Earthquakes and Seismic Design

Transcription

1 Earthquakes and Seismic Design

2 Cross-sectionofEarth Though we have explored space above ground extensively, we could go only about 7.6 miles below ground! Russian geologists started drilling into the Kola Peninsula, near Finland, in 1970 and after 22 years could not proceed further.

3 Plate tectonics (Alfred Wegener1912)

4 Plate tectonics (PT) Earlier theories assume gradual shrinking (contraction) or gradual expansion of the globe. PT is based on continental drift& developed in early 20th century Lithosphereis broken up into 7-8 major tectonic plates, and numerous smaller plates Tectonic plates move because lithosphere has a higher strength and lower density than the underlying asthenosphere-dissipation of heat from the mantle is the source of energy Lateral relative movement of the plates-0 to 100mm annually

5 Three types of plate boundaries exist

6 Global earthquake epicenters,

7 Aerial view of San Andreas Fault in the Carrizo Plain, northwest of Los Angeles

8 What is an earthquake? An earthquake is a sudden release of energy due to shifts in the earth s plates that has been stored in the rocks beneath the earth s surface which causes a trembling or shaking of the ground. The energy that is released from the ruptured rock travels in waves which are known as seismic waves. There are two types of seismic waves; body waves which travel through the interior of Earth and surface waves which travel on Earth's surface. The two body waves are primary waves (p-waves) and secondary waves (s-waves).

9 The compressional (push-pull) wave will vibrate parallel to the direction that the wave is traveling up to speeds of 4 to 8 km per second (2.49 to 4.35 miles per second). The S-wave vibratesperpendicular to the direction of travel and can travel up to speeds of 2 to 5 km per second ( 1.24 to 3.11 miles per second). Love waves and Raleigh waves are known as Surface waves. Surface waves are the slowest of the seismic waves, but because they travel near the surface of Earth and contain a range of oscillating frequencies they often cause the most damage

10 RayleighSurface Waves Animation 10/64

11 Earthquakes Around 500,000 earthquakes occur each year, detectable with current instrumentation. About 100,000 of these can be felt. Human activities that produce minor earthquakes: Storage of large water behind a dam, Injecting liquid under high pressure into wells (frackingto extract natural gas), Coal mining Oil drilling

12 Recurrence of Earthquakes Average recurrence of Earthquakes are: Earthquake of M every year, Earthquake of M every 10years, Earthquake of 5.6 or larger every 100years. The United States Geological Survey estimates that, since 1900, there have been an average of 18 major earthquakes (M ) and one great earthquake (M 8.0 or greater) per year.

13 Aftershocks An aftershock is an earthquake that occurs after a previous earthquake, the mainshock. It occurs in the same region of the main shock but always of a smaller magnitude. If it is larger than the main shock, the aftershock is redesignated as the main shock, and the original main shock is re-designated as a foreshock. Formed as the crust around the displaced fault plane adjusts to the effects of the main shock They are dangerous -usually unpredictable, can be of a large magnitude, and can collapse buildings that are damaged from the main shock

14 Earthquakes-Epicenter Some Definitions Epicenter is the point on the Earth's surface that is directly above the hypocenter(where the Strain energy stored in the rock is first released)

15 EARTHQUAKES

16 Seismograph is used to measure wave amplitude

17 Can we predict earthquakes correctly? Long ago, Catholic Church in Romecondemned Galileo Galilei and put him under house arrest for teaching Earth revolves around the sun! Oct 22, 2012 an Italian court convicted seven scientists and experts for 6 years in prison for failing to adequately warn citizens before an after shock struck central Italy in 2009, killing more than 300 people.

18 Can we predict earthquakes correctly? Scientists generally cannot predict the time, location and magnitude of EQ -But they did it once! On Feb. 4, 1975, seismologists issued a warning to residents of Haichengin northeastern China, prompting people to seek safety outdoors. A M7.3-EQ struck that evening, killing more than 2,000 people and destroying more than 90 percent of the city. Without the warning, about 150,000 people would have died!

19 Characteristics of an Earthquake

20 As the quality of the sediment decreases, the amplitude of the waves increases

21 Wave Propagation Rock Soft Soil 21/64

22 Magnitude: Richter scale-californian seismologist Charles F. Richter, in 1930s

23 P and S waves and Magnitude P waves are the first to arrive due to their high displacement speed, Followed by the S waves. Two parameters that determine magnitude: The time delay between the arrival of the first P waves and S waves(proportional to the distance between the seismograph and the hypocentreof the earthquake), and Their amplitude.

24 P and S waves

25 Graphical solution of the mathematical formula for determining magnitude on the Richter scale

26 Intensity of earthquakes Modified Mercalli Intensity scale (MMI) and MSK scale Initially developed early last century by Giuseppe Mercalli based on observations and how it felt by people Both have twelve levels of intensity LevelI leastperceptive LevelXII mostsevere

27 What do Architects do? Architects design buildings and structures. They advise individuals, property owners and developers, community groups, local authorities and commercial organizations on the design and construction of new buildings, the reuse of existing buildings and the spaces which surround them. Architects work closely with other members of the construction industry including engineers, builders, surveyors, local authority planners and building control officers.

