Download Building Performance in the Taiwan Earthquake

Building Performance in the
Taiwan Earthquake
A Review of
Nantou
County
by Peter Yin
March 2000
City of Los Angeles
Department of Public Works
Bureau of Engineering
City of Los Angeles
Mayor Richard Riordan
Board of Public Works
Ellen Stein, President
Valerie Shaw, Vice-President
Maribel Marin
Tod Burnett
Woody Fleming
Bureau of Engineering
Vitaly B. Troyan, City Engineer
Acknowledgments
The author wishes to express his appreciation to the City of Los Angeles
Board of Public Works and the Bureau of Engineering for their support of
the relief mission to the Taiwan Earthquake. He is also grateful to the
Taipei Economical and Cultural Offices in Los Angeles for their coordination of all arrangements for the trip.
Particular acknowledgment is due to fellow relief team members: Mr.
Henry Huang of the Los Angeles County Public Works Department; Mr.
Albert Chen of Black & Veatch; and Mr. Albert Tsai of W. E. Moscicki and
Associates, and to Dr. Jing-Wen Jaw of the City of Los Angeles Bureau of
Engineering for their review and critique of this paper.
Thanks also go to Winifred Harano and the Administrative Services
Division of the Bureau of Engineering for assistance in the editing, layout
and production of this report.
Building Performance in
the Taiwan Earthquake
A review of Nantou County
by Peter Yin, PE, SE
Preface
The devastating earthquake that struck central Taiwan on September 21, 1999, was the
largest in recent Taiwan history. Measured at
7.6 on Richter Scale, it killed more than 2,300
people, destroyed 10,000 buildings, seriously
damaged many streets, bridges, public facilities, power distributions, and industrial operations. The initial estimate of reconstruction
cost was well beyond one hundred billion U.S.
dollars.
In responding to the disaster, structural
engineers in Southern California formed a relief team to assist the Taiwanese people. With
the emphasis on the experience in reinforced
concrete structures and high-rise building design background, a group of eight structural
professionals from public agencies and the pri-
vate sector across the Southland, were chosen.
I was one of the members. The team was given
one of the most challenging tasks: Assigned
to Nantou County, the epicenter to evaluate
mid-rise/high-rise buildings, examine governmental buildings, schools, hospitals, libraries, and review low-rise residential buildings.
From October 6 to October 10, 1999, we visited six hard-hit cities or townships, reviewed
and inspected more than one hundred buildings. We eyewitnessed the ground rupture, the
wiping out of entire blocks, and the destruction of all kinds of structures, large and small,
public and privately-owned. We also saw
many buildings with minor or no damage, in
the same neighborhoods as those that failed.
The difference in these buildings may have
been just due to engineering, but the results
between those that succeeded and those that
failed was life or death.
This report, reflecting my personal observation and opinion, is focused on buildings
in Nantou County. No detailed geotechnical
data or seismology theory will be discussed
here, except for a few instances where it is
closely associated with the building case studies included in this report. By reviewing these
cases and discussing their successes and failures, it is hoped that we
would have a better idea on
what worked and what did
not in the Taiwan earthquake.
And we all know that what
happened in Taiwan yesterday, might happen to California tomorrow.
Buildings in Taiwan
Taiwan is an island country
situated in a humid subtropical region approximately
8,000 miles west of Los Angeles. It is one fourteenth the
size of California but has a
population of 22 million, or
about two thirds of California. Almost two thirds of the
1
island are mountainous area or hillside, thus
land is extremely valuable. Because of the economic benefits, an “Open Space” policy was
initiated by the government which allowed
building owners to build more square footage vertically as long as the space on the street
level was open to the public. This, however,
created a weak-story for a vast majority of
buildings. Further, because of the humid environment and cost considerations, excepting
high-rise buildings in major cities, almost all
buildings are made of rigid concrete frames
with non-reinforced brick in-fills. These in-fills
played a crucial role in the Taiwan earthquake.
They might have either saved lives or destroyed the building, depending on how they
were used.
Building design and construction in Taiwan
Taiwan’s building code closely resembles the
UBC and ACI practiced in the United States.
