seismic hazard and state of the art seismic design practice in … 0... · 2019. 9. 4. · seismic...

Current Seismic Design Practice in Korea and Engineering Implications of Recent M5.8 Gyeong-Ju Earthquake

4 6 8-0.5

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4 6 8 10-0.5

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Cheol-Ho LeeDept. of Arch. and Arch. Engrg., Seoul National University

Introduction to Earthquake Engineering and Dynamics of Building Structures_ 1

Steel Structures & Seismic Design Lab, Dept. of Arch and Arch Engrg, SNU

I. Introduction

II. Ground Shaking for Seismic Design of Building Structures in Korea

III. Brief Summary of Current Seismic Design Practice in Korea

IV. Preliminary Engineering Analysis of 2016 M 5.8 Gyeong-JuEarthquake

VI. Summary and Conclusions


Outline of Presentation

Kobe EQ, Japan (1995.1.17)

* M = 6.9

* Casualties 6,000

* Economic loss

200 trillion USD

Ji Ji EQ, Taiwan(1999.9.21)

* M = 7.7,

* Casualties 2415

* Taiwan’s economic basis

severely shaken

Northridge EQ, Calif.

(1994.1.17)

* M = 6.7

* Casualties 57

* Economic loss 22 trillion USD

“Some recent strong EQ events and Losses”

Sichuan EQ, China (2008.5.12)

* M= 8.0

* Casualties and loss 70,000

Christ Church, NZ

(2011.2.22)

* M = 6.3

* Shallow epicenter (5km)

East Japan Great EQ (2011.3.11)

* M = 9.0

* Casualties 15, 000

* Catastrophic tsunami

* Hukushima nuclear power plant

explosion due to tsunami

* Strongest since 1900 in Japan

Haiti EQ (2010.1.12)

* M= 7.0

* Casualties 100,000

* More than 250,000

dwellings damaged

Nepal EQ(2015.4.25)

* M= 7.8

* Casualties and injuries 210,000

* Economic loss

50% Nepal’s GDP

Tainan EQ, Taiwan (2016.2.6)

* M= 6.4

* Casualties 117

Kumamoto EQ (2016.4.15)

* M= 7.0

* Casualties 49

* Shallow epicenter (10km)


I. Introduction

Date 2008.6.14 2008.5.12

Magnitude

(16 times difference in energy)

7.2 8.0

Focal depth (km) 10 19

Casualties/losses/injuries 10/12/ 231 69,180/17,406/

374,431

Notes:

* Drastic difference in terms of casualties/losses/injuries

* Seismic provisions often exist in seismically fragile

countries (apparently very stringent)

* Effective seismic design down to the grass root level is

critically important

Seismic hazard (or activity)

Casualties/losses/injuries

Comparison of Iwate (Japan) and Sichuan (China) EQs in 2008

“The critical importance of implementing seismic design effectively down to the grass root level”

Average expectation

Seismically fragile

countries

Seismically

well-prepared

countries

“Our hopeful positioning should be here”

“The core of seismic resilience: effective implementation of seismic design and the emergency management and recovery actions”


“The First Defense Line”


II. Ground Shaking for Seismic Design of Building Structures in Korea

“Infrequent but large earthquakes can occur at the faults within the plate (intraplate earthquakes); for example, Tang-San M 7.8 EQ in 1976, China”

• Set to account for “infrequent but large earthquakes” probable in low-to-moderate intraplate EQ regions (like Korea and eastern US…)

• Infrequent but large EQs: so called MCEs (max. considered earthquakes) for building structures with 2400-year return period

• We don’t care about more stronger shaking beyond 2400-year EQ (or we don’t care about geological scale EQs).

• DBE (Design Basis Earthquake) in Korea• = (2/3)*MCE • = approx. 1000 year EQ


Design Basis Earthquake (DBE) in Korea

Intraplate EQs

Possible Standard Seismic Performance Levels

DBE MCE

BSO (Basic Safety Objective) for general bldg. structures (Importance Class= 2) implied in 2016 KBC

(Korean Building Code)

(70yr EQ) (200yr EQ) (1000yr EQ)(2400yr EQ)

OperationalImmediate Occupancy Collapse PreventionLife Safety


Significant structural and non-structural damage permitted, able to resist after-shocks, generally costly repair needed to re-occupy

Lateral stiffness and strength almost lost and barely supporting gravity load, very dangerous for after-shocks

Brief Review of Seismic Hazards in Korea

• Instrumental EQ data collected since late 1970’s_ now fairly dense strong motion instrumentation arrays managed by the government agencies

• A fairly long historical EQ data including 600-year good records in Cho-Sun Dynasty

• Engineering quantification of such historical EQ data: very important especially for low-to-mid seismicity regions like Korea because of lack of more reliable instrumental EQ data

• Huge uncertainties inevitable to quantification of historical EQ data_ the importance of engineering seismic design to overcome such uncertainties involved

Seismic Hazard from Historical Earthquakes_ dominant

Historical earthquake data in Korea

Period Total no. of quakes No. of quakes with MMI> VII

Total

0

0

*

1 (2 / 3) [Gutenberg-Richiter 1956]

log( ) 0.5 [Gutenberg-Richiter 1956]

log( ) 0.014 0.3 [Trifunac-Brady 1975]

