Base Isolation strategies for steel frame structures CE510 Presentation By: H- Adib CONTENT

1- Seismic base isolation & the philosophy behind it

2-Types of conventional seismic base isolation

3- seismic base isolation in steel frame structures & rehabilitation of existing structures

4-Base isolation advantages & defficiencies

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Seismic Base Isolation technology could effectively prevent disaster like this worldwide (Iran Roudbar-Manjil Earthquake 1990 40000 killed M=7.4)

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We (Structural Engineers) are responsible to make the world safe to live 1990 Earthquakes with Magnitude 7.0 and Greater

Vanuatu 7.1

Fiji Island 7.6

Costa Rica 7.3

Mariana Island Region 7.4

Minahassa , Indonesia 7.6

Sakhalin, Russia 7.2

Sudan (2 EQ) 7.1-7.2

Luzon, Philippines 7.7

Flores Sea 7.1

Romania 7.0

Panay, Philippines 7.1

Iran (Rudbar-Manjil) 7.4

Vanuatu 7.2

Papua New Guinea 7.5

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SEISMIC BASE ISOLATION GENERAL DIFINITION

Structural vibration control technology (at foundation )in seismic design of structures is called Base Isolation.

A base isolation device is a flexible interface between the superstructure and ground foundation that can considerably reduce the level of earthquake forces transmitted to the building

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Deficiencies in non-isolated(Fixed Based) design of structures

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If ductile large inter story drift and possible residual deformation after intensive earthquake

If stiff and rigid high floor acceleration will create damage to the sensitive contents & none-structural components and

possible brittle failure in the structural element

SEISMIC BASE ISOLATION TECHNOLOGY

One 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 ISOLATION CAPABILITIES Conventional (Fixed base) VS Base Isolated Structures

During an 8 Richter Magnitude Earthquake a Seismically Isolated Building Will Behave as if it Were Experiencing a 5.5 Earthquake

Forces reduces by 3 to 6 across the isolator in BI structure

Forces reduces by 8-12 At the roof in a BI structure

Ground forces will amplify by a factor of 3 to 4 In conventional (Fixed Base) structures

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Isolated structure

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IT REALLY WORKS ! Kobe Earthquake Japan 1995 FB vs. BI structure (Recorded acceleration & forces)

Conventional Building Kobe Earthquake Recorded acceleration amplification

Steel frame structure

Superstructure consist of braced frames with fixed-based period of 0.68 sec.

The target period of the isolation system is up to 3.3 sec. for displacement of 240mm

Isolators are lead-plug rubber bearing and high-damping rubber bearing with diameter of 1.2m

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Philosophy behind the base isolation system

(Acceleration & Displacement Response Spectra)

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Type of Seismic Base isolation

Elastomeric isolators (natural rubber or high-damping elastomer & steel reinforced plates)

Sliding isolators (Teflon- Stainless Steel)

Combination system Sliding for friction & Elastomeric for re-centering capabilities

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Types of Seismic Base Isolation Device

Laminated rubber bearing with lead core.Elastoeric bearing

Friction pendulum system FPS specifications

Friction Pendulum system combines sliding action and re-storing force by geometry

The effective stiffness of the isolator and the isolation period of structure is controlled by radius of concave surface

The coefficient of friction depends on pressure and sliding velocity

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Base isolation Strategies

Friction Pendulum Bearing

SECTION

STAINLESS STEELCONCAVE SURFACE

SELF LUBRICATINGCOMPOSITE LINER

ARTICULATINGSLIDER

PLATECONCAVE

HOUSINGPLATE

Basic requirements of seismic isolation system

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Sufficient horizontal flexibility to lengthen the building period, while maintaining the vertical stiffness

Sufficient damping to restrict the relative deformation at the plane of isolation & limit it to bearing capacity

Sufficient stiffness to prevent damage under service load (wind load)

Small residual horizontal displacements after an earthquakes( Re-centering capability)

International terminal at Sanfrancisco airport largest base isolated steel structure in the world

276 Seismic isolators

Friction Pendulum sliding (F

Cast steel bearing consist of a stainless steel spherical surface with articulated slider

Allowable lateral isolator displacement of 20 in.

Provide isolation of 3 sec

Reduces Earthquake force demand on supper-structure by 30% of the demand for the fixed-base structure

BI structure Remain elastic & undamaged under PGA=0.6g

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Base Isolation strategies in rehabilitation (retrofit) of existing structures(Oakland city hall)

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Work maybe Confined to

Basement

Building Can Continue to be

Occupied

Often the Most Economic

Solution

Base Isolation with so much capability why not implemented world wide?

Seismic base isolators are large and heavy some could weight 1 ton or more

Expensive design-fabrication & construction cost from $5000 and up

Too much design provision to be implemented(eg.2 specimen to be tested for each BI type)

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Farther research and experimental studies must be conducted on production & implementation of low cost base isolators

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To extend this effective earthquake-resistant strategy to housing and commercial buildings specially in developing country the cost and weight of the isolators must be reduced

Design provision and requirements should be simplified

Steel plate reinforcement layers in the isolator elastomer may be replaced by high strength low weight reinforcement fibers like carbon or glass fibers

REFERENCESS

Dynamics of structures –third edition-Anil K. ChopraEarthquake resistance design with rubber-second edition-James M . KellyEarthquake Engineering 2004-Yousef Bozorgnia & Vitelmo V. BerteroDesign of seismic isolated structures - Farzad Naeim & James M. Kelly

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