NTSB Chairman Robert Sumwalt briefs media on the Miami, FL bridge collapse investigation

March 16, 2018: NTSB Chairman Rober Sumwalt briefs the media on the status of the 3/15/2018 Miami, FL bridge collapse.

[keithpedersen]

In my opinion, NTSB Chairman Robert Sumwalt is the best public servent that we have.

[screamdoctor]

The bridge was designed by amateurs. Nobody designs all reinforced concrete truss bridges, especially those with a long span to shallow depth ratio. Concrete cannot carry the shear forces through the diagonal joints. Concrete cannot transfer the top chord forces either across a cold joint at brace top or into a brace cap, and deliver that load into a concrete diagonal. Bridges like this can only be designed by using steel wide flange struts that are welded together at all of the joints. and have massive embedments into the concrete roof and floor slabs, in order to transfer huge forces. The initial failure occurred at the roof slab to upper brace intersection at the first intersection in from the north end. The post tensioning in the diagonals was for crack control only and not strong enough to handle the tension loads in the highly stressed diagonal tension members at each end of the bridge. The two most likely failure mechanisms were a shear failure through the cold joint on top of the end diagonal compression brace, or a vertical shear failure in the middle of the joint between the two end diagonal braces. Whomever designed this bridge never checked shear stresses through the joints.

[PeterFalconII]

I am a in the UK. I can see the bridge failed when the cable they were tightening snapped. You can see the typical hydraulic tensioner sticking out on the end of the cable over 2 meters long on the north end. When that cable went slack with a noise that happened a few hours earlier making the crack appear in the bridge the road should have been closed. When a hydraulic tensioner fails it had hundreds of foot pounds of torque on it and shoots out like a bullet. In the UK no cars are allowed to travel under a bridge that is under construction. As an additional point the top section was part of the structure not just a decorative section it was the section that made the bridge stable it would have been attached to the tops of the bridge and connected to the bottom section via the cables they were tightening at the time of the failure.

I feel for the victims of this tragedy.

[mikeklaene]

I wonder who did the ORM on the day when the bridge collapsed? As in who thought it was a good idea to be doing adjustments to the bridge without closing the roadway?

[Nicholas-f]

It's blamed on a rush to not disrupt traffic :/

[KrustyKlown]

The questions are painful ! ... nobody asked anything about the investigation process. Instead they mostly asked questions nobody could possibly answer at the start of an investigation... basically all asking: "tell me what failed, so I can publish it?"

[jeremyl]

They keep asking the same questions over and over

[cretivplace]

He (guy in ntsb hat)states he's not a bridge engineer or a bridge expert but He's the one in charge of the investigation. Wouldn't you want a bridge EXPERT in charge of the investigation to make sure it's investigated accurately? The other guy couldn't even remember what the question asked was 30 seconds after it was asked. One reporter asked "what is the normal procedure or type of tests they do to make sure the bridge is safe to have traffic under?" He answered as if she had asked what they did. She was asking for bridges like this what would be the tests they should perform and were those done and passed in order for traffic to pass under.

[rowdyropp]

Who in the name of safety approved this bridge type engineered design? Why would they not have used preformed, prestressed concrete? Looks like a poorly engineered and designed structure! It has too many assembly flaws! Where the alleged (non suspension of the structured walkway) cables tie into the pretension cables of the angled structure spanned crossbraces, the angle of which these are pulling is perpendicular to the crossbraces and there tension devices! It may be sumwhat copeasthetic if both sides where using the same span cable. Given during assembly this was not occurring from your video indication. There is so many issues, its almost as if the Whitestar Titanic is being built again! It was doomed before it was being laid out and drafted! Then it being the first of this design that you were aware of? Does anyone ever test these situations out, and follow protocol applying safety guidelines during assembly and building phases while they were being evolved and composed? Boy, what a sad, sad, situation as well as a tragic travesty!!! Best of luck fellas! I will bet they over tensioned a cable to aggressively as well as not doing all pretension and suspension devices that applied to the phase sections. Were laser deflection measuring devices used to monitor changes in structural integrity? How warm was it when the forms were poured and how warm was it at the time the neglegent incident occurred? How much time had transpired in the curing process? Was the pours tested for proper component measurements as well as proper strength? I could go on and on. Yet, this whole project looks like a trainwreck! A man would have been foolish to have taken a ride! Be safe!