MMI M

M MMI

MMI PGA

PGA MMI

PGA MMI

*

(1/3)


• Korean peninsular seismically active during the 15th

~18th century; seismic loading in KBC governed bythe historical earthquakes

• Max. historical MMI (Modified Mercalli Intensity)~ VIII~IX

• Empirically converted M~ 6.2

• Thirteen (13) M6.2 quakes in our history

“MMI Rating= VIII”

An Example of MMI Rating from a Historical Record (1518)

• Different rating possible among different evaluators • Tends to be conservative because local extreme damage is extrapolated to wider areas (Bolt 1978)

“Thundering sound heard; people not able to stand up well ; castle walls fell down…”


Large Events AD 27 and 89 Events (Baek Jae Dynasty, 2000 years ago)

Note that no epicenter information is reported at all

“How to locate the epicenter needed for seismic hazard analysis?”

“Nominally assign the epicenter to the ancient capital location when no epicenter information is reported_ one of the intensity rating rules often used”

Then, where is the location of the ancient capital?; NamhanSansung or Mongchon Tosung or else where?


KMA official historical EQ report (2012)(KMA= Korea Meteorological Agency)

Rated MMI (not JMA scale)-- VIII

Rated MMI (not JMA scale)-- VIII~IX

“M~ 6.2”Based on very empirical MMI0-M conversion

Construction quality 2000 years ago: worse or better than the 1930’s ?

Recall: MMI rating is based on the building construction quality of 1930’s US west coast practice

Assigned to Mongchon Tosung area; surely, this would artificially increase the seismic hazard of downtown Seoul

“Based on the very brief damage description”


0

0

*

1 (2 / 3) [Gutenberg-Richiter 1956]

log( ) 0.5 [Gutenberg-Richiter 1956]

log( ) 0.014 0.3 [Trifunac-Brady 1975]

MMI M

M MMI

MMI PGA

PGA MMI

PGA MMI

*

(1/3)

Another huge uncertainties in empirical conversion formula among MMI-M-PGA

widely used


MMI (epicenter) M

MMI PGA

Recorded maximum accelerations vs.

reported intensities for the period

1933-1943

Recorded maximum accelerations Vs. reported intensities for the period

1933-1954

“Log scale”

Recorded PGA vs.

Reported MMI

(Ambraseys 1974)


MMI PGA Conversion

Recorded maximum accelerations vs.

reported intensities for the period

1933-1973

Gutenberg-Richter 1956

“Log scale”

“Log scale”

“Already 10 times different!”

What if for 1933-2016?

Recorded PGA vs. Reported

MMI (Ambraseys 1974) Steel Structures & Seismic Design Lab, Dept. of Arch and Arch Engrg, SNU

Recorded PGA vs.

Reported MMI

(Ambraseys 1974)

MMI PGA Conversion

Accept over- or under-estimation of historical seismic hazards

All these uncertainties should be absorbed in seismic design

Hazard Map in KBC (NEMA 2013)

2400 year return period

• Incorporates the historical seismic hazards and prepared through the consensus of the domestic representative seismologists, and should be respected and conformed by the law.

• The EPA of 2400-year EQ (MCE, for general building structures) thus obtained: about 0.22g

• The DBE (design basis earthquake) is defined in KBC (Korean Building Code) as (2/3) of MCE in order to take into account of the infrequent but large EQs in low to moderate seismicity of Korea


High uncertainties involved should be admitted

Comparison with Japanese Design Spectrum (in terms of PSV spectrum_ soft rock site)

Japanese traditional “dual spectrum approach” highly evaluated personally

Japanese 500yr EQ (for ultimate strength)

Japanese 50yr EQ (for serviceability)

Korea:13,000yr EQ5,000yr EQ1500yr EQ

• DBE in KBC slightly stronger than Japanese serviceability EQ by about 10% or more

• Japan is seismically real strong DBE= 1000yr EQ

“EQ ground motion: a stochastic (random) process: high variability in amplitude, strong motion duration and frequency contents”

No same EQs occur even at the same site

Response spectra from three EQs at the same site (Imperial valley station, southern Calif.)

• Factors on the details of EQ ground motions:

1. local site conditions; only available usually

2. hypocentral distance,

3. source mechanism,

4. wave transmission path geology, etc.

All from southern California

Base shear

Structural period

Usually unknown


III. Brief Summary of Current Seismic Design Practice in Korea


“Avoid socio-economically unacceptable elastic design,,and permit damage strategically; resist strong EQswith cyclic ductility Engineering Compromise inmodern seismic design

Strength demand for no damage or elastic response

Actual strength supply= 1/8~1/1.5

저층건물

고층건물

지진하중/ 건물자중

“Averaging, smoothing_ highly variable in fact

“Socio-Economic Reasons”

Base shear ratio


• Possibility of strong EQs, whether DBE or MCE, very low for a nominal useful life of buildings (say 50 years) but could be catastrophic once they occur

Adopted the R-factor approach similar to the US practice but in modified form with considering our design and construction practice

• One thing sure as a result of the reduction: strong EQs would drive building structures beyond elastic range, or would demand seismic energy dissipation

• Control the seismic behavior reliably only through design, not by analysis which has to base very uncertain input

• The capacity design concept has to be resorted to which ensures stable seismic energy dissipation irrespective of the details of EQ ground motions usually known to us

• Now well established after the 1994 Northridge EQ and incorporated in KBC seismic provisions since 2009.


