the central pier of the west span of the bay bridge

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the central pier of the west span of the bay bridge

Postby >-) » Tue Aug 22, 2017 5:20 am UTC

the west span of the bay bridge in san francisco looks like this

https://en.wikipedia.org/wiki/San_Franc ... n_Span.jpg

as the image shows, it is a double suspension bridge, with a central pier in the middle of the two suspension bridges. in addition, the central pier is used to anchor the cables from each of the suspension bridges. it may not be obvious from the image, but the distance between all 4 towers is the same.

my question: it seems that the central pier serves very little purpose, other than acting as an anchorage, and serves no load bearing purpose (except to bear its own weight). why did the original builders not simply use one long cable across all 4 towers, therefore removing the need for an anchorage and making the central structure unnecessary?

some possible hypotheses:
1. mabe if the bridge collapsed, the independence of the two spans would make it more likely that only one suspension bridge would collapse, halving the reconstruction cost.
2. possible to reduce the forces on the tower, particularly shear forces (this seems rather unlikely to me)

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Re: the central pier of the west span of the bay bridge

Postby morriswalters » Tue Aug 22, 2017 10:01 am UTC

Possibly an expansion point to let the bridge move without breaking the deck.

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Re: the central pier of the west span of the bay bridge

Postby Soupspoon » Tue Aug 22, 2017 10:36 am UTC

Did a convenient (maybe subsurface) rock exist, to build on, to usefully add to the support and stability, but ill-placed in fractional position across the crossing for use as a suspension tower? Or, alternately/additionally, it was an awkward rock (once the spans were built) that, without hyperobvious marking, would have been the doom of many a vessel drawn to take its passage through the natural centre of the suspensive span.

(Is it of the same architectural style as the landed suspension-end footings, top to bottom? If it was the remains of a prior project, with much work gone into it, it could also have just been appropriated into the new design.)

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Re: the central pier of the west span of the bay bridge

Postby p1t1o » Tue Aug 22, 2017 11:39 am UTC

It is explained right there in that very wiki page:

"The chief engineer was Ralph Modjeski, a Polish-American. Construction began on July 9, 1933.[23] Ultimately, twenty-four men would die constructing the bridge.[24] The western section of the bridge between San Francisco and Yerba Buena Island presented an enormous engineering challenge. The bay was up to 100 feet (30 m) deep in places and the soil required new foundation-laying techniques.[21] A single main suspension span some 4,100 feet (1.2 km) in length was considered but rejected, as it would have required too much fill and reduced wharfage space at San Francisco, had less vertical clearance for shipping, and cost more than the design ultimately adopted.[25] The solution was to construct a massive concrete anchorage halfway between San Francisco and the island, and to build a main suspension span on each side of this central anchorage."

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Re: the central pier of the west span of the bay bridge

Postby Soupspoon » Tue Aug 22, 2017 12:29 pm UTC

My reading of that (and looking at the picture) continues to make me as puzzled as the OP.

"Single" suspension bridge:
(In-ground anchor)-LandedFooting-UpCurve-TowerTop-DownCurve-(pulls-against)-UpCurve-TowerTop-DownCurve-LandedFooting-(anchoring)

Add another tower to hold another -(pulls against)-UC-TT-DC that fits between the original last DC and shifts the LandedFooting-(anchor) further, and it still balances. And this bridge appears to be:
(a)-LF-UC-TT-DC-(pa)-UC-TT-DC-(pulls against*)-UC-TT-DC-(pa)-UC-TT-DC-LF-(a)

This central (pulls against*) also seems to be supported, in ways not seemingly necessary in the other -DC-(pa)-UC- parts of the spanning, despite equality in all other ways.

My newer idea, though, is that (despite modern suspension bridges avoiding this) the fear was that this most vital central suspension span (of the three 'full' catenaries, and two half-catenaries at each end) would slump exactly above the central channel where vessels would want to pass. So the middle of it was propped up (by a purely compressive tower that did not 'feel' any of the sideways pull that the continuous cable safely transmits between the two sides) to ensure extra height that, for some reason, could not be achieved merely by good pre-planned tensioning of the central catenary.

