Comparison of block-shear and whitmore section methods for load rating existing steel truss gusset plate connections

Many transportation agencies are now evaluating and rating their inventory of steel truss bridge gusset plate connections after the collapse of the I35W Bridge in Minneapolis. Modern design methodologies are being used to evaluate inventory gusset plate connections that were designed using allowable stress methods. One such implementation is evaluation of gusset plates using block shear, an approach that was developed well after many of the nation's truss bridges were designed. Vintage designs commonly used the Whitmore section to proportion gusset plate connections. Differences between rating outcomes from block-shear analysis at strength conditions with designs that employed allowable stresses on the Whitmore section method were identified and expected outcomes for rating of gusset plate connections are proposed. Examples are shown to highlight the similarities and differences in these two approaches to assist rating engineers in implementation and expected variabilities on the outcomes based on connection geometric proportions and material properties.