Ultrasound Method Diagnoses Stress Before Bridges Crack

In a typical situation, a pin and hanger connection is used to suspend an inner span from an outer support span; it also accommodates thermal expansion. But because the connection is located at the expansion joint, road deicing salts may wash down through the joint and cause corrosion at the pins. If the corrosion is sufficient to lock up the pins, it can cause additional forces on the connection, leading to fatigue cracking and eventual failure.

NIST is experimenting with an ultrasound technique, which has the potential to determine connection stress prior to development of fatigue cracks. To evaluate its method, NIST designed and constructed a pin and hanger simulation facility at its Boulder, Colo., laboratories. Strain gauges were mounted on the hangers, and the strains were measured as load was applied. At the same time, ultrasonic measurements were made at the gauge locations.

Strains were predicted from the ultrasonic data and compared with values measured by the strain gauges. "Overall good agreement was obtained between ultrasonic and strain gauge results," the scientists report.