Miami Pedestrian Bridge, Part V 71

[dik]

This continues the discussion from Part I, II, III, and IV. Please read the other threads first:

Part I
thread815-436595
Part II
thread815-436699
Part III
thread815-436802
Part IV
thread815-436924

 

[jasm]

There is new information. FDOT has just released the bridge construction plans. They are at

https://cdn2.fdot.gov/fiu/13-Denney-Pate-signed-and-sealed-FIU-bridge-construction-plans.pdf

 

[jasm]

FYI, connection details in the construction plans are on pages 86 and 88 of the pdf (pages B-61 and B-63)and post-tensioning forces on p. 94 (page B-69).

For comparison, the original bridge proposal is at

http://facilities.fiu.edu/projects/BT_904/MCM_FIGG_Proposal_for_FIU_Pedestrian_Bridge_9-30-2015.pdf

The PT forces in the proposal are on page B-17 (page 115 of this pdf)

Changes over the initial design: stressing forces have been increased in most PT rods in the truss that failed, and decreased in all rods in the truss not yet built. (That is, the section that was to be on the other side of the central tower.) Among these changes, the stressing force on each rod in member 2 has been taken from 200 to 280 kips. The new PT rods in member 11 are also at 280 kips. The differences in the connection details of member 11 and the now-thicker member 2 are shown on Page B-61 of the construction plans.

 

[SFCharlie]

Anybody got press credentials?
The following memo was sent to the university community April, 26.
MEMORANDUM
To: University Community
From: Carlos B. Castillo, Esq., General Counsel
Date: April 26, 2018
Dear members of the university community,
- This is an update to my April 6, 2018 memorandum concerning FIU’s participation in the National Transportation Safety Board’s ongoing investigation of the bridge collapse and how that participation affects FIU’s ability to respond to public records requests.
- I write to advise that pursuant to FIU’s request that the NTSB consider authorizing the release of additional records, the NTSB has now authorized FIU to release records that existed as of February 19, 2018, in accordance with Florida law. As a result of this authorization, FIU is now processing pending public records requests in accordance with our existing procedures and will respond accordingly.
- As always, if you receive a request for public records, please forward it to the Office of General Counsel at generalc@fiu.edu. As permitted by the NTSB, we will continue to provide updates when additional information becomes available. We appreciate your patience and continued support.
- Please do not hesitate to contact our office if you have any questions about this communication.

https://news.fiu.edu/2018/04/update...y-to-ntsb-investigation-public-records/122452

 

[JohnRBaker]

Anybody got press credentials?

Not for about 50 years now (I had a 'Press Pass' from the my university's school paper when I was doing some freelance photo work).

John R. Baker, P.E. (ret)
EX-Product 'Evangelist'
Irvine, CA
Siemens PLM:
UG/NX Museum:

The secret of life is not finding someone to live with
It's finding someone you can't live without

 

[SFCharlie]

JohnRBaker (Mechanical)
Thanks, good to know the old sense of humor is in good working order.
Charlie

 

[bimr]

Anybody got press credentials?

One doesn't need press credentials to make a FOIA request. If the information you want is not publicly available, you can submit a FOIA request to the agency's FOIA Office. The request simply must be in writing and reasonably describe the records you seek.

 

[jrs87]

An existing example of a concrete open web truss pedestrian bridge with asymmetrical members:

Alfenz Bridge, Austria

https://www.archdaily.com/385226/al...&ad_name=selected-buildings&ad_content=385226

(Covers what designers did to address shear)

https://www.designboom.com/architecture/marte-marte-architects-alfenz-bridge-is-an-alpine-landmark/

https://www.youtube.com/watch?v=g8nhjwrtePA

I have to say, this bridge looks nice.

 

[hokie66]

Good new posts. I doubt this was just a column failure of member 11. Like The Mad Spaniard, I question the lack of directness in the connection of struts and ties. The load path of the strut forces into the deck tie tendons is very tenuous. As with many structures, the key is in the connections.

