 0.0275 0.03 0.0325 0.0350

10

20

30

40

50

60

70

80

90

Transverse Reinforcement Ratio

Nu

mb

er

of C

olu

mn

s

Rectangular

Circular

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 20

10

20

30

40

50

60

70

80

90

Vy/V

o

Num

ber

of C

olu

mns

Rectangular

Circular

V

Drift

Vy

Vy>Vo

Vy<Vo

MP for RC columns - Dataset

Data in form of lateral force-deformation plots

Buenrostro, E. (2013). “Deformations in Non-Seismically Detailed

Concrete Columns." MS Report, University of Texas at Austin

MP for RC columns –

Parameters

Most influential parameters for a were found to be

Axial load ratio

Transverse reinforcement ratio

Vy/Vo

Parameters

Table bounds need updating

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.80

0.005

0.01

0.015

0.02

0.025

0.03

0.035

Axial Load Ratio

tra

nsve

rse

Re

info

rce

me

nt R

atio

Circular columns

Rectangular columns

Table 10-8. Modeling Parameters and Numerical Acceptance Criteria for Nonlinear Procedures—Reinforced Concrete Columns

Conditions

Modeling Parametersa Acceptance Criteriaa

Plastic Rotations

Angle (radians)

Residual

Strength

Ratio

Plastic Rotations Angle

(radians)

Performance Level

IO

a b c LS CP

Condition i.b c

0.1 0.006 0.035 0.060 0.2 0.005 0.045 0.060

0.6 0.006 0.010 0.010 0.0 0.003 0.009 0.010

0.1 0.002 0.027 0.034 0.2 0.005 0.027 0.034

0.6 0.002 0.005 0.005 0.0 0.002 0.004 0.005

Condition ii.b

c

d

0.1 0.006 3 (0.25) 0.032 0.060 0.2 0.005 0.045 0.060

0.1 0.006 6 (0.5) 0.025 0.060 0.2 0.005 0.045 0.060

0.6 0.006 3 (0.25) 0.010 0.010 0.0 0.003 0.009 0.010

0.6 0.006 6 (0.5) 0.008 0.008 0.0 0.003 0.007 0.008

0.1 0.0005 3 (0.25) 0.012 0.012 0.2 0.005 0.010 0.012

0.1 0.0005 6 (0.5) 0.006 0.006 0.2 0.004 0.005 0.006

0.6 0.0005 3 (0.25) 0.004 0.004 0.0 0.002 0.003 0.004

0.6 0.0005 6 (0.5) 0.0 0.0 0.0 0.0 0.0 0.0

Condition iii.b c

0.1 0.006 0.0 0.060 0.0 0.0 0.045 0.060

0.6 0.006 0.0 0.008 0.0 0.0 0.007 0.008

0.1 0.0005 0.0 0.006 0.0 0.0 0.005 0.006

0.6 0.0005 0.0 0.0 0.0 0.0 0.0 0.0

Condition iv. Columns controlled by inadequate development or splicing along the clear heightb c

0.1 0.006 0.0 0.060 0.4 0.0 0.045 0.060

0.6 0.006 0.0 0.008 0.4 0.0 0.007 0.008

0.1 0.0005 0.0 0.006 0.2 0.0 0.005 0.006

0.6 0.0005 0.0 0.0 0.0 0.0 0.0 0.0

P

Ag fc-----------

Av

bws--------=

P

Ag fc-----------

Av

bws--------=

V

bwd fc-------------------

P

Ag fc-----------

Av

bws--------=

P

Ag fc-----------

Av

bws--------=

MP for RC columns – a Regression analysis results

0.0023.063.0043.0042.0'

o

y

t

cg

RV

V

fA

Pa (rad)

Rectangular columns

Circular columns (spirals or hoops)

0.0037.03.1058.006.0'

o

y

t

cg

CV

V

fA

Pa (rad)

0.0005 ≤ ρt ≤

0.0175

Vy/Vo ≥ 0.2

P/(Agf’c) ρt Vy/Vo aR (rad) aC (rad)

