Design fabrication of a prestressed concrete beam

Since many of you seem interested In this concrete bridge failure i will quickly go thru the basic points:

Deign---the span and loading are determined and the appropriate beam is selected--box beam,i beam,t beam,flat slab.
Most highway bridges are built to support the maximum semi load of 40 tons per lane
The beam chosen is anaylized as to how much prestreesing is needed and the number and location of the cables are placed
Most cable patterns are on a 2 inch grid and the CG of the cables is toward the lower part of the beam--so to impart a high compression force in the bottom of the beam and minimal tension at the top.

Fabrication--a flat casting bed is used,either concrete or steel plate,4 to 8 ft wide, and 100 to 300 ft long. forms are made to form the sides of the beam. Rebars,called stirrups, are bent and place inside the forms.
The number and spacing of the strands are placed inside the stirrups for the length of the bed,or beam, or multiple beams in a row.
the strands are pulled to a calculated force, which was determine on length of beam,shrinkage,creep,strand relaxation,temperature drop,etc which all will reduce the strand tension over time--30,000 lb pull is fairly typical

Concrete of the 28 day required strength, 5000 typical,is pored into the form around the stressed strands. It is covered and stem cured overnight to get to 80% of the 28 day strength and check by breaking concrete test cylinders.

The design engineer has determined a cutting pattern and each strand is cut at each end of each beam simultaneously--even with that its amazing to see a 50 ton beam jump all over the casting bed, The high compression forces in the bottom of the beam and minimal tension on the top makes the beam arch upward in the middle--called camber,

The forms having been stripped already the beam is lifted gently and set in a storage area, covered so it can continue moist curing for 7 days and then not shipped until more concrete test cylinders show the 28 day strength

Interesting stuff.

Was sure surprised that the walk bridge that failed was a simple prestressed, and they put so much into making it look like a hanging bridge.

Over the next 2 years, starting in April, they are going to remove a large bridge over a rail line and valley, and a bridge over the Minnesota river. Replace them and add a third new bridge as an overpass at a congested dangerous intersection. All in a mile or less of road.

Going to be an interesting couple of years, that is the main artery into this town, detours are, um, ugly.

Hope they got a better design/ construction crew for these bridges!

Paul

well it appears it was a bit more exotic and complicated--they had post tensioning concrete struts for the web truss and now it appears that the pipe stays may contribute to the overall strength and stability--time will tell as more details come out

Braum's built one of their little ice cream stores in our little town. Immersed inside the forms was a grid-work of plastic tubes with steel cables, several feet each way on center, threaded ends on the cables. After the concrete had set, they came back in and set the ferrules and nuts and washers, and tightened the cables up. In sequence. As I recall there were no footings under the edges f the slab, and I do know the contractors told me that ore side of the slab could wash out in a flood, and it'; just sit there, no collapse. Don't know WHY they did it that way, but they did. This thing sis on the crest of a hill, far form any water.

yes --that is called post tensioning --seems the diagonal trusses on the Miami bridge were post tension

Yea, the more I would say the more I would show I don't know what I'm talking about.

It sure looked like a complicated design, but probably not in the scope of designing bigger buildings and bridges and so forth.

As well seemed that moving it and supporting it in odd places would add to the potential for failures.

Really, combining concrete and steel to build anything freestanding is an odd deal, one is so brittle and one is so bendable, putting them together and making them work with each other to do anything for 50 years is a silly idea, if we hadn't been doing it for so long already.

Paul

if steel and concrete are put together correctly they work well--Just about all the highway bridges and pedestrian bridges I designed were steel beams and concrete slab called composite design ---the steel beam was designed to not only carry the dead load of the concrete slab and then a composite action to carny the live load, trucks or pedestrians. the steel beam had stud shear connectors welded to the top flange, so after the top slab hardened the compression stresses were shared by the top steal flange and concrete slab--made a much stronger beam and the compression in the slab helped to reduce shrinkage cracks

Actually the tension cable stuff is harder for an average person to understand.

On the walkway that failed, I noticed the angled, vertical (sorta) beams between the walkway and the roof were all angled to look like they matched the tall suspension cables. But it sounds like there was little to no strength from all that overhead structure.

So those angled beams were not really angled for 'best use' of a concrete and steel/ tension cable design that sits on each end.

It would be like designing a roof truss of wood (more familiar material to me...) and running the diaganals in the wrong directions. You can do that, but you have to over build and look at the pressure points all differently.....

