What Will a Large Earthquake be

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26 Νοε 2013 (πριν από 3 χρόνια και 8 μήνες)

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What Will a Large Earthquake be
Like?

Tom Heaton

Caltech

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.”

Magnitude Paradox … Engineer


Perhaps there can be larger motions, but these
are extreme examples of extraordinary events
that shouldn’t 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!



Current model says
most of the threat to
downtown LA is not
from the San Andreas
fault

1906 M 7.8 San Francisco earthquake rupture with large ground displacement.

Notice that the farm buildings were largely intact.

Pt Reyes Station 1906

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 you’ve got a good building code, who
needs a seismologist?




How do buildings resist earthquake forces?

Front View

Top View

Image: Courtesy EERI

Tall Building Anatomy

stiff

Wooden Houses

7
-
11 stores

Caltech

High
-
rise buildings

flexible

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.

Lateral stiffness relies on flexing beams

Large deformation should result in
bent steel beams

From the lab of Chia
-
Ming Uang, UC San Diego

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.


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

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).


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

1906 g
round motion simulation from
Brad Aagaard (USGS)



Simulated 20
-
story steel
frame for a M
7.8 1906
-
like
earthquake


Yellow and red
are damaged
beyond repair


Pink is
simulated
collapse

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


Isolator
displacements
(m)


Most isolated
buildings cannot
exceed 0.4 m

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 brittle


Potential 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

Northridge 118 FWY

Example of failure of a brittle concrete column (pre
-
1975 code)

Example of “ductile” behavior of concrete columns. Although the parking
structure performed poorly, the exterior columns did not fail.

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.

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 doesn’t burn.