Just a guess.
Spoiler:
The diagram here (the larger resolutions crash the browser tab, so I can't read the dense text!) doesn't really help.

I presume from what is said, and the foreshortened photo given as the original evidence, that the unseen W1 is the west footing, W2 is a tower identical to W3, there is apparently no W2.5 (of a W4 kind) on the way onto the image, W5 also has no 4-like W5.5 on its way to the W6 tower and probably W7 is the east footing.

The distances between Ws 2…3, 3…5 and 5…6 are likely nearly identical. W1…2 and W6…7 are roughly half that, much as W3…4 and W4…5 halving the W3…5 span. Certainly when reduced to simple vulgar fractions, give or take a few percentage points of differences.

If the half-catenaries at each end are as long as the full-catenaries 2…3 and 5…6 and the two half-catenaries 3…4 and 4…5, then maybe the reason is something to do with the technical hurdles of maximum wire-bundle lengths, but I definitely don't get that feeling from the imagery I've so far seen. I'll look at some alternate pictures when I'm not on this device, nor on this metered data connection.

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Re: the central pier of the west span of the bay bridge

Postby p1t1o » Tue Aug 22, 2017 1:51 pm UTC

Yeah it does say the design for the single bridge was too low in the centre.

But it also said that the single bridge design would have required too much space at the San Francisco side, and "too much fill" - I take this to mean that it would have required a lot more excavation, as it was also deemed too expensive.

Ergo, two bridges with an anchorage in the middle.

You've gotta imagine that the cables and tension for a single-span bridge would have been heavier and more expensive than those for two bridges of half the length, so adding the central island not only "props up" the centre of the bridge, but allows the entire rest of the bridge to be build in a far more lean manner due to smaller forces.

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Re: the central pier of the west span of the bay bridge

Postby HES » Tue Aug 22, 2017 2:18 pm UTC

p1t1o wrote:You've gotta imagine that the cables and tension for a single-span bridge would have been heavier and more expensive than those for two bridges of half the length, so adding the central island not only "props up" the centre of the bridge, but allows the entire rest of the bridge to be build in a far more lean manner due to smaller forces.

Pretty much. I'd imagine there's very little direct vertical load on the pier from the bridge, rather that it's restraining the cables (less "propping up", more "anchoring"). I have since lost the links I was reading, but I did only google "bay bridge central anchorage".

Soupspoon wrote:Did a convenient (maybe subsurface) rock exist, to build on, to usefully add to the support and stability
Interestingly, yes. There was conveniently* accessible bedrock at that location.

*relatively. Still needed significant underwater excavation, but it was at least in the realms of feasibility.
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Re: the central pier of the west span of the bay bridge

Postby Soupspoon » Tue Aug 22, 2017 2:46 pm UTC

That still doesn't help explain, in my mind. (@p1t1o, this is!)

They avoided a single span by having three (and two halves) spans*. Then additionally supported the middle one anyway. For reasons the OP, and now me, cannot fathom.

Each free-suspending tower-to-tower catenary (once the cable and tower is designed sufficient to hold its own, assuming neighbouring structures keep everything in mutual longitudinal balance, which they do - and free reign is given to make the grounded anchorings as strong and taught-to-the-cable as needed to 'pretend' to be another full link) cares not whether it's a joined to a terminal half-catenary with sufficient footing or first to another catenary (balanced across the tower) before the footing. Or to another catenary just one of many in a row, if you so like.

* - Yet to confirm this to myself. But the original photo has a half-span in the distance, a full one, two-halves-of-a-full, and another full span disappearing off the edge of the photo, presumably with a footing-ended half to complete. With no obvious reason why the two-halves have forces upon them different from the non-halved spans either side. And no obvious extra arching, but then sometimes it is just an additional few feet of clearance at a high tide that matters, at design time.

