A curious paradox lies at the heart of trying to understand why the Miami bridge collapsed. The more details you learn of what happened, the harder it is to understand why it happened, writes forensic engineer Sean Brady.

If this were a movie you wouldd say it was unrealistic, you would say engineers would never behave so nonchalantly in the face of significant structural cracking. (You probably spent most of Part 2 shouting 'stop, stop' while you read about the growing cracks.)

But engineers did behave this way. In fact many engineers involved in the project behaved this way.

Why wasn't FIGG more concerned about the cracks?  Why didn’t the Florida Department of Transport (FDOT) stop construction? Why did the parties believe retensioning Member 11 would return the bridge to a satisfactory condition?

These whys go on and on, and in this instalment we will answer a key one: why did gross errors in the bridge’s design go unidentified, despite the design being subject to peer review by an independent engineering firm.

A close examination of why this peer review process failed shows how the systems we put in place to prevent engineering collapses can fall apart just as easily as the bridge itself.

The peer review requirements

The FDOT set out its peer review requirements in its Plans Preparation Manual. These requirements demand that an independent firm – not the firm that designed the bridge – check that the bridge plans are compliant with the relevant design requirements. The independent firm must be prequalified with the FDOT to undertake this work, and the cost of the review is borne by the design and construction firm.

In theory, this is a robust system to ensure a design is error free, carried out by an independent firm whose expertise has been confirmed by the FDOT.

But this is what happened in practice.

The peer review in practice

On January 14, 2016, MCM was engaged in a design-build contract to deliver the bridge. As we discussed in Part 2, MCM engaged FIGG to act as both engineer of record and to provide bridge design and engineering services for the project.

But in FIGG’s technical proposal to MCM, it stated that it, not an independent firm, would undertake the review.   

Fast forward to a meeting on June 30, 2016, where the FDOT informed FIGG that it couldn’t undertake the peer review – an independent peer review would be required by an external firm. FIGG didn’t factor this situation into its fee with MCM but, despite that, it put out requests for bids. 

At this point, the final player in our story makes their appearance: Louis Berger.

On July 5, 2016, Louis Berger submitted a bid for the peer review, and the following day it confirmed to FIGG it was prequalified with the FDOT.

The scope of the review was not up for discussion – this was a peer review in accordance with the FDOT’s Plans Preparation Manual – Louis Berger is required to ensure that the bridge, during all stages of construction and its service life, meets the relevant standards. (It not only needed to check that the structure was compliant in its final (service) state, it also needed to check each stage of construction, e.g., casting, transportation, tensioning and detensioning.)

Louis Berger submitted its scope of work, along with a proposed fee of $110,000.

But a month later Louis Berger appeared to get nervous. On August 10, it asked FIGG to: “Please note the quote we have is for a very thorough scope and creation of independent models. Please inform FIU [Florida International University] on evaluating bids, as a lesser fee may be associated with less effort/value.”

This email also stated:  “We would appreciate an opportunity to respond with a BAFO (Best and Final Offer) if necessary to be fair and level the assumptions.”

It was right to be nervous – FIGG had received three bids from consultants and Louis Berger’s was the most expensive at $110,000. The other two firms came in at $85,000 and $63,000.

The NTSB report doesn’t spell out what happens next, but it looks like Louis Berger was given the opportunity to come back with, in its words, its best and final offer.

The following day Louis Berger confirmed to FIGG that it had revised its fee from $110,000 down to $61,000. This brought it in lower than the bid of $63,000.

But the scope of work remained the same, despite the fee decrease. Louis Berger also agreed to a reduced timetable of seven weeks for the work, down from the 10 weeks originally proposed. This was to meet FIGG requirements.

The contract between FIGG and Louis Berger was dated September 16, 2016, and it specifically stated: “Louis Berger will perform Independent peer review for the concrete pedestrian bridge plans in accordance with the project and request for proposal requirements and FDOT Plans Preparation Manual (Chapter 26).”

Without knowing it, the parties had sown the seeds that would culminate in the collapse 18 months later.

The (not quite) peer review

FIGG made errors in the design of the bridge, which resulted in the joint between Member 11 and the bridge deck having insufficient strength. This was the joint that blew out on the day of the failure and led to the collapse.

