What Will a Large Earthquake be Like?

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What Will a Large Earthquake be Like?

• Tom Heaton
• Caltech

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Magnitude Paradox Seismologist

• Radiated energy increases by 32 times for each
  unit of magnitude.
• The number of earthquakes decreases by 10 times
  for each unit of magnitude.
• In California, most of the energy is in
  earthquakes with magnitudes larger than 7.4.
• Large earthquakes do most of the work of plate
  tectonics. Although they are infrequent, they
  are inevitable.
• After the M 6.7 1994 Northridge earthquake
  seismologists said, this was only a moderate
  earthquake wait till you see a great one.

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Magnitude Paradox Engineer

• Perhaps there can be larger motions, but these
  are extreme examples of extraordinary events that
  shouldnt be used for building design.
• Eyewitness reports of the 1906 earthquake
  indicate that the shaking was comparable to that
  in 1994, but it lasted longer and occurred over a
  larger area.
• Computer models shows that most of the risk comes
  from more frequent moderate size events.
• This building is designed for a M 8, the largest
  possible earthquake!

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Current model says most of the threat to downtown
LA is not from the San Andreas fault
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1906 M 7.8 San Francisco earthquake rupture with
large ground displacement. Notice that the farm
buildings were largely intact.
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Pt Reyes Station 1906
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Current Building Code

• Current building codes are mostly prescriptive
  rules based on the building type and seismic
  zone.
• Codes have been developed by fixing deficiencies
  from past earthquakes.
• If youve got a good building code, who needs a
  seismologist?

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How do buildings resist earthquake forces?
flexible
stiff
Front View
Top View
Wooden Houses 7-11 stores Caltech
High-rise buildings
Tall Building Anatomy
Image Courtesy EERI
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Flexible or Strong?

• Stiff buildings tend to have high stresses, and
  must therefore be strong.
• Making a building strong increases the stiffness,
  which increases the stresses, which increases the
  required strength of the building (a vicious
  circle).
• Making a building flexible tends to decrease the
  stress, but it also decreases the strength of a
  building (another vicious circle).
• Tall buildings are always designed to be flexible.

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Lateral stiffness relies on flexing beams
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Large deformation should result in bent steel
beams
From the lab of Chia-Ming Uang, UC San Diego
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Tall buildings cannot withstand large Tilts

• Integrity of the columns is critical.
• Gravity loads are normally axial compressional
  loads on the columns.
• Tilted columns result in bending forces on the
  columns caused by the weight of the building.
• Drift (i.e. column tilt) should not exceed 0.03
  for tall MRF buildings.

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• John Halls design of a 20-story steel MRF
  building that meets California 1994 code (zone
  IV, site class C)

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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).

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• Simulated deformation of 20-story steel frame for
  the M6.9 1989 Loma Prieta Earthquake
• JBC is Japanese building code
• UBC is 1994 California code

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1906 ground motion simulation from Brad Aagaard
(USGS)
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• Simulated 20-story steel frame for a M 7.8
  1906-like earthquake
• Yellow and red are damaged beyond repair
• Pink is simulated collapse

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Large displacements can overwhelm base isolation
systems

• 2-meter displacement pulse as input for a
  simulation of the deformation of a 3-story
  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

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• Isolator displacements (m)
• Most isolated buildings cannot exceed 0.4 m

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Frame buildings can have also be built with
concrete columns and beams (as opposed to
steel)1971 San Fernando earthquake showed that
many concrete frames were brittlePotential for
collapse at drifts of about 0.01 (lower than for
steel buildings)There are thousands of these
buildings in California and occupants have not
been notified
Olive View Hospital M 6.7 1971 San Fernando
Earthquake
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Northridge 118 FWY
Example of failure of a brittle concrete column
(pre-1975 code)
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Example of ductile behavior of concrete
columns. Although the parking structure
performed poorly, the exterior columns did not
fail.
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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.
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Conclusions

• Current probabilistic hazard analysis may
  seriously underestimate the importance of large
  earthquakes.
• Flexible buildings that rely on high ductility
  will be damaged beyond repair in large
  earthquakes and many may collapse.
• Fix the pre-1994 steel welds!
• Notify occupants of pre 1975 brittle concrete
  frame buildings!
• Base isolation systems may be overdriven by large
  near-source ground motions.
• Strong shear-wall construction is best suited to
  resist large-magnitude earthquakes (your wooden
  house will perform well).
• If it doesnt burn.