Bridge failure near Albany NY 15

[bridgebuster]

I think they forgot some stiffeners

Link

 

[spsalso]

So the bridge pieces (kinda difficult to call it a bridge, since it wasn't) will be removed.

Maybe they should forget the bridge, and just have the path make a turn and parallel Route 85 until 85 is again at ground level, cross over, and come back to the walking path using a similar method. Good news is that this will create a longer walking path--more healthy! And they don't have to pay for the bridge!


spsalso

 

[azcats]

I'm also, really not buying that this would have been solved by shoring. Nary a headed stud in sight. Concrete at or below CL of section. What mechanism for transferring any compressive force into/out of the slab?

Maybe fail slightly differently but still fails in spectacular buckling at that same spot.

 

[waross]

Nary a headed stud in sight.

You're correct.
I have just looked at all of the pictures posted in this thread.
I can not find a picture of the top of the cross beams (where studs would be expected).
The few pictures of the top were taken post collapse and the view is obscured by the rebar.
That doesn't mean that the cross beams were not well studded, just that there is no photo confirmation of studs.
As long as the concrete is above the lower flange it will provide a reaction force.

To me, the question is:
Would the bridge stood as constructed after the concrete was set?
That begs another question;
Is this a design error or a construction error?
(Or a failure to provide adequate construction instructions.)

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Lomarandil]

If the bridge flanges are at all sized with economy in mind (e.g. not grossly oversized), simply by nature of their scale there is a significant force needing to transfer from the top flange in the deeper midspan section, to the top flange in the shallower end sections. This would be similarly true during concrete placement, and after the bridge is open to the public.

The detailing at the transition is obviously and woefully inadequate to transfer significant forces. Concrete deck providing stiffness, composite action, or not, as AZCats mentions, it will buckle spectacularly as soon as the girders are loaded in flexure.

This one doesn't require much detective work.

 

[3DDave]

Rough order of magnitude - ignoring the upper flange and looking only at the deck and lower flange:

Assuming the deck is 6 inch * 15 feet wide by 120 feet long, simply supported. At a density of 140 lbf/ft^3 - that is a weight of 130,000 pounds, give or take.

The moment on a uniformly loaded simply supported beam at the center is wl^2/8 => 130,000*120^2/8 = 2.3E8 ft-lbf.

The majority of the moment of inertia would be the slab = wh^3/12 = 15*.5^3/12 = .156 ft^4; c, to the outer fiber of the lower flange is about 1 foot so the stress at the outer fiber = Mc/I or 1.5e9 PSF. Divide by 144 in^2/ft^2 => 10 million psi.

Even if I'm off by a factor of 10, that would reduce the stress to 1E6 psi, which is high for steel.

I think the math is right, but it's worth an independent check.

 

[waross]

Hi 3DDave.
Does that imply that had the deck been supported until the concrete set, it is possible that when the first heavy vehicle crossed the bridge, as well as buckling, the bottom flange may have failed as well?

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[3DDave]

Err - it means that the top flange wasn't decorative and depending solely on the combination of deck and lower flange would not work.

I'd have to figure out the moment of inertia for the full depth beam - it should be far larger than a thin concrete deck - but I expect the bridge company at the least did the calculation for maximum bending on the full depth beam.

 

[ACtrafficengr]

A "preliminary engineering opinion" has been released regarding the failure. It pretty much lines up with y'alls opinions.

Report details reason behind Rail Trail bridge collapse

The abrupt compression flange transition in the step pattern at the bridge can result in a concentration of stresses being imparted into the web at the location of the transition. This often requires a detailed analysis along with providing additional strengthening of the girder web – either through the placement of additional web stiffeners in the step area (e.g. radial, vertical or horizontal) and/or providing additional web thickness. It does not appear this additional stress concentration in the webs at the transition locations were considered.

My glass has a v/c ratio of 0.5

Maybe the tyranny of Murphy is the penalty for hubris. -

http://xkcd.com/319/

 

[spsalso]

"...CAN [my emphasis] result in a concentration of stress..."

Is there ever a case where it will not?


spsalso