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

• 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

Accelerographs are placed on man-made structures to measure performance during an earthquake

• 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

Freeway collapse

Simi Valley freeway collapse due to high accelerations.

Northridge Earthquake

• Increase with building height

Acceleration

San Jose High School, 1906 earthquake: stiff building material and increase acceleration with height

• Decreases with distance from epicenter

Acceleration

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

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

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: 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?

• Soft story: inadequate lateral bracing

Structural failure, Northridge Eq

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 7.68:50 AM, local time80,000 fatalities200,000 injuries

Unreinforced Masonry

Wall Failure

Traditional structures failed- unreinforced brick

L’Aquila, Mw 6.3April 4, 2009

• Seismic waves travel horizontally and vertically

• Failure occurs at the connections

• Increase in acceleration with height

Irregularly shaped buildings

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

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

Cripple wall failure

• The wall between the sill plate and the house

Mexico

Silent earthquakes

• 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

Mexico City: Building Failures

• Hammering

Soft story collapse

Chile, Mw 8.8, 2010

Uplifted terrace with lighthouse

Intertidal fauna exposed

3-6 feet of uplift along the coast

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