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Re: the central pier of the west span of the bay bridge

Postby morriswalters » Tue Aug 22, 2017 3:01 pm UTC

Go out to that location on Google maps and look. Suspension bridges are symmetrical. They built two bridges tied together with an expansion joint. The complement to that pier is on the shore. The same is true on the larger end. I just watched them build a cable stayed bridge and do exactly this on both ends. There is an expansion joint at all the endpoints to allow the bridge to move without breaking. The last cable on each span at each end is two halves. These stop rotation at the top of the center columns. Each tower is half a bridge. Watch them build one. :D

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Re: the central pier of the west span of the bay bridge

Postby p1t1o » Tue Aug 22, 2017 3:31 pm UTC

Soupspoon wrote:That still doesn't help explain, in my mind. (@p1t1o, this is!)

They avoided a single span by having three (and two halves) spans*. Then additionally supported the middle one anyway. For reasons the OP, and now me, cannot fathom.



As far as I am understanding it -

Its not a "support", its like an artificial island so they could build 2 lightweight bridges instead of 1 heavyweight one.

It provides lateral anchorage to the cables from opposing bridges rather than vertical support to a singular span. Although technically yes it is vertically supporting the "middle" of the bridge, however it is not the middle of a bridge but the end of two bridges.

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Re: the central pier of the west span of the bay bridge

Postby Soupspoon » Tue Aug 22, 2017 5:26 pm UTC

There's only one bridge, though. Just counting the one pictured, not either the old or replacement continuation bridge going east from the central (non-artificial) island onto Oakland, connected to this mostly by tunnel.

It has six spans (yet surely needs only five!). Or else is three main spans (one of which has a useless central prop) and two end-spans. Or is four towers each supporting two half spans, that are helpfully held out by neighbouring spans except for the ends that are held by the ground and one other that has the mystery tower - whilst two of those half-span-to-half-span joins seemingly do not need such a prop, despite an identical set of forces.

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Re: the central pier of the west span of the bay bridge

Postby >-) » Tue Aug 22, 2017 5:34 pm UTC

yes, i'm away of the reasons they couldn't build a single span, as well as the fact that the central pier happened to rest nicely on shallow water. but as soupspoon pointed out, this doesn't actually explain the presence of the central tower. in the image below, i have drawn out the forces on the cables and the towers of the bridge.

each of the three "U" sections between the four towers should experience the same tensions on the cable. yet, in the middle U section, they decided to anchor both halves of the U to a concrete pier in the center (the cables are not connected!). as some have pointed out, the really strange thing is that the huge concrete pier seems to have no vertical force loading on it

why did they not simply use the bridge design on the bottom half of my drawing, where the cable is connected, making an anchorage unnecessary?

soupspoon said something about maximum wire length, but i'm not sure that's very convincing. in suspension bridge construction, i believe the main cables are constructed over many iterations, they aren't preconstructed and just draped across the towers. then, it seems unlikely that a single strand of the main cable couldn't have been made arbitrarily long

morris walters said something about expansion (thermal expansion?). However, I find it hard to believe that the mechanism to allow thermal expansion couldn't have been implemented at the towers and needed a central pier instead. or perhaps, it could even have been implemented in the center of the U shapes, considering the deck of the bridge experiences minimal lateral forces.

HES mentioned propping up the center, but I imagine this could have been also easily accomplished by shorting the vertical cables on the middle U span, causing the deck to rise.
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Re: the central pier of the west span of the bay bridge

Postby HES » Tue Aug 22, 2017 6:52 pm UTC

>-) wrote:HES mentioned propping up the center,

No he didn't. Less "propping up", more "anchoring".

The vertical component of the forces in the cable is relieved by the towers, but the lateral component is only relieved at the ends of the bridge. If the bridge is too long, the tension in the cables becomes too great. The "center anchorage" provides a point to transfer those forces to bedrock.
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Re: the central pier of the west span of the bay bridge

Postby Soupspoon » Tue Aug 22, 2017 7:17 pm UTC

Before he posted his diagram, I did my own back-of-the-envelope diagram…

(Which seems to be sideways. But is on an envelope!)
Spoiler:
IMG_20170822_180336.jpg

For any given length of cable, the longitudinal force1 pulls at both ends against a force they would encounter whether (supposedly) anchored or else run on from a counterpart length of cable. That's true at both Tower Top and at the low-slung point. (It's true at every point, or else something would be moving. But the symmetry is more obvious at those points.) There's no more and no less longitudinal force from a cable going back up from a minima than the anchoring force of an end-block, surely… West-4 couldn't even conceivably anchor one half (two towers-worth, and one and two-half catenaries) of the bridge, without the other half pulling the other way (and, in turn, being pulled back), right?