“Control yield mechanism by design, not by analysis, and provide ductile details at the predetermined locations”

Brief History of Seismic Design in Korea

• Enacted for the buildings more than six-story high since 1988 because of the 1978 Hong-Sung EQ that caused some un-negligible damage and now about to enforce seismic design for all new construction

• Large-size cyclic testing started since late 1990’s, meaning researchers in Korea started to appreciate the importance of experimental evidence for reliable seismic design

• Now large-size testing active for academic and commercial purposes


The early full-scale testing setup by Lee, CH and Park, JW (1997)

Lee et al. (2001)

Cyclic testing conducted for some proprietary connection and its field

application (Lee and Park et al. 2011)

-400

0

400

-300 0 300

Beam tip displacement (mm)

Beam

tip

forc

e (

kN

)

aa

a

Test

Analysis

• Nice large-size testing labs in some universities and public/private institutes

• Even PBSD is tried for some important building projects or to circumvent prescriptive R factor approach


An example of nonlinear dynamic analysis input for seismic performance evaluation

“De-aggregation of seismic hazard at the site may be used to select candidate input motions”

An 80-story high-rise building located at soil condition SD


KBC design spectrum

“Our seismic design force level is not low at all in fact when the soil and/or importance factor is involved; especially for long-period (tall building) structures”


IV. Preliminary Engineering Analysis of 2016/9/12 M5.8 Gyeong-Ju Earthquake


•Gyeong-Ju Quake M= 5.2/5.8, 09/12/2016

The first (pre-) shock: ML 5.2, 19:44The main shock: ML 5.8, 20:32Focal depth: 13km (relatively deep)


The 912 Gyeong-Ju EQ, strongest ever recoded instrumentally, has strongly shaken the south-eastern part of Korean peninsular and drove us into panic

Magnitude reported

11.8 1.5

16.05 1.5

0 0

log( ) 11.8 1.5 ; 10 ( ) (1)

(2 / 3) log( ) 10.7; ( ) 10 ( ) (2)

L

W

M

L

M

W

E M E ergs

M M M seismic moment dyne

The main shock: ML 5.8Focal depth: 13km

The main shock: Mw 5.36Focal depth: 15km

(per Prof. YH Kim, SNU)


22 km

8 km5.9 km

Three accel. records near the epicenter available

“SB (MKL, DKJ) or SC (USN) soil condition speculated”

SC (USN)


USN-NS

Processed Time Histories and Spectra(prepared by CH Lee, JH Park, SY Kim and TJ Kim)

But strong motion duration (SMD) is very short, just about 3 seconds; or damage potential is weak.

Apparent PGA is as high as 0.39g

Nyquist freq.= 25 Hz


Ground motion maxima ratio (stiff soil site)PGA: PGV: PGD= 1g: 1.22 m/sec: 0.91m (Calif. EQ)

PGA: PGV: PGD=1g: 0.16 m/sec: 0.015 m (GJ USN EQ)

“1/8 and 1/60”

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0

0.5

4 6 8 10-0.5

0

0.5

6 8 10 12-0.5

0

0.5

Nyquist freq.= 25 Hz

MKL USN

DKJ

PGA= 0.39gPGA= 0.26g

PGA= 0.09g

Arias

Intensity

(m/s)

PGA

(g)

0.18 0.257

0.70 0.351

0.05 0.092

MKLUSNDKJ

“Apparent PGA is a weak damage indicator”

“EPGA~ 0.18g (진앙지 부근)”


“Amplitude, strong motion duration and frequency contents should considered all together”

10-1

100

101

102

10-4

10-3

10-2

10-1

100

Frequency (Hz)

Magnit

ude (

g2-H

z)

Kyung-Ju 2016-MKL.HGN (raw)

Kyung-Ju 2016-MKL.HGN (filtered)

Kyung-Ju 2016-USN.HGN (raw)

Kyung-Ju 2016-USN.HGN (filtered)

Kyung-Ju 2016-DKJ.HGN (raw)

Kyung-Ju 2016-DKJ.HGN (filtered)

Fourier Amplitude Spectrum

“Energy was concentrated in high frequency band over 10 Hz; these high frequencies can not excite multi-story building structures effectively”


MKL-EW

33

Design Sa = 0.37g at 0.3 sec for site class S_B

Design Sa = 0.15g at 1.0 sec for site class S_B

Pseudo-acceleration Response Spectrum

“Damage of short-period structures (1~3 story buildings) with poor or brittle construction highly probable as really observed”

“Damage of building structures of average construction with periods longer than 0.3~0.4 sec. difficult to occur”

Comparison of elastic spectral acceleration caused by 912 Geong-JuEQ with KBC (SB) spectrum


EPGA (effective peak ground accel.)