 

[TheGreenLama]

Two things that trouble me about what I see within the design drawings posted last week (sheets B-39, 40):

1) Shear reinforcement called out for ALL diagonal truss members (members 2-11) was identical, # 4 @ 12", almost as if this was detailed as an afterthought. As you move from center of bridge span towards pier supports I would expect to see either the spacing of stirrups to decrease, or else bar size should increase (or both).

2) No longitudinal reinforcement was placed at the corner bends of the shear reinforcement in the truss members. In some instances PT ducts occupy this space, but in others nothing is there.

 

[Cutterhead]

Hi,
1st timer here, so I'll cut to the chase...
The short fillet block between #11, & #12, = 10" tall x 24" = 240sq inch cross sectional area.
#11 diagonal = 21 x 24 = 504 sq inch cross sectional area.
#11 is in the range of 4,000psi
The fillet potentially = 8,000 psi load. The fillet was undersized by a factor of 1/2 x #11.

#11 to deck is a somewhat flexible "construction" joint by design. The resistance to movement is by rebar attached to 2 separate pours, even worse than what a cold joint would be. #12 to deck is more like a mortise & tenon joint, because of the deck pour boxed out area, & following column pour into boxed out area. The net result is #12 is a solid connection to deck, & #11 is not.

I see this as a situation where the short spacer fillet was between a rock, & a hard place. Since concrete failure only needs .003/inch compression for failure, the amount #11 would need to slide across deck is only 8" x .003" = .024", Or for a visual, the equivalent of 7 sheets of 20# printer paper. Result, .025 travel = 8,000 psi to fillet = explosion.
If you want to adjust the block to a horizontal vector value, you are wrong. That doesn't happen until #11 force is transferred to the deck. It would also be wrong to use #11 to deck friction as a reduction in design factor on this.

A few tidbits of time line...
The tendon broke 1/2 way through the failure cycle...
When the Fillet exploded, the bottom #11 tendon started to zipper out. This proves the bottom tendon was initially intact.
When the canopy karate chopped the upper 1/3 of #11, it resulted in a humongous increase in tension (think 2 ft tall beam chopped from the top. The tendon then snapped, & jettisoned the jack past worker. (God Bless Navarro Brown & family, & may he rest in peace) It is in the videos if you look closely. It's definitely there, at the right time, & traveling out of the blister.
Thanks for letting me post on this great site!
Joe

 

[hokie66]

TheGreenLama,

I doubt that the designer considered the ties to be shear reinforcement, but rather just column ties. It appears that axial forces only were considered, not frame action. I think that assumption, along with the lack of discrete chords, meant the structure was doomed by its concept.

 

[epoxybot]

I think it is fair to assume the strength development of the concrete in the truss, column & diaphragm were past the minimum 28 day strength of 8500psi when the bridge was positioned on the SPMTs. The deck & truss were poured prior to the shoring & form work for the canopy. The only concrete that might have been less than 28 days could be the canopy & the blisters but not by very much. 8500psi is also a FDOT Bridge Spec. Type VI concrete. It would be a mix a ready-mix plants would have dialed in.

Something to keep in mind regarding cracking in high strength concrete, is that the cement paste & in this case silica fume bond to the aggregate is greater than the fracture strength of the aggregate itself. There is much less roughness to the plane/prism of the crack than with 3000 to 6000psi concrete. Indeed the photographic evidence of the failed concrete from the FIU bridge suggests aggregate fracture rather than cement to aggregate bond failure.

 

[MOJOJOHN]

I suggest consideration of an anchor failure at the base of member 11. Anchors are shown to be 8 x 12 which is ok, but no thickness shown. Left to pt supplier. Any thoughts?

 

[hokie66]

Please discontinue posting in this thread. Go to Part VI for any further posts.

Part I
thread815-436595
Part II
thread815-436699
Part III
thread815-436802
Part IV
thread815-436924
Part V
thread815-437029
Part VI
thread815-438451