I am still assuming that, without the overhead cables and mast actually doing anything, they were at least using the walkway floor, and the overhead concrete roofline, and the angled vertical beam backbone, as integral support of the whole thing? Maybe I'm wrong on that too.

Paul

dpendzic,
The two trusses on the end that collapsed (especially the far left one) are not symmetrical V.
Is that proper engineering?

This "Accelerated Bridge Construction" technology seems to come from a Dr. Atorod Azizinamini.

The TIGER program was also involved in this project.

(quoted from post at 15:28:29 03/19/18) Actually the tension cable stuff is harder for an average person to understand.

On the walkway that failed, I noticed the angled, vertical (sorta) beams between the walkway and the roof were all angled to look like they matched the tall suspension cables. But it sounds like there was little to no strength from all that overhead structure.

So those angled beams were not really angled for 'best use' of a concrete and steel/ tension cable design that sits on each end.

It would be like designing a roof truss of wood (more familiar material to me...) and running the diaganals in the wrong directions. You can do that, but you have to over build and look at the pressure points all differently.....

I am still assuming that, without the overhead cables and mast actually doing anything, they were at least using the walkway floor, and the overhead concrete roofline, and the angled vertical beam backbone, as integral support of the whole thing? Maybe I'm wrong on that too.

Paul

Click to expand...

an't speak for you being right or wrong, Paul, but for sure somebody on that bridge deal was definitely wrong!

I think you may be correct there Paul--many structures are designed a certain way just to sustain the dead load weight--and then further additions to carry any live load added--in this case the pipe stays for further support and stability--however this failure shows that the basic structure wasn't capable to hold its own load--it did for 5 days but fiddling with the post truss tensioning did them in--unfortunately while traffic was underneath!

Doh,
I missed Paul's mention of the odd truss shapes.

My opinion is that the ?fast track? everybody is taking about has very little to do with the collapse. That was only the erection method itself. You still need quality control and proper design, not to mention sufficient shoring until the superstructure and support cables were finished.
I wonder if they had a bad spot where the concrete didn?t have sufficient compressive strength; and when they retensioned the cables, that bad spot crushed and caused a chain reaction.
They will find out what happened, eventually.

I believe that interstate bridges are designed for far greater loads than the standard 40 ton freight hauler. I have hauled loads in excess of 200,000 pounds over 1,000 of miles of interstate highways legally with " special haul permits" issued by various states D.O.T. departments. Very occasionally was a detour required due to a weak bridge, so I assume the state engineers that approve the superload permits have confidence that the structures will support that weight. It was much more common to have speed restrictions on bridge structures. Detours off the interstates are much more common for over height loads, than heavy loads!

yes by individual overload permits a much heavier load is allowed because it happens infrequently and there is residual strength over normal strength needed. typical is a R permit which allows the 40 ton vehicle to go up to 60 tons.
I once permitted a 300 ton load go over a drawbridge posted at 20 tons. It all depended on the number of axles--distance apart, speed, and location of truck on the bridge..That permit took me a week of calculations to determine the stresses.
Interstate highways were designed for heavier loads and higher clearances--mainly due to anticipated military movements

dpendzic: Thanks for sharing information about beam/bridge loading; I thought you might find the attached video interesting with respect to your comments.
Untitled URL Link

Interesting.

The whole point of mixing steel and concrete is that concrete has great strength in compression but very little in tension. Pre-tensioning the steel ensures that the concrete will always be in compression, and the steel will always be in tension.

Thanks--that was very interesting and the specialized trailer to spread the load was something else!!

By code we were allowed to go to 212 psi tension in the bottom of the beams, but being in a salt air environment we chose to only go to 0 psi. Much less chance of cracks in the bottom of the beam which would let moisture in and possibly corrode the prestress strands.

I did a lot of fabrication work in the past for a prestressed plant. the amount of arch in a beam was determined by the tension of the cable and how much it was deflected after it was tensioned.

Jim

The overhead structure was not installed.

Yea, it's confusing......

There is the bridge deck, where people were to walk.

Then there is the overhead part, which is where the roof or awnings we're going to attach, and the strong angled (cable tensioned?) columns go between the deck and that overhead part.


Then there is the mostly decorative tall column and the angled long pipes that are too look like cables supporting the whole bridge, but are there for looks; the bolted on pipes offer only modest stiffening of the whole structure to lightly control deck bounce?

My description/ references are probably poor - I am a simple dirt farmer.

Paul