POST-EVENT PLASTIC FAILURE ANALYSIS OF SHEAR FRAMES FROM THEIR PRE-EVENT ELASTIC RESPONSE

We demonstrate that plastic failure loads of shear frames can be inferred from their elastic ambient response. The interstory plastic mechanism force is derived for moment-resisting (rigid) frames as a function of two measured elastic (low-amplitude) frequencies. Structural health monitoring techniques are traditionally devised for “post-event” assessment of structures after exposure of a facility to a potentially damaging loading event such as strong earthquakes or blasts. The knowledge of induced damage, its location, and severity in an otherwise functioningstructure, as important as it is, may be too late for precautionary preparations. Naturally, one is interested in identification of potential failure mechanisms and indicators prior to damaging events when a structure is responding to environmental loads elastically. Are post-event plastic failure loads identifiable from the pre-event ambient response? We answer this question by first deriving interstory shear stiffness values from a set of measured ambient frequencies that are then incorporated into post-elastic equilibrium equations for a closed-form expression of failure loads as a function of measured frequencies. We test our procedure using a typical shear frame example as proof of concept. To extend the relevance and applicability of the proposed procedure we consider uncertainties associated with the measured and estimated quantities and assess their effects in our model output. The closed-form solutions presented allow study of fully-stressed designs and we present the optimal stiffness distribution for such designs as another example. It is anticipated that temporal relevance of structural health monitoring techniques to “pre-event” assessment will be extended in the near future to such promising technologies as earthquake early warning systems.