Yesterday the National Transportation Safety Board (NTSB) said interim findings point to a design error that led to undersized gusset plates used in the I-35W bridge in Minneapolis, which killed 13 people and injured more than 100 when it collapsed.

Gusset plates are the large, flat steel connectors that hold load-bearing columns in place, tying together the angled steel beams of the bridge's frame.

As Minnesota's Pioneer Press reports:

The Interstate 35W bridge might have been doomed all along. Federal investigators pinpointed a flawed design that dates to when engineers first drew up blueprints in the mid-1960s as the primary reason the Minneapolis bridge fell into Mississippi River on Aug. 1, killing 13 and injuring 145.

More than a dozen steel support plates suspected in the bridge collapse were deficient in size, according to safety officials yesterday, and a routine inspection would not have uncovered the problem. The NTSB will not reach a final conclusion until autumn on why the I-35W bridge in Minneapolis collapsed into the Mississippi River, as the board's investigation is still on-going, but preliminary findings show 16 gusset plates were too thin to support the weight they held.

"During the wreckage recovery, investigators discovered that gusset plates at eight different joint locations in the main center span were fractured," safety board Chairman Mark Rosenker said at a news conference (via Reuters).

Safety board investigators discovered the original design process of the bridge led to a serious error in sizing some of the plates of the bridge's main skeletal structure.

According to a press release at the NTSB's Web site:

Undersized gusset plates were found at eight of the 112 nodes (joints) on the main trusses of the bridge. These 16 gusset plates (2 at each node) were roughly half the thickness required and too thin to provide the margin of safety expected in a properly designed bridge.

Other gusset plates on the bridge were designed adequately, according to Rosenker.

Rosenker said investigators were "shocked" 16 large steel gusset plates were fractured during the collapse. Those plates are normally the strongest points on a bridge, the Pioneer Press reports: "In the history of this organization, we have never seen anything like this before," the Minnesota press reports Rosenker as having said.

Indeed, there have been hundreds of gusset plates on the bridge since its 1967 completion.

Investigators have not pinpointed why the apparent sizing issue occurred but suspect a calculation error. Rosenker all but ruled out maintenance issues such as rust and corrosion noted in annual bridge inspections, or the age of the infrastructure, as factors in the collapse.

The 16 broken plates were located at four paired locations on either side of the bridge. The location where the plates broke first is known as U10. Click image for larger view.
Credit: NHSB

The NTSB emphasizes that there is no evidence to suggest that the deficiencies in the various design review procedures associated with this bridge are widespread or even go beyond this particular bridge. There are 13,000 steel truss bridges in the United States. Investigators are not aware of any similar bridge failures of this type.

The safety board has issued a safety recommendation that addresses the design issue, recommending the following to the Federal Highway Administration (FHWA):

[For] all non-load-path-redundant steel truss bridges within the National Bridge Inventory, require that bridge owners conduct load capacity calculations to verify that the stress levels on all structural elements, including gusset plates, remain within applicable design requirements, whenever planned modifications or operational changes may significantly increase stresses.

Transportation Department officials, immediately following the safety board recommendation, advised states to go beyond usual practice and include gusset plates in their bridge strength calculations.

Despite focusing on the gusset plates, Rosenker said the board still hasn't determined why the bridge held up for four decades and what exactly happened during the afternoon rush hour on Aug. 1 to bring it down.

NTSB investigators are expected to reveal a final report this year. A replacement bridge is expected to be completed by Christmas.

Resources

National Transportation Safety Board, 1 and 2

Pioneer Press

Federal Highway Administration

Reuters

Associated Press