Scour Team Helps Avert Catastrophe
Early spring flooding threatened the stability of bridges in Minnesota and North Dakota, endangering the lives of anyone using them. In response to these potential catastrophes, the FHWA-sponsored bridge scour research team was deployed on April 1 from the U.S. Geological Survey (USGS) District in Louisville to the upper Midwest.
Record flooding on the Minnesota River undercut this bridge abutment on a U.S. Highway 212 bridge. Two hours after this photo was taken, the abutment collapsed. Two hours after the inspection found scour under the abutment foundation, the approach span of the Highway 212 bridge over the Minnesota River collapsed. Another inspection found the Swift County highway bridge over the Pome de Terre Creek in similar danger of collapsing. It was immediately closed.
Using state-of-the-art instrumentation, the team helped local researchers in North Dakota collect bridge scour data, Minnesota DOT bridge inspectors make underwater inspections, and USGS and Army Corps of Engineers offices complete evaluations. They were able to measure discharges that would have been extremely difficult or impossible to measure with conventional equipment. This particular flood made the equipment even more necessary because of the intense cold combined with record flooding.
(Top) Harry Hitchcock, Dave Mueller, and Steve Darnell of the USGS scour inspection team prepare to take measurements at a bridge over the Mississippi River in St. Paul, MN. (Bottom) Wood and debris gather in the flood plain of a bridge over the Mississippi River. The unseen damage at the river bottom is far worse.
The team was equipped with a high-tech Acoustic Doppler Current Profiler capable of measuring three dimensional velocities along an entire water column and flow depths, a digital echo sounder, an OmniStar Global Positioning System (GPS) capable of constantly tracking boat locations with 1-m accuracy, a remote controlled boat for monitoring under bridges with low clearance, a scour board for quick and easy visually positioned measurements, a boom truck with 50 k sounding weight and Price AA velocity meter, and a hand-held laser range finder capable of calculating horizontal distance and vertical displacement of objects within sight.
Even with this equipment, the elements ruled the research. On one occasion the ropes that were used to deploy some of the instrumentation froze in such a solid loop that they had to be thawed in a warehouse before they could be used.
This flood survey trip was especially productive because catastrophes were averted. It also brought FHWA-sponsored researchers together with State DOT bridge inspectors. The bridge inspectors benefited from the well-equipped research team's capability to make underwater measurements, and the research team benefited immensely from the State DOT's direction of bridges to select for scour measure- ments.
-- Sterling Jones.
Eddy Currents Keep It Together
Can cracks in welded steel structures be detected with out taking the structure apart? The Nondestructive Evaluation (NDE) team has been developing a new method for using eddy currents to detect cracks in welded steel structures without taking anything apart. The eddy current method is a traditional NDE technique that is typically used on non-ferromagnetic materials in the aerospace and power industries. The method involves inducing a magnetic field in the surface of a metal, and measuring perturbations in the field caused by cracks or other defects.Research to date examines use of a bi-directional differential probe specially designed for use on ferromagnetic weld metal. The probe has been tested to determine its effectiveness for detecting surface breaking cracks in the weld metal and heat affected zones. Research has also been done on the effects of conductive and nonconductive coatings used on bridges. It has been shown that the eddy current method can detect transverse cracks in weld crowns and longitudinal cracks at the toe, as well as other defects.
The method has been field tested on a welded steel box girder. The field tests showed that the method could effectively penetrate a three component bridge coating (primer, intermediate, and top coat) and detect defects in the weld metal, with no surface preparation required. Further tests to determine if the method can be used to measure crack depth are planned for this summer. Findings to date are available in the book, Nondestructive Evaluation of Infrastructure, published by the American Society for Nondestructive Testing.
-- Glenn Washer.
