Seismic assessment of unreinforced masonry structures : an investigation of the Longfellow Bridge masonry piers

Abstract

Infrastructure in the United States is comprised of numerous structures that are decades old. The Longfellow Bridge is one of the oldest pieces of this infrastructure that is still in use and has become one of the most historic structures of the Boston area. Currently, the steel superstructure of the bridge is in poor condition while the masonry piers and abutments remain in good condition. In the near future, a major replacement of the superstructure will be required. In order for the masonry elements to be approved for a renewed design life, they must be assessed for their ability to withstand seismic loads. This assessment presents an investigation of original construction documents and identifies critical components of the bridge that require more in depth analysis. It also shows a qualitative review of expected seismic activity for the region surrounding Boston. A review of current theory related to unreinforced masonry structures is introduced and analysis is then performed on critical wall sections. Specific ground motions are applied using both constant acceleration and impulse loadings to the structure. The results of the analyses reveal a need for further investigation into retrofitting schemes as there is not a sufficient factor of safety that exists with certain pier elements. Furthermore, a failure envelope is developed and presented for several types of impulse loads in order to serve as a basis for understanding the behavioral response to potential earthquake loading.