Post on 18-Dec-2015
transcript
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
• Added strength is needed to maintain a structure’s integrity when subjected to lateral accelerations
Structures are built to maintain their integrity due to gravity
• Acceleration readings vary with earthquakes
• What type of fault would produce the highest accelerations?
Acceleration
-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
• Increase with building height
Acceleration
San Jose High School, 1906 earthquake: stiff building material and increase acceleration with height
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
• 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
Period and Resonance
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
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
1950 structure: lacked seismic design
Earth material: bay mud
Seismic waves amplified
Liquefaction
Marina District
Earth material: unengineered fill
Liquefaction
Seismic waves amplified
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
LiquefactionUnreinforced masonry (URM) failed
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 Collapse
• Parking garage is a soft story
• Scenes like this were familiar near the epicenter
• Where have you seen this type of structure?
Kobe, Collapse of 5th Story• Another example of
soft story collapse• 5th floor restaurant• Open structure• Stories above and
below have more support
• Seismic waves travel horizontally and vertically
• Failure occurs at the connections
• Increase in acceleration with height
Irregularly shaped buildings
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
Foundation
Sill plate
House attaches to the foundation through the sill plate
HOG: house over garage
Open, weakly supported garage fails with heavier and sturdier structure above
• Yellow: GPS data– Slow slip or silent
earthquakes– Early- 2002, mid-2006
• Red/Green: seismic stations– Circled area,
earthquakes
Silent earthquakes: indicative of earthquakes
• Shallow and then becomes more steep under Mexico City
Mexican subduction zone
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
• Earthquakes are more shallow than other subduction zones
Mexican subduction zone
Mexico City
• Drained Lake Texcoco• Clay sedimentary layers• Low frequency surface
waves amplified• 1-2 second frequencies• Matched the periods of
buildings 6-16 stories
Mexico City: Common Building Failures
• Top floors fail-resonance• T-shaped structures• Flexible structures
between stiff structures
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