(See Figure A). In fact, many of the university
professors, engineers, architects, and governmental officials studied and even worked in
the U.S. before they went back to Taiwan.
During our visit to Taiwan, our team had opportunities to talk to professionals in Nantou
County and the City of Taipei. With the qualification of our team having two members who
were in fact chairs of subcommittees of the
Structural Engineers Association of Southern
California, we were impressed with the
knowledge and ability of the professionals we
met. It is obvious that there are many qualified professional engineers and geologists in
Taiwan, practicing a code similar to the building code in the United States.
However, in Nantou County the architect
is in charge of the structural design for any
building whose height is less than 35 meters.
He/she employs the engineers and geologists
to perform structural calculations or soil reports, sometimes under cutthroat type bidding wars. The structural engineer and the soil
engineer have no authority in building design
and construction in these cases. (In most metropolitan areas in Taiwan any building taller
than 35 meters, or seven-stories, are mandated
to be designed by structural engineers and
reviewed by a structural review committee
formed of local professionals.)
The architect would incorporate the
sketches and details from the engineers to his/
her plan, and apply for building permits from
the government.
Figure A (Data source: UCSD Ji-Ji, Taiwan Earthquake of September 21, 1999.)
Recommended design base shear by building code.
2
Figure B (Data Source: Central Weather Bureau,
R.O.C.)
After the permit was obtained, the
owner/developer took over and hired a contractor. The developer and the contractor both
should have licensed engineers to interpret the
plan and execute the project by regulation. The
owner normally hired his architect as
representative.
The local enforcement agency
would inspect the construction at few
key stages, though in most cases, it
was never as thorough, nor with the
follow-through as practiced in the
United States. Construction quality
control has been largely the responsibility of the architect, who might or
might not receive cooperation from
the contractor.
Another common practice was
the use of subcontracts. Taiwan is no
exception in this regard. After the contract was awarded, the general contractor would sub the project out to
several low bid subcontractors, whose
performance would really determine
the quality of the construction. In such
cases, if a stringent inspection system
was not implemented or enforced by
the government, the quality of the construction most likely would be in serious jeopardy.
The Earthquake
Taiwan is situated in a subduction zone between the Philippine Sea Plate and the Eurasian Plates. (See Figure B) The East Coast experienced most of the serious ground motion
Figure C
3
in the past, while the Nantou County at the
central part of Taiwan, was considered a moderate seismic zone. The strong ground motion
that ruptured at Hsuangtung Fault and the
Chelungpu Fault, (See Figure C) had the initial rumble measure 7.6 on the Richter Scale,
followed by three aftershocks bigger than the
Northridge Earthquake (6.8) in the first 48
hours, and 9,000 aftershocks in the next two
weeks. In the cities we visited, many of the
areas experienced Peak Ground Acceleration
of 0.5g in the horizontal direction, and a 0.3g
in the vertical direction with the duration exceeding 30 seconds, according to data from
the Central Weather Bureau of Taiwan.
The relief team visited three cities:
Nantou, Puli, Tsao-Tun, and three townships:
Ming-Jen, Chung-Liao and Ji-Ji. The last one
was the quake’s epicenter.
How the buildings were evaluated
This report discusses the performances of
buildings in Nantou County in 8 categories:
1. 3 to 4 story non-ductile, concrete frame
buildings.
2. Low-rise commercial buildings.
3. High rise buildings
4. School buildings
5. Municipal Buildings
6. Hospitals
7. Libraries
8. Convalescent Homes
Since there are so many buildings, only a few
examples will be given of each. Each instance
will refer to the five principles in the buildings seismic design: building configuration;
structural redundancy; structural continuity,
ductile detailing and construction quality.
4
Building Case Studies
Section 1
Three to four story, non-ductile, concrete framed
buildings.
These types of buildings are commonly built for residential/commercial use and are most
popular in Taiwan. Usually a family occupies the upper floors of the unit with a high percentage using the ground level for commercial purposes. The following were observed:
Features:
Outcomes:
1. Wide open store front at street level.
1. Creates weak story/soft story on the first
floor.
