Transcript Tall Building Collapse Simulations in the near

Future Earthquake Shaking in the
Los Angeles Region
Thomas Heaton (Caltech)
Anna Olsen (Univ. of Colorado)
Masumi Yamada (Kyoto Univ.)
Key Issues
• I will concentrate on ground shaking and its impact on
buildings. Stiff buildings vs. flexible buildings
• Modern high-rise buildings and base-isolated buildings
have not yet experienced large long-period ground motions
(pgd > 1 m).
• But they will
• Is statistical prediction of long period ground motions
technically feasible?
• Maybe … but it will look very different from psha for short
periods
• Will the design of long-period buildings change dramatically
in the next 100 years?
• “Excuse me senator … could you repeat that question?”
How do buildings resist earthquake forces?
Front View
Top View
Image: Courtesy EERI
Tall Building Anatomy
Different Types of Buildings Have very
Different Characteristics
• Wood frame houses are light, stiff, strong, and ductile (very good in
earthquakes).
• Un-reinforced brick buildings (pre 1935) are heavy, stiff, weak, and brittle
(very poor in earthquakes).
• Concrete shear-wall buildings are heavy, stiff, strong and fairly ductile.
Can be very good, but must be strong and ductile enough to handle high
accelerations.
• Moment-resisting frame buildings (most high-rises) are light, flexible, and
weak. Pre 1975 concrete frames and pre 1995 steel frames are far less
ductile than intended.
• Pre 1975 concrete frame buildings are potentially very dangerous in
moderately strong shaking.
• Modern high-rise construction performs well for high-frequency motions,
but is vulnerable when ground displacements are large, or when the
ground “resonates” at the natural frequency of the building.
Flexible, but brittle, pre 1975 concrete frame
One of the great disappointments is that there has been little progress in
the retrofitting of “nonductile” concrete frame buildings. Most people who
live or work in them are not aware of the serious risk involved.
Flexible Steel Frame
Plastic Hinge of Steel Beam
special post 1994 connection
Cucapah-El Major M 7.2 earthquake
Easter 2010 in Mexicali
John Hall’s design of a 20-story steel
MRF building
•Building U20
1994 UBC zone4
Stiff soil, 3.5 sec. period
•Building J20
1992 Japan code
3.05 sec period
Similar to current IBC with
highest near-source
factor
•Both designs consider
Perfect welds
Brittle welds
Pushover Analysis
Pushover Curves
40
U20B
U20P
U6B
U6P
J20B
J20P
J6B
J6P
35
Base Shear (frac of building weight) %
•Special attention to Pdelta instability
•Story mechanism
collapse
•Frame 2-D fiberelement code of Hall
(1997)
•2 m roof displacement
is near the capacity of
any of these designs
30
25
20
15
10
5
0
0
50
100
150
200
Lateral Roof Displacement cm
250
300
20-story steel-frame building subjected to a 2-meter
near-source displacement pulse (from Hall)
• triangles on the frame indicate the failures of welded column-beam
connections (loss of stiffness).
Ph.D. Thesis of Anna Olsen, 2008
• collected state-of-the-art simulations of crustal
earthquakes
• 37 earthquakes, over 70,000 ground motions
– 1989 Loma Prieta (Aagaard et al., 2008)
– 1906 San Francisco, with alternate hypocenters (Aagaard
et other al., 2008)
– 10 faults in the Los Angeles basin (Day et al., 2005)
– Puente Hills fault (Porter et al., 2007)
– TeraShake 1 and 2 (K. Olsen et al., 2006, 2007)
– ShakeOut, from Chen Ji
• Moment magnitudes between 6.3 and 7.8
• Long-period (T > 2 s) and broadband (T > 1 s)
• PGD and PGV calculated from vector of northsouth and east-west components
• Synthesized
ground motions
• 30% probability
of collapse
contours
• pgv > 1 m/s gets
the building to
yield
• and pgd > 1 m
collapses it
From Anna Olsen
•Severe damage or
collapse in many areas
•Stronger, stiffer building
(J20) performs better than
more flexible building (U20)
•Brittle weld buildings 5
times more likely to
collapse than perfect-weld
buildings
•Results summarized in
Olsen and others (BSSA,
2008)
Large displacements can overwhelm base
isolation systems
•
•
2-meter displacement pulse as input for a simulation of the deformation of a 3story base-isolated building (Hall, Heaton, Wald, and Halling
The Sylmar record from the 1994 Northridge earthquake also causes the building to
collide with the stops
3-sec spectral displacement
• Typical base isolator is
3 sec with a maximum
allowed displacement
of 40 cm
• Nonlinear isolator
displacements exceed
linear by 20% to 40%
(Ryan and Chopra)
• Described in Olsen and
others (BSSA, 2008)
• Anything in yellow or
red would exceed
current typical base
isolation system
meters
Maps of Building Responses
• M 7.15, Puente Hills fault
• 6-story, more flexible design
with sound welds
• Colors follow FEMA 356:
–
–
–
–
–
Blue: peak IDR < 0.007
Green: peak IDR > 0.007
Yellow: peak IDR > 0.025
Red: peak IDR > 0.05
Pink: simulated collapse
Shakeout Simulation (Aagaard and
Graves)
Response of three different
20-story buildings with and without
Brittle welds to the Shakeout motions
(Swaminathan Krishnan
•
•
•
•
PEER Tall Building Initiative to conduct performance based analysis of three 40story buildings in downtown LA (5 ½ to 6 s fundamental periods).
