In Turkish design practice, slab on steel composite I-girder bridges have been designed to span between 50 and 80 meters. To date, modified versions of the AASHTOLFD (Load Factor Design) or ASD (Allowable Stress Design) requirements are adapted in Turkey. The recent switch of the U.S. bridge codes to Load Resistance Factor Design (LRFD) method also necessitates the calibration of the Turkish LRFD design code that is under development. In this on-going research, it has been determined that the current Turkish design truck is not very appropriate to be used in design of bridges with span lengths in excess of 50 meters. The main aim of this study is to define a new type of live (truck) load to be used in the basic gravity load combination, as well as to develop the corresponding load factors to be implemented in the design of slab on steel composite I-girder bridges. In this scope, usually a target reliability index is selected to reflect the safety level of current design practice based on the uncertainties associated with the design parameters. For the basic gravity load combination, which includes the dead and live loads, a minimum target reliability of 4.00 is selected, instead of 3.50 that have been used in the U. S. In the statistical computations of the reliability index, the quantification of uncertainties is made based on local data supplemented by information compiled from relevant international literature.