1 You called it lateral, but I'd like to reserve that for in/out-of paper forces across the deck. Which might be a valid argument for that pillar, actually. Battling wind forces perpendicular to the roadway, and their transmission to bend the towers in a direction unopposed except by intrinsic strength.

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Re: the central pier of the west span of the bay bridge

Postby HES » Tue Aug 22, 2017 7:52 pm UTC

So I pulled out one of my old textbooks, and now I remember why I barely scraped a pass in structural mechanics.

(To reassure those living in England, I do the easy bits of highway engineering. The clever people downstairs do the bridges)
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Re: the central pier of the west span of the bay bridge

Postby >-) » Tue Aug 22, 2017 8:01 pm UTC

just to make it explicit, i believe that the force on the cables should be the same as long as the distance between two adjacent towers remains constant, no matter how many towers there are. it is for this reason that it's possible to hang electrical cables across many many pylons and the tension does not increase for every additional tower we add.

the perpendicular wind forces argument is unconvincing to me because the central pier seems to be a poor way of accomplishing it. the central pier reduces the force on the central U shape, but not on the other two, which presumably experience about the same amount of force. also, it seems likely that strengthening the towers would be more economical than building a massive pier in the center, which probably weighs about the same as all 4 towers combined.

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Re: the central pier of the west span of the bay bridge

Postby freezeblade » Tue Aug 22, 2017 10:51 pm UTC

Ok, so here's my two cents (bear in mind that it's been awhile since my structures class, and I was an Architect major, not a structural engineer):

When the bridge was built, in the 1930's, it was believed that the total span of the western part of the bridge was too long for a single suspension bridge design (given the budget). Each section of the western span is 2,310 feet (704 meters), putting the total span at 4,620 feet. Looking at the list of longest suspension bridges, there wouldn't be one built spanning 4,000+ feet until the Verrazano-Narrows Bridge in the 1960's* (which was the longest suspension bridge when it was built), and another one wouldn't' be until the 1980's, and most in the 4,000+ club were constructed in the late 90's and early 00's.

*I'm discounting another bridge in the bay, the golden gate, which is a 4,200 foot bridge. The decision for this one was more due to logistics, as the strong current and deep waters made such an anchorage impractical (as well as being "visually unacceptable" to the city).

Just my thoughts, it's not that it wasn't possible, it's that it would be more costly and require more demolition on the wharf-side of the bridge.
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Re: the central pier of the west span of the bay bridge

Postby >-) » Tue Aug 22, 2017 10:58 pm UTC

i think we're all in agreement it is reasonable to have a 4 tower double suspension bridge over a 2 tower single suspension bridge

the question is, what purpose does the middle pier serve in this 4 tower suspension bridge? we know it serves as an anchorage for the cables, but that reason doesn't make much sense, for the same reason that a typical suspension bridge does not need an anchorage pier in the middle of its single span.

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Re: the central pier of the west span of the bay bridge

Postby morriswalters » Tue Aug 22, 2017 11:06 pm UTC

>-) wrote:i think we're all in agreement it is reasonable to have a 4 tower double suspension bridge over a 2 tower single suspension bridge
If you build bridges in my town let me know.
>-) wrote:the perpendicular wind forces argument is unconvincing to me because the central pier seems to be a poor way of accomplishing it.
Wind loads do bad things, if you don't account for them you get Galloping Gertie. That has nothing to do with this. Draw this from it what you will. When they build a suspension bridge the cable goes in first. A cable stayed bridge is self supporting at the tower during construction. A suspension bridge must be tied at the endpoints by the cables first. If you tried building two spans in one the bridge would fail. The center span on the second span would be pulled up in the center to counter the forces on the first span. because, well, there ain't nothing holding it down. Put big ole force arrows pointing toward each other at the top of the span. :twisted: Somewhere on YouTube is a video if you don't want to do science. Oh here it is, science fair time.. Watch the scales.

