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  #21  
Old 23 March 2018, 02:04 AM
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Originally Posted by BoKu View Post
I have since encountered detail drawings of the attachments for the "cables," (they're actually 16" diameter hollow tubes), and their rather superficial anchors to the canopy back up the idea that the pylon and "cables" are just for aesthetics, and add relatively little strength or stiffness.
Yes, if the tubes are hollow they're definitely not structural in any real sense. If so they would be full of cables. This is a series of pics from construction of the Ravenel Bridge in Charleston, SC:

http://ravenelbridge.net/wrapper_mob...004_cable.html

About half way down the page they show them pulling the cables through a tube (cable housing).
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  #22  
Old 23 March 2018, 09:33 AM
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So, mea culpa, I was wrong on this one. I looked at the structure and was completely confident from the arrangement of the truss members that I understood the reasoning behind them. I felt that they had an intrinsic logic that revealed the structure beneath. And I was wrong.
Off-topic, but I just have to say - BoKu, I believe this is the most gracious self-correction I've ever read. All class.
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  #23  
Old 23 March 2018, 04:58 PM
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...My inclination at this very early stage is that the construction did not follow the design...
My inclination is the other way around, but I rather hope to be proven wrong.

I predict that as the NTSB investigation proceeds, we're going to see finite element analysis (FEA) screenshots with lots of red near the intersections of the truss members with the deck and canopy.

I think that one of the central issues will be the degree to which the structure is determinate; that is, such that the stresses in the various members can be predicted and modeled without having to consider the elastic properties of the members.

The classic determinate structure is a triangulated truss where all the members are joined with pivots so that the members carry only tension and compression loads; there is no way to put bending moments into them through the pinned joints. With such a structure, if you know the loads applied to the structure you can easily find (or at least closely approximate) the loads in any member without having to consider the elastic properties (how much it stretches or compresses) of any of the members.

And, by the way, steel trusses tend to act like the joints are pinned even when they're rigid. Steel has enough ductility that it flexes elastically or bends plasitically when applied loads distort the geometry of the structure.

With an indeterminate structure, there are additional constraints such that you don't know the exact loads in each member unless you also know the how much each member compresses or stretches per unit of loading. For example, a truss where instead of a single diagonal member there is an X consisting of two diagonal members that cross. If both members are capable of reacting both tension and compression, you have to know how much one stretches in order to calculate how much the other compresses.

Consider also a square with rigid bolted corners and a diagonal member between two opposite corners, and tension loads applied to the other two corners. That is also indeterminate, since you have to consider the bending loads reacted between members at the corners in order to understand the loads and stresses in the diagonal member. And that's basically what we have here.

Unlike steel, concrete structures tend to have very little ductility once they're set, so they tend to behave very differently, and be much less forgiving, than steel.

What I think we'll learn in retrospect is that concrete trusses such as this one are too nearly indeterminate to reliably predict their behavior without very granular analysis. I predict we'll learn that the analysis brought to bear here was not granular enough, and did not embody an adequate depth of understanding about how complex concrete structures behave.

In specific, I also think that we'll learn that the design did not adequately address the reaction between the outermost diagonal members and the bridge deck. But that's really getting down into the weeds.

--Bob K.
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  #24  
Old 23 March 2018, 10:25 PM
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Originally Posted by RichardM View Post
... The engineer reported that the cables were not tightened to spec after the bridge was set in place. I question why this observation wasn't made before it was allowed to set the bridge in place.
And I would question how the tension was determined and if the error in that determination was in the first or second measurement.
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  #25  
Old 23 March 2018, 10:31 PM
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BoKu, My inclination that the contractor did not follow the plans comes from the cynicism of 40+ years as a design engineer. I frequently say that any resemblance of the final project to the plans is strictly by coincidence. And I apologize to any contractors out there as this is not a true statement. Also, I often tell the contractor that the plans are perfectly constructable on paper. In metal, maybe not so much.
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  #26  
Old 23 March 2018, 10:44 PM
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Default Design change put [bridge] project behind schedule, millions over budget

Florida bridge collapse: Design change put project behind schedule, millions over budget

Construction of the pedestrian bridge that collapsed and killed six people in the Miami area was behind schedule and millions over budget, in part because of a key change in the design and placement of one of its support towers. Documents obtained by The Associated Press through a public-records request show that the Florida Department of Transportation in October 2016 ordered Florida International University (FIU) and its contractors to move one of the bridge's main support structures 11 feet north to the edge of a canal, widening the gap between the crossing's end supports and requiring some new structural design.

https://www.cbsnews.com/news/florida...es-2018-03-20/
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  #27  
Old 24 March 2018, 05:08 AM
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Originally Posted by BoKu View Post
I looked at the structure and was completely confident from the arrangement of the truss members that I understood the reasoning behind them. I felt that they had an intrinsic logic that revealed the structure beneath.
Seems like they suceeded in designing the visual element to duplicate an engineering element.
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  #28  
Old 24 March 2018, 03:58 PM
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BTW the discussion about this on the eng-tips.com forums is fascinating and intense. It's also civil and constructive.
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  #29  
Old 24 March 2018, 04:23 PM
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Thanks, I'll go look.
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  #30  
Old 24 May 2018, 02:24 PM
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The NTSB released the preliminary report yesterday. It doesn't contain much new information that hasn't already been reported:

https://www.ntsb.gov/investigations/...009-prelim.pdf

~Psihala
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  #31  
Old 15 June 2018, 12:31 AM
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Default FIU had grand plans for 'signature' bridge. But the design had a key mistake...

http://www.miamiherald.com/news/loca...212571434.html

Quote:
As a consequence of the apparent design error, the diagonal support at the span's north end was so overloaded that additional stress put on it by construction crews tightening internal support rods on March 15 likely caused it to separate from the walkway deck, instantly sending the entire 950-ton span crashing to the roadway in a chain reaction of structural failure, the engineers said. The accident killed one construction worker and five people sitting in cars at a stoplight below.
I'm calling this one:

Quote:
Originally Posted by BoKu View Post
...In specific, I also think that we'll learn that the design did not adequately address the reaction between the outermost diagonal members and the bridge deck. But that's really getting down into the weeds.
Bob "BoKu" K.
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