Corrections and clarifications: This story has been corrected to describe the pedestrian bridge as a truss design, despite its cosmetic appearance as a cable-stayed bridge.
The central column and cables that appeared in the design of a collapsed pedestrian bridge in Miami were decorative and not needed to support the span, federal investigators and bridge designers said Friday.
Bridge engineers had questioned why the column designed to rise above the bridge deck to a height of 109 feet wasn’t erected before a large section of the bridge deck was hoisted into place Saturday.
The 174-foot section of deck collapsed Thursday, killing at least six people.
But while the column would have been needed as part of the support for a cable-stayed bridge, it was not needed for a truss bridge.
Cheryl Stopnick, an outside spokeswoman for FIGG Bridge Engineers, which designed the bridge, said the structure was “truss bridge with above-deck truss elements.”
Robert Accetta, the National Transportation Safety Board investigator in charge, said diagonal elements between the bridge’s canopy and deck worked like a truss bridge. But the cables designed to fan out from the column weren’t needed to support the bridge deck, he said.
“As I understand it, these were cosmetic,” Accetta said. “They were not structural members.”
NTSB Chairman Robert Sumwalt led a team of investigators Thursday to determine what went wrong and what could prevent similar collapses in the future
The $14.2 million Florida International University bridge was designed under a process called “accelerated bridge construction” that allowed for larger sections to be built and then lifted into place. When finished, the bridge would have been 289 feet long.
But a 950-ton section of bridge collapsed for reasons still under investigation.
NTSB investigators revealed Friday that a FIGG engineer left a message with state officials two days before the collapse warning of cracks in the structure that weren’t considered life-threatening. But the message wasn’t retrieved until Friday, after the collapse.
FIU posted pictures of the bridge as envisioned, with a tall central column and cables stretching down to hold the bridge, shaped like a sailboat. The design gave the appearance of a cable-stayed bridge, which is a type of suspension bridge.
Cable-stayed bridges have cables attached directly from the column to the span, while suspension bridges string cables between towers and have other cables descend to the span.
But the lack of central tower when the deck was hoisted into place raised questions among bridge experts anticipating a cable-stayed bridge.
Amjad Aref, a professor at University of Buffalo’s Department of Civil, Structural and Environmental Engineering, said a suspended bridge is typically built gradually, with the center tower or towers erected early.
“I don’t want to speculate,” Aref said. “From a structural-engineering point of view, the forensic engineers won’t take long to figure out what happened.”
Andrew Hermann, past president of the American Society of Civil Engineers, said when bridges are built in stages, the supporting piers must be able to bear the weight placed on them at each phase.
“When you’re doing staged construction like this, what you have to make sure is that at each stage that the structure is strong enough for the loads that are on the bridge,” Hermann said. “The engineering, both design and the construction engineering, should have taken that into account with the bridge in that condition.”
Munilla Construction Management, a Miami-based construction management firm, won the bridge contract with FIGG Bridge Engineers of Tallahassee. Munilla said it would cooperate with the investigation. FIGG said in a statement “in our 40-year history, nothing like this has ever happened before.”
But FIGG was fined in 2012 after a 90-ton section of bridge collapsed on railroad tracks in Virginia. Munilla was accused of substandard work in a lawsuit filed this month after a makeshift bridge collapsed at Fort Lauderdale International Airport.
Occupational Safety Health Administration records show fines totaling more than $50,000 against Munilla for 11 safety violations in the past five years for complaints about unsafe trenches, cement dust and other problems.
Contributing: Alan Gomez and The Associated Press
More on Miami bridge collapse: