# Florida International Bridge Collapse

Discussion in 'Architecture & Engineering' started by hardalee, Mar 18, 2018.

1. ### Peter DowRegistered Senior Member

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215
OK I have a way to estimate the effect of the lost concrete from the 2 x 3" diameter ducts of cross sectional area of about 7 square inches each - 14 square inches for the 2 of them.

14 square inches = 21" x (14/21)" = 21" x 0.66"
so we can estimate the strength loss by subtracting 0.66 from 24 inches
So recalculate for a 21" x 23.33" column
Other values the same
Maximum allowable design (factored) load or capacity, Pu: 1112 kips

Actual load 1615 kips was 145% of the maximum allowable design load of 1112 kips before they added even more load with the post-tensioning bars!

With something like another 2 x 200 kips from the 2 x 1.75" PT bars, the actual compression load would have been about 1915 kips or 172% of the maximum allowable design load of 1112 kips.

Last edited: Mar 23, 2018

3. ### Peter DowRegistered Senior Member

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215
It's OK I have the estimate I wanted thanks to that great wee online calculator thanks to Jonathan Ochshorn of Cornell University.

5. ### hardaleeRegistered Senior Member

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383

Ok, Have a haggis for me in Deacon Brodies for me and I'll eat some southern fried chicken for you.

7. ### Peter DowRegistered Senior Member

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215
Oh I'd prefer to eat southern fried chicken with Dr Rice, please.

8. ### Peter DowRegistered Senior Member

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215
I found clearer video of that PT bar from member #11 to extract better quality still photographs.

So yes there does seem to a thread but it appears to be somewhat of a fine thread, not cut very deeply into bar, compared to some other threads on post-tensioning bars for sale which I have seen.

However if the bar failed by snapping then presumably the thread did its job, which is more than be said for the rest of the bar or I suppose it could have been over-jacked by a careless jack operator.

9. ### Dr_ToadIt's green!Valued Senior Member

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2,527
Dude, everyone has known for days that it was a post-tensioning fault and a lift in the wrong place for the jacking. Why are you not on Facebook with this elementary shit, or even an engineering forum?

You get no chops from this shithole.

10. ### sculptorValued Senior Member

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7,548
(but perhaps, not altogether inaccurate?)

11. ### hardaleeRegistered Senior Member

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383
Peter,

This is why I quit participating in this forum. Unfortunately, the trolls show up who know nothing about what's being discussed and weigh in with their meaningless opinions and bad manners. That's why I jumped at you earlier before I knew you were seriously looking for answers.

It's a shame they have nothing better to do.

Pat your selves on the back trolls. You've shown your true colors again.

12. ### Peter DowRegistered Senior Member

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215
Quiet at the back of the class, please.

The online calculator's middle values for f'c(ksi) - concrete grade - 4 ksi which was an unfair assumption because the FIU FIGG-MCM proposal specifies a higher (the highest) grade of concrete - grade VI - 8.5 ksi.

Unfortunately, the calculator does not allow the user to select such a high grade of concrete so I had to knock up a spreadsheet calculator of my own to calculate the Maximum allowable design factored load for 8,500 psi concrete which I have now done.

- which information is better digested graphically, as follows.

- which tells us that the concrete has to be fully up to the grade VI specification just barely to hold the bridge up with no additional load from post-tensioning bars or from any pedestrians on the bridge. Anything less than top notch concrete and that bridge is coming down.

This tells us that only calculating with a risky safety factor of only 1.2 can we assess that the truss member #11 is just barely strong enough to hold the bridge up with no additional load from post-tensioning bars or from any pedestrians on the bridge. Using anything more cautious for a design safety factor would warn that the bridge is at an unacceptable risk of coming down.

So we can see that the bridge designers were gambling with people's lives even before a single bar was post-tensioned.

Last edited: Mar 24, 2018
13. ### Peter DowRegistered Senior Member

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215
The rabble have nothing better to do it seems.

14. ### Howard WilleyRegistered Member

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2

Is it possible that we are looking at instrumentation cable for the smart sensors? I read that the span was supposed to have numerous embedded stress sensors, and this looks like the size of 16 channel signal cable.

Great detective work.

15. ### Peter DowRegistered Senior Member

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215
I don't know exactly what you are looking at?

Is it appropriate for you to name one of my labels, "B", "R", or "T", or can you post your own image and label exactly what you are looking at? Only then can we be sure that we are looking at the same thing.

