21 December 2017

Performance-Based Seismic Design in real life: The good, the bad and the ugly

Dimitrios Vamvatsikos, Assoc. Professor N.T.U.A.

Summary:  Designing a structure to deliver the desired performance under the uncertainties of hazard, materials and questionable models, is largely the Holy Grail of structural engineering. This is the premise of performance-based seismic design whereby prescribed levels of structural response, loss or casualties are allowed to be exceeded at a maximum rate less than a given mean annual frequency. Even at its simplest structural response basis, the inverse probabilistic nature of the requirements has not yet allowed a satisfying solution. Resolving a problem of design, where the functional relationship between the design variables and the performance objectives is not invertible, essentially requires iteration. Each iteration for a nonlinear structure means a cycle of design and analysis, where the latter is a comprehensive performance-based assessment involving nonlinear static or dynamic runs. Despite the apparent usefulness of such approaches, their implementation is not trivial. Moreover, the link between a performance objective and the resulting design becomes fundamentally obscured within the outcome of a series of numerical analyses. A number of simpler methods have appeared in the literature claiming to offer this coveted prize. To promote adoption they often display some degree of compatibility with the current seismic codes and yet they may require heavy computations or strict assumptions, sometimes offering a useful but partial solution, perhaps delivering something other than what the user expected, or even failing to deliver altogether. This does not necessarily detract from the usefulness of each method, but it does certainly mean that some differentiation among approaches should be maintained, despite all of them being bundled underneath the moniker of “performance-based”. During the presentation we shall attempt to shed some light on the viability of different approaches and help interested users decide on what may best suit their needs.