28 What do Structural Engineers do? Structural engineers combine their knowledge of science and design as they construct better framework for buildings and other structures to safely resist natural and mademade forces. They are involved in physical testing, mathematical modeling, computer simulation all of which support decisions that aid in the creation and maintenance of safeand economical structures. /

29 What is Earthquake Engineering? Earthquake engineers are concerned with creating earthquake resistant designsand construction techniques to build all kinds of bridges, roads and buildings. Earthquake engineers are faced with many uncertainties and must be smart in their decisions in developing safe solutions to challenging problems. They rely on state-of-theart technology, materials science, laboratory testing and field monitoring.

30 A Famous Quote Structural engineering is the art of using materials that have properties which can only be estimated To build real structures that can only be approximately analyzed To withstand forces that are not accurately known So that our responsibility to the public safety is satisfied!

31 Earthquake-Resistant Structure Building designed to prevent total collapse, preserve life, and minimize damage

32 Factors Influencing Seismic Damage The following factors influence the seismic damage: Peak Ground Acceleration (PGA) Duration and frequency of ground vibration, Magnitude, Distance from epicenter, Geographical conditions between the epicenter and the site, Soil properties at the site and foundation type, Building type and characteristics. Damage to a Steel building in Mexico City, 1985

33 Lateral Force Resisting Systems

34 Better Performance in Earthquakes Have simple and regular Plans

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43 Collapse of L-shaped building in Ahmedabad, 2001

44 Avoid Irregular Configurations

45 Avoid Novel Structural Features (If their EQ behavior is not known)

46 Geometric Vulnerabilities CCTV Tower, China

47 Response Spectra for Different Strong Earthquakes

48 Response Acceleration Coefficient Smoothened Elastic Design Acceleration Response Spectrum (SEDRS) for 5% damping. For Steel structures use 2% damping

49 Fundamental natural periods of structures differ over a large range Adapted from: Newmark, (1970), Current trends in the Seismic Analysis and Design of High Rise Structures, Chapter 16, in Wiegel, (1970), Earthquake Engineering, Prentice Hall, USA.

50 Distribution of Base Shear to Different Levels of the Building After the base shear force V B is determined it should be distributed along the height of the building (to the various floor levels) using the following expression: After the base shear is distributed, the frames may be analyzed by any standard computer program to get the internal forces!

51 Dynamic Analysis The dynamic analysis methods are grouped into: Response spectrum method(multistory buildings, irregular buildings, overhead water ranks and bridge piers are often designed using this method) Time-history response analysis (most important structures such as nuclear reactors, large span structures or very tall buildings are designed using this method).

52 EARTHQUAKE DESIGN PHILOSOPHY The seismic design philosophy: Minor and frequent earthquakes should not cause any damage to the structure Moderate earthquakes should not cause significant structural damage but could have some non-structural damage Major and infrequent earthquakes should not cause collapse Hence design is done for much smaller forces than actual seismic loads. Note that this approach is different than that adopted in the case of wind, dead, live and other loads, where the structure is designed for the actual loads.

53 Earthquake design philosophy

54 Seismic Design Philosophy Though the structure is designed for reduced earthquake loads, the following contributing factors will prevent the collapse of the structure: Over-strength, Redundancy, Ductility

55 Ductile and Brittle Performance

56 CURRENT DESIGN CODES Expected Performance: The design requirements primarily are intended to safeguard against major failures and loss of life, NOT to limit damage, maintain functions, or provide for easy repairs.

57 Performance Based Design (PBD) Future (PBD) Codes will be based on: Desired performance chosen by owner Reduced business interruption Reduced damage costs Current Performance based design documents Vision 2000 FEMA 356/273 ATC 40 FEMA 310

58 Load Combinations In general consider the 8- load combinations: (1) 1.5 (DL + IL) (CL or SL) (2) 1.2 (DL + IL) (CL or SL) ±0.6(WL or EL) (3) 1.2 (DL + IL ±WL or EL) (CL or SL) (4) 1.5(DL ±WL or EL) (5) 0.9 DL ±1.5 (WL or EL) (6) 1.2 (DL + ER) (7) 0.9DL ER (8) DL (IL + CL or SL) + AL Where, DL = Dead load, IL = imposed load (live load), WL = wind load, SL = snow load, CL = crane load (vertical / horizontal), AL = accidental load, ER = erection load and EL = earthquake load.

59 Loading Combination for Nonorthogonal buildings Eight additional possibilities should also be considered. (1) EL x EL y (2) 0.3EL x + EL y (3) EL x 0.3EL y (4) 0.3EL x -EL y (5) (EL x + 0.3EL y (6) (0.3EL x + EL y ) (7) (EL x 0.3EL y ) (8) (0.3EL x EL y )