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Re: the central pier of the west span of the bay bridge

Postby >-) » Tue Aug 22, 2017 11:38 pm UTC

some more research:

first, there seems to be multiple two-span suspension bridges (meaning 3 towers) in china:
https://en.wikipedia.org/wiki/Taizhou_Y ... ver_Bridge
https://en.wikipedia.org/wiki/Ma%27ansh ... ver_Bridge

but i was unable to find any three-span bridges, which did not make use of a central anchorage.
https://en.wikipedia.org/wiki/Great_Set ... eSmall.jpg

morriswalters wrote:The center span on the second span would be pulled up in the center to counter the forces on the first span. because, well, there ain't nothing holding it down


i believe the weight of the central span would be holding it down. given that the central span weighs just as much as the right and left span, it seems like the amount of tension on the cable on each span should be the same.

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Re: the central pier of the west span of the bay bridge

Postby morriswalters » Wed Aug 23, 2017 12:10 am UTC

Look at the Wikipedia entry for the Bay Bridge, it has drawings. They'll say it better but here it is. The forces at the top are moment arms. For the towers not to rotate the load across the tops must be equal. The purpose of the bridge is to carry the deck. The cable hangs one way when it's the cable alone, but it stretches as its loaded. The center span divides the load between towers. Each tower is half a bridge. The counter weights are massive. They are either buried or anchored in bedrock. Do the force equations.

Soupspoons drawing are close. His vectors however are wrong. Or perhaps it would be better to say incomplete. All the forces at the top of the tower are downward. One vector towards the center of the span in both directions. And another down the tower. Each tower is more or less a triangle with the tower at the apex. That should make it apparent that half of the center span is carried by each tower. That's why the can make long span bridges with no support in the center. The tower bases handle the tower vertical loads and the cables. The first drawing is shows forces. The second is your bridge section.

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Re: the central pier of the west span of the bay bridge

Postby freezeblade » Wed Aug 23, 2017 12:17 am UTC

Found an interesting scanned document that goes into the design and specs in some amount of detail here (https://cdn.loc.gov/master/pnp/habshaer ... 52data.pdf)

There is a few passages of interest in the huge document. First is a description of the center anchorage: (spoilered for length)
Spoiler:
Center Anchorage
- this anchorage is unique among U.S. suspension bridges, and even among world suspension bridges until
construction of the back-to-back Minami Bisan-Seto and Kita Bisan-Seto bridges on the Honshu-Shikoku
corridor in Japan in the 1980s. The center anchorage serves to connect the ends of the bridge cable together
and to anchor both bridges. Many aspects of the unit are unusual, it is the A-frame system at its top that
is unique. This fabricated and riveted steel assembly transfers balanced cable forces directly from one cable
to the other, and transfers unbalanced cable forces into the concrete box pier beneath. The A-frame is post-tensioned
to the pier with eyebars, providing a sturdy and constructable system.


If you look at page 121 of the document, you can see that there was a plan at one point for a double-span bridge without the center anchorage, but it was rejected, although I was unable to find explanation why in this case. It also appears that this may have been the first double-span suspension bridge, so perhaps the engineers weren't fully confident in preforming the design without the anchorage, which from what I can tell is designed to hold lots of tension between the two bridges.
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Re: the central pier of the west span of the bay bridge

Postby Soupspoon » Wed Aug 23, 2017 12:27 am UTC

morriswalters wrote:If you tried building two spans in one the bridge would fail. The center span on the second span would be pulled up in the center to counter the forces on the first span. because, well, there ain't nothing holding it down. Put big ole force arrows pointing toward each other at the top of the span. :twisted:
I don't understand this at all. There are at least three (and technically five) spans here, and "centre span on the second span" is… I don't know.

(Construction of decks beneath cables of a suspension bridge tends to be symmetric between and across all spans. Either from tower-adjacent points out to eventually meet each neighbouring deck (or the end-anchors) at the minima, or from each minima out in all directions to meet decks at the tower-adjacent points. Sometimes both, but that adds even more measurement precision during construction, or possibly bespokely creating the very last deck units to be inserted only as soon as the final gap size is known.)

Somewhere on YouTube is a video if you don't want to do science. Oh here it is, science fair time.. Watch the scales.

