FIU pedestrian bridge collapse. What we know and what we shall know.

Last Thursday March 15, 2018 will be sadly remembered and mourned because of the shocking collapse of the under construction Florida International University (FIU) pedestrian bridge, onto Southwest Eighth Street in Miami, Florida. As of today, 6 victims have been confirmed[1] and there are several others injured. This dreadful aftermath has to make us all think and learn.

When a terrible event like this one happens, the technical community has the obligation of analyzing what caused the failure, explain the conclusions to the community and propose actions intended to prevent any other similar catastrophe in the future. Lessons have to be learnt and widely communicated so Engineers and Construction professionals can take them into account from now on, it is our duty with the society and with the victims and their families. This has also to be done properly, rigorously, technically, methodically and without delay, avoiding however premature or precipitous judgements or appraisals that can distort the picture of the events and cause confusion. What a difficult task to accomplish in nowadays fast-food-knowledge society where the immediate has to be told, no matter how inaccurate, vague or merely speculative are the pretended facts presented.

Just a few moments after the heartbreaking collapse happened, I began to receive “information” about it, and people on social networks began to react. I was astonished by the senselessness of some of the comments I read –some of them, even from pretended Engineers and experts-, not to mention the almost immediate point-a-finger campaign that started in the networks. In such an ambiance, I felt it was my duty to try and investigate about what we know for a fact, try to separate it from opinions, guessing and gambles and expose them to my network, so we can have a clearer picture. All the information I gathered is publicly available on internet, and there is a huge amount. Do not expect to find out the reasons why it happened, because it is simply impossible for a person not familiar to the project and from the distance to find out. All I want is to expose facts and refute some of the comments I have read so far. I will quote statements and information gathered from Project participants’ sources, and I will identify my comments by using italics.

WHAT WE KNOW

DESCRIPTION OF THE PROJECT.

FIU’s Intent (FIU acts as the owner of the Project). [2]

Florida International University issued a Request For Proposals (RFP) for the Design and Build of the Bridge, describing it as follows:

“The OWNER seeks America’s best designers and builders as members of a design-build team to design and build an innovative signature bridge that will become a respected and valued design landmark in Miami. It will serve as the critical element of a pedestrian-oriented shared-use corridor between FIU and Sweetwater, igniting the development of UniversityCity. We envision a wide pedestrian bridge (20’ minimum width to perhaps even more than 30’) that would serve not only as a means to cross from one side to the other, but would become a destination in its own right where community members might linger, gather, and create an urban social space — a linear park. We expect that the bridge might even be used as an event venue.”

“Innovation – A balance must be achieved between the desire for innovative design and the realities of proven operating and maintenance practices. A commitment to innovation and design excellence as well as to durability, ease of maintenance, and material innovation are encouraged”

“J. Sequence of Construction:

The Design-Build Firm shall construct the work in a logical manner and with the following objectives as guides:

1. Maintain or improve, to the maximum extent possible, the quality of existing traffic operations, both in terms of flow rate and safety, throughout the duration of the Project.

2. Minimize the number of different Traffic Control Plan (TCP) phases, i.e., number of different diversions and detours for a given traffic movement.”

“3. Traffic Control Restrictions:

Lane closures shall occur only during non-peak hours on non-event nights. Non-peak hours are 9:00PM to 5:30AM Sunday through Thursday and 11:00PM to 7:00AM on Friday and Saturday Nights. A lane may only be closed during active work periods. Pacing Operations will be allowed during the approved lane closure hours. All lane closures, including ramp closures, must be reported to the local emergency agencies, the media and the OWNER. Also, the Design-Build Firm shall develop the Project to be able to provide for all lanes of traffic to be open in the event of an emergency.”

The Pedestrian Bridge Design Criteria.[3]

TY Lin International, a prestigious Engineering Firm, redacted the Design Criteria to be taken into account by all the proponents. Here I include some interesting excerpts:

“A thorough study has been conducted that analyzed the practicality of multiple structural typologies including cable stayed, arched, stress ribbon, tensile nets, trusses, typical box girders, and various others. In the end it was been determined that a truss or a hybrid of sorts was the best typology for the site given the budget, site constraints, and desired aesthetic level. One of the major parameters governing the selection of a truss typology was the ability to seamlessly integrate the required 8 foot missile fence over the roadway into the structure and skin of the bridge. The missile fence should not stick out as its own discrete component but should contribute as a feature that is woven into the holistic design and as such function for the sake of providing shade, safety, reinforcement the geometry, and so forth.” (emphasis added)

“The crossing is to be a single span bridge over the US41 right-of-way. The overall length is approximately 250 ft with supports located a minimum of 35’south of the back of sidewalk on SW 8th street on the south side and on SW 7th Terrace on the existing northern canal bank. A support pier may be located on the southern canal bank between US41 and the canal.”

