Florida International Bridge Collapse

[Peter Dow]

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.

 

[Peter Dow]

Sorry, I'm now away from my office and programs. Will not likely return until next Saturday before Easter. That's why I sent you the info so you can play with it.

I don't have much doubt that by the time I return, the problem will have been solved by many. I will be out of internet contact from Tuesday until late Saturday, deep in the dark woods of Georgia. Time to get away for a while.

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

 

[hardalee]

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.

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

 

[Peter Dow]

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

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

 

[Peter Dow]

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.

 

[Dr_Toad]

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.

 

[sculptor]

You get no chops from this shithole.

Jeez Dr_Toad that's a tad rad
(but perhaps, not altogether inaccurate?)

 

[hardalee]

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.

 

[Peter Dow]

Quiet at the back of the class, please.

This kind of thing - an online calculator, saves so much time. A great wee tool. I assume you are using something like this yourself?

Design and analysis of reinforced concrete column calculator
by Jonathan Ochshorn
https://courses.cit.cornell.edu/arch264/calculators/example7.5/index.html

Using method A
I chose middle values for
f'c(ksi) = 4,
f'y(ksi) =60
shape - rectangular (tied)
I've typed in W = 21 inches. L=24 inches
14 x #7 rebar

The result the calculator gives is
Maximum allowable design (factored) load or capacity, Pu: 1138 kips

So the maximum design load for the column they actually built (never mind that it has 2 more holes for the 3" ducts and therefore must be a bit weaker still) is only 1138 kips, but the bridge load is 1615 kips!
So the actual load was 141% of the allowable load for member #11.

It was a wonder that the mainspan didn't fall apart when they made it at the side of the road. It was living on borrowed time.

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.

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.

 

[Peter Dow]

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.

The rabble have nothing better to do it seems.

 

[Howard Willey]

So this is not "all-thread-bar" then?

Well it is not a very clear picture so it is hard to see what it is.


Ah, thanks. I've taken some stills from that video too.

What do you reckon to that bar labelled "B"? So do you think that looks more like ribs on a rebar than an all-thread-bar now or what?


No problem I have got a picture of the post-tensioning bar - or "rod" as you and NBC 6 are calling it.

Well there doesn't seem to be such an obvious thread on that bar "R" but there might be a thread on it but the video doesn't have the resolution to show the thread clearly. What we'd need is good quality photograph.

If the duct of diameter "T" has a 3" diameter that would make the bar / rod of diameter "R" to be 1.75" as expected.

That would make the threaded / ribbed bar "B" from the previous picture above of diameter about 0.875" if anyone is interested.

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.

 

[Peter Dow]

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.

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.

Great detective work.

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.

 

[Howard Willey]

The rabble have nothing better to do it seems.

Quit sit bastardus carborundum

 

[hardalee]

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.

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
Add post tensioned rods 400 kips service load
Diagonal service load= 2015
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.
Add 2 -1 3/4” pt= 480 ult load Check for load factor, 1.2 used. Add post tensioning
Diagonal design load= 2741 Kips ultimate design load
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
Load fact= 1.09

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

 

[RainbowSingularity]

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 ?

 

[DaveC426913]

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 ?

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.

 

[sculptor]

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

 

[iceaura]

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.

 

[RainbowSingularity]

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.

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

 

[Peter Dow]

If 10 number 7 bars (unknown) I think 14 or so:

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

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.

If the duct of diameter "T" has a 3" diameter that would make the bar / rod of diameter "R" to be 1.75" as expected.

That would make the threaded / ribbed bar "B" from the previous picture above of diameter about 0.875" if anyone is interested.

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!