2. Brick in fill between units.
2. Very strong resistance to earthquake forces
in the longitudinal direction.
3. Reinforced concrete frame and pour-inplace slab.
3. Load paths from floors to frames have no
problem.
4. Insufficient column size, low brick wall 4. Brick wall above beam creates strongadded to the beam at building front.
beam/weak-column system; column will
fail by shear or flexural forces.
5. Beam reinforcement extends to the face of 5. Insufficient lap length and concrete cold
building for future add-on neighboring
joint on concrete beam bad for load paths.
building.
Re-bar splice at joint against ductile detailing. Buildings banged to each other due to
these weak links; impact was greater when
adjacent buildings were of different
heights.
6. Illegal addition to the top.
6. Further burdens the structural system
which was not built seismically sufficient
to start with.
7. Concrete stairway in the middle of floor 7. Acting as K-truss to resist transverse (seisplan.
mic) forces.
8. Brick in-fill wall at the rear of building.
8. Acting as shear wall may have saved many
buildings from collapse.
5
1-1 (Puli) Beam reinforcements extended for future neighboring building. Noted column width (parallel to the street) reduced to the minimum to increase the store open
space.
Typical 3-4 Story Concrete
Buildings.
1-2. and 1-3 (Puli)
Weak story/soft story
and lack of ductile detailing made these columns vulnerable,
which in turn could
cause building collapse.
6
1-4 (Puli) Stairway at
middle floor plan provides lateral support to
the building. See typical
floor plan
1-5 (Puli) Though not by design,
the brick wall at rear was the actual lateral support to the building. See typical floor plan.
3-4 Story Building Typical Floor Plan
7
1-6 and 1-7 (Puli) Ductile detailing was absent.
8
1-8 above (Chung-Liao, 10km from epicenter) All buildings on the block lost their first floor. On
average, every family lost a member.
1-9 (ChungLiao) Despite
the loss of the
first floor, this
was the only
building on the
block still
standing.
9
1-10 above (Nantou) Five-story building showed column flexural failure. Note
masonry wall added to top of girder.
1-11 right (Nantou) Stirrup space too
large, electrical conduit reduced the column effective area.
1-12 below (Nantou) Shear failure at
column and wall in this five-story building.
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1-13 (Nantou) No
seismic detailing.
1-14 right (Chung-Liao)
Column flexural failure in
this three-story building.
11
1-15 Three-story apartment at Ji-Ji, the epicenter.
1-16 Detail of flexural failure at column. No ductile detailing, weak-story.
12
Section 2
Low-Rise Commercial Buildings
Due to Taiwan’s “open space” policy and for commercial reasons, most buildings have wideopen space on the street level. This, along with in-fill-brick walls added to the floor beam and
lack of ductile detailing, caused numerous commercial buildings to collapse when their first
floor failed. Unlike residential buildings, which have stairways and brick walls (in the rear)
as redundancies, commercial buildings were mostly supported by columns.
2-1 and 2-2 (Puli) Eightstory twin buildings lost first
floor. Note that the columns
had no seismic detailings.
13
2-3 and 2-4 (Puli) Detail of the twin buildings showed
lack of seismic ties. Many required detailings were missing in the columns. Concrete quality was questionable.
14
2-5 Commercial buildings at Puli. Like many others, these two buildings lost their first floor due to
weak story/soft story.
2-6 Market at Puli. The buried first floor was normally jammed with people during business hours.
Luckily, the quake occured at 1:47 am.
15
2-7 (Puli) Another example of the devastating effect of the “strong beamweak column” frame system.
2-8 This street in Puli is a perfect example of structural continuity problems. All buildings in this
block lost the first floor due to weak story. Meanwhile, the shorter building (built at a different time) hit
the adjacent taller building when movement occurred in a different direction. The deformation of the
taller building demonstrates this. (Also see Section 1, Feature 5 for comments.)
16
2-9 Seven-story commercial/residential building.
See details below.
2-10 and 2-11 Column shear failure in this building. The
building did not collapse due to the large amount of structural redundancies.
