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aostling
05-08-2009, 12:06 AM
This photo (which I took in December) currently serves as the wallpaper image on my monitor. Now that I am staring at it a lot I can't help wondering about the arch on the SF end of the bridge.

The arch prevented having to raze the Civil War-era fort, located beneath it right where a lattice tower would have had to go. But the arch looks over-designed.

What loads require so much more steel-work here, compared to the span at the far left which is supported by a much leaner tower?


http://i168.photobucket.com/albums/u183/aostling/GoldenGateBridge.jpg

interrupted_cut
05-08-2009, 12:22 AM
Just a guess, but I think the arch and the 2 sets of piers on either end of it form the cable anchorage on that end. I think they would have had a monolithic chunk of concrete there if not for their desire to save the fort. The archwork has to carry some of the loads from the outermost pier to the near one, and not just the span load of that section of roadway.

aostling
05-08-2009, 01:33 AM
I don't see the cable loads as being transferred to the arch. The cable is embedded into the concrete bunker on the ground. Here is a closer look:

http://i168.photobucket.com/albums/u183/aostling/FtPointArch.jpg

Evan
05-08-2009, 01:55 AM
For that bunker to move it will have to push the arch and piers along too. It's pretty obvious that the arch is transferring load from the bottom of the near piers to the far piers. Normally that would be the weak direction to load an arch but it isn't just an arch. It is tied to the truss span above.

RancherBill
05-08-2009, 01:57 AM
It is to make it symmetrical with the solid other end, or the other end was made to match the structure over the fort.:) I was surprised to learn the bridge is metric. :eek:

http://upload.wikimedia.org/wikipedia/commons/0/02/Golden-Gate-Bridge.svg

loose nut
05-08-2009, 08:49 AM
Could be that the extra "engineering" mass is to dampen out vibration that would have shaken the fort to pieces as the vehicles move over it.

Evan
05-08-2009, 09:28 AM
I have removed the perspective compression to give a better idea of the relative dimensions of the arch. It is really fairly shallow, just enough to clear the fort. Looking closely the structure is braced, cross braced and fully triangulated. The intent is to make it effectively react as a solid mass which is in keeping with a transfer of force between the pillars and also the rock on which it is placed. There is no need to triangulate and brace an arch to such an extent that supports the usual downward compression load. This arch is intended to resist a lateral compression load.

http://ixian.ca/pics6/ggarch.jpg

aostling
05-08-2009, 10:49 AM
This arch is intended to resist a lateral compression load.


Yes, I can see that now. The Marin end of the bridge is solid between pillars, supporting your explanation. Since an arch is almost always used to support vertical loads, I was fooled.

[p.s.] interrupted_cut, you too saw this.

Evan
05-08-2009, 12:31 PM
It's also probably a good place to park when the "Big One" strikes. :eek:

ckelloug
05-08-2009, 02:50 PM
See http://www.nps.gov/fopo/historyculture/upload/SentryatGoldenGate%20for%20web.pdf

The document cites the chief engineer of the bridge, Joseph Strauss, as having built the arch to span the area of the Fort Point Fort without placing bridge supports in the area where the fort was. The originally planned southern support was moved 200ft further south and the arched section used to span the area of the fort.

--Cameron

J Tiers
05-08-2009, 11:38 PM
Assuming the truss portion is appropriately sized for the span to the left of the arch, the span is longer for the arch area. This would have required a considerably deeper section truss, OR the center support, if the arch were not there.`