Achieving safer, longer life steel bridges and other
structures is now a reality with the advanced technology of Ultrasonic Impact
Treatment (UIT).
UIT can help arrest and prevent fatigue cracking in welds
and base metals of members by enhancing the geometry of the weld, introducing
favorable compressive stresses and relieving residual tensile stresses caused
by the weld process that are internal to the weld. Eliminating the fatigue
cracking increases the service life of the structure and improves performance,
while reducing the overall lifetime cost of maintenance.
UIT is performed using a hand tool that weighs approximately
8 lb. The tool is attached to an electronic control generator and water cooling
system.
Treatments can be applied under normal operating conditions,
causing minimal or no traffic disruptions. And unlike other weld treatments,
UIT results in little noise.
The procedure can be carried out in approximately one-tenth
the time needed for conventional retrofit procedures.
The use of UIT can allow structural design engineers to
employ fatigue-prone details that they would otherwise not be able to use, such
as socket joints.
UIT can be applied to the socket joint welds, for example,
to prevent fatigue. Because fatigue-resistant designs are more difficult to
construct and more expensive, UIT is a solution that allows engineers to use
more cost-effective designs that are still fatigue-resistant.
Invented by Dr. Efim Statnikov in 1972 and used by the
Soviet Union in its Naval and Aerospace programs, UIT was first demonstrated in
the U.S. in 1995.
In 1996, full-size girder members were tested at
FHWA’s Turner-Fairbank Highway Research Center in McLean, Va., which led
to an FHWA-funded research program at Lehigh University.
This research program has studied the effects of UIT on more
than 23 full-size structures. The studies conclusively demonstrated that the
use of UIT technology significantly extends the fatigue life of dynamically
loaded members.
In 2002, FHWA extended the program’s funding to cover
further tests on the use of UIT on bridge components made of advanced,
high-strength steels.
Additional research on UIT also has been sponsored by the
Texas Department of Transportation. The research study at the University of
Texas-Austin examined the use of UIT on socket joints, which are found on
thousands of fatigue-prone traffic sign and light pole structures nationwide.
The testing demonstrated that through the use of UIT the life expectancy and
fatigue characterization of components such as mast arms in traffic signal
poles and the area at the base of light and sign structures can be
substantially improved, significantly increasing the safety and reliability of
these structures.
To date, UIT has been applied on bridges in Georgia, Texas
and Virginia. These projects demonstrated that UIT could easily be used in not
only a manufacturing environment but also in the field as a standard
maintenance and repair technique.
The Virginia Department of Transportation (VDOT) used UIT on
four bridges on I-66 in Warren County that were experiencing fatigue cracking.
The fatigue cracks were occurring at the end of the fillet welds at the top of
the connection plates. VDOT decided to weld the connection plates to the
compression flanges and perform UIT on the weld terminating areas to prevent
fatigue cracking in these areas in the future.
In Georgia, UIT was used on a highway bridge over Allatoona
Lake that had been in service for 20 years. Numerous fatigue cracks were found
at the main girder beams. Repairs were performed by hole drilling or welding,
followed by application of UIT. Some undamaged parts of the bridge also were
treated with UIT to reduce the chances of future cracking.
As Virginia’s and Georgia’s experiences show,
UIT is both a viable and economical repair technique that can substantially
enhance the service life of steel bridges and other structural components that
are prone to fatigue, resulting in safer and more reliable structures for all
of the traveling public.
