SUMMARYIn many parts of the world, the repetition of medium-strong intensity earthquake ground motions at brief intervals of time has been observed. The new design philosophies for buildings in seismic areas are based on multi-level design approaches, which take into account more than a single damageability limit state. According to these approaches, a sequence of seismic actions may produce important consequences on the structural safety. In this paper, the e ects of repeated earthquake ground motions on the response of single-degree-of-freedom systems (SDOF) with non-linear behaviour are analysed. A comparison is performed with the e ect of a single seismic event on the originally non-damaged system for di erent hysteretic models in terms of pseudo-acceleration response spectra, behaviour factor q and damage parameters. The elastic-perfect plastic system is the most vulnerable one under repeated earthquake ground motions and is characterized by a strong reduction of the q-factor. A moment resisting steel frame is analysed as well, showing a reduction of the q-factor under repeated earthquake ground motions even larger than that of an equivalent SDOF system.

Abstraet:The present work reports an investigation on the rheological behaviour of fresh cement pastes. Three types of Portland cement were used. The water/cement ratio was varied in the range 0.35 + 0.40. The rheologieal tests were performed using the coaxial-cylinder vis¢ometer Rotovisko-Haake RVll. The material to be tested was subjected immediately after mixing to a constant shear rate until a steady value of shear stress (equilibrium value) was attained. All the pastes tested exhibited a flow behaviour of the partially thixotropic type. A rheological model consisting of a sole constitutive equation ~ = ~(~, t) was defined according to the Cheng and Evans approach. The parameters of the constitutive equation were correlated with the cement specific surface and the water/cement ratio.

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SUMMARYThis paper concerns the design of passive base isolation systems characterized by a bilinear hysteretic behaviour. The study refers to the case where the structure to be isolated (superstructure) vibrates according to the ÿrst mode. In this case the whole isolated structure can be modelled by a two-degreeof-freedom system. The base isolation e ectiveness has been evaluated for di erent characteristics of the device, namely mass, strength, elastic and plastic sti ness, by using mainly energetic quantities.The optimum values for the base device have been obtained by minimizing the input energy and the displacement of the superstructure. Conclusions are drawn for superstructures with a fundamental period of 0:5 s, a damping ratio of 5% and for three di erent kinds of earthquake ground motions. The study showed that the seismic input greatly a ects the behaviour of the isolated structure, and therefore the design ground motion must be carefully chosen, dependent on the characteristics of the site. A simple procedure that involves mainly linear dynamic analyses is proposed for the design of base devices used in conjunction with superstructures of any fundamental vibration period. The procedure produces good results in spite of its simplicity, and therefore it is suitable for practical use by design engineers.