I shall watch this tomorrow, when I'm not on the road, and hope that this explains the above.




(?Six?-span suspension bridge: https://commons.m.wikimedia.org/wiki/Fi ... rLoire.jpg - no non-tower central anchorage, seemingly. Additional cables, but no obviously different function to them, just pre-CAD construction caution/overkill, maybe.)


((More posted since I started the above, then went looking for multispan image. Note that my force arrows are forces on the towers by the cables (separated into orthogonal forces), except for the minima points where a 'virtual meeting point' was being shown. Tried to put more detail in, buf my four-coloured pen and relatively small drawing area put paid to doing everything I wanted. Anyway, more reading later. Late-night eating first, having had little to eat all day, possibly with sleep (and strange dreams!) to follow soon after.)

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Re: the central pier of the west span of the bay bridge

Postby morriswalters » Wed Aug 23, 2017 1:02 am UTC

Look closer at the Bay Bridge. From left to right. Anchor(on shore), tower, tower, anchor(Out in the bay), tower, tower, anchor(on shore).

Your example bridge has two cables. One for the hangers, one over the top. A Frenchman(woman) showing his/her love for being different maybe? Here are some more views. Read that top cable as always in tension along the top of the piers. Only its weight pulling it down. Damn,that is one of a kind! :D

Look up see saw.

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Re: the central pier of the west span of the bay bridge

Postby >-) » Wed Aug 23, 2017 1:37 am UTC

https://www.slideshare.net/DavidColling ... on-bridges

there is a journal paper behind this presentation, which is behind a paywall, but nonetheless, this seems to pinpoint exactly the issue behind multi-span suspension bridges, which is the fact that apparently, multiple spans results in greatly increased flexibility and more deflection in the case of a non-static load (for example, a truck crossing the bridge).

there are various techniques to stiffen the bridge and mitigate this problem, but it looks like the approach taken for the bay bridge was simply to build a central pier and structurally separate the two suspension bridges completely.

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Re: the central pier of the west span of the bay bridge

Postby Soupspoon » Wed Aug 23, 2017 1:39 am UTC

I know the BB's configuration already, morris:
Soupspoon wrote:And this bridge appears to be:
(a)-LF-UC-TT-DC-(pa)-UC-TT-DC-(pulls against*)-UC-TT-DC-(pa)-UC-TT-DC-LF-(a)

This central (pulls against*) also seems to be supported, in ways not seemingly necessary in the other -DC-(pa)-UC- parts of the spanning, despite equality in all other ways.

a=anchor, part of LF=Landed Footing; UC=Upwards Cable arc to and over TT=Tower Top then down DC=Downwards Cable arc through the minima (pa=pulling against, with or without *=possibly unnecessary anchor) and so on.

Also I already noted the 'additional' cable on the French bridge
Soupspoon wrote:([…] Additional cables, but no obviously different function to them, just pre-CAD construction caution/overkill, maybe.)
Other examples I have seen have cross-bracing (Tower Top to Tower Deck-Level), purely connecting the towers, without supporting the deck, but the deck-supporting cables do this (TT-to-TT, at least) whilst taking deck-supporting tension, too. Seems like a primitive protection against cable (or more fragile chain) failures, preserving the pylons even if a deck section fails/falls and ends up taking the suspension elements out of service, no longer balancing adjacently intact suspensions.

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Re: the central pier of the west span of the bay bridge

Postby p1t1o » Wed Aug 23, 2017 10:21 am UTC

A lot of the arguments/counter arguments being mentioned assume that the bridge is constantly in a state of steady equilibrium.

It is true that a suspension bridge can be made arbitrarily long without increasing the tension in the cables.

But this is only true in a steady-state.

Consider a 4-tower bridge with 3 main central spans and two half-spans on each end.

Now put a very heavy truck in the centre of the first and third main spans.

Because the central main span is empty, there is a tension imbalance.

The weight of the trucks is adding to the tension in the central span.

If we put an anchorage in the middle of the central span, creating two half-spans, now each half-span only suffers increases tension from one truck.

So, the anchorage could be deleted with no problems, if you never drove any traffic across the bridge.

