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Ground motion characteristics and scaling: how far can intensity measures go?

Amplitude scaling of ground motion records is a widely used, yet admittedly controversial approach, to globally modify natural accelerograms to match any level of seismic intensity. While it is relatively easy to consider low scaling factors of the order of 0.5 or 2, it is quite more difficult to accept the large factors of 4 or 8 (and above) that may be needed to force a well-designed structure to collapse. Ignoring issues of spectral shape (or frequency content) and duration as larger magnitudes and smaller distances are involved, essentially introduces unwanted bias into seismic assessment whenever scaling grows out of bounds. Still, significant research so far has shown that this error is heavily dependent on the choice of the conditioning intensity measure. Generally, improvements in performance are gained as the intensity measure becomes more and more structure-specific. Thus, the peak ground acceleration can introduce sizeable bias when used for moderate or long-period structures, the spectral acceleration at the first mode period can offer reasonable performance for first-mode dominated buildings up to ductilities of 2-3, while a geometric mean of several spectral acceleration values can significantly reduce the magnitude of errors even for large scaling levels. All in all, a hierarchy of different levels of intensity measure complexity is available, each offering a different compromise of accuracy versus ease-of-application. Pertinent choices are presented and their pros and cons are discussed with an outlook for practical application in performance-based assessment and design.

Author(s):

Dimitrios Vamvatsikos    
National Technical University of Athens
Greece

Athanasia Kazantzi    
J.B. ATE
Greece

Agathoklis Giaralis    
City University London
United Kingdom