This steel
through-truss bridge had a "
fracture-critical" design with non-redundant load-bearing beams and joints that were each essential to the whole structure staying intact. An initial failure (perhaps by cracking) of a single essential part can sometimes overload other parts and make them fail, which quickly triggers a chain reaction of even more failures and causes the entire bridge span to collapse. In 2007 the
I-35W Mississippi River bridge in Minneapolis collapsed suddenly from slow cracking of a single undersized and over-stressed
gusset plate. In steel, these initial fractures begin small and take years to grow large enough to become dangerous. Following the Minneapolis incident, such age-related disasters in fracture-critical bridges are now avoided by finding and repairing cracks in a required thorough inspection every two years. Eighteen thousand bridges in the United States are labelled fracture-critical (from their design) and require crack inspections. The Skagit River bridge had last been inspected for cracks in August and November 2012 with only minor work needed. Besides fracturing, some bridges with critical non-redundant parts can also suddenly fail from buckling of compressive members (the opposite of cracking of tensile members). In through-truss bridges the critical compressive parts are the
top-chord beams running horizontally along the top of the bridge, parallel to the roadway edges. They carry most of the weight of the bridge and traffic. The chords are normally kept aligned and held in place by vertical posts, diagonals, and sideways sway struts. Top chords will quickly fold if their joints somehow become misaligned. Buckling damage is cumulative, but mostly happens from collision damage or overstresses rather than from age and corrosion. The vertical clearance for vehicles is limited by the portals and sideways sway struts. These are relatively low in older bridges. In Washington State bridges, the sway struts are often curved downwards at the outer ends, with less clearance above the outer lanes and outer shoulders. Tall loads then need to use the inner lanes for maximal clearance. These bridges are vulnerable to impacts by overheight vehicles, and such impacts were common. There was a known strike on this bridge that occurred on October 22, 2012, and investigators found evidence of several other impacts in years past. Bridge inspection reports dating back to 1979 frequently note damage caused by over-height vehicles, and an inspection report from late 2012 noted a three-inch gash in the steel. According to Charles Roeder, a professor of civil engineering at the
University of Washington in Seattle, through-truss bridges were a common bridge design in the 1950s (there are 10,200 through-truss bridges in the US), but "[i]f you take out some of the top framing, you set that bridge up for a stability failure." Before computers, bridge engineers analyzed truss forces by
slide rule, with each calculation being time-consuming. Although the
finite element method and
plastic design theory, both capable of analyzing redundant structures, had recently been formulated and had seen occasional use, they required significantly more calculation than the simple calculation methods for
statically determinate structures, which precluded the use of redundant structural members. A great number of bridges were being designed at that time, and there were insufficient design engineers available to design many bridges as
indeterminate structures. Nowadays, through-truss and other fracture-critical designs are avoided in most new bridges for moderate-sized spans. Using three or more parallel main beams or trusses allows the structure to survive a single component failure. ==Incident==