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 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 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 Liquefaction Unreinforced 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 7.6 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

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 zonezone

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 zonezone 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 zonezone

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

EarlyEarly seismic construction Huaca Pucllana Lima, Peru 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