The usual method for determining the in-plane buckling strength of steel trusses consists of determining the buckling loads for the individual compression members with an assumed degree of fixity at the ends of each member. The paper shows that a steel truss can buckle not only through the buckling of one of its compression members but also through the instability of one of its joints. It also shows that the end fixities of various members of a truss, which are functions of applied loads, cannot be determined in isolation, i.e., without considering the whole truss.A computer based iterative technique is presented for determining values of rotational restraints at the ends of truss members, and also for determining the buckling loads for individual compression members and joints of the truss. The method, which is implemented through a computer program, has been validated against model test results. It is expected that by using this method the reserve of strength in existing steel truss bridges, which results from designs based on approximate methods of analysis, could be realistically evaluated and utilized to allow the bridges to be used to their full capacity.