FIU Pedestrian Bridge — A Miscalculated Node That Sheared Off Over Live Traffic

On 15 March 2018, at about 1:47 p.m., the partially constructed main span of the FIU-Sweetwater UniversityCity pedestrian bridge crossing the eight-lane SW 8th Street in Miami, Florida fell onto live traffic stopped at a red light, killing six people and injuring ten. The 174-foot concrete truss span dropped roughly fifteen feet onto the vehicles below. The National Transportation Safety Board (NTSB), the federal investigating body, identified the probable cause without hedging: load and capacity calculation errors made by the bridge designer, FIGG Bridge Engineers, in the design of the main span truss. The failure began at a single location — the nodal connection where diagonal truss member 11 and vertical member 12 met the bridge deck.

The mechanism was an under-designed connection, not an exotic one. The bridge was a concrete truss, and like any truss its loads concentrated at the nodes. At node 11/12 the steeply inclined diagonal pushed a large horizontal force into the deck, and that force had to be resisted by shear along a construction cold joint between the diagonal and the deck slab. FIGG underestimated the demand on that interface and overestimated its capacity to resist sliding. The NTSB found the actual demand on the node was nearly double the designer’s calculated value, while the calculated shear resistance was too high. The connection was overloaded from the moment the span carried its own self-weight; it had no reserve at all.

The warning was visible and ignored. After the span was set in place on 10 March, severe cracks opened at the north end, precisely at node 11/12, and grew over the following days. Photographs were emailed to the engineer of record. On the morning of the collapse, the project team met and concluded the structure was not compromised and there were no safety concerns. The roadway was never closed. Hours later, a post-tensioning crew followed instructions to re-tension the rods inside diagonal member 11 — re-clamping the very joint that was already failing. The re-tensioning broke the last of the connection, the diagonal slid off the deck, and the span came down.

What makes the FIU collapse a permanent case file is that nothing about it was hidden. The error was a routine interface-shear calculation on a non-redundant structure. The cracks were documented, measured, and discussed. An independent peer review existed but missed the error. A road full of motorists sat beneath a connection that the designer’s own arithmetic had under-built by a factor of two, and which was visibly tearing apart in the days before it fell.