“Apparent PGA by high frequency pulse not damaging”

“Repetitive (effective) PGA in SMD more damaging and meaningful”

0 0.5 1 1.5 2 2.5 30

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8Spectra in a single direction

KBC 2016 SB

MKL E-W

MKL N-S

USN E-W

USN N-S

DKJ E-W

DKJ N-S

MKL E-W MKL N-S DKJ E-W DKJ N-S

EPGA (g) 0.1706 (S_B) 0.184 (S_B) 0.0436(S_B) 0.0993 (S_B)

4 6 8-0.5

0

0.5

4 6 8 10-0.5

0

0.5

6 8 10 12-0.5

0

0.5(per KBC 2016)

EPGA (effective peak ground accel.)

MKL(near epicenter)

DKJ

PGA= 0.26g

PGA= 0.09g“EPGA~ 0.18g (near epicenter)”


“1000yr EPGA~ 0.15g”

PGA= 0.26g -- EPGA= 0.18g (70%)MKL(near epicenter)

교수님,

적분 시 수치를 다르게 집어 넣어서 틀린 값이 나왔었습니다.아래 값이 ePGA입니다.

MKL.KG.BGE = 0.1533gMKL.KG.BGN = 0.1639gUSN.KS.BGE = 0.2925gUSN.KS.BGN = 0.1918gDKJ.KG.BGE = 0.0386gDKJ.KG.BGN = 0.0839g

그전 값에 비해 10%가량 낮은데, 전에 지반조건을 이리저리 다르게하다 보니 다소 오차가 있었던 것같습니다.

Comparison with 1940 El Centro strong motion record

0 10 20 30 40-0.4

-0.2

0

0.2

0.4

Time, sec

Accele

rati

on

, g

El Centro 1940 S00E

Kyung-Ju 2016 USN.HGN

Comparison of two time histories; El Centro

1940 S00E and Kyung-Ju 2016 USN

El Centro 1940 S00E record_ one of the representative strong motion records most frequently used among researchers worldwide

M Epicentraldistance

(km)Comp

PGA (g)

PGV (m/s)

PGD (m) Site

Imperial Valley 5/18/1940, El Centro site 6.9 11.5 S00E 0.348 0.335 0.109 Alluvium

30 times stronger than Gyoeng-Ju EQ

• Demands more for very stiff and brittle structures (with ductility capacity less than 2) • Strength supply required by Geong-Ju USN is much lower for the velocity and

displacement regions

10-1

100

101

0

0.2

0.4

0.6

0.8

1

Period Tn , sec

f y/w

=A

y/g

El Centro 1940 S00E


=1

8

4

2

1.5

8

4

2

1.5

1

Constant Ductility Spectrum

Yield base shear (divided by building weight)

0.2 sec


0 5 10 15 20 25 30

-100

-75

-50

-25

0

25

50

75

100

Time, sec

No

rmali

zed

dis

pla

cem

en

t u

/uy

El Centro 1940 S00E


Yield base shear ratio

fy= 5% W (bldg. weight)

When building period is as short as 0.1sec

Ductility demand ~ 90

Ductility demand~ 12

0 5 10 15 20 25 30-60

-40

-20

0

20

40

60

Time, sec

Norm

ali

zed d

ispla

cem

ent

u/u

y

El Centro 1940 S00E


Ductility demand~ 7

Ductility demand ~ 60

Yield base shear ratio


0 5 10 15 20 25 30

-4

-2

0

2

4

Time, sec

No

rmali

zed

dis

pla

cem

en

t u

/uy

El Centro 1940 S00E



When building period is 1.0 sec (say, 10-story bldg.)

DD~ 4

“Elastic”


-4 -2 0 2 4

-2

-1

0

1

2

Normalized displacement u/uy

No

rmali

zed

rest

ori

ng

fo

rce

f S/f

y

El Centro 1940 S00E


“Elastic”

Failures reported

VI StrongFelt by all, many frightened. Some heavy furniture moved; a few insta

nces of fallen plaster. Damage slight.

VII Very strongDamage negligible in buildings of good design and construction; slig

ht to moderate in well-built ordinary structures; considerable damage

in poorly built or badly designed structures; some chimneys broken.

Intensity Shaking Description/Damage


• Most of average buildings successfully resisted the EQ without any significant structural damage and casualties

• Most of failures occurred in low-rise non-engineered/poor construction and corresponded to the MMI V~VII damage


Show window shattered Typical corner cracking at

opening

Failure in an already poor (non-engineered) construction

Unreinforced block wall fallen down


Damage observed in a 3-story RC Building (Ulju, Ulsan)_ ceiling and brick wall failure


• One of the well-known seismic failure modes observed in a Buddhist temple: so called “short-column” shear failure

• Never imagined to see….in Korea

“Virtually devoid of hoops”

The most impressive failure mode_ “short-column” shear failure


“Two-way shear test”

• Similar short-column shear damage in the piloti column

• Found on September 30 (2 weeks later)• More close investigation needed for

possible structural damage unrevealed yet


Some observations and probable reasons for relatively less seismic damage of building structures in the 912 Gyeong-Ju EQ

1. Apparent magnitude of PGA is a weak damage indicator2. The epicenter was relatively deep (approx. 13~15 km)3. Surface soil layer is generally shallow (about 20 m-deep) and the bedrock motion is

not much amplified4. Energy concentrated in high frequency band over 10 Hz; these high frequency can

not attack building structures with long periods effectively 5. The strong motion duration was very short; 6. Because of the reasons above, damage was mostly restricted to short-period

structures (1~3 story buildings) with non-engineered/ poor or brittle construction.