“Working with engineering consultants and experts experts at SCEC, we selected
records to represent frequent (25-yr) and extremely rare (4975-yr) shaking. The
latter is well beyond the shaking level commonly considered.”
“the code-designed cases (2006 IBC, 2008 LATBDC) have acceptable performance
under the 475-yr motions, and survive the 2475-yr motions. Under the 4975-yr
event, however, some elements may fail.”
Similar study of One Rincon Hill in downtown San Francisco for similar building
design.
Spectral acceleration, g
•
•
•
15 realizations of spectrum compatible motions used by PEER Tall
Building Initiative for 40-story (6 second) building analysis in
downtown LA
These are the Maximum Considered Earthquake Spectra (MCE) for a
2,476-year repeat (life safety level
This project is considered as a PEER/SCEC collaboration
PEER Spectrum Compatible 2,500-yr Ground Velocities
for 40-story 6-second Building in Los Angeles
1.0 m/s
-1.0 m/s
PEER Spectrum Compatible 2,500-yr Ground Displacements
for 40-story 6-second Building in Los Angeles
1.0 m
-1.0 m
PEER Spectrum Compatible 2,500-yr Ground Displacements
for 40-story 6-second Building in Los Angeles
1.0 m
1 meter pgd is
considered as
extreme ground
motion
-1.0 m
1906 San Francisco Ground Motions
•Magnitude 7.8
•Same slip distribution, three hypocenter locations
•Long-period PGD exceeds 2 m near the fault
•Long-period PGV exceeds 1.5 m
•Simulations by Aagaard and others (BSSA, 2008)
All strong motions recorded
at less than 10 km from
rupture from M>6
From Masumi
Yamada
•Near-source
pga’s are lognormal
•Same distribution
will apply 100
years from now
Short periods are Gaussian statistics
• Can reliably determine the mean and standard
deviation
• How many people will die in auto accidents?
• How many people will suffer a heart attack?
• How many buildings will experience some
level of pga?
•Long-period ground motions
are not log normal
•A few large earthquakes can
completely change the
distribution
•Cannot predict what the
shape of this distribution will
look like 100 years from now
•Area(M)~10M10-bM=constant, if
b=1
•i.e., given that a fault slips, all
values of slip are equally likely
•The small pgd’s will come in a
few at a time as smaller but
numerous eq’s occur
•The large pgd’s will arrive in a
large clump when infrequent
large eq’s occur
Long Periods are power law statistics
• Probabilities are difficult to estimate for
power law.
• How many people will die in
• A war?
• A pandemic?
• What will your 401k look like in 20 years?
• 40 years of
strong motion
recording
• What will this
distribution
look like in AD
4,500?
• Have SCEC
scientists really
meant to say
that the 5,000yr pgd is 1.2m
in LA?
PEER LA 5,000 yr
Graves near-source
PEER LA 2,500 yr
Existing Near-source
records
PGD per unit fault slip
Near-Source PGD’s
are roughly 2/3 of
the fault slip in
nearby segments.
1
0.9 Y=0.7/(1+(0.13X)1.6)0.5
Displacement
Slip (m)
(m)
0.8
0.7
model
dip=45
dip=60
dip=75
dip=90
But what will
the fault slip
be?
0.6
0.5
0.4
For strike-slip fault,
0.3
0.2
-30
-20
-10
0
10
20
Distance from the fault (km)
(Aagaard et al. 2001)
30
Predicting Near-Source pgd’s
• Must predict fault slip amplitude on known and unknown
faults
• Use of magnitude desensitizes the analysis to fault slip … even
smaller events may have large slips
Concluding Remarks
• 2,500 yr probalistic prediction based on 40-years of collection of strong
motion data
• Implies that we know the rules of the game and that we will not change
our minds
• Can save a bunch of money by stopping funding of PSHA research … the
problem is basically already solved
• Votes from a committee of experts … If we took a vote of a such a
committee in 1980, would it reach the same conclusion as a committee
today? How about in 2040? What about 3040?
• Scenarios are criticized because no one knows how to assign a probability
… somehow with PSHA this problem magically goes away
• If you believe that, then I’ve got some “highly rated” mutual funds that I’d
like to sell you … the probability that they’ll crash is negligible
• The honest answer is … “WE DON’T KNOW” … there should not be any
long-period Natl. PSHA maps, they are only misleading
What Government Policy Actions are
Needed?
• Government should ask three professional communities
(earth science, academic engineers, practicing engineers) to
jointly provide a frank assessment of the impact of plausible
future earthquakes (e.g. Puente Hills Thrust M 7.0).
• Occupants of buildings should be provided with a mechanism
for recognizing deficient buildings.
• Non-ductile concrete buildings are a widely recognized major
hazard, and there should be a program to deal with them.
• There should also be a program to deal with brittle welds in
pre-1995 steel frame buildings.