The new overcrossings, designed in the framework of the A14 widening, are investigated taking into account the soil-structure interaction and evaluating site amplification effects on the definition of the seismic input. Bridges with almost the same geometry have been used within a distance of about 50 km: the overcrossings have a total length of about 80 m and are divided into three spans. The deck has a steel-concrete composite cross section, with variable high I-shaped girders. The isolation system is constituted by elastomeric devices over the two piers and by sliding bearings at the abutments. Piers are founded on pile groups and bank seats are carried on piles driven through fill.
A large number of laboratory and in situ tests are available for the area from which the soil profile and properties are deduced. The seismic action is defined at outcropping bedrock by means of real accelerograms, consistently with the area hazard level. Local response analyses are performed to determine the site amplification effects.
Soil-structure interaction and site effects are analysed with reference to five soil profiles, representative of the area. Responses of isolated and non-isolated solutions are compared. Soil-structure interaction may be significant in both cases: with reference to the non-isolated solution, the soil-foundation flexibility may sensibly modify the dynamics of the structure by lengthening the fundamental periods while in the isolated case, the increment of the fundamental periods may produce greater structural displacements which may compromise the efficiency of the isolation system. In both cases, site effects have to be considered to account for amplification effects at the deposit fundamental frequencies.