Simple Probability-Based Assessment of Bridges under Scenario Earthquakes

A simple probability-based method is presented for assessing the state of existing bridges under scenario earthquakes in order to define their current safety level and eventually decide the need for strengthening interventions. The method is based on the probabilistic description of the bridge capacity, expressed in terms of input peak ground acceleration, to fulfill a predefined performance level. All uncertain quantities influencing the bridge response are treated as random, so that the bridge response is also random. A simple damage function is introduced that yields the actual damage level, which is then to be compared to a target performance level (PL). Three PLs have been defined, in terms of requested/available ductility ratio, that are to be checked for, depending on the bridge importance. The probability of exceeding each PL as a function of the peak ground acceleration (PGA) gives rise to a so-called “fragility curve” that is compared with a target fragility curve, here defined as the fragility curve pertaining to an optimally designed bridge built at the same site and having the same typology and materials. Thus, by comparing the exceedance probability of the bridge under the scenario PGA with the corresponding target, the capacity of the bridge to fulfill the requirements of each PL can be assessed. An application of the method has been presented, relative to the road network of the city of Catania, in Sicily, South Italy, that allows one to identify the bridges for which strengthening interventions are necessary in order to avoid functional disruption of the network under a scenario earthquake.