The design of the Florida International University (FIU) bridge which collapsed onto a live highway killing six people, could be seen in hindsight as a potential “recipe for disaster” according to one bridge expert.
Speaking during a panel discussion on the collapse at the NCE100 breakfast club in London, Mott MacDonald senior project manager metros and civil Andrew Bance said the over complicated nature of the design could have been a factor in the collapse.
“It was a very unique structure which could be viewed as a bit of a fake,” he commented in relation to his review of the information available following the collapse. “It was meant to look like a cable stayed structure, but it isn’t and if you were to design a truss by itself then you wouldn’t arrange it like that. Then you’ve potentially got a new concrete and an accelerated programme, it’s a recipe for disaster.”
There is currently an investigation into the collapse by US construction firm MCM, which was constructing the bridge. Miami-Dade Police and the National Transportation Safety Board are conducting separate investigations. Figg Bridge Group designed the bridge and the panel at the NCE 100 event said that the engineering firm was well known in the bridge design community and had respected expertise.
Bance’s comment came in response to news that, although the bridge appeared to be a cable stayed design, the cables did not support its structure. Tender documents which came to light last week said the cables were in fact hollow tubes designed for architectural effect and to meet the natural frequency requirements of the bridge.
Tony Gee & Partners group director Akram Malik said the addition of non-structural “cables” was an “odd decision”.
“It’s not by any means a unique decision, I’ve seen it quite a few times where you add something that is purely aesthetic and just makes life more difficult, rather than actually adding to the structure.”
Malik added that in general when clients want a “statement structure” designers might twist the concept to fit the client’s brief.
“The client is there for something but then the engineer should be there to put the right elements in,” said Malik.
The FIU bridge collapsed onto traffic, killing six people on 15 March during work to post-tension bars in a diagonal member in the bridge’s concrete truss .
Engineers have suggested the point of collapse was an explosive failure of the concrete in the joint at the north side of the bridge which occurred during the post tensioning works.
Independent bridge consultant Simon Bourne said this type of failure could be caused by poor concrete compaction due to too much reinforcement in the joint.
“What you need in that node is good quality concrete and to detail it well with just the reinforcement that you barely need,” he said. “Additional reinforcement can just cause problems later on with compaction.”
Temporary Works Forum chair Tim Lohmann said that often an issue was that each designer would add in their own reinforcement, making joints heavily congested.
“Frequently you get multiple designers involved in connections and everyone wants to put their little bit in to be sure and you end up with too much,” said Lohmann.
Malik agreed, adding: “That’s one thing is often missing across the board, we just accept bursting steel [added in for each pre-stressing bar] from sub-contractors, but there’s no overview to rationalise that reinforcement to only make sure you’ve got what you need.”
Lohmann went on to stress the importance of an independent check on the design and said it was unclear from the available documents how the bridge design was to be checked.
“Who were the checking authorities?” Lohmann asked. “The checking period was four weeks, and for a structure of this unusual nature, that doesn’t sound like a very long time to me.
“[In the documents] It said it was one round of checks and it’s not clear what the independence of that check was because it was being managed by the bridge designer.”
Figg has been contacted for a response to the points raised at the New Civil Engineer event. In general statements it has said it is working with authorities on their investigations. “This is an unprecedented event – no other bridge designed by Figg Bridge Engineers has ever experienced such a collapse. Figg-designed bridges have proven to be incredibly durable. For example, we have worked on more than 230 bridges throughout the United States and have designed nearly 56km of bridges in the southeastern Atlantic and Gulf of Mexico that have withstood multiple hurricanes,” it said.
MCM said in a general statement: “We will conduct a full investigation to determine exactly what went wrong and will cooperate with investigators on scene in every way.”