Earthquake Resistant Design Philosophy and Approach in New Zealand Donald Kirkcaldie
The 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.
New 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
New 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.
Traditional 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
Traditional Performance Principles • 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)
Traditional Performance Principles • 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
Updated 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:
Draft 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
Implementation - 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
Implementation – 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)
Implementation – 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.
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
Implementation - 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
Implementation – 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
Implementation - 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.
Implementation - 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
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
Detailing 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
Detailing Examples • Tight and loose linkage of spans to supports • Span – support overlap • EQ ULS & SLS movement & knock-off device • Settlement slabs behind abutments
Detailing Examples • Linkage to a pier • Deck slab diaphragm action
Base 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
Base Isolation with Mechanical Energy Dissipation • 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
Base Isolation with Mechanical Energy Dissipation • 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.
Conclusion • 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