Green Line Extension to Medford & Union Sq

Arlington

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Me thinks we see the New Lechmere viaduct threading underneath the old Lechmere viaduct??
I think what we see in this photo (reposted below) is the new viaduct (center) stopping short (but on the same elevation as) the existing viaduct (far right). Specifically the new steel set in the running direction is "pointing at" the similar-height old steel (set in the "cross" direction)

This photo is taken from the crosswalk at the "open" end of the current bus berths, and is looking East toward Boston. Roughly from here (google maps https://goo.gl/maps/qC2rx5cmqGAucZkr8)

It looks to me like they have placed the last columns that they can, spanned it with the last steel that they can, leaving half of the concrete column-top empty (ready for the future spanning steel).

The thinner columns appear to be set up to hold the center platform. And I'm assuming that later they will lay another (parallel) triplet of steel beams to span on the "NorthPoint" side of that platform.

From here, the drilling of the next columns (that'll march where the current steel El is) will have to wait until the El comes down. And they will later span with steel between the new column you see and the first that is waiting to be built.

 
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Equilibria

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I realize they're going for green because it's the Green Line, but I feel like it's going to inspire some Central Artery comparisons as well.
 

HelloBostonHi

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I realize they're going for green because it's the Green Line, but I feel like it's going to inspire some Central Artery comparisons as well.
 

Equilibria

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Interesting! It's not like green blends into this setting, though. Nor did it blend into the Artery's surroundings (nor the Tobin's, really). Actually, MassDOT's steel bridges (Whittier, Braga, the one between Worcester and Shrewsbury) tend to be blue.
 

atlantaden

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Many of the newer interchanges in South Florida paint the steel sides of their overpasses various colors and they look great, just as the lighting on the underside of the Longfellow Bridge is gorgeous!
9A70CC19-CF70-4AE8-AFC1-70687719E86B.jpeg
3F6DC0BF-8B4B-4B72-94B8-2721AC44002F.jpeg
 

RandomWalk

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Steel is cheaper and faster to erect. The self-launching gantries for the segmental concrete viaducts on the Big Dig weren’t cheap.
 

kingofsheeba

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I realize they're going for green because it's the Green Line, but I feel like it's going to inspire some Central Artery comparisons as well.
In what capacity do you think that this future section of the GL will draw Central Artery comparisons? The neighborhood will ask for it to be grounded after all this work?
 

whighlander

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Steel is cheaper and faster to erect. The self-launching gantries for the segmental concrete viaducts on the Big Dig weren’t cheap.
Fundamentally -- Steel offers longer unsupported spans and more opportunities to span with non-linear segments

When you see concrete horizontal structures you are actually seeing steel with concrete surrounding it as concrete is great as long as it stays in compression. However, if you take a pure concrete horizontal beam and load it -- the natural cattenary shape sags in the middle making the upper surface compress while the lower surface wants to stretch putting it into tension.



Concrete without reinforcement is lousy in tension and shear. So when you see a really big concrete beam traveling along a road to a site to be erected -- you are actually seeing a clever composite mechanical structure designed to keep the concrete in compression under all circumstances.





Typically for long beams in addition to the normal rebar mesh at the local level -- the compression of the concrete is accomplished by embedded steel cables under substantial tension [either pre or post tensioned] this enables the system as a whole to remain in a state of compression -- as long as the load on the beam doesn't exceed the engineering limit.


Modern sophisticated Finite Stress Analysis codes running on super computers has allowed the design of amazing concrete beams [including curves in 3D] -- but there is an Achilles heal -- corrosion of the steel.

The steel embedded in the concrete is constantly a risk of corrosion due to rain, snow, ice and in particular snow / ice mixed with salt [designed to keep the roads driveable]. While massive steel beams slowly corrode from the outside inward -- being homogeneous -- the cracks can be tracked over time and fixed as necessary.

However -- Water that seeps into a crack in the concrete can attack critical steel reinforcement [almost invisibly] and with freeze-thaw cycling chipping on the concrete the crack can grow admiring more water with more steel being corroded until serious damage has been done internally.



Go to Alewife Parking Garage to see the result of 30 years of corrosion and what is now being done to fix it.

As for me - give me steel beams sitting on concrete posts [steel caged for earthquake scenarios] with proper monitoring [Internet of Things] and necessary maintenance and we'll be happy for many decades.
 

kc

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The bridge schedule has been delayed again. This e-mail was sent out to city residents this afternoon.

We are writing to let you know of important schedule changes for the ongoing GLX-related bridge closures.
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On Monday, January 27, the Green Line Extension Project released an updated schedule for bridge closures in Somerville associated with GLX construction.

Most notably, the revised timeline estimates that the reopenings of all three bridges that are currently closed will be delayed. The Broadway bridge in Ball Square is now expected to open in July 2020, about four months later than anticipated, the Washington Street bridge will now reopen in July 2020, and the Medford Street bridge will now reopen in October. The School Street Bridge has not yet been closed, and the new schedule now shows it and the Medford St. bridge around the corner being closed concurrently for approximately four months in the summer and fall of 2020.

In his testimony before the MBTA’s Fiscal Management Control Board today, Mayor Curtatone made it clear that our community has had to shoulder a difficult burden during construction and that the City of Somerville is not only deeply disappointed by these delays but that additional assistance will be necessary to address the impacts. In response to today’s announcement, the City is working closely with the MBTA and MassDOT on strategies and resources to reduce construction impacts, including advocating for assistance with public safety and traffic management, support for local businesses affected by construction, and other measures to mitigate the effects of construction on the community.

More information about traffic detours and mitigation efforts will be provided when available. Constituents are encouraged to contact the 24/7 GLX hotline at (855) 459-4636 or info@glxinfo.com with questions or comments.


New GLX Bridge Closure Schedule

Washington St.: Expected to reopen in May 2020
Broadway: Expected to reopen in July 2020
School St.: Expected to close after Washington St. bridge is reopened in May 2020 and reopen in June 2021 (the School St. bridge will not close until the Washington St. bridge is reopened.)
Medford St.: Expected to reopen in October
GLX officials still expect the project in full to be completed on time, by the end of 2021.
The slides from the FMCB meeting today didn't seem to have much additional info beyond this: https://cdn.mbta.com/sites/default/files/2020-01/2020-01-27-fmcb-G-green-line-extension-update.pdf
 

kingofsheeba

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So is the Washington Street bridge reopening in May or July? This update seems to indicate both
 

kc

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Good catch, it looks like a typo in the paragraph text. I would trust the FMCB slides and they say May.
 

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