“The bridge will carry pedestrian and bicycle traffic and shall provide a minimum of 20’-0” total clear walkway (selection committee may grant more credit for wider bridge widths), with access provided through a combination of elevators (single) and stairs.”

“5.18 Accelerated Bridge Construction (ABC). For bridges constructed in a staging area and launched, slid, or otherwise transported into final location, provide the following items to the OWNER, stamped by a professional engineer licensed to practice in the State of Florida:…”

 

Design-Builder Proposal[4]

The Construction Process Proposal[5]

 

 

THE MAIN SPAN BRIDGE MOVE (ACCELERATED CONSTRUCTION).[6]

“The bridge’s main span has been built on temporary supports (temporary shoring) on the south side of Southwest 8th Street. Self-Propelled Modular Transporters (SMPTs) with pull-up gantries will be positioned under the main span, lift it from its temporary shoring, and then carry it along an arc-shaped path to the northeast roughly 90 degrees and lower it onto its permanent position. It will be the largest pedestrian bridge move via a Self-Propelled Modular Transportation (SPMT) in U.S. history.”

“Length of section being moved: 174 feet. Weight of section being moved: 950 tons”

THE CONCRETE CRACKING.[7]

According to information released by the Florida DOT, ON Tuesday March 13 one of the FIgg’s Engineers called FDOT representatives for the Project leaving a voice mail describing “some cracking that’s been observed on the north end of the span, the pylon end of that span we moved this weekend” the message continued “obviously some repairs or whatever will have to be done but from a safety perspective we don’t see that there’s any issue there so we’re not concerned about it from that perspective although obviously the cracking is not good and something’s going to have to be, you know, done to repair that.”

THE COLLAPSE.

· On Thursday 15, 2018 (approximately at 12:50 h ET), works were underway on the main span deck. Some traffic lanes of the SW 8 Street were closed (those closest to the Pier 2 – the pylon). Workers from the company Structural Technologies (designer and manufacturer of VSL post-tensioning and construction systems in the United States) were involved in the works carried out.

· At least 5 lanes of traffic were at that moment occupied by stopped vehicles below the construction area, waiting for the green light.

· All of a sudden, the concrete deck collapses and falls to the ground, trapping several vehicles beneath it and throwing away some of the workers. The first sections affected by the collapse are the two truss joints closest to Pier 2 (the future pylon). As it can be observed in the pictures below, 2 kinks are detected in the geometry of the upper and bottom chords of the truss in the very moment of the collapse.

 

 

·        All of a sudden, the concrete deck collapses and falls to the ground, trapping several vehicles beneath it and throwing away some of the workers. The first sections affected by the collapse are the two truss joints closest to Pier 2 (the future pylon). As it can be observed in the pictures below, 2 kinks are detected in the geometry of the upper and bottom chords of the truss in the very moment of the collapse.

·        Marco Rubio, Florida’s Governor, tweeted after the collapse that “The cables that suspend the #Miami bridge had loosened & the engineering firm ordered that they be tightened. They were being tightened when it collapsed today.” Later on, he added “More details about my tweet last night. Construction crews were working at north end of bridge applying a “posttensioning force” when #MiamiBridgeCollapse occurred. The @NTSB safety investigation is now fully underway”. Finally, he posted “New video released of @FIU #miamibridgecollapse from the dashboard view of a driver who witnessed it. At the time of the collapse construction crews were working on the diagonal beam at the north end of bridge applying posttensioning force to strengthen it”. Some pictures and videos of the site after the collapse show a PT jack applied to a PT bar in a truss member:

 

SOME (PERSONAL AND SUBJECTIVE) REMARKS:

· Figg, the Design Engineering Company for the Project, has designed several World Class bridges in the US, such as the Leonar P. Zakim Bridge in Boston, MA. http://www.figgbridge.com/portfolio.html. They are involved in several ongoing major projects, such as the New Harbor Bridge in Corpus Christi, TX.

