a b s t r a c tA linear oscillator simultaneously subjected to stochastic forcing and parametric excitation is considered. The time required for this system to evolve from a low initial energy level until a higher energy state for the first time is a random variable. Its expectation satisfies the Pontryagin equation of the problem, which is solved with the asymptotic expansion method developed by Khasminskii. This allowed deriving closed-form expressions for the expected first passage time. A comprehensive parameter analysis of these solutions is performed. Beside identifying the important dimensionless groups governing the problem, it also highlights three important regimes which are called incubation, multiplicative and additive because of their specific features. Those three regimes are discussed with the parameters of the problem.

a b s t r a c tThis work focuses on the stochastic version of the linear Mathieu oscillator with both forced and parametric excitations of small intensity. In this quasi-Hamiltonian oscillator, the concept of energy stored in the oscillator plays a central role and is studied through the first passage time, which is the time required for the system to evolve from a given initial energy to a target energy. This time is a random variable due to the stochastic nature of the loading. The average first passage time has already been studied for this class of oscillator. However, the spread has only been studied under purely parametric excitation. Extending to combinations of both forcing and parametric excitations, this work provides a closed-form solution and a thorough analytical study of the coefficient of variation of the first passage time of the energy in this system. Simple asymptotic solutions are also derived in some particular ranges of parameters corresponding to different regimes.

Recent events such as natural catastrophes or terrorist attacks have highlighted the necessity to ensure the structural integrity of buildings under exceptional events. For more than 10 years, the University of Liege is strongly involved in researches on the response of structures further to such exceptional events through participations to national or European research projects. In particular, the University of Liege has contributed very recently to a European research project entitled ROBUSTIMPACT investigating the behaviour of steel or steel-concrete composite building structures subjected to impact loading and proposing simplified procedures to predict the response of such structures under the considered scenario. As a contribution to this research project, an experimental test campaign was realised in Liège; the objective was to study the behaviour of beam-to-column joints, of column bases and of most of their constitutive components under impact loading and, in particular, to highlight the influence of the strain rate effects on the response of the studied joints and components. The proposed paper first describes the conducted test campaign with all the required details about the tested specimens and the testing setup; then the analysis of the so-obtained experimental results is presented. Finally authors' views on how to account for the strain rate effects in analytical approaches for joint design is briefly expressed. .