Bay Bridge - We'll fix it better this time

[MintJulep]

I haven't really spent too much time trying to figure this out, and very likely the picture is wrong

http://www.sfgate.com/cgi-bin/objec...009/10/28/ba-bridge1029_gr_SFCG1256780933.jpg

but can anybody say "Holy indeterminate load path Batman!"

Doesn't this clap-trap add significant resistance to the pin joint's ability to rotate?

Doesn't that introduce shear and moment loading into the eye bars (that they probably weren't designed to deal with)?

 

[ishvaaag]

Certainly and friction causing rhomboidal distortion making the clamps themselves indent highly stressed rods.

 

[miecz]

I have my doubts about friction being the cause. If you take a long, thin element, like a 2" diameter rod, fix it against rotation at each end, and displace the one end [Δ], you get a smooth curve and a bending moment at the support M=6EI[Δ]/L[²]. Now, if you put that rod in tension, the smooth curve becomes a straight line, and the bending moment at the support goes up dramatically. In other words, the rod tends to kink at the support. A classic set up for a fatigue failure. I hope the engineers ar not overlooking that.

 

[ishvaaag]

Certainly, it could be even worse, from fabrication or placing in situ, right from the start.

 

[MikeHalloran]

I'm sorry to say it, but that's the most awful band-aid fix I've ever seen.

Is there some reason they couldn't just replace the cracked eye-bar?



Mike Halloran
Pembroke Pines, FL, USA

 

[MikeHalloran]

Oops, spoke too soon; of course they don't have spare eyebars in stock.

It's still an ugly repair of an ugly band-aid.

It can't be the first eyebar ever to crack.

Has anyone ever done it better?
Temporarily take the load off a cracked eyebar?
Without welding to adjacent eyebars?






Mike Halloran
Pembroke Pines, FL, USA

 

[HgTX]

Keeping them in stock is not a big deal; you could cut something shaped like that out of plate in no time. The problem is installation. You'd have to get the new eyebar onto the pins.

Hg

Eng-Tips policies: faq731-376

 

[MintJulep]

You'd have to start with a mighty big plate. I doubt that anyone just happens to have a sufficiently large plate of the appropriate material lying around just waiting for the Bay Bridge to fall down.

Mills don't inventory anything anymore. Every run is to-order and done just-in-time (or just a bit late).

 

[HgTX]

Last week, A 709 plate was on a 9-week backorder. Not too bad. But they'd still need to get it installed. If total replacement were a viable option, that steel would have been ordered already and expected in a few weeks--and Caltrans would have been taking the opportunity to tell everyone about how their brandy-new eyebar is on its way. Since that announcement hasn't been made, my guess is they're not planning that kind of solution, that the installation problems are prohibitive.

Hg

Eng-Tips policies: faq731-376

 

[MintJulep]

Yeah, installation would be a bitch. You'd have to figure out a way to hold up the entire bridge section while you take out the old eye. Maybe come up with a way to offset the resultant unbalanced horizontal loads on the tower too.

 

[MikeHalloran]

About getting the eyebar onto the pins:

Bridges have been built with eyebars long before field welding was an option. So how were they replaced?

They seem to always take the same shape; a circular eye with smallish fillets between the eye and the bar proper, instead of the gradual taper in section that one might expect based solely on stress considerations. From that standpoint, eyebar ends in profile should look like swaged cable terminations. So why don't they?

The relative size of the fillets suggests that perhaps, in days gone by, the way to take the load off a given bar was to support its neighbors by paired rounds tied to each other across the fillets, and tied to the other, far distant, neighbor, by long chains or cables with turnbuckles to take up the tension.

Maybe they need to find some really, really old bridgebuilders, or a really, really old book.


Mike Halloran
Pembroke Pines, FL, USA

 

[MintJulep]

Eye chiwawa!

I hadn't seen this picture of the crack before:

 

[ishvaaag]

It seems also to have degraded the pin.

 

[HgTX]

Mike, I don't think they really have replaced the eyebars much. It's not a standard frequent-replacement maintenance item, any more than a girder would be. I'm no great authority, but I've been peripherally involved with a few historic bridge rehabs (a lot smaller than that one), and from what I've seen, when we get to it that's the first time anything's been replaced. Along we come 100 years later, figure out what parts are dead, replace them, paint the whole thing, hope it lasts another 50 years before we turn it into a quaint pedestrian bridge. But I haven't been involved with a project yet where what was being replaced was a significant structural member, just bracing and the occasional floorbeam.

With my most recent eyebar experience, they were located such that they could all be pulled off and replaced. No one was trying to preserve a joint while replacing a single bar. And, by the way, the decades-old retrofits that had been in place to strengthen the old eyebars looked mighty like the 4-rod repair that failed on the Bay Bridge.

There was a project where they recabled a small suspension bridge. That was a very, very serious undertaking.

One more thing--don't read too much into the shape of the eye. It could be as much an artefact of the fabrication process as a well-thought design reflecting stresses or rehab avenues.

Hg

Eng-Tips policies: faq731-376

 

[crossframe]

I used to work on that bridge. The eyebars are definitely 1936 original.

Closing a bridge carrying 300,000 cars a day (each way) will do a lot to the congestion on the other Bay Area bridges. I haven't seen it mentioned above, but only a "temporary" is warranted.

The "permanent" fix is 100 or so feet to the north.