· Questions have been asked about the bridge’s cable-stayed system (not present at the time of the failure). Analyzing the Design-Builder’s proposal, it is clear that the proposed construction phases were being followed, and the main span was designed to be transported using SPMT’s and then erected without the cable-stayed system. The D&B’s proposal also makes clear that “The size of the stay pipes and spacing were designed to provide additional stiffness. The structure meets strength design criteria without the stays; they are an additional structural feature provided to meet the natural frequency requirements.” (emphasis added). In fact, the stays were designed as simple steel pipes, instead of using the common multi-strand post-tensioning strand stays, as it can be seen on the following detail:

· Regarding the typology of the bridge, it is clear that the Design Criteria designated the truss as the preferable option. The material to be used for it was not clearly defined, but steel and concrete composite solutions were mentioned “The bridge superstructure should be primarily structural steel with concrete walking surface.”. However, the selection of the post-tensioned concrete solution is widely justified by the D&B in their proposal. It is a heavier solution (the 174 ft – 53 m long main span weighted 950 tons, which is a significant self-weight for a pedestrian bridge), but it has some advantages such as the improved anti-vibration properties or the noise isolation.

· Some comments have been also raised regarding the materials used for the project and their origin. The only information we have is that the Project was under the Buy-America regulations, “Materials must comply with the “Buy America Act”, 49 U.S. code Section 5323”, and therefore all materials shall be produced and manufactured within the US.

· Obviously, the appearance of cracking in the area nearby the pylon has to be investigated, but cracking is a very common phenomenon when dealing with concrete -due to its reological properties- and can be allowed, subjected to certain restrictions. The information available does not offer details about the extent, dimensions and location of the cracking, and therefore no conclusion can be extracted.

· Marco Rubio’s tweets apparently point to the diagonal post-tensioning. According to the Design Preliminary Drawings, the truss members have the following PT system:

In PT concrete structures, tension adjustments are common. In this case, additionally, the supporting system of the main span truss changed: the SPMT’s temporary supports were located beneath the bottom inner joints, and when erected, the truss was supported at its ends, in the permanent piers. Such change in the support conditions had an impact on the forces withstand by the extreme diagonals, and in them even the external axial force may have shifted from tension (when being supported by the SPMT) to compression (when supported by the permanent piers). The external forces are to be faced by the concrete and its post-tensioning system, so an adjustment is a common requirement.

·  With all that said, and assuming Marco Rubio’s statements are true, a question is making me mad: If you are acting in any way on a fracture critical element (such as the diagonal of a simply-supported planar truss, which is an isostatic structure), why is the traffic permitted below the structure? Why are the temporary deck supports completely removed if there are adjustments to be made on some of the critical structural members?

[1] http://www.miamiherald.com/news/local/community/miami-dade/article205721884.html
[2] Florida International University. DESIGN-BUILD MAXIMUM PRICE REQUEST FOR PROPOSAL For UniversityCity Prosperity Project Miami-Dade County, Florida. June 2014. Published on https://facilities.fiu.edu/projects/BT-904.htm
[3] TY Lin International FIU-UniversityCity Prosperity Project – Pedestrian Bridge Design Criteria. April 2015. Published on https://facilities.fiu.edu/projects/BT-904.htm

[4] MCM+FIGG Design Build Team. Proposal for the BT-904. FIU UniversityCity Prosperity Project. September 2015. Published on https://facilities.fiu.edu/projects/BT-904.htm
[5] MCM+FIGG Design Build Team. Proposal for the BT-904. FIU UniversityCity Prosperity Project. September 2015. Published on https://facilities.fiu.edu/projects/BT-904.htm

[6] FIU. https://news.fiu.edu/wp-content/uploads/17703_EXT_FIU_Bridge_Move_Fact_Sheet_030918_DIGITAL.pdf
[7] http://www.miamiherald.com/news/local/community/miami-dade/article205627294.html

 

Spain

 

Spanish version

 


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Autor de este artículo. Ingeniero de Caminos, Canales y Puertos por la Universidad de Granada. Fundador y Presidente de PONTEM ENGINEERING SERVICES, S.L. y ESCAMILLA INGENIERÍA S.L.U. Mi vida profesional se ha orientado desde el inicio al mundo del diseño, construcción, asesoría técnica y aseguramiento de calidad de estructuras singulares (puentes, edificios, estructuras offshore, túneles). Colaboro con la oficina Carlos Fernández Casado, S.L. (www.cfcsl.com) desde 2007, y con la firma estadounidense CALTROP (www.caltrop.com) desde 2012. Proyectos activos: Viaducto del Tajo para la Línea de Alta Velocidad Madrid-Extremadura (Vano principal: Arco de hormigón de 324 m de luz), New Gerald Desmond Bridge (Primer Puente Atirantado para vehículos de California), varios proyectos de estructuras Offshore. Y ahora, orgulloso colaborador invitado del blog Estructurando.net!

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