M 6.2 EQ near Pyung-Yang in 1952 or “during Korean War”

• Known during the technical meeting between south and north Korean seismologists for the KEDO program (supporting program for north Korea’s NPP) once promoted but now halted

• Seismic design is one of the top priority issues in any nuclear power plant construction

• The information following is based on the presentation made by Drs. TS Kang and MS Jeon (former KIGAM researchers) at the EESK symposium last year: “Seismological evaluation of major earthquakes in Korean peninsular and seismic safety of building and civil structures”, Feb. 23, 2016

• The occurrence of this major EQ was not well recognized probably due to the turmoil during the war, but… < 1952 평양 인근 지진의 진앙 (USGS) >Epicenter location

reported by USGS


• Date and Time

• 1952-03-19 09:04:18 (UTC)

• 1952-03-19 18:04:18 (Local Time)

• Epicenter (USGS)

• 38.872N, 125.834E

• Nearby cities

• Chung-HWA: 3 km

• Pyung-Yang: 19 km

• Sariwon: 41 km

• Focal depth : 35.0 km (estimate; appear to be rather deep, less damaging)

• Magnitude reported by various researchers based on measured records: M= 6.2~6.5

RUSSIA_ Rustanovich et al.(1963): M=6.3

CHINA_中国国家地震局科技情報中心(1987) Ms=6.5

Yuche Li (2001): M=6.5

JAPAN_Ishikawa et al.(2008): Md=6.5

USA_ USGS Mw 6.3

KOREA_ Kang (2011) Mw 6.2

(Ishikawa et al., 2008)

1952

19781978

1936

2004

1980

20071996

1982

SOURCE: the presentation made by Drs. TS Kang and MS Jeon at the EESK symposium last year: “Seismological evaluation of major earthquakes in Korean peninsular and seismic safety of building and civil structures”, Feb. 23, 2016

“100 or 1000 times more meaningful than historical EQs since this is instrumental”


V. Summary and Conclusions

• i) The very high uncertainties associated with modern engineering seismology should be and can be absorbed through the engineering means like the capacity design principle.

• ii) The capacity design method, underlying domestic design codes since KBC2009, should be faithfully implemented down to the grass level (or from design to construction) through the participation of structural engineers well equipped with seismic deign.

• iii) The 912 Gyeong-Ju EQ has effectively invalidated the misbelief among many people, even structural engineers, that only weak EQs around M~5 would occur in Korea.

• iv) However, the overall damage caused by the Gyeong-Ju EQ was minor compared to the magnitude because the strong motion duration was short and the seismic energy was mainly concentrated in the high frequency range, thus causing seismic damage mostly in low rise and/or poor building construction.

• v) PGV and PGD observed in the 912 Gyeong-Ju EQ were much lower, compared to the ground motion maxima ratio in California, thus causing much less spectral demand in the velocity and displacement regions

• vi) It is speculated that the 912 Gyeong-Ju EQ records may represent typical characteristics of EQs in Korean peninsular; relatively short strong motion duration dominated by high-frequencies. However ductility demand can be substantial for short-period structures even with a yield base shear ratio as high as 10% building weight

vii) Further extensive studies from the perspective of both seismic design and engineering

seismology are warranted to identify the damage potential factors inherent in the strong EQs

expected to strike our building structures.

viii) The increase of the seismic force level, in spite of 1952 Pyung-Yang and 2016 Kyung-Ju

EQs, is not needed. Seismic capacity against strong EQs lies in well-implemented seismic

design, or ductility design rather than strength or stiffness design.

ix) Proper and cost-effective measures should be taken to salvage poorly constructed and inherently brittle low-rise buildings. Nonstructural damage which may cause injuries as well as malfunctioning due to typical larger amplitude of motion of the upper part of high-rise buildings should be properly addressed.

x) The 912 Gyeong-Ju EQ should be considered as the Early Warning EQ for Korea. All the

lessons learned from this EQ should be reflected in future actions, technological and

institutional, to establish seismically resilient Korea in a socio-economically acceptable way.

xi) We do not live in southern California nor in Japan. Over-threatening of seismic hazard

without clear physical/scientific evidences can lead to the abuse of our valuable national

resources.

