earthquake resistant design philosophy and approach in new zealand donald kirkcaldie

Earthquake Resistant Design Philosophy and

Approach in New Zealand

Donald Kirkcaldie

Earthquake Resistant Design Philosophy and

Approach in New Zealand

Donald Kirkcaldie

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The Development of Earthquake Resistant DesignThe Development of Earthquake Resistant Design

1931 Napier Earthquake – Design lateral loading of 0.1g introduced

1964 Anchorage and Niigata Earthquakes – major damage to infrastructure due to soil liquefaction

1971 San Fernando Earthquake in California:• Serious damage to major highway structures

• Provided the impetus for a concerted international research effort

• Led to the initiation of NZ’s seismic design philosophy and approach – MWD Highway Bridge Design Brief published, with updates in 1972, 1973, and 1978.

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New Zealand’s SeismicityNew Zealand’s Seismicity

NZ’s seismicity

• Comparable to California, and areas adjacent to the Himalayan system, e.g. Burma, Bangladesh

• Major earthquakes in recent times have included:• 1855 Wairarapa - ~8.1 magnitude• Since 1885: • ~15 7.0 - 7.9 magnitude & 17 6.0 – 6.9 magnitude• I.e. > two major events every ~10 years

• Seismicity varies throughout the country as shown in the next slide

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New Zealand Seismicity ZonationNew Zealand Seismicity Zonation

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New Zealand’s Earthquake Resistance PhilosophyNew Zealand’s Earthquake Resistance Philosophy

Forces imposed on bridge piers can exceed 1g for the more rigid structures, scaling down to ~0.2 g for more flexible structures, if the bridges remain elastic.

Costly and seldom economically justifiable

Build energy absorbing capability into the structure and use this to reduce the earthquake forces to be resisted.

Energy absorption provided through ductile inelastic action, focused to occur only in members specifically detailed for the purpose.

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Traditional Performance PrinciplesTraditional Performance Principles

(a) After a design return period event:• Damage may have occurred and temporary repairs may be required

• Bridge is to be useable by emergency traffic

• Bridge is to be able to be reinstated to full capacity for traffic and seismic loads

(b) After an event of return period significantly less than the design value:• Damage should be minor

• No disruption to traffic

(c) After an event of return period significantly greater than the design value• The bridge should not collapse

• The bridge should be useable by emergency traffic after temporary repair

• Repair should be feasible, albeit to a lower level of load capacity

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The design earthquake adopted intensity is a function of the importance and design life of the structure.

A bridge is categorised for its structural action under horizontal seismic load.

Maximum ductility factors are assigned based on the manner in which and extent to which plastic mechanisms can form and their accessibility for repair. (See next slide)

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Categorisation of Structures – Maximum Ductility Factor µCategorisation of Structures – Maximum Ductility Factor µ

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In practice, design is only carried out for (a)

Bridges well designed for (a) are expected to also satisfy (c) due to:

• Greater actual member strength

• Greater damping due to more of the response being inelastic

• Reduced structural stiffness due to softening of the structure

• Greater structure displacement ductility capability

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Updated Required Bridge Performance LevelsUpdated Required Bridge Performance Levels

NZTA Bridge Manual has traditionally aligned with the NZ Loading Code

NZS 1170.5:2006 has introduced explicit consideration of SLS requirements in addition to the ULS.

Corresponding draft amendments have been prepared for the Bridge Manual as follows:

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Draft Revised Bridge Performamce RequirementsDraft Revised Bridge Performamce Requirements

SLS1 (100 year return period*)

• No damage to structural or non-structural components

SLS2 (500 year return period*)

• No more than minor damage to primary structural members• Damage to secondary or non-structural components shall not

significantly impede the operational functionality of the bridge

ULS (2500 year return period*)

• Structural stability is maintained• Access available for emergency vehicles after temporary repairs• Damage may be extensive, but structure should be repairable to

full capacity

* Return periods are those applying to most state highway bridges

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Implementation - OverviewImplementation - Overview

Earthquake resistant design is more an art than a science

• The designer needs to develop an understanding of the likely structural behaviour

• Choices may be necessary in order to optimise the structural form

• Analysis is only a guide to likely behaviour, not an accurate representation. Some significant assumptions are involved

• Robust detailing is required to cater for the unexpected and the possibility of larger than design events

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Implementation – Site AssessmentImplementation – Site Assessment

Site assessment

• Categorisation of the site subsoil

• Proximity of active faults

• Need for a site specific hazard study (- active fault proximity, bridge of high value)

• Development of site hazard spectra

• Selection of appropriate earthquake records if time-history analysis is to be undertaken

• Site stability (- mass ground movement, potential for soil liquefaction)

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Implementation – Choice of Structural FormImplementation – Choice of Structural Form

Considerations in the choice of structural form include:• Bridge form (simply supported v continuous

superstructure, support on bearings v build superstructure integral with substructure, etc.)

