Earthquake Design Considerations

By Dr. N. Subramanian3rd Nov. 2012

Dr. N. Subramanian

Pounding between adjoiningbuildings due to horizontal vibrations

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More mass onone side causes the floors to twist

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One-side open ground storey buildingtwists during earthquake shaking

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Unequal vertical members cause building to twist about a vertical axis

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Waves of different periods

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If the ground is shaken by earthquake waves that have short periods, then short period buildings will have large response.Similarly, if the earthquake ground motion has long period waves, then long period buildings will have larger response.

Soil condition at site may influence damage

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Different Buildings Respond Differentlyto Same Ground Vibration

Design Codes

IS 13920, 1993, Indian Standard Code of Practice for Ductile Detailing of Reinforced Concrete Structures Subjected to Seismic Forces

IS 1893 (Part I), 2002, Indian Standard Criteria for Earthquake Resistant Design of Structures (5th Revision)

IS 4326, 1993, Indian Standard Code of Practice for Earthquake Resistant Design and Construction of Buildings (2nd Revision)

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Longitudinal steel in Beams

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Stirrups as per IS 13920

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Stirrups with 135 degree hooks at the end are required

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Lapping of Longitudinal bars

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Soft storey created by open GF car park

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Earthquakes do not kill people;man in his role as a builder, kills people.

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Total Horz. EQ Force increases downwards along its heightCollapse of partially open GF building in Bhuj EQ, with vertical split at the middle!

Possible plastic collapse mechanisms

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Strong-Column Weak –beam Principle

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Circular spiral columns Vs Rect. Columns in the same building during 1971 SFO EQ

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Column reinforcement

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Detailing of columns in seismic zones

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180° links are necessary to prevent the 135° tie from bulging outwards

Shear failure of column

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Large spacing of ties and lack of 135° hook ends caused brittle failure of columns during 2001 Bhuj earthquake

Buckling of column bars

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Confinement steel in columns

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Correct location for column splices

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Short Column effect

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Short columns are stiffer and attract larger forces during earthquakes – this must be accounted for in design

Short column effect

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Short column effect

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Detailing of short columns

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Beam-Column joints should be designed and detailed properly

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Detailing of beam-column joints

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Ties with 135 degree hooks resists the ill effects of distortion of joints

Pull-Push forces cause two problems

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Three stage procedure to provide horizontal ties in joints

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Anchorage of beam bars in interior joints

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Anchorage of beam bars in exterior joints

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Shear walls are to be placed symmetrically to avoid twist

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Detailing of shear walls as per IS 13920

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Collapse of nominally connected water tank

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IS 1893 – Connections designed for five times the design horizontal acceleration coefficient

Bare Vs infilled frame

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Predominant frame action Predominant shear action

Effect of infill walls

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Collapse of intermediate storey in 6 storey building, Bhuj, 2001

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Improper Anchorage into stiff RC elevator core walls in Ghandhidham

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Effect of Staircases

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Diagonal slabs or beams in staircases attract large seismic forces-sliding supports limits the seismic forces

Brick Buildings- Horz. Bands

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Base isolation of buildings to reduce shaking

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Base Isolation TechnologyOne of the most

significant developments in earthquake engineering in the past 35 years.

It provides the design profession the ability to design a building that is “operational” after a major earthquake

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Base isolated structure Conventional structure

BASE ISOLATOR

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Base isolators may beeither coiled springs or laminated rubber-bearing pads, made of alternate layers ofsteel and rubber, and have a low lateral stiffness.

Examples of Base Isolated Systems

Base Isolated LA City Hall

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San Francisco Airport International Terminal is the World’s Largest Base Isolated Building

Base isolator being installed. during a seismic event. Every isolator will extend in any direction 21 inches.

Energy Absorbing Devices

• “Passive energy dissipation is an emerging technology that enhances the performance of buildings by adding damping to buildings.”

• (ASCE/SEI 41-06, pg 280)

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Commonly used dampersViscous dampers (They consist of a piston-cylinder

arrangement filled with a viscous silicon based fluid, which absorbs the energy)

Friction dampers (energy is absorbed by the friction between two layers, which are made to rub against each other).

Hysteretic dampers (energy is absorbed by yielding metallic parts)

Visco-elastic dampers (containing visco-elastic material, sandwiched between two steel plates, which undergoes shear deformation, thus dissipating energy.

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Other Types Of Dampers

Tuned mass dampers (TMD)- They are extra masses attached to the structure by a spring-dashpot system and designed to vibrate out of phase with the structure.

Tuned liquid dampers (TLD) – They are essentially water tanks mounted on structures and dissipate energy by the splashing of the water.

Hydraulic activators- They are active vibration control devices and have a sensor to sense the vibration and activate the activator to counter it. -Require external energy source and are expensive.

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Why Use Dampers?Dampers dramatically decrease earthquake induced

motion . Less displacement : over 50% reduction in drift in many

cases Decreased base shear and inter-story shear, up to 40% Much lower “g” forces in the structure. Equipment keeps

working and people are not injured Reduced displacements and forces can mean less steel.

This offsets the damper cost and can sometimes even reduce overall cost.

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Viscous Damper

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Example- Viscous damper

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Tuned Mass Damper (TMD)

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Taipei 101, the world's second tallest skyscraper is equipped with a tuned mass damper. This 18 feet dia.,730-ton TMD acts like a giant pendulum to counteract the building's movement--reducing sway due to wind by 30 to 40 %. Cost: $4 million

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