Engineers Understanding of Earthquakes Demand and Structures Response

Abstract

In this paper the author discusses the engineers understanding of the strength and displacement demands imposed to structures by earthquake motion, and of the structures capacities to withstand these demands, in a historical perspective. Without any claim of completeness or accurate critical assessment of the significance of various seismic events or of the scientific development of knowledge, the essential relevance of the lessons learnt from some seismic events is critically examined in parallel with the development of structural dynamics. The story moves from a claimed use of some base isolation measure in the temple of Diana at Ephesus in the sixth century B.C., continues with the renaissance treaties, where in one case only some emphasis is placed on how to build a safe structure and passing through the first understanding of dynamic equilibrium arrives to the ages of enlightenment and the Lisbon earthquake of 1755. The breakthrough towards modern seismic analysis is clearly identified with the two earthquakes of San Francisco (1906) and Messina (1908). In particular it is discussed how most of the fundamental principles used for a century had already been stated after the second one. Spectra, ductility and performance based design are then identified as further milestones derived from earthquake evidence, to conclude with a critical appraisal of some major misunderstanding of structural response, with the merits of displacement based approaches and to eventually close the circle opened with the temple of Diana with modern base isolation techniques.

Graece magnificentiae vera admiratio exstat templum Ephesiae Dianae CXX annis factum a tota Asia. In solo id palustri fecere, ne terrae motus sentiret aut hiatus timeret rursus ne in lubrico atque instabili fundamenta tantae molis locarentur, calcatis ea substravere carbonibus, dein velleribus lanae ¹.

Pliny, Naturalis Historia, Liber XXXVI, xxi, 95 [Something that should be really admired of the Greek magnificence is the temple of Diana at Ephesus, constructed in 120 years with the contribution of all Asia. It was built on a marshy soil, locating charcoal and wool furs under its foundation, to reduce its sensitivity to earthquakes and to avoid locating such a big mass on unstable soil].

This paper was the subject of the inaugural lecture for the beginning of the academic year at the Università degli Studi di Pavia on 18 January 2010, (1185th year from the Capitolare of Lotario, 649th from the Studium Generale institution), “without any claim of completeness or accurate critical assessment of the significance of various seismic events or of the scientific and technical development of knowledge, but instead illustrating a history of events and ideas seen from the point of view of the author, influenced by his training, the places in which he has lived, the master teachers he had”