Structures
Design
Age
Building materials
Measuring force on structures
Acceleration
Resonance
Acceleration
• A measurement made on structures
relative to gravitational force
• 1 g = 32 ft/sec squared or 9.8
meters/second squared
• Building codes are at about 40-60
percent of that or written as .4 to .6
Acceleration
Structures are built to maintain their integrity due to gravity
• Added strength is needed to maintain a
structure’s integrity when subjected to
lateral accelerations
Accelerographs are placed on man-made structures to
measure performance during an earthquake
Acceleration
• Acceleration readings
vary with earthquakes
• What type of fault
would produce the
highest
accelerations?
Freeway Collapse
-Horizontal accelerations
reached 1.19 and 1.02 g at the
base and 1.8 g on the roadway.
-The Los Angeles River
sediments underlie this bridge.
-What happened?
Freeway collapse
Northridge Earthquake
Simi Valley freeway collapse due to high
accelerations.
Acceleration
• Increase with building height
San Jose High School, 1906
earthquake: stiff building
material and increase
acceleration with height
Acceleration
• Decreases with
distance from
epicenter
Period and Resonance
• Period is the amount of time it takes
one wavelength to pass a point
• Seismic waves with a long wavelength
have a larger period (2-4 seconds)
• Seismic waves with a short wavelength
have a shorter period (1/2-20
cycles/sec)
Wavelength
Period and Resonance
• Buildings also have a period
• The period (long or short) is determined by
the number of stories
• Resonance occurs when the seismic waves
pass through the earth material producing a
particular wavelength and this wavelength
matches the buildings period (wavelength).
• Remember: frequency is inverse of period
Period and Resonance
• Resonance causes the motion of the
bldg to increase
• 0.1 second for a one-story building
• 1-2 seconds for a 10-20 story building
Common Building Failures
• Resonance:
when the period
of the seismic
wave matches the
period of a
structure
• 30 seconds of
shaking put the
structure into
resonance
Wood Shear Wall Construction
Structural Failure associated with
the Loma Prieta Earthquake
Bay Bridge failure
• First pier into bay
mud off Yerba Buena
Island (bedrock)
• Connection failed due
to low frequency
seismic waves (mud)
and high frequency
seismic waves
(bedrock)
Cypress structure, Oakland
Reinforced
concrete
failure
Earth
material:
bay mud
Seismic
waves
amplified
Liquefaction
1950 structure: lacked
seismic design
Marina District
Earth
material:
unengineered
fill
Seismic
waves
amplified
Liquefaction
Soft story: a floor of a multiple story
building that lacks the structural strength
or symmetry of the other floors
Downtown Santa Cruz
Earth material:
unconsolidated
sediments
deposited by
the San
Lorenzo River
Seismic
waves
amplified
Unreinforced masonry (URM) failed
Liquefaction
Structural Failure
Man-made structures:
– Structural design and age
– Building materials
– Fire
– Infrastructure failure: gas lines, water lines,
electrical wires or transformers, cell phone
towers
Soft Story: one floor has
less support than the
adjacent floors
Soft Story Collapse
• Parking garage is a
soft story
• Scenes like this were
familiar near the
epicenter
• Where have you seen
this type of structure?
Structural failure, Northridge
Eq
• Soft story: inadequate lateral bracing
Kobe, Collapse of 5th Story
• Another example of
soft story collapse
• 5th floor restaurant
• Open structure
• Stories above and
below have more
support
Stiff building material
Pakistan, 2005:
Mw 7.6
8:50 AM, local time
80,000 fatalities
200,000 injuries
Unreinforced Masonry
Wall Failure
Traditional structures failed- unreinforced brick
L’Aquila, Mw 6.3
April 4, 2009
Irregularly shaped buildings
• Seismic waves travel horizontally and
vertically
• Failure occurs at the connections
• Increase in acceleration with height
Irregularly shaped structures
Irregularly shaped buildings
• T-shaped structure
• Communication
center in Mexico City
• The city lost
international
communication after
the 1985 earthquake
Resonance
• Resonance: when
the period of the
seismic wave
matches the
period of a
structure
• 30 seconds of
shaking put the
structure into
resonance
Mexico City, 1985
Earth material
• Loosely consolidated sediments and water
saturated mud or sand amplify seismic shaking
• Liquefaction often occurs
• Failure at connections where earth material
varies
Bay Bridge
Cypress Structure
Moss Landing
House falls off foundation
House attaches to the foundation
through the sill plate
Foundation
Sill plate
HOG: house over garage
Open, weakly supported garage fails
with heavier and sturdier structure above
Cripple wall failure
• The wall between the
sill plate and the
house
Mexico
Silent earthquakes
Silent earthquakes: indicative of
earthquakes
• Yellow: GPS data
– Slow slip or silent
earthquakes
– Early- 2002, mid-2006
• Red/Green: seismic
stations
– Circled area,
earthquakes
Mexican subduction zone
• Shallow and then
becomes more steep
under Mexico City
Mexico City Earthquake
• 50 x 170 kilometers of displacement along
the subduction zone
• M 8.1
• Mexico city is 400 kilometers away
• City was built on the sediments of Lake
Texcoco
Mexican subduction zone
• Cocos tectonic plate is
subducting under the
North American Plate
• Two plates lock
• Stress builds and energy
is stored
• Stress exceeds frictional
force
• Release of energy in
terms of an earthquake
Mexican subduction zone
• Earthquakes are
more shallow than
other subduction
zones
Mexico City
• Drained Lake
Texcoco
• Clay sedimentary
layers
• Low frequency
surface waves
amplified
• 1-2 second
frequencies
• Matched the periods
Mexico City: Common
Building Failures
• Top floors failresonance
• T-shaped structures
• Flexible structures
between stiff structures
Mexico City: Building Failures
• Hammering
Soft story collapse
Chile, Mw 8.8, 2010
Uplifted terrace with lighthouse
3-6 feet of uplift along the
coast
Intertidal fauna exposed
Conception:L-shaped structure
failure
Failure of URM and soft story
Balcony beams and weak
internal wall caused buckling
of building.
Conception: failure of concrete
walls
Liquefaction induced failures
Early seismic construction
•
•
•
•
Huaca Pucllana
Lima, Peru
200-700 CE
Bricks built in a
trapezoid pattern with
spacing
• Accommodates
seismic shaking
Which structural designs tend to fail
during ground shaking?
• Soft story
• Structures constructed from stiff building
materials
• One weak point initiates other failures
• Irregularly shaped structures
• Structures that move into resonance
• Earth material fails
• Hogs
• House off foundation
• Cripple wall failure
Constructing model buildings
and subjecting to shaking
• Building must be:
– At least 30 cm high
– At least 3 stories
– No central post or uprights
• Materials are limited
• Complete construction in limited time