Miami Pedestrian Bridge, Part II 55

[dik]

Nobodyimportant:

Your arrows are drawn to show the forces in the members and not the forces at the panel points; is that correct? Nice drawings...

And add to that the forces induced by the 'fake' cable stays, it gets more interesting. These additional forces should be included in the design for the blocks at the tensioning cable intersections.

Just because the cable stays are fake, doesn't mean they don't take load or influence forces in other members.
Dik

 

[hokie66]

Tomfh,

I was just referring to the deflected shape of the truss as it fell. You posted it. Both the top and bottom chord kinked at the ends of Member 10. But I don't know where a failure first occurred.

 

[Nobody important]

Just a rough average spread of weight across the span to give a rough view of either tension or compression in both load scenarios.

 

[Ingenuity]

Wonder if VSL were involved with stressing the cables...

Yes, VSL (USA licensee is STRUCTURAL Group) did the supply, install and stressing of the PT - $440K subcontract sum.
Source: Link

And one of their field crew members died in the collapse - RIP.

 

[hokie66]

If member 11 failed first, why not member 2, which would have had more force and was longer?

 

[dik]

Ingenuity: Thanks for the info... and, as you noted in an earlier post, one of their employees lost his life in the collapse. RIP, too.

Dik

 

[dik]

Ron:

Were you able to determine if the failure occurred at the ends of the web members or at the panel point?

Dik

 

[Settingsun]

In an ideal world, the DOT would take no responsibility if it was neither the designer or the client (ie the commissioner of the bridge design and construction). Its review would be reviewing the road with a new obstruction/hazard built by others. Eg is the bridge high enough for legal overheight trucks, allows for future resurfacing, doesn't block planned widening, piers are protected if within the traffic hazard zone.

In reality, it has deep pockets and Florida probably has joint and several liability (haven't checked), so it will be drawn in and may pay up if the insurance of the real culprit doesn't.

 

[hokie66]

dik,
As to your question of Ron, what's the difference? The ends of the web members and the panel points are the same thing.

 

[dik]

I was wondering if the temporary rod was acting as confinement in some way

If it were acting in compression, it would be aggravating the problem, I would think.

Dik

 

[dik]

The ends of the web members and the panel points are the same thing.

A failure at the end of a member may be different than the panel point 'crushing' or 'exploding'.

Dik

 

[Nobody important]

Does anyone else not like the V shaped profile under the deck, and the SPMT using quite narrow wedge shaped load spreaders on top the shoring?
Hope they loaded it in the right places too, that truss arrangement has some significant load complexities.

 

[dik]

From FOX news (I'm not a subscriber), "The two firms responsible for building Florida International University’s "instant bridge," which suddenly collapsed Thursday and left six people dead, are coming under increased scrutiny as details emerge of past engineering failures and inspection fines -- including a recent accusation that one hired “unskilled” and “careless” workers."

Dik

 

[epoxybot]

Nobody important - This was one of my concerns, particularly after re-watching the the full length bridge move & the pause, that took place. Driving over the center divider did tilt the bridge. Reading up on the SPMT vehicles, some of them are built with telescopic axles to accommodate uneven ground. Still, it has to be done correctly.

 

[jharkey4]

Okay, this has been really eating me up... #11 is clearly a compression member in the configuration at failure. So, why was it not detensioned immediately after/during placement? Or if it was, why was it (presumably) being tensioned after the fact?

This is the single biggest mystery to me.

 

[Lnewqban]

Does anybody have experience with the capability of the self-propelled modular transporters to keep the bridge lifted at a more or less constant plane, avoiding transferring ground irregularities, like cross slope of the road and road separator, to the structure?
It seems to me that the I-beam shape of the bridge made it especially vulnerable to twisting loads and deformations, which could deteriorate the nodes of the web.

"Where the spirit does not work with the hand, there is no art." - Leonardo da Vinci

 

[oldestguy]

Not knowing, but wouldn't the jack on the rod have to work up to the load in the rod and a little more so the bracket on the rod cold either be taken off permanently or at least until some of the load was released and then re-set. Maybe it got away with them on the pull to come up to the load in the rod, just enuf extra to tear the end anchor loose. Or sufficient pull to cause a compression failure at a joint.

 

[waross]

Re: The blue box.
It appears that hydraulic lines are leading to the box and there appears to be a pressure gauge attached to the box.
I'm betting that it is the hydraulic pump for the tensioning jack.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Tomfh]

If member 11 failed first, why not member 2, which would have had more force and was longer?

It's a larger member

 

[Ingenuity]

Not knowing, but wouldn't the jack on the rod have to work up to the load in the rod and a little more so the bracket on the rod cold either be taken off permanently or at least until some of the load was released and then re-set.

Yes, to achieve 'lift-off', whereby the coarse-thread nut "just" achieves 'daylight' from the bearing plate, the stressing load is a small increment above the pre-existing PT bar load.

For manual systems, usually the 'stressing stool' has an opening on one face enabling a technician to have a open wrench on the nut with a human-effort to verify that lift has been achieved (when the nut part-turns). Other systems use a displacement gauge to verify lift-off.

For more sophisticated systems, the stressing stool has a in-built, chain-driven wrench, however, the principle is the same.

Keep mind that the hyd pump operator is monitoring the applied force via gauge pressure. I have often used a load cell 'in-line' to verify applied loads with a bit more accuracy - especially true in proof or performance testing of rock/soil anchors.