This paper presents an approach to use strain data from a multi-girder, composite steel bridge for long-term Structural Health Monitoring (SHM). The bridge being studied is part of a research project at the University of Connecticut in which long-term SHM systems are being installed on a series of bridges throughout the State of Connecticut. Strain data is collected from normal truck traffic to determine live load stresses, load distribution factors, and the location of the neutral axis in each girder. Known weight trucks were used along with a finite element analysis for verification of the behavior. The long-term monitoring approach is based on determining the live load distribution factors, peak strains, and the neutral axis locations. The goal is to use existing, readily applied technology for SHM for long-term use on bridges that have raised concerns, due to corrosion noted in routine visual inspections, overloading, or fatigue sensitive details. The SHM system proposed can be used on a continuous basis to determine if there are significant changes in the structural behavior that would be indicative of major damage to either the girders or the concrete deck.