Structural design approaches

Two new approaches to structural design are gathering momentum within the structural engineering profession. Of the two, ' displacement-based design' has fewer implications for architects although it does provide a more rational basis for achieving acceptable levels of performance in design earthquakes. Rather than a structural engineer commencing a seismic design by calculating the stiffness and natural period of a building and then determining inertia forces, displacement-based design begins by specifying a maximum acceptable displacement and then determines the forces that condition causes.16 Displacement-based design will probably eventually replace the current method of force-based design.

Performance-based Seismic Design, which has considerably more ramifications for architects and their clients, is the second new design approach. FEMA 349 offers an explanation:

The concept of Performance-based Seismic Design (PBSD) is to provide engineers with the capability to design buildings that have a predictable and reliable performance and permit owners to quantify the expected risks to their buildings and select a level of performance that meets their needs while maintaining a basic level of safety. PBSD uses the concept of objectives, allowing the owner to specify an acceptable level of damage to a building if it experiences an earthquake of a given severity. This creates a 'sliding scale' whereby a building can be designed to perform in a manner that meets the owner's economic and safety goals.17

Performance-based Seismic Design, which takes a far more holistic view of seismic design than most current approaches that concentrate on life-safety performance, is currently under development.1 8 When applied to a particular building, the method begins by subjecting the building computer model to a series of previously recorded earthquake events approximating a design-level earthquake. Then, by utilizing a database that relates structural and non-structural damage and repair costs to peak drifts and accelerations, the total repair costs incurred can be determined. Eventually, losses such as incurred by down-time and casualties may be calculable.

With such a sophisticated and powerful tool to model damage and down-time costs, architects and engineers will be able to quantify many

▲ 14.25 A carbon fibre strip glued onto an unreinforced masonry wall to increase its shear strength. Hotel, Wellington.

of the issues that clients need to be informed of and make decisions about (Chapter 13). It offers potential for far more detailed explorations of design alternatives with respect to the cost implications of their seismic performance. Even if this design approach is not used in structural engineers ' design offices on a day-to-day basis due to its current complexity, it will be useful in comparing the performance of different structural systems and will inevitably influence seismic code developments.

Performance-based Seismic Design concepts have been applied to improving the seismic resilience of the University of California Berkeley campus. A seismic performance goal was to limit a post-earthquake campus closure to thirty days. With reference to one major laboratory building, Mary Comerio outlines a very detailed assessment of the vulnerability of lab equipment and other building contents. After computer modelling the building and shake-table tests of selected lab contents, equipment damage was determined and measures to mitigate it and consequent down-time were taken.19

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