END OF PRESENTATION

Analysis of Structural Damage Observed in the 2016 Gyeongju and 2017 Pohang Earthquakes in Korea

Introduction to Earthquake Engineering and Dynamics of Building Structures_ 2

Cheol-Ho LeeDept. of Arch. and Arch. Engrg., Seoul National University


I. Introduction

II. Brief review of 2016 Gyeongju EQ

III. Comparative analysis of damage potentials

IV. Some notes on the 2017 Pohang EQ damage

V. Conclusions

Outline


I. Introduction

• The 2016 Gyongju EQ M5.8_ the first modern damaging instrumental EQ in seismic history of Korea

• Effectively invalidated our misbelief

• The epicenter_ the impact widespread and profound

• Thought to be a rare event

• A big surprise_ M5.4 Pohang EQ occurred just one year later; much more damaging in spite of its lower magnitude M5.4.

Primary objective_ provide probable explanations and reasons for the more severe damage observed in the 2017 Pohang EQ.


“Provided strong ground motion records for the first time and enabled meaningful engineering analysis to be started in Korea”

II. Brief review of 2016 Gyeongju EQ damage and its engineering impact


The most severe damage reported

Damage in non-structural elements prevalent

School building damage reported_ minor and just several cases

Damage observed

The overall damage caused by the 2016 Gyeongju earthquake_ minor,mostly nonstructural


Probable reasons for the relatively minor damage in the 2016 M5.8 Gyeongiu EQ:

The strong motion duration was very short.

Seismic energy was concentrated in the high frequency band over 10 Hz; the surface soil layer in Korea is generally shallow (about 20 m-deep or less) and the bedrock motion is not much amplified

Thus, damage was mostly restricted to low-rise (1 or 2 story high) non-engineered and poor construction.


The impact of the 2016 Gyeong-ju EQ

• The 2016 Gyeongju EQ effectively invalidated the misbelief among many peoplethat only weak EQs around M5 would occur in Korea.

• This earthquake has made Korean government and people admit that earthquake isa real and effective threat.

• The Earthquake Disaster Mitigation Task Committee formed on Sep. 22, 2016 by theMinistry of Public Safety and Security and the experts from academia andprofessional societies: proposed short-term and long-term actions and measures tobe taken


Recommended major measures and actions

1. Overall re-evaluation of governmental EQ emergency management plans2. Enforcing seismic design to all the new housing and small-sized buildings 3. Modernization of seismic codes for some facilities in civil and infra side 4. Improvements of strong motion instrumentation program and early warning system 5. Earthquake response manuals for various purposes and sectors 6. Seismic retrofit for public and private buildings accelerated/encouraged 7. Long-term large-scale investigation of faults and construction of active fault map8. Re-checking seismic safety of NPPs, major industrial facilities and historical buildings 9. Earthquake-preparedness education and drills for the public 10. EQ-related man power and budgets in the government much expanded

(about 100 expects newly hired by the government) 11. Establishment of earthquake refuge facility and rescue system 12. Laws and regulations revision if needed for enhancing national seismic safety, etc.

The major beneficiaries from the Gyeong-ju EQ: the seismologists group; four-stage 20-year project to construct comprehensive fault map under way.


III. Comparative analysis of damage potentials

Source: KIGAM (Korea Institute of Geoscience and Mineral)

ML5.8 (MW 5.4), Focal depth: 13km ML5.4 (MW 5.4), Focal depth: 4km

2016 Gyeongju EQ 2017 Pohang EQ

Real Time Shake Map (PGA)


: Piloti building damage observed in the 2017 Pohang earthquake

Building damage much more severe in the Pohang EQ

: The most serious damage reported in the 2016 Gyeongju earthquake

Damage in the Phang EQ

• Direct damage cost_ 50 million USD (5 times GJ EQ)

• Recovery cost_ 150 million USD (10 times GJ EQ)

• No. of completely-damaged housing_ 331

• No. of Half-damaged housing_ 228• No. of slightly-damaged housing_

25,362

• Public facility damage_ 27 million USD

• School building damage_ 13 million USD

• Seaport damage_ 2.4 million USD• Cultural heritage damage_ 1.4

million USD


ML 5.8 (MW 5.4)

Focal depth: 13km

ML 5.4 (MW 5.4)

Focal depth: 4km

Gyeongju Pohang

Focal Depth_ shallow vs. deep

Shallow EQs produce structurally destructive surface waves more.

“One probable explanation for the more severe damage in Pohang EQ”


SD soil site

Pohang EQ

Gyeongju EQ

Rock-site recording stations (KIGAM/KMA, dense)

Free-field recording stations (Min. of Public Administration and Security, sparse/difficult to access)

SC soil site

USN

PHA2

Rock

Soil

6 Records Measured Yellow box

White box

* Gyeongju USN and Pohang PHA2 records measured on Sc and SD soil site at about 8km epi-central distance_ the most strong and meaningful for engineering analysis


20

d2

T

AI a t t

g

Summary of ground motion characteristics from the 2017 Pohang and 2016 Gyeongju earthquakes

Earthquake 2017 Pohang 2016 GyeongjuStation CHS HAK DKJ PHA2 MKL USN

Epi-distance, km 25 23 28 9 5.9 8Soil condition SB SB SB SD SB SCComponent EW NS EW NS EW NS EW NS HGE HGN