I didn't see any references to "instrumentation" or "sensor" in the FIU FIGG MCM proposal pdf.

Signal cable doesn't need ribs, ridges or threads like we see in the bars in my images.

Forensic scientists make the greatest detectives, certainly.

However, I'd be able to be a greater detective if I could get more information. It is somewhat frustrating not getting definite answers to some questions.

Is the P.T. bar which I have labelled "R" - and / or the P.T. bar which the jack is still connected to - severed or not?

Here is this still image

taken from this NTSB video at about 2 minutes 20 seconds -

- where the P.T. bar is curving down into the rubble but it is not clear to me if the P.T. bar is severed there or if it is still connected to the anchor which is buried in the rubble?

If that P.T. bar is not severed and there are no two loose ends to be found in the rubble anywhere - simply an intact P.T. bar which has been ripped out of the concrete - then we must junk the snapped P.T. bar explanation and adopt the alternative explanation of a pure concrete compression failure.

Last edited: Mar 25, 2018
16. ### Howard WilleyRegistered Member

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2
Quit sit bastardus carborundum

17. ### hardaleeRegistered Senior Member

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383
If 10 number 7 bars (unknown) I think 14 or so:
Bridge wt= 950 Tons = 1900 kips A little less as ½ of top cord and north column weight would not apply.
Note: 950 ton weight should be checked.
North side= 950 kips vertical component
Diagonal factor= 1.7
Diagonal force= 1615 kips service load What the diagonal should have carried
Ultimate factor= 1.4 Note: 1.2 is only in combination with other loads. If dead load by itself, use 1.4
Ultimate load= 2261 kips ultimate What the diagonal should have been designed for if no post tensioning.
Rectangular column

Column ID Pu Phi Fy f'c D B Ag # bars Bar size As % Pu
Diagonal 2741 0.65 60 8.5 21 24 504 10 7 6.01 1.19% 2187.19 Kip

Still guessing on number and size of bars, probably more but same range.

Last edited: Mar 26, 2018
18. ### RainbowSingularityValued Senior Member

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what is amazing me while i look at the pictures is the way the concrete appears to have just fallen apart like powder on the upright collums.
why is that ?

19. ### DaveC426913Valued Senior Member

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15,407
Because, while concrete has an incredibly high compression strength, it doesn't have all that high a tension strength. Where it's not reinforced with rebar, it can disintegrate when the beam is subjected to unintended bending. That's not a flaw.

The engineers can probably explain it better.

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20. ### sculptorValued Senior Member

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7,548
It seems to me:
That they could have moved the damned stop light.

Peter Dow likes this.
21. ### iceauraValued Senior Member

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30,848
When I was working on a crew putting in stage curtains and other theatrical stuff, the load factor for anything over people's heads was a lot higher than 1.2.

Peter Dow likes this.
22. ### RainbowSingularityValued Senior Member

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6,124
steel is safer ?
steel cable and steel ... ?
burried concrete counter weights...? no canterleavers!

23. ### Peter DowRegistered Senior Member

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215
The number of rebars is difficult to estimate but we can get an idea by looking at this picture which shows the smashed bottom end of member #11 with the exposed ends of the rebars and ties sticking up in the air now -

- by assuming that the ties are #4 (0.5" diameter) bar as per the engineering drawings and trying to count the thicker, 0.875" diameter, rebars we can see on one side and doubling up to get the total number of rebars, while not miscounting a thinner, 0.5" diameter, tie as a 0.875" diameter rebar.

Unfortunately, there is not enough resolution in the photograph to be sure of what is a rebar and what is a tie.

What would help would be new high resolution photographs of that exposed end of member #11. If all the bars we can see sticking out are 0.875" diameter rebars then there could well more than the 10 I assumed.

My method for estimating the size of the rebars I explained earlier in the following quoted post.
To explain again - one image tells me that the ratio of the diameter of the rebar "B" to the diameter of the plastic duct "T" is about
T/B =3.4
and another image tells me that the ratio of the diameter of the P.T. bar "R" to the diameter of the plastic duct "T" is about
T/R = 1.7
and by substituting for T we can deduce
R/B = 2
meaning that the diameter of the P.T. bar is about twice the diameter of the rebar.

If the diameter of the P.T. bar is 1.75" throughout as per the engineering drawings then the diameter of the rebar is half that or 0.875", number #7 rebar.

I would welcome a more accurate estimate - or better still an actual measurement!