Here’s video from a security camera that was across the street and captured part of the bridge as it collapsed:
According to a bridge specialist I saw on Fox last night, this bridge was built using the pin and hanger method. Well, in this piece about a bridge in Texas, the pin and hanger method was described at “old.”
Innovative techniques in precast concrete bent cap connections are being used by the Texas Department of Transportation as it replaces an antiquated bridge over Lake Belton along State Highway 36 in Bell County.
Groundbreaking for the $20 million project took place in September 2002 to replace the existing bridge that was constructed in the early 1950s. “The existing structure is 45 to 50 years old,” said Paul Hoffman, Belton area construction inspector for TxDOT. “It was built using the old pin-and-hanger method, and it’s just tired.”
I don’t know if this method was considered outdated back in 1967 when the bridge was first constructed, but in 2005, inspectors said the 35W bridge structure was insufficient. Here’s more information about the bridge:
The National Bridge Inventory contains a report on this bridge from 2003. It reports the following items:
* Deck Condition: Fair.
* Superstructure Condition: Poor.
* Substructure Condition: Satisfactory.
* Scour: Foundations determined to be stable.
* Bridge Railings: Meets currently acceptable standards.
* Structural Evaluation: Meets minimum tolerable limits to be left in place as-is.
* Water Adequacy Evaluation: Superior to present desirable criteria.
* Bridge Sufficiency Rating: 50%
A University of Minnesota Civil Engineer in a report to MN-DOT recently noted that this bridge is considered to be a non-redundant structure. That is, if any one member fails, the entire bridge can collapse. A key factor is that there are only four pylons holding up the arch. Any damage to any one pylon would be catastropic. The textbook example of a non-redundant bridge is the Silver Bridge over the Ohio River. It failed shortly before Christmas in 1967 resulting in 46 deaths. A single piece of hardware failed due to a tiny manufacturing defect. But that piece was non-redundant, and the entire bridge collapsed into the icy river. Today, bridge engineers design bridges so that any single piece of the bridge can fail without causing the entire bridge to collapse. It is tragic that the I-35W bridge was built a few years too early to benefit from that lesson.