Structures Design Age Building materials Measuring force on

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 5 th Story • Another example of soft story collapse • 5 th 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