17
Section 3
High-Rise Buildings
Most of these buildings were built over the past 5 to 10 years. Reinforced special moment
resisting frame system was supposed to be used. However, no ductile detailing was found in
many columns abutting the street. The unfavorable locations on the floor plan and poor detailing made these columns vulnerable. Lack of quality control also contributed to many column failures
3-1 and 3-2 Twelve-story commercial building in Nantou.
Due to wide use of brick in-fill
walls throughout every floor,
this building only suffered
damage to columns on the first
floor, street front.
18
3-3 and 3-4 (Nantou) Columns showed crowded reinforcements. Utility and
drainage conduit took away
further space. There were no
seismic ties, and improper
lap splices. Also, concrete
quality was questionable.
19
3-5, 3-6 and 3-7 (Nantou) Similar problems occurred in this building as those in 3-1 and 3-2. Columns showed poor construction as well.
20
3-8 at right (Puli) Sixteen-story Kuo-Pao
Plaza
3-9 Largest drift occurred at the east
corner. Southeast column damages
should not be surprising. Note that stirrups were missing. (See Typical Floor
Plan)
3-12 Column at middle of south wing
was damaged due to bad detailing.
Shear failure occurred above the short
brick wall.
21
3-10 Wall elements suffered most
damage in the north wing where
the floor plan bent. Note that the
shear wall on the first floor was
severely damaged and removed.
In the photo a worker is starting
to rebuild the wall. (See Typical
Floor Plan.)
3-11 Though the concrete
peeled off, this link beam in the
north wing was considered to
be successful.
22
Typical Floor Plan
Indicates beam column rigid frame system with brick-in-fill at all places. “C” shape configuration and bend at plan layout caused plan irregularity. The areas that suffered damage
were predictable.
23
Section 4
School Buildings
Tsau-Tun Trade School
The school construction has several features:
a.
b.
c.
d.
Non-ductile concrete frame throughout.
Brick in-fill wall at transverse direction full weight.
Short brick wall above girders at longitudinal direction.
Column weak axis bends along longitudinal direction. Building has no shear wall in this
direction.
4-1 Three-story moment frame system. All columns became “short column” due to the built-up of
short brick walls on girder. Structural system had no chance to deform and to absorb energy before the
sudden shear failure at columns. These short brick wall was never considered in the design analysis.
24
4-2 A close up of the first
column of building on
previous page.
4-3 Show failure at “d” from joint face.
4-4 A combination of shear and compression failure on columns.
25
4-5 The result of a “weak story”.
4-6 Note the short wall above girder, second-floor made column flexural failure inevitable.
26
4-7 Shear failure at “short column.”
4-8 Deformed columns.
Ductile detailing would
have mitigated the damage.
27
Section 5
Municipal Buildings
Almost all governmental buildings were non-ductile concrete moment frames with brick infill walls. Poor construction quality and “short column” were common problems at these
buildings.
5-1 Puli City Hall. This three-story reinforced concrete building pancaked. The high brick in-fill wall
made the column very short. Columns were sheared off before any frame deformation could start.
5-2 Tsau-Tun City Hall had a better fate. Structural redundancy and flexibility made the difference.
28
5-3 Longer columns
between floor (deformed
before shear failure) and
redundancy was observed at this building.
(Tsau-Tun City Hall)
5-4 Brick in-fills
were the first
defense of this
building. (TsauTun City Hall)
5-5 Ground
rupture in front
of Tsau-Tun City
Hall. An even
larger rupture
tilted the building
in the back of this
picture.
29
5-6 and 5-7 Puli Police Headquarters had the same column shear failures as City Hall.
30
Section 6
Hospitals
Puli Veterans Hospital
This building, completed within 10 years, was not a success. It was constructed as a dual
system, but neither the shear wall nor the frame has a ductile detailing. The shear wall was
lightly reinforced with only one curtain rebar and no boundary member. This building did
not collapse due to the large amount of structural redundancy.
6-1 A vertical irregularity
(Two-story high lobby at entrance) caused all shear walls
at the first and second floors
to crack. No diagonal reinforcement was provided at
shear wall between windows.