But putting an anchorage in means that the bridge suffers only half the dynamic stresses of unequal loads on the spans.

This is borne out by this statement, mentioned by freezeblade:

"This fabricated and riveted steel assembly transfers balanced cable forces directly from one cable
to the other, and transfers unbalanced cable forces into the concrete box pier beneath."

speising
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Re: the central pier of the west span of the bay bridge

Postby speising » Wed Aug 23, 2017 11:17 am UTC

Just had a look at google maps.
Can somebody explain to me why there are 2x5 lanes on the east span, and only 1x5 on the west? Is all the extra traffic swallowed by treasure island? and what is the purpose of the 3rd bridge which suddenly ends over water? (only visible on satellite view)

morriswalters
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Re: the central pier of the west span of the bay bridge

Postby morriswalters » Wed Aug 23, 2017 11:35 am UTC

p1t1o wrote:Because the central main span is empty, there is a tension imbalance.

The weight of the trucks is adding to the tension in the central span

They need to understand what that means. The center of the middle span goes up as the tension increases. It tries to rotate. They keep looking at the bridge and not the roadbed. The Tacoma Narrows bridge collapse is the extreme case of this, caused by wind. The center span is translating up and down. Evidently the main span acted like a violin string in resonance.

@Soupspoon
They make multiple pylon bridges all the time. I call them squirrel bridges. Wires between telephone poles. Any suspension bridge is two telephone poles holding up the sidewalk. Why would you put in telephone poles you don't need? This is the point of suspension bridges. The spans can be long. All bridges are a product of their restraints, either physical limits or financial. One suspension bridge might have been cheaper. It would have still had just one long span.

Thanks for that French Bridge. If you talk about bridges you should keep that one in your pocket, it neat. :D I'm sorry for not seeing your point, and for not being clear.

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HES
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Re: the central pier of the west span of the bay bridge

Postby HES » Wed Aug 23, 2017 11:58 am UTC

speising wrote:Just had a look at google maps.
Can somebody explain to me why there are 2x5 lanes on the east span, and only 1x5 on the west? Is all the extra traffic swallowed by treasure island? and what is the purpose of the 3rd bridge which suddenly ends over water? (only visible on satellite view)


The west bridge is double-deck, still 2x5. The east bridge has been replaced, and the old structure is still being dismantled.
He/Him/His Image

morriswalters
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Re: the central pier of the west span of the bay bridge

Postby morriswalters » Wed Aug 23, 2017 12:36 pm UTC

As sometimes happens, I saw the trees but not the forest. I forgot to look at the calendar. The Bay Bridge

Image
Until the 1920s, experts regarded building a bridge across the bay — a stretch of water nearly three times longer than that crossed by the then-longest bridge in the world, the 1890 Firth of Forth bridge in Scotland — as unfeasible. It was not until 1928 that the idea for a bay bridge was taken seriously, when President Herbert Hoover and California Gov. C.C. Young appointed a commission to explore the feasibility of building one.

Biggest challenge

The commission green-lighted the bridge. It would be planned and built by the state, with a loan from the federal government. The massive volume of ferry traffic across the bay gave Uncle Sam confidence the bonds would be repaid.

The biggest challenge was the western span, between San Francisco and Yerba Buena Island. This section would have to cross 2 miles of deep water — too long for a suspension bridge. But engineers discovered that there was a fortuitous ridge of bedrock midway between the island and the Embarcadero. If they could figure out a way to build an island on that ridge, they could build two suspension bridges, each a mile long.

But building this island would be no easy feat. It would have to be anchored to the bedrock so it could withstand the unthinkable tension generated by the bridge’s cables. And the bedrock was 200 feet below the bay’s surface. No such structure had ever been built.
The center anchorage, or Moran’s Pier as it is sometimes called, contains more concrete than was used in the Empire State Building and is larger than the Great Pyramid of Giza. It is one of the engineering marvels of the world.

Four mighty steel towers were built on smaller piers to hold the cables that supported the western roadway. Each could hold 65 million tons of weight. The tallest rose 519 feet above the water. Each tower represented a construction job equal to building a 60-story skyscraper.


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