Is it convenient to retrofit structures for earthquake protection?

Under which conditions is it economically convenient to seismic retrofit a structure? This paper present a procedure whose results allow to give a simple answer to the above question, central in earthquake engineering and, in a broader context, in any man-made activity. The procedure uses, as a starting point, the results of a standard reliability analysis conducted on the structure in its present state and after retrofitting. Once these are known, expressed in terms of mean rate of exceedance of specified limit states, it is shown that it is possible to compute whether the upgrading should be made after all and how convenient it is. The assumptions to make the problem tractable are clearly listed and appear, in the authors's viewpoint, quite reasonable. The final results can be expressed with a simple formula. The method is finally applied to an example case, the bridges on an the Naples - Bari Italian highway stretch [Donferri et al., 1998] already studied and presented by the authors, which had been developed up to the computations of the mean rate of failure.