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Mexico highway network has more than 14,000 bridges. Most of them are reinforced concrete (RC) structures. The bridges design process incorporates the use of an overstrength factor that is not justified and has received little attention in published works. Mexican regulations allow using an overstrength factor for buildings in the range of 2-3, to reduce the design spectra as a function of the selected seismic behavior factor. However, for bridges, a single factor of 1.5 is proposed independent of any design parameter. The bridges in Mexico are mostly simply supported structures with maximum span lengths of 50 m. A relevant and distinctive aspect of the bridges designed in Mexico is the large load amplitudes of the trucks used to define the live load and the high seismic activity in the country. This study determines overstrength factors of a family of medium-length RC bridges composed of simply supported superstructures and substructure made up of single and multi-column RC piers. Non-linear dynamic analyzes using a set of 80 accelerograms were carried out. The results show that the height of the bridges and their seismic location are relevant parameters in the overstrength of the structures. Finally, analytical expressions are proposed to assess the overstrength factors of a very common bridge typology in Mexico and the world. K E Y W O R D Seffect of pier height on overstrength factors, empirical equations to obtain overstrength factors in rc bridges, influence of the type of substructure on overstrength factors, nonlinear dynamic analysis of RC bridges INTRODUCTIONBridges are essential structures for the economic development of a country. During an earthquake occurrence, it is important to limit damages in bridges to reduce the economic impact of the event. Despite the great seismic activity in Mexico, bridge design regulations in Mexico are scarce and the generation of bridge standards follows a much slower process than the standards for other structures as buildings.The national highway network in Mexico included, at the end of 2014, 16,656 bridges (8493 in toll-free roads and 8163 in toll roads) located mostly in highly seismic or intermediate seismic areas. 1 About 31% of the structures are in areas of high seismicity and 29% in areas of intermediate seismicity.Bridge regulations recognize that the real strength of the structures when subjected to lateral loads beyond the material yielding, is greater than the nominal design strength, due to various factors that include the design methodology, the

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