Improving the safety of steel bridges through more accurate and affordable modeling of connections

This thesis develops an improved and affordable method to model the connections of steel truss bridges with a view of improving their design, analysis and safety. The issue came to the fore when the I35-W Bridge in Minnesota collapsed on August1, 2007 [NTSB, 2008]. After many Finite Element Analyses, the National Transportation Safety Board (NTSB) found the buckling of an under-designed gusset plate to be the main cause of the disaster. After this tragedy, the Federal Highway Administration (FHWA) focused its attention on all the 465 steel deck truss bridges present in the National Bridge Inventory [NTSB, 2008], and provided guidelines to bridge owners to verify the safety of these kind of bridges. The present work focuses on the means to assess the structural safety for these particular types of steel truss bridges, and proposes a method for the correct and efficient modeling of the connections. It starts with the basic question: “how safe is it to consider all the connections in these types of bridges as rigid joints?” The work is organized in three parts: • the first gives an overview of the problem of the structural safety of complex structure such as bridges, and proposes the use of the sub-structuring method, [Przemieniecki, 1968], [Bontempi and Arangio, 2008]; • the second part reviews the relevant literature, standards and codes. Both the Eurocodes and the American codes are missing a way to assess the stiffness and strength of gusset plate connections. This work aims at filling the gap between advanced computing methods that can be brought to bear on a failure investigation, and the rigid connections, linear beam analysis typical of routine design, [AASHTO, 1994], [Astaneh, 2010], [Ballio and Mazzolani, 2005], [Crosti and Duthinh, 2010], [Chambers and Ernst, 2005], [EN 1993-1-8, 2003]; • in the third part, the proposed method is applied to the I-35W Bridge. The I-35W is classified as a Fracture Critical Truss Bridge, meaning that the failure of one major element would cause the collapse of the whole structure. The method makes use of the detailed finite element models of the NTSB and FHWA to find the strength and stiffness of the joint in question and replace it with five spring elements. The method provides accuracy and substantial computational savings for repeated load cases, particularly if many joints in the structure are similar, [NTSB, 2008], [FHWA, 2009]. The goal of the thesis then is to develop accurate but computationally affordable connection models to improve global analysis and thus allow bridge owners to predict the effects of joint deterioration, design deficiencies and to guide the requirements for structural monitoring..