PGA, g 0.018 0.014 0.023 0.035 0.017 0.036 0.131 0.189 0.285 0.351

EPGA, g 0.010 0.012 0.020 0.028 0.006 0.018 0.073 0.098 0.151 0.189

EPGA/PGA 0.547 0.864 0.871 0.807 0.371 0.498 0.555 0.517 0.530 0.538

IA, m/s 0.002 0.002 0.005 0.007 0.002 0.004 0.080 0.158 0.225 0.675

D5-75, sec 6.000 3.650 3.600 1.500 5.750 1.600 0.900 1.400 0.760 1.890

D5-95, sec 16.150 15.800 11.700 8.550 20.150 9.900 2.950 2.450 1.800 10.130

• 2 time in EPA• 4 times in IA• Longer SMD

“Gyeongju USN much stronger”

USN

PHA2

2018 SEEBUS, Kyoto University, Japan, November 2-3

: Comparison of the 2017 Pohang and 2016 Gyeongju earthquakes

Frequency spectra Response spectra

Black line_ Gyeongju recordsRed line_ Pohang PHA2

“High frequency contents over 10 Hz dominant”

“Spectral peak occurred around 2 Hz”

Gyeongjurecords

Pohang PHA2


Record SI (m/s×s)

USNEW 0.1873

NS 0.1196

PHA2EW 0.2319

NS 0.4442

𝑆𝐼 = න0⋅1

2.5

𝑆𝑣 𝜉,𝑇 ⅆ𝑇

Spectrum Intensity(defined by Housner)

* The area under the pseudo-velocity spectrum with periods between 0.1 and 2.5sec _ related to the elastic strain energy absorbed by almost all the building structures

* The best scalar measure for evaluating comprehensive damage potential of a ground motion

Pseudo-velocity= elastic strain energy-equivalent velocity

“4 times larger value”

𝐸𝑠, 𝑚𝑎𝑥 =1

2𝑚𝑠𝑣

2


μ=1

1.5

2

4

8

1

1.5

2

4

8

1

1.5

2

4

8

1

1.5

2

4

8

Comparison of constant ductility Spectrum

5% damping

USN

PHA2

El Centro

Mexico City

• The base shear demand of Gyeongju USN is very high only for very stiff and brittle structures, and becomes almost null in the velocity region.

• PHA2 record is still demanding for mid-period range


IV. Some notes on the 2017 Pohang EQ damage

Source: AIK (2018). a study of damage prevention strategies for earthquake-vulnerable building structures including piloti construction: a report submitted to the Ministry of Land, Infrastructures and Transport.

• Major damage concentrated_ Heung-hae and Jang-sung dong, northern Pohang part, within 10km epi-central distance

• Low-rise piloti housing buildings in Jang-sung dong severely damaged


Taiwn

Southern Calif.

“Relevant Keywords”

• Vertical irregularity• Horizontal irregularity• Torsion• Design/construction errors

Japan 2017 Pohang earthquakeKorea

Piloti building failure_ always warned after any damaging EQ but always repeats itself in next EQ everywhere


Critical design error revealed in damaged piloti structures at Jang-Sung dong

(US approach adopted since 2009 KBC)

“The amplified seismic load combination to supplement current elastic-analysis based R factor approach, or to estimate probable force demand on critical elastic elements at M point”

“M point”

• The amplified seismic load combination for the transfer elements and piloti columns not required before 2009,

• Neglected in design after 2009 in spite of the relevant provisions enforced in KBC 2009

𝜴 = 𝟐. 𝟓

“This load combination not considered in design at all”


Other errors or deficiencies revealed

• Severely eccentric core (staircase structure)

• Non seismic details_ missing cross ties, 90-degree hooks

• Insufficient hoop rebars• and others….

“Damaging earthquakes always reveal design and construction errors hidden”


Epicenter

Jang-sung dong site_ 3km away from epicenter

: Data base on Geotechnical Information Portal System, Korea.

: PGA attenuation observed in Gyeongju EQ (KIGAM 2016)

Generation of Jang-sung dong ground motions for nonlinear dynamic analysis (AIK 2018)

AIK (2018): a study of damage prevention strategies for earthquake-vulnerable building structures including piloti construction: a report submitted to the Ministry of Land, Infrastructures and Transport, Korea.

“Three measured rock motions as seeds”


Base shear level exerted on Jang-Sung dong pilotibuildings

Gyeongju USN_ SC soil, 8km epi. distance

Jang-Sung Dong (avg)_ SD soil, 3km epi. distance

Pohang PHA2_ SD soil, 9km epi. distance (it’s fair to scale up PHA2 by 1.25 for motion at Jang-Sung dong considering attenuation)

Range of “elastic” period of damaged piloti buildings at Jang-Sung dong

“The base shear level exerted on the pilotibuildings_ only 0.50g”


Comparison with Sylmar record

Korean records_ moderate tremors in a stable continental region (SCR), much less damaging than strong interplaterecords like Sylmar

Sylmar record

Gyeongju

Pohang

“Level of base shear_ 3.0 g”

Range of “elastic”period of damaged low-rise piloti housing at Jang-Sung dong

Sylmar NS record from the 1994 Northridge EQ, M6.7, 16km hypo-distance, site class D (deep alluvium over rock)

“Level of base shear_ 0.50g”


A case study_ seismic vulnerability of a low-rise piloti housing_ seriously damaged Crystal Villa

• No amp. seismic load applied in design

• Severely eccentric core (staircase) • Non seismic details_ missing cross

ties, 90-degree hooks• Insufficient hoop rebars• and others….

