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Details earthquakes and their effect on Structures
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Earthquakes and Seismic Design
By Dr. N. Subramanian3rd Nov. 2012
Cross-section of Earth
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.
Plate tectonics- Alfred Wegener , 1912
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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
centuryLithosphere is broken up into 7-8 major tectonic plates, and
numerous smaller platesTectonic 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 100 mm annually
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Three types of plate boundaries exist
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Aerial view of San Andreas Fault in the Carrizo Plain, northwest of Los Angeles
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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 (fracking
to extract natural gas), – Coal mining– Oil drilling
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Recurrence of Earthquakes
• Average recurrence of Earthquakes are:– Earthquake of M3.7–4.6 every year, – Earthquake of M4.7–5.5 every 10 years,– Earthquake of 5.6 or larger every 100 years.
• The United States Geological Survey estimates that, since 1900, there have been an average of 18 major earthquakes (M 7.0–7.9) and one great earthquake (M 8.0 or greater) per year.
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AftershocksAn 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 redesignated 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
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Earthquakes-Epicenter
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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)
EARTHQUAKES
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Earthquake Prediction Instrument... from ancient China
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Seismograph is used to measure wave amplitude
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Can we predict earthquakes correctly?
• Long ago, Catholic Church in Rome condemned Galileo Galilei and put him under house arrest for teaching ‘Earth revolves around the sun’!
• Now, (Oct 22, 2012) an Italian court convicted seven scientists and experts for 6 years in prison for failing to adequately warn citizens before an earthquake struck central Italy in 2009, killing more than 300 people.
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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 Haicheng in 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!
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Characteristics of an Earthquake
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As the “quality” of the sediment decreases,the amplitude of the waves increases
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Magnitude: Richter scale- Californian seismologist Charles F. Richter, in 1930s
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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 hypocentre of the earthquake), and
• Their amplitude.
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P and S waves
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Graphical solution of the mathematical formula for determining magnitude on the Richter scale
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Intensity of earthquakes
• Modified Mercalli Intensity scale (MMI) and MSK scale (Appendix D of Draft IS 1893)
• Initially developed early last century by Giuseppe Mercalli.
• Both have twelve levels of intensity– Level I – least perceptive – Level XII – most severe
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Factors Influencing Seismic Damage
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Damage to a Steel building in Mexico City, 1985
The following factors influence the seismic damage: Peak Ground Acceleration (PGA) Amplitude, 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.
Lateral Force Resisting Systems
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Better Performance in Earthquakes
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Have simple and regular Plans
Collapse of L-shaped building in Ahmedabad, 2001
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Avoid Irregular Configurations
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Avoid Novel Structural Features(If their EQ behavior is not known)
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Geometric vulnerabilities- CCTV Tower, China
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Response Spectra for Different Strong Earthquakes
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Response Acceleration coefficient as given in IS 1893 (Part 1)-2002
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Smoothened Elastic Design Acceleration Response Spectrum (SEDRS) for 5% damping.For Steel structures use 2% damping
SEISMIC ZONES OF INDIA
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Probabilistic Seismic Hazard Map (PSHM) of India
• The National Disaster Management Authority, Govt. Of India, New Delhi has also developed a Probabilistic Seismic Hazard Map (PSHM) of India
•http://ndma.gov.in/ndma/disaster/earthquake/India-psha-finalreport.pdf
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Equivalent Lateral Base Shear Force Procedure
Equivalent Lateral Base Shear Force :
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Where Z= zone factor, I = importance factor, and R= Response reduction Factor
I = 1.5 for largely crowded and imp. Buildings, and equal to 1.0 for other buildings.
Approximate Fundamental Period
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The approximate fundamental natural period of vibration for a moment resisting frame without brick infill panels is : Ta = 0.085 h0.75 in seconds
where h = height of the building in m For all other buildings, including moment resisting frame buildings with brick in-fill,
Ta = 0.09h / √d in seconds
where d = base dimension of the building at the plinth level, along the considered direction of the lateral force, in meters.
Equivalent Static Method (seismic coefficient method)
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Total design seismic base shear if determined by
VB = Ah W
Ah = Design horizontal acceleration spectrum value W = Seismic weight of the building
Fundamental natural periods ofstructures differ over a large range
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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.
Distribution of Base Shear to Different Levels of the Building
After the base shear force VB is determined it should be distributed along the height of the building (to the various floor levels) using the following expression:
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After the Base shear is distributed, the frames may be analyzed by any standard computer program to get the internal forces!
Dynamic AnalysisThe 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).
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EARTHQUAKE DESIGN PHILOSOPHYThe seismic design philosophy as per IS 1893(part 1) is: Minor and frequent earthquakes should not cause any damage to the
structureModerate earthquakes should not cause significant structural damage
but could have some non-structural damageMajor 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.
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Earthquake design philosophy
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Seismic Design Philosophy
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Though the structure is designed for reduced earthquake loads, the following contributing factors will prevent the collapse of the structure:
Over-strength, Redundancy, Ductility
Ductile and Brittle performance
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CURRENT DESIGN CODES
Expected Performance:
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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.
Performance Based Design (PBD)Future (PBD) Codes will be based on:Desired performance chosen by owner.Reduced business interruptionReduced damage costs
Current Performance based design documents:Vision 2000FEMA 356/273ATC 40FEMA 310
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Load CombinationsIn general consider the 8- load combinations: (1) 1.5 (DL + IL) + 1.05(CL or SL)(2) 1.2 (DL + IL) + 1.05(CL or SL) ± 0.6(WL or EL)(3) 1.2 (DL + IL ± WL or EL) + 0.53 (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 + 1.2 ER(8) DL + 0.35(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.
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Loading Combination for Non-orthogonal buildings
Eight additional possibilities should also be considered.
(1) ELx + 0.3 ELy
(2) 0.3ELx + ELy
(3) ELx – 0.3ELy
(4) 0.3ELx - ELy
(5) – (ELx + 0.3ELy
(6) (0.3ELx + ELy)
(7) – (ELx – 0.3ELy)
(8) – (0.3ELx – ELy)
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Seismically Active regions in India
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Past Earthquakes in India
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Four Great earthquakes (M>8) occurred in a span of 53 years from 1897 to 1950; the January 2001 Bhuj earthquake (M7.7) is almost as large
Past Earthquakes in India
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