0 0.0005 0.2 0.038 0.058

0 0.0005 2.0 0.0 0.0

0 0.0175 0.2 0.048* 0.071*

0 0.0175 2.0 0.007 0.0

0.7 0.0005 0.2 0.008 0.016

0.7 0.0005 2.0 0.0 0.0

0.7 0.0175 0.2 0.018 0.029

0.7 0.0175 2.0 0.0 0.0

Values of a at parameter boundaries

MP for RC columns – a Analysis of fit

-0.04 -0.02 0 0.02 0.04 0.06 0.080

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Error in aR

= Experiment - Estimate (rad)

Cu

mu

lative

Dis

trib

utio

n

Number of Tests = 319

Standard Deviation Equation = 0.015

Standard Deviation ASCE41-06 = 0.015

Equation estimates

ASCE 41-06 estimates

-0.04 -0.02 0 0.02 0.04 0.06 0.080

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Error in aC

= Experiment - Estimate (rad)

Cu

mu

lative

Dis

trib

utio

n

Number of Tests = 171

Standard Deviation Equation = 0.019

Standard Deviation ASCE41-06 = 0.022

Equation estimates

ASCE 41-06 estimates

• Median estimate achieved

• Improved fit for circular columns

MP for RC columns – a Analysis of fit

0 0.2 0.4 0.6 0.8 1 1.2 1.4-0.04

-0.02

0

0.02

0.04

0.06

0.08

Rectangular column s/d

Err

or

in a

R =

Experim

ent -

Estim

ate

(ra

d)

135o hooks

90o hooks

Unknown

0 0.2 0.4 0.6 0.8 1 1.2 1.4-0.04

-0.02

0

0.02

0.04

0.06

0.08

Circular column s/d

Err

or

in a

C =

Experim

ent -

Estim

ate

(ra

d)

spirals

lapped

Unknown

• Little bias with respect to s/d

• Little bias with respect to tie hooks

MP for RC columns – b Behavioral model

Due to the scarcity of columns tested to axial collapse a

behavioral-model is selected instead of a regression-

based model

Drift ratio at axial failure

Simplified Elwood-Moehle axial failure model

∆

𝐿=

0.5

5 +𝑃

𝐴𝑣𝑓𝑦𝑡

𝑠𝑑𝑐

MP for RC columns – b Analysis of fit

• Including a loading protocol

factor of 0.5 on the drift at

axial failure for columns

tested under 6 load cycle

per drift level or bi-axially

Model Average

Measured/

Calculated drift

ratio at axial

failure

Standard

Deviation

Coefficient of

Variation

ASCE 41-06 Suppl. 1 1.97 0.93 0.47

Elwood-Moehle 0.97 0.44 0.46

Simplified Elwood –

Moehle with loading

factor

1.13 0.38 0.33

MP for RC columns – b Analysis of fit

Using full database

0 0.02 0.04 0.06 0.08 0.1 0.12 0.140

0.02

0.04

0.06

0.08

0.1

0.12

0.14

Experimental b2R

(rad)

Pro

posed b

ehavio

ral m

odel b (

rad)

Tests not to axial collapse

Tests to axial collapse

0 0.02 0.04 0.06 0.08 0.1 0.12 0.140

0.02

0.04

0.06

0.08

0.1

0.12

0.14

Experimental b2C

(rad)

Pro

posed b

ehavio

ral m

odel b (

rad)

Tests not to axial collapse

Tests to axial collapse

• Model produces median estimates for rect. and circular columns tested

to collapse

Acceptance Criteria

AC defined as percentiles of MP a and b

Fixed probability of exceeding a threshold behavior

defined by MP a and b

IO LS Primary CP Primary

LS

Secondary

CP Secondary

Criteria 10% of

MP a

20th percentile

of MP a

35th percentile

of MP a

10th percentile

of MP b

25th percentile of

MP b

Rectangula

r

0.1

aR

0.50 aR 0.75 aR 0.60 bR 0.75 bR

Circular 0.1

aC

0.30 aC 0.65 aC 0.60 bC 0.75 bC

Summary

1. Seismic hazard in the Middle-East is high and

mitigation is needed

2. ASCE 41-13 is the standard used in the U.S. for

seismic evaluation and retrofit of buildings

3. Document used in many other applications as it

contains MP and AC for a wide range of members and

materials

4. ACI 369 standard will be the source document the

concrete provisions of ASCE 41.

5. Wide ranging updates to concrete provisions are under

way

6. Discussions on altering procedures in ASCE 41 also

under way