Nationally, concerns have been raised regarding the relatively new design approach of combining the use of integral abutments with horizontally curved steel I-girder bridges. In order to address concerns regarding the superstructure behavior, this research experimentally and analytically investigated four in-service, horizontally curved, steel I-girder bridges with integral and semi-integral abutments. These bridges are located at the major interchange of Interstates I-235 and I-80. For the research, a monitoring system was installed on the bridges using an array of strain gauges. The implications of the critical data that the monitoring system produced will enable further development of design specifications for similar bridge types, particularly with respect to thermal effects. In addition to the measured field data, an analytical model for one of the instrumented bridges was established using a commercial finite element analysis software package. Several conclusions were formed from both of the experimental and analytical results. First, the short term experimental results produced moment distribution factors that were most heavily influenced by the degree of curvature. Also from the short term results, a simplified analysis method, referred to as the V-Load method, provided only an approximate preliminary assessment of the lateral bottom flange bending based on the degree of curvature with minimal skew. Next, the long term experimental results indicated that an effective thermal range of 100⁰F may cause up to 12 ksi of additional stress in the girders due to restrained expansion and contraction of the bridge. Lastly, results from the analytical investigation indicated that the stresses in the lower flange of the girder, due to applied thermal loads, were greatest at the fixed pier locations. These stresses were mostly due to lateral flange bending caused by the fixed pier restraining lateral movement of the curved girder. Based on the experimental and analytical investigations, the findings within this research suggest that similar bridges require a refined method of analysis when incorporating integral abutments and fixed piers. More importantly, bridges with increased curvature and skew may require special attention in future practice as lateral bending stresses may increase due to temperature loads.