• Materials to be used, the structure’s flexibility, and ductility attainable with a particular configuration

• The number and location of supports used to resist seismic forces, and relative distribution of mass and stiffness within the structure

• The balance between displacement and strength requirements. • Flexibility decrease the seismic response accelerations but

increases the movement at joints, and their cost.

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Implementation – Choice of Structural Form (contd.)Implementation – Choice of Structural Form (contd.)

Considerations in the choice of structural form include (contd.):

• The number, location and form of energy dissipation points. • Are they to be plastic hinges in structural members or energy

dissipation devices, and their accessibility for repair.

• The number location and function of movement joints

• Maintenance of structural integrity and prevention of collapse of spans off supports

• The bridge response to EQ in all directions

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Implementation - AnalysisImplementation - Analysis

Selection of the analysis method should take into account:• The complexity of the structural form

• The nature of the anticipated response to ground motions

• The degree of inelastic behaviour

Three forms of analysis are available:• Equivalent static force analysis

• Elastic modal spectral analysis

• Inelastic dynamic analysis by numerical integration

Analytical results only provide an indication of a structure’s likely behaviour. The structure must be capable of surviving deformations exceeding what analysis might predict

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Implementation – Capacity DesignImplementation – Capacity Design

Capacity Design entails the following:

• The predetermination of locations in the structure at which plastic yielding will be designed to occur.

• The design of those plastically yielding locations for yielding to initiate at an acceptable intensity of earthquake ground motion

• The detailing of those plastically yielding locations to maintain their ability to yield in a controlled ductile manner throughout the response

• Other structural elements, intended not to yield, are detailed with sufficient reserve strength to remain elastic and so as to maintain their integrity

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Implementation - DetailingImplementation - Detailing

Appropriate and robust detailing is required to ensure:

• That energy dissipation throughout the yielding of elements can occur

• The integrity of the structure will be maintained during response to earthquakes of greater than design intensity and to effects not predicted by the analysis

• In particular, that the collapse of spans is avoided

• No significant damage or loss of serviceability is incurred during earthquakes of SLS design intensity.

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Implementation - DetailingImplementation - Detailing

Some critical areas of detailing to note include:

• Plastic hinge zones:• No lapping of primary flexural reinforcement in these zones• Increased confinement to prevent bar buckling• Increased shear reinforcement, due to reduced contribution

from the concrete

• Design of Non –Yielding Elements • These must withstand the forces induced by yielding elements

developing their “overstrength” capacity

• Prevention of span collapses• Linkage of spans to supports and to each other• Adequate overlap of girders with supports

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Implementation – Detailing (contd.)Implementation – Detailing (contd.)

Some critical areas of detailing to note include (contd.):

• Freedom to Displace• The structure must have the freedom to displace to develop

the design ductility• Knock-off elements at the abutments and maybe elsewhere,

(unless designed as locked-in or with integral abutments)

• Deck Joints and Bearings• Ability to accommodate the SLS seismic movements• Security of bearings against being displaced

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Detailing ExamplesDetailing Examples

Plastic hinge accessible for inspection

Resisting elements designed for PH overstrength

Reinforcement laps positions

Column & pile confinement

Joint shear reinforcement

Bar terminations

Pier top rotational inertia effects

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Tight and loose linkage of spans to supports

Span – support overlap

EQ ULS & SLS movement & knock-off device

Settlement slabs behind abutments

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Linkage to a pier

Deck slab diaphragm action

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Base Isolation with Mechanical Energy DissipationBase Isolation with Mechanical Energy Dissipation

Base isolation with mechanical energy dissipation is a special case of the ductile capacity design approach

• The bridge superstructure is isolated from the worst effects of the ground motion by means of flexible mounting, usually elastomeric bearings.

• The effect is to increase the period of response of the structure into a period range of the response spectrum where the earthquake shaking is less intense

• Increased equivalent structural damping is achieved through the use of special devices which rely on the cyclic yielding of steel or lead components

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Base isolation with mechanical energy dissipation has been found to be most effective in situations where:

• The seismicity is high

• The substructures are stiff or are required to remain elastic

Note however, that foundation flexibility increases the section ductility demand on any form of plastically yielding element

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Potential advantages of base isolation with mechanical energy dissipation lie in:• Reduced displacements compared to structures with elastomeric

bearings only – enabling simpler deck joint details

• Favourable reduction in the level of seismic forces imposed on the structure

• Redistribution of seismic forces on the substructure and foundations

• Enabling the use of non-yielding structural forms or components intended to remain elastic in the substructure

• Greater control over earthquake induced damage – Under design intensity earthquakes structural damage should be completely avoided.

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ConclusionConclusion

Earthquake resistant design is not an exact science. Skilled judgement based on experience is a vital ingredient

Experience has shown that by using the ductile capacity design approach, bridges can be economically constructed to resist severe earthquakes

The philosophy and approach is equally applicable to areas of significant but lesser seismicity than NZ

In any area of potential seismic activity, it is essential lifeline bridges be of appropriate form, well tied together longitudinally, and detailed to ensure tolerance to seismic overload and unexpected effects

earthquake resistant design philosophy and approach in new zealand donald kirkcaldie

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