Tennessee highway officials believe an impact from a vehicle’s oversized load is likely to blame for the April 1 partial collapse of a ramp structure at the I-75/I-24 interchange in Chattanooga. The impact caused the outer box beam and railing of the 148-ft-long bridge’s nearly 51-ft main span to fall onto an access ramp, injuring a motorist whose vehicle collided with the debris.
A forensic investigation conducted by the Tennessee Dept. of Transportation suggests a hardened piece of steel or similar material sliced through five 3⁄8-in.-dia reinforcing strands in the beam, which had a vertical clearance of 16 ft, 10 in.
TDOT Chief Engineer Paul Degges said in a press briefing that the interval between impact and collapse is uncertain, as a concrete railing may have held the damaged beam in place for several hours or even days. Given the absence of rust on the steel, “we believe it was a short timeframe,” Degges said, adding that contraction from changing temperatures may have ultimately led to the beam’s failure.
The 61-year-old bridge was widened in 2017 and received satisfactory to good condition ratings when it was last inspected in June 2018. Design and maintenance were not factors in the collapse, Degges added.
The bridge is scheduled to be removed within the next three years as part of a full reconstruction of the I-75/I-24 interchange scheduled to begin later this year. C.W. Matthews Construction Co., which was awarded the $132.6-million design-build contract in December, is also handling emergency repairs to the bridge, which TDOT inspectors determined to be structurally sound following the impact. The agency says the $300,000 repair work is expected to be complete by mid-April.