• Commercial code ETABS used

• Medelling_ as-built conditions_ ASCE 41 (hinge parameter etc)_ ACI 318 (shear strength)

• Input_ original PHA2 record having freq. contents as given by the nature

• Bi-directional input 3D analysis

All design/construction errors and deficiencies:

“Hoop spacing too wide”


Wall_ fiber elements and elastic shear spring Simple DCR analysis

“All the columns modelled as brittle in shear_ Condition III”


Name DCR(= Vu/Vn)

C1 1

C2 1

C3 0.86

C4 0.96

C5 0.32

C6 0.83

C7 0.11

C8 0.59

C9 0.09

C10 0.58

[Column shear DCR]

Name DCR(=Vu/Vn)

W1 1.22

W2 1.06

W3 1.47

W4 0.91

[Wall shear DCR]

80% PHA2 Intensity

= Column failure in shear

uy,max=9.58 mm

[1st story displacement time history_ Y-axis]

Max. SDR= 0.37%

At 80% PHA2 intensity, the columns farthest from the core started to fail in shear


Name DCR(=Vu/Vn)

C1 1

C2 1

C3 1

C4 1

C5 0.42

C6 1

C7 0.14

C8 0.75

C9 0.11

C10 0.73

Name DCR(=Vu/Vn)

W1 1.50

W2 1.34

W3 1.84

W4 1.24

100% PHA2 intensity

[Column shear DCR]

[Wall shear DCR]

= column shear failure

Wall cracking observed

Column shear failure predicted

Column shear failure reported (AIK 2018)

C2

C1

C4

C3

C6

[1st story displacement time history_ Y-axis]


[Column shear responses]

C2 C4

C1 C3

Predominantly bi-axial

Predominantly uni-axial

100% PHA2 intensity


* Shear hinge parameters calculated per ASCE 41-13

Name DCR(=Vu/Vn)

C1 1

C2 1

C3 1

C4 1

C5 0.76

C6 0.87

C7 0.30

C8 0.60

C9 0.09

C10 0.59

Name DCR (=Vu/Vu)

W1 0.71

W2 0.70

W3 0.81

W4 0.67

“Wall_ cracked, but shear demand less than ultimate strength”

= Column shear failure predicted

100% PHA2 analysis with including wall nonlinearity in shear

Including wall nonlinearity did not affect the overall column shear failure pattern.


C2

C1

C4

C3

C2

C1

C4

C3

As a result of adding wall nonlinearity in shear, the bi-axial behavior weakened


Source: KIGAM (Korea Institute of Geoscience and Mineral)

KIGAM Report (2018)on Large-Scale Soil Condition Survey Results in Pohang Area

Sedimentary soil thickness

Landfill thickness Weathered rock

thickness Sedentary deposit

thickness

Depth to bedrock Vs30 Site period (Tg)


The fatal piloti building damage at Jang-Sung dong was a result of unfortunate marriage of bad structure and bad soil.

The severely damaged Jang-Sung dong area_ a spot of deeper alluvium with resulting longer ground period and low shear wave velocity

Jang-sung dong

Jang-sung dong

Jang-sung dong


2017 Pohang EQ (ML5.4, MW5.4)

2016 Gyeongju EQ (ML5.8, MW5.4)

Well-known among geotechnicians and geologists:• Gyeongju_ generally rocky (granite) region• Pohang_ largely of sedimentary area (sea about

10 million years ago)

Overall Ground Soil Conditions of the Two Cities

Much more extensive damage in the Pohang EQ more easily understandable when considering urban-scale soil conditions

2018 Hualian Earthquake


Gyeongju?

Pohang?

From Gyeongju?

From Pohang?


: Geographical distribution of characteristic ground period (estimate per microtremor)

“No. of stories of collapsed buildings:11~12

Characteristic ground period:

0.8~1.2 sec

“Several piloti multi-story buildings collapsed”

High correlation between the characteristic ground period and the fundamental period of collapsed piloti buildings


“Almost the same phenomenon occurred at two different places of this globe”


V. Summary and Conclusions

This studied comparatively analyzed the damage potentials of the two recent Gyeongjuand Pohang EQ based on simple frequency and time domain analysis of measured ground motions.

Some seismo-geotechnical perspectives were also presented to better understand the more severe damage observed in the Pohang EQ in spite of its apparent lower magnitude than the Gyeongju EQ

The 2017 Pohang earthquake clearly showed that fatal damage to poorly engineered and constructed piloti buildings is highly probable, when subsoil is soft, the epicenter is close, and the hypocenter is shallow, although the earthquake is just moderate (M 5+) and of short duration.


seismic hazard and state of the art seismic design practice in … 0... · 2019. 9. 4. · seismic...

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