Why didn’t Louis Berger’s peer review identify these design errors?

Firstly, Louis Berger only analysed the structure in its service state. It didn’t analyse its performance during the construction or transportation stages, despite the FDOT requirements stating the review had to be comprehensive. It was during these stages when the cracking initiated and began to grow.

Secondly, Louis Berger didn’t analyse the complete structure. It only analysed and checked the design of the structural members, it didn’t check the design of the joints between members.

If you don’t check the joints, you won’t identify design errors in these joints.

But if a peer review is required to be comprehensive, the reviewer doesn’t get to pick and choose which parts of a structure to check – a bridge is either compliant in its entirety, for the duration of its construction and service life, or it’s not. 

So why did Louis Berger pick and choose what to check?

When asked about the peer review of the bridge’s construction stages, a Louis Berger engineer told investigators: “My model was for the structure as one structure. Doing construction sequence staging analysis was not part of our scope. And again, doing such an analysis requires much more time than what we agreed about [with FIGG]”.

With respect to checking the joints it said: “... in the beginning, I suggested to do this kind of analysis, to analyse the connections. I’m talking about the nodes, or the joints to analyse the connections. However, the budget and time to do this actually was not agreed upon with the designer”.

Louis Berger had formally cut its budget which, in turn, appears to have driven cuts to the scope of work when it came to the practicalities of the review. These cuts were inconsistent with what it had contracted to do for FIGG, and were inconsistent with the comprehensive peer review required by the FDOT.

And the story of the peer review doesn’t end there, it gets worse.

The peer reviewer’s prequalification

In order to undertake a peer review, a firm has to be prequalified by the FDOT, and on July 6, 2016, Louis Berger informed FIGG it was.

In order to obtain this prequalification it would have needed to apply to the FDOT and demonstrate it had the required expertise – in this case, at least three professional engineers, each having a minimum of five years' bridge design experience.

But the 2013 to 2018 FDOT records show Louis Berger held no such prequalification. (Further, in 2013, Louis Berger applied for prequalification, but was turned down because it lacked engineers with the required experience.)

And there’s a further sting in the tail.

At the time when FIGG was procuring the independent peer review, the FDOT website did list Louis Berger as prequalified. And following the incident, investigators requested that the FDOT check the accuracy of its website.

The FDOT admitted Louis Berger was listed as prequalified, but claimed this was due to a technical error that occurred when processing its physical records.

It then defended its position by saying that the website was for informational purposes only and was not intended to be used as a substitute for due diligence by FIGG. (Thus attracting the wrath of Bruce Landsburg, the vice-chairman of the investigation, who said:  “FDOT claimed a technical error on the FDOT website and then, after the collapse, fabricated a disclaimer that they are not responsible for the data that they post. That’s unacceptable in my view – either ensure the information is accurate or don’t post it”.)

FIGG could have asked Louis Berger or the FDOT for a letter confirming its prequalification. It appears it didn’t.

The how and the why

So the how is straightforward. FIGG made design errors, and these errors were not identified in the peer review.

The why is a tangled web of cascading decisions that began with FIGG assuming it could undertake the peer review internally. Once the FDOT vetoed this idea, cost was thrust into the spotlight as a major consideration, resulting in Louis Berger drastically reducing its fee.

This fee cut led, apparently informally, to a reduction in review scope that culminated in the design errors going undetected and the bridge collapsing. And at the centre of this review was a firm not qualified to be there, watched over by a department whose website said it was, engaged by a designer who never confirmed it.

If you’d set out to ambush a peer review, you couldn’t have done a better job.

The design error

In Part 4 we will take a detailed look at how the design error occurred. For now I’ll leave you with more wisdom from Bruce Landsberg:

'A bridge-building disaster should be incomprehensible in today’s technical world. We have been building bridges in this country for over 200 years, and long before that in other parts of the world. The science should be well sorted out by now and for the most part, it is.'

But if the science was so well sorted out, why wasn’t it applied to this bridge?

Author: Sean Brady is the managing director of Brady Heywood, which provides forensic and investigative structural engineering services. Web: www.bradyheywood.com.au; Twitter: @BradyHeywood