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Capacity spectra for bi-linear models and a procedure for constructing capacity design spectra

J.X. Zhao and J.J. Zhang

A very useful tool for the preliminary design of structures is the elastic capacity spectrum. A pseudo-acceleration relationship has to be assumed for constructing a capacity spectrum and this assumption results in large errors for long period structures with large damping ratios. In the present study, capacity spectra for bi-linear models are presented.  Pseudo-acceleration is also assumed which results in acceptably small errors when a viscous damping ratio of 5% proportional to the tangent stiffness of a single degree freedom structure is used. For nonlinear structures with a small damping ratio and tangent stiffness dependency of damping force could be acceptable because energy adsorption is primarily the result of structural nonlinear deformation. Capacity spectra of bi-linear models for a number of near-source records from large earthquakes and spectral ratios of two horizontal components are presented. A procedure for constructing capacity spectra of bilinear models from an elastic 5% damped design spectra is also presented.

Paper P8: [Read]

Capacity spectra for lead rubber bearing seismic isolation systems

J.X. Zhao and J.J. Zhang

In this study, we present a capacity spectrum method for the design of seismically isolated structures using lead rubber bearings or other types of isolators with bilinear hysteresis loops. The capacity spectra are very useful for visual selection of optimal isolation parameters and eliminate the use of an equivalent linear-elastic substitute structure. The responses of seismically isolated structures subjected to near-source ground motions with either a large forward-directivity pulse or a fault-fling pulse are presented. Our analyses suggest that seismic isolation can be used to protect structures subjected to any near-source ground motions that are available to us, with acceptable levels of base shear coefficient and isolator displacement, except for one component of the TCU 068 record from the 1999 Chichi Taiwan earthquake that contained a large permanent displacement of nearly 10m. Some strategy in the design of seismically isolated structures using lead rubber bearings will be discussed.

Paper P9: [Read]

Base isolation of an existing 10-storey building to enhance earthquake resistance

W.D. Clark and J.E. Mason

The Rankine Brown Building at Victoria University of Wellington, New Zealand houses the University Library and is pivotal to the operation of the University. Built in the early 1960s, the building was of innovative form and construction for its time with long span precast concrete waffle slabs over 10 floors supported on 16 main columns. 3 years ago a review of the building structure suggested that increased protection of the building would be required to meet the University’s expectation for operational continuity after a moderate or major earthquake event in the Wellington area.

This paper outlines:

• The background to the decision to base isolate the building to enhance its resistance to earthquake effects.
• Form and deficiencies of the original building construction.
• Development of Design Criteria.
• Modified earthquake records used in the time history analysis.
• Analysis of building structure with and without base isolation.
• Reduction achieved in ductile demand on building elements due to base isolation.
• Construction requirements for the installation of the base isolation bearings.

Paper P10: [Read]

Value of assessing seismic vulnerability

P.N. Davenport

Studies of seismic damage to a variety of property types in New Zealand have found that the data fit well to a lognormal distribution. In this study, this distributional form is used in a numerical simulation of seismic damage to a number of portfolios of property. The value of various levels of knowledge about the vulnerability of the property is assessed. In some situations, better knowledge of the vulnerability of the property can be of value to the owners and insurers in assessing the seismic risk.

Paper P11: [Read]

Learning from Earthquakes - Looking back and looking ahead

A.B. King and D.C. Hopkins

A review of the impact and application of lessons learnt during the Society’s past 17 earthquake reconnaissance missions has been completed as part of a Society Technical Group study. This has involved a detailed examination of the reports, findings and recommendations from missions mounted since 1985 including those to Mexico (1985), Armenia (1993), Northridge (1994), Kobe (1995), Turkey (1999) and Taiwan (1999)

The two primary objectives of the review were to ascertain the degree that findings and recommendations evolving from the various missions have influenced earthquake risk mitigation measures in New Zealand and to highlight areas where the reconnaissance procedures should be modernised to meet current needs and expectations.

Coincidental with the review was the establishment of the Natural Disaster Investigation Coordination Group (NaDIG). This group operates under the auspices of the Ministry of Civil Defence and Emergency Management recognising the potential benefits to be gained from first-hand exposure to natural disasters both here and offshore. The protocols developed by NaDIG will be discussed and its influence on the Society’s reconnaissance scheme considered.

Paper P12: [Read]

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