The aim of this paper is to discuss the seismic improvement of a RC building according to the new Italian Code: the building was designed according to an old seismic code, was detected to be realized with poor reinforcement details and with low strength concrete. It is placed in a sloping and sliding ground and relevant differential vertical displacements are detected too. For these reasons it appears as a very interesting case study.

The RC structure was design according to allowable stress method without capacity-design criteria both in elevation and in foundation (RC pile foundation). It was built in 80's in the little town of Offida (in Marche Region) and it is about 70 kilometers from epicentral area of 2009 L'Aquila Earthquake. The site is a sloping and sliding one and some nearby RC buildings of similar design approach collapsed due to foundation movements. Deep pile foundations have varying lengths due to the stratification of the soil.

Due to L'Aquila Earthquake (01:32:39 UTC 06/Apr/2009 ML = 5.80) it suffered a relevant non-structural damage in spite of a low estimated maximum acceleration (a_g/g = 0,03). This estimated acceleration is lower of both the original design ground acceleration (a_g/g = 0,07 - almost equal to actual Serviceability Limit State) and the actual Ultimate Limit State ground acceleration (a_g/g = 0,23). A comprehensive in situ tests on concrete and steel reinforcement was carried out (Scam Laboratory of Chieti-Pescara University) in order to understand the origin of this damage: poor reinforcing detailing, low strength concrete (about an half of design strength) and relevant differential vertical displacements are detected. In particular different inclination of columns to the vertical are detected due to building phases differential vertical displacements.

This unacceptable horizontal deformability needs an improvement of horizontal stiffness of RC frames. On the other side the new Italian Seismic Code demands that ultimate failure loads in columns can be assigned in the elastic range to the foundation system. The design of the seismic improvement of the supported structure, according to the actual foundation characteristics, is the topic of the paper.