Structural Design Against Earthquakes

Structural Design against Earthquakes

Manoj Kumar Scientist, Structures Division, ADE, Bangalore

Noise, Vibration & Harshness: Theory and Practice Proficience Hall, Indian Institute of Science, Bangalore December 10, 2008

What is an earthquake? z

The shaking or trembling of earth caused by the sudden release of energy.

Causes: z Rockslides z Landslides z Volcanic eruption z Nuclear explosions z Forces which are released along fault z Reservoir Induced seismicity

Earthquake waves Body Waves: P and S waves

Surface Waves: R and L waves

Body Waves: P and S waves z

Body waves

•

P or primary waves

• • •

•

fastest waves about 5.4km/s travel through solids, liquids, or gases compressional wave, material movement is in the same direction as wave movement

S or secondary waves

• • •

slower than P waves, 3.3km/s travel through solids only shear waves - move material perpendicular to wave movement

Surface Waves: R and L waves

z

Surface Waves

• • •

Travel just below or along the ground’s surface Slower than body waves; rolling and side-to-side movement Especially damaging to buildings

Measure of an Earthquake/ Energy released Two earthquake size measurements Magnitude – estimates the amount of energy released at the source of the earthquake Intensity – a measure of the degree of earthquake shaking at a given locale based on the amount of damage TABLE : 1 MAGNITUDES OF EARTHQUAKES AND ENERGY RELEASED M (Richter)

5.0

6.0

6.5

7.0

7.5

8.0

8.4

8.6

E (1020 ergs)

0.08

2.5

14.1

80

446

2500

10000 20000

Measure of an Earthquake/ Energy released An increase in magnitude (M) by 1.0 implies 10 times higher waveform amplitude and about 31 times higher energy released. The energy released by a M6.3 earthquake is equivalent to that released by the 1945 Atom Bomb dropped on Hiroshima.

Destructive energy

India, 2001

Pakistan, 2005

Destructive energy

Turkey, 1999

Destructive energy

Taiwan, 1999

India, 2001

Aseismic design of structures z

Earthquake causes the ground to move in all three directions.

z

The various parts of structure move differently with respect to the foundation, and due to this relative deformation, additional forces are exerted on the structure.

General Principle and Design Criteria (IS 1893 : 2002) The design approach adopted in this standard is to ensure that structures z

possess at least a minimum strength to withstand minor earthquakes (
z

Resist moderate earthquakes ( DBE ) without significant structural damage though some non-structural damage may occur

z

and aims that structures withstand a major earthquake ( MCE ) without collapse,

General Principle and Design Criteria z

The magnitude of the forces induced in a structure due to a given ground acceleration will depend, amongst other things, mass of the structure, the material, and the damping, ductility and energy dissipation capacity of the structure.

z

By enhancing ductility and energy dissipation capacity in the structure, the induced seismic forces can be reduced.

Design Philosophy z

The guiding principles governing the conceptual design against seismic hazard are: • Structural simplicity; • Uniformity, symmetry and redundancy; • Bi-directional resistance and stiffness; • Torsional resistance; • Adequate foundation.

Analysis Methods

From this: V(x), M(x)

(nonlinear) time step analysis for MDOF, Response spectrum

F

u F

Fb

Δt

t

equivalent static force,

Koyna, Maharashtra 1967, M6.5

u

How to deal with huge earthquake forces z

Allow inelastic response (Ductile Behavior)

z

Increase damping (Energy dissipation devices)

z

Increase natural period (Base Isolation)

Energy dissipation by Ductile Behavior z

Structures subjected to several cycles of cyclic loading.

contd…

Energy dissipation by Ductile Behavior z

Under seismic loading, for a given energy input, elastoplastic response differs from elastic response in following ways:

• • •

the energy gets dissipated; The induced force is less; and the maximum deflection is more.

Thus, while ductility helps in reducing induced forces and in dissipating some of the input energy, it also demands large deformations to be accommodated by the structure.

Increase damping (Energy dissipation devices) z

Another approach for controlling seismic damage in structures and improving their seismic performance is by installing Seismic Dampers in place of structural elements, such as diagonal braces. These dampers act like the hydraulic shock absorbers in cars - much of the sudden jerks are absorbed in the hydraulic fluids and only little is transmitted above to the chassis of the car. When seismic energy is transmitted through them, dampers absorb part of it, and thus damp the motion of the structure.

Energy dissipation devices

Energy dissipation devices (Types of dampers) z

Viscous Dampers (energy is absorbed by silicone-based fluid passing between piston cylinder arrangement)

z

Friction Dampers (energy is absorbed by surfaces with friction between them rubbing against each other)

z

Yielding Dampers (energy components that yield)

z

Visco-elastic dampers (energy is absorbed by utilizing the controlled shearing of solids)

is

absorbed

by

metallic

Thus by equipping a structure with additional devices which have high damping capacity, we can greatly decrease the seismic energy entering the structure.

Increase natural period (Base Isolation) z

As the name suggest, in this approach the structure (building, bridge or a machine/equipment) is separated from its foundation.

Principle of Base Isolation The fundamental principle of base isolation is to modify the response of the structure so that ground can move below it without transmitting these motions into the structure.

Structure acceleration and displacement

Rigid Structure

zero period

Flexible Structure

Infinite Period

Base isolation or seismic isolation z

z

The energy input from an earthquake is proportional to velocity squared. Implementation of base isolation is based on the assumption that over mid frequency range, for a period of 0.5 sec to 4 sec the energy input is constant, that is, velocity is constant. For a constant velocity, and

displacement α period, acceleration α (period)-1

if the period is doubled, the displacement will double but the acceleration (hence force induced) will be halved.

Types of Isolators z z z z

Sliding systems Elastomeric Rubber Springs Roller and ball bearings

Thank you!

References z

Base Isolation of Structures: Design Guidelines 2001, Travor E Kelley, Holmes Consulting Group Ltd.

z

Base Isolation of Structures, Seminar Report June 2006, Manoj Kumar, Deptt. of Civil Engg., G B Pant University, Pantnagar. Dynamics of Structures: Theory and application to earthquake engineering, Anil K Chopra, Prentice-Hall, New Jersey.

z

z

Elements of Earthquake Engineering, Jai Krishna, A V Chandrasekaran 1994, South Asian Publishers, New Delhi.

z

IS 1893: 2002, Criteria for Earthquake Resistant Design of Structures Part 1, General Provisions and Buildings (Fifth Revision), Bureau of Indian Standard, New Delhi.

z

RC Design, S U Pillai, D Menon, 2005 Tata McGraw-Hill Publishing Company Ltd, New Delhi

z z

www.iitk.ac.in www.nicee.org

z

www.nisee.berkeley.edu

www.manojsaini.co.cc