Collapse of Lightly Confined Reinforced Concrete Frames during Earthquakes

Jack Moehle

Pacific Earthquake Engineering Research Center

University of California, Berkeley

NATO SfP Project 977231:Seismic Assessment and Rehabilitation of Existing Buildings

International Closing Workshop Istanbul Turkey 30 May-1 June 2005

Collapse of Buildings in Past Earthquakes

Column failures Joint failures

Shear-critical columns

Drift at shear failure

Axial Load Failure

Shake Table Tests

4’-10”

6’

Transverse torsional beam

5’ wide beam designed to model bending stiffness of typical spandrel beam(masses not shown for clarity)

Force transducers to measure reactions at base of columns

Shear-Critical Column

Designed to support gravity loads after failure of center column A B

9”

9”

W2.9 wire @ 6” o.c.

#4 corner bars

#5 center bars

Section A

10”

#3 spiral @ 2”

#4

Section B

Shake Table Tests

Top of Column - Total Displ.

Relative Displacements

Specimen #2 – Moderate Axial Load

Axial Load Capacity versus Shear Damage

Tasai, 2000

Shear-Friction Model for Axial Failure

P

s

Aswfy

Aswfy

N Vsf

θθθθ

Elwood, 2002

Implementation in OpenSees

Lateral response

Axial response

P

∆∆∆∆horz

Axial limit curve

Shear limit curve

Analytical modelapplication

sw

Axial Load

analysis

test

Case study frame

Calculated static nonlinear response(pushover curve)

0

20

40

60

80

100

120

0 1 2 3 4 5 6 7 8 9

Drift (%)

Base S

hea

r F

orc

e (

KN

)

Long. steel

yielding

Shear failure

initiation

Residual shear

strengths

Spalling

Non-Ductile

Outer Col.

Ductile

Outer Col.Non-Ductile

Inner Col.

Ductile

Inner Col.

First Floor Columns

Axial failure

initiationProjected behavior

X

Collapse

FEMA 356

Collapse Prevention

Limit

xx

Calculated dynamic response

1994 Northridge earthquake recording stations

PGV = 80 to 110 cm/s

Response spectra of selected motions

0.0

0.5

1.0

1.5

2.0

2.5

0.0 1.0 2.0 3.0

Period (sec)

Sp

ec

tra

l A

cc

ele

rati

on

(g

)

Mean spectrum

Sylmar - Converter Sta East

Sylmar - Olive View Med FF

Sylmar - Converter Sta

Jensen Filter Plant

Newhall - W Pico CanyonRd.

Newhall - Fire Sta

Rinaldi Receiving Sta

Initial period of case-study building

(PGV = 80 to 110 cm/s)

Calculated response to Northridge ground motions

-10

-8

-6

-4

-2

0

2

4

6

8

10

0 1 2 3 4 5 6 7Time (sec)

1s

r F

loo

r D

rift

(%

)

RRS228

JEN292

SCS142

NWH360

SCE011

SYL360

WPI046

SCS142neg

X

Initiation of axial load loss at +7% drift

Initiation of shear failure at +/-2% drift

Long. bar yielding at +/-0.8% drift

X

Collapse of Concrete Buildings in Past Earthquakes

OperationalHeavy

Collapse

Mexico City, 1985

Erzincan, 1992

0

20

40

60

80

100

Percentage

Kobe, 1995

Luzon, 1990

Otani, 1999Building Damage Rating

California hospital retrofit program

San Jose Mercury News, 10 Feb 2005

“Hospital retrofit delays sought - …The price tag for all this retrofitting has been estimated between $21 billion and $41 billion. Of about 430 California hospitals, 366 did not meet standards set for either 2008 or 2030…. The price tag for hospitals through[out] the state is pretty large, and it’s probably going to force some hospitals to close.”