Earthquakes and Seismic Design

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)

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

� Lithosphere is 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 100 mm annually

Three types of plate boundaries exist

Global earthquake epicenters, 1963–

1998

Aerial view of San Andreas Fault in the Carrizo

Plain, northwest of Los Angeles

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).

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

vibrates perpendicular 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

Rayleigh Surface Waves

Animation

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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

Recurrence of Earthquakes

• Average recurrence of Earthquakes are:

– Earthquake of M 3.7–4.6 every year,

– Earthquake of M 4.7–5.5 every 10 years,

– Earthquake of 5.6 or larger every 100 years.

• The United States Geological Surveyestimates 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.

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 re-designated 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

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)

EARTHQUAKES

Seismograph is used to measure wave

amplitude

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’!

• 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.

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!

Characteristics of an Earthquake

As the “quality” of the sediment decreases,

the amplitude of the waves increases

Wave Propagation

Rock� Soft Soil

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Magnitude: Richter scale- Californian seismologist

Charles F. Richter, in 1930s

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.

P and S waves

Graphical solution of the mathematical formula for

determining magnitude on the Richter scale

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

– Level I – least perceptive

– Level XII – most severe

What do Architects do?

� Architects design buildings andstructures. They adviseindividuals, property owners anddevelopers, community groups,local authorities and commercialorganizations on the design andconstruction of new buildings,the reuse of existing buildingsand the spaces which surroundthem.

� Architects work closely withother members of theconstruction industry includingengineers, builders, surveyors,local authority planners andbuilding control officers.

www.architecture.com

www.cnn.com

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 made-made forces.

� They are involved in physical testing, mathematical modeling, computer simulation all of which support decisions that aid in the creation and maintenance of safe and economical structures.

http://www.seaint.org /

http://cee-ux49.cee.uiuc.edu/strweb/home.html

www.earthscience.org/.../geopro/seismic/seismic.html

What is Earthquake Engineering?

Earthquake engineers are concerned with creating earthquake resistant

designs and 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-the-art technology, materials science,

laboratory testing and field monitoring.

www.sciencedaily.com

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!

Earthquake-Resistant StructureBuilding designed to prevent total collapse, preserve life, and minimize damage

http://nisee.berkeley.edu/elibrary/getpkg?id=GoddenD50-69

http://www.infinityfoundation.com/mandala/t_es/t_es_agraw_quake.htm

Factors Influencing Seismic Damage

Damage to a Steel building in

Mexico City, 1985

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.

Lateral Force Resisting Systems

Better Performance in Earthquakes

Have simple and regular Plans

Collapse of L-shaped building in

Ahmedabad, 2001

Avoid Irregular Configurations

Avoid Novel Structural Features

(If their EQ behavior is not known)

Geometric Vulnerabilities

CCTV Tower, China

Response Spectra for Different Strong

Earthquakes

Response Acceleration Coefficient

Smoothened Elastic Design Acceleration

Response Spectrum (SEDRS) for 5% damping.

For Steel structures use 2% damping

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.

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:

After the base shear is distributed, the frames may be

analyzed by any standard computer program to get

the internal forces!

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).

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.

Earthquake design philosophy

Seismic Design PhilosophyThough 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

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.

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

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.

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)