MnDOT steps up


MnDOT has closed the Highway 23-Division Street bridge after three gusset plates were found bulging.

The bridge is designed along the same vein as the ill-fated I-35W bridge. Shortly after the I-35W bridge collapsed, Gov. Pawlenty ordered bridges in the state inspected. A day later, inspectors looked at the bridge (photo above).

They looked at the cracks on the surface:


And the rust:


And determined that the bridge could stay open.

At a previous inspection in 2005, it was given a sufficiency rating of of 56.3 out of 100. Anything lower than 50 and a bridge is considered to be in need of replacement. After the inspection last August, the bridge rating was pegged at 57.3. The bridge was opened in 1959, eight years before the I-35W bridge.

But in January, the National Transportation Safety Board said the design of the bridges, or more specifically the gusset plates, was inadequate.

On Thursday, Jim Povich, an assistant district engineer for the Minnesota Department of Transportation, told the St. Cloud Times that the gusset plates were found distorted during a recent inspection.

“We decided to err on the side of safety,” Povich said.

In the wake of the I-35W bridge collapse, that seems to be exactly what Minnesotans want to hear, and the best way to restore confidence in the embattled agency.

  • Bridge Collapsed by Pavement Expansion

    Q: What force caused the collapse?

    A: Structural Engineers consider gravity and environmental forces in the design of a bridge. Those forces are well-known, were accounted for and were not causative. A much less-known force, P.E.F. or Pavement Expansion Force was present at the site for at least 20 years. The unique feature of this force is that it grows larger with time, often reaching the compressive limit of concrete.

    Q: How did this P.E.F. act on the bridge in order to cause the observed damage and finally separate the continuous steel truss?

    A: P.E.F. worked at the level of the bridge deck. When portions of the deck were removed, this force moved down into the steel truss through steel “shear connectors” embedded in the concrete deck. Over a month of deck removal, the force in the compression members of the truss grew larger, causing these members to gyrate under the combined gravity and P.E.F. When they reached their limit, say 14,000 p.s.i., the gyrating members broke the connections at their ends. Since the largest compression force on this truss is just past the river piers, that is where the separation occurred.

    Q: What is P.E.F.?

    A: P.E.F. only occurs in concrete pavement. Fine material from the supportive sand, works its way up into the joints in the pavement due to thermal changes. This causes the original width of the joint to widen over time. This increase in width produces P.E.F . if a reacting force is available in opposition to it .

    In conformance with the American Society of Civil Engineer’s Ethical Code, which requires a civil engineer to consider the public his/her client, the author is publishing his findings to the public so that this tragedy need not occur again.