6-2 Columns at lobby
area were sheared off. No
seismic details were provided.
31
6-3 Building side view. Veneers were peeled off after concrete wall cracked.
6-4 There is no boundary member noted at the inner side of shear wall. Light reinforcement has
only one curtain re-bar.
32
6-5 The interior shear wall has no required seismic detailing.
6-6 Columns (no ductile detailing) damaged after shear wall failed.
33
Section 7
Libraries
Nantou County Library
This eight-story reinforced concrete special moment resisting frame building was still waiting for final inspection and approval when the earthquake struck. All interior and exterior
walls were non-structural and were not designed for seismic resistance. Plan irregularity indicated that torsion would govern (see page 37). Despite walls cracked at south and east
sides, this project was considered a success. Quality of construction was good.
7-1 Cracks are evident in south elevation of the Library wall.
34
7-2 and 7-3 Damaged walls were not designed as
lateral force resisting elements.
35
7-4 A look at the same damaged
wall from the inside. Note that the
columns next to the wall panels
have no damage.
7-5 Northeast corner of building.
The only other place that exhibited significant damage (nonstructural).
36
7-6 Even though the ceiling fell apart, no cracks
at the column or beam
were observed.
Plan
This special moment resisting frame building has plan irregularity. Centroid of rigidity is at
northwest portion on the plan, thus the south and east exterior walls suffered the most damage.
37
Section 8
Convalescent Homes
Jeri Retirement Center - A successful example
Jeri Retirement Center, in Nantou City, has three large non-ductile reinforced concrete frame
buildings with brick in-fills. It suffered no damage in this earthquake. The construction quality was good (the owner was familiar with construction practice), the building had regular
configuration, and brick walls provided extra redundancy.
The brick in-fills shown on the plan
were constructed from ground to
roof. Regular configuration and excellent wall layout made the building resistent to damage.
Plan
Typical floor plan
38
8-2 and 8-3 Elevation views of the other two buildings.
39
Observations and
Conclusions:
During our team’s stay in Nantou, only a limited number of buildings could be observed.
However, based upon the magnitude of the
devastation and how wide spread the damaged area was, it was clear that the Taiwan
earthquake was much stronger than the
Northridge quake.
The following was observed:
6.
7.
1. Near the fault line rupture, ground motion
could vary significantly within a very short
distance. If a building performed well, it
does not necessarily mean that building
was designed and built better than those
that collapsed at nearby sites.
8.
2. Despite the magnitude of the seismic force,
well-designed shear wall and special moment frame system proved to be most effective. Except for those near the fault line,
no single structures designed and built per
9.
building code was found to have collapsed
in Nantou County.
3. Partial height wall/brick in-fill significantly changed building behavior. One of
the most damaging factors in this quake
was the failure of “short columns” created
by these low walls. The building failed instantly due to a frame system that had no
chance to deform and absorb the energy.
4. Full height non-reinforced brick in-fills, despite it’s low engineering value, clearly
prevented many buildings from collapse
and thus saved lives. The importance of
redundancy in building design can not be
overemphasized.
5. Ductile detailing is essential for rigid frame
buildings in seismic design. Many building would have survived had the code requirements been followed. The low-rise
residential/commercial concrete buildings
40
not built according to code suffered the
most casualties.
Lack of structural continuity also caused
many building failures of 3 to 4-story structures, especially where the buildings were
of different heights. The impact of building collisions was tremendous, as illustrated in Section 2 of this report.
Liquefaction caused foundation settlement
in a significant number of buildings. Many
buildings were built around fault lines and
the failure of these structures was inevitable. Geotechnical data and strict regulation regarding these issues should be established and strictly enforced.
The authority and responsibility of structural and geotechnical engineers should be
established. Recognition of the importance
of these disciplines is essential to public
safety, as demonstrated in this earthquake.
Ignorance in design practice and poor
quality in construction was observed.
Implementation of a plan review, permit
processing and inspection system, similar
to the one in the City of Los Angeles is recommended.