Question For You Bridge Builders

[teflon6string]

From the hideous salt mine with windows where I toil in downtown Portland, I've been watching the new light rail bridge (Tilikum Crossing) going up across the Willamette River from both sides, to eventually join in the middle. Viewed from a mile away, it appears that the two sides have finally met. So here's what I've been wondering:

With all kinds of lasers, GPS and technology that I can't even begin to understand, shall I assume the two spans joined perfectly mid-river when they finally met? Even with precision aiming gear, shift happens, yes? Did they need some bending/fudging to line up when it was time to touch? If they weren't right on, how far off where they?

Was anyone here standing right there when they came together?

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[ZigZagZeke]

I was an instrument technician for many years. We once had an engineer submit drawings for a calibration jig, wherein we were supposed to machine the parts to 1/10,000th of an inch....AND THEN WELD THEM TOGETHER!!!!????

 

[Provincial]

It is generally the lowest-paid worker who makes the parts fit together. In the field. With no technical support. With only the tools at hand.

In the real world, structures like this are adjusted as they get closer together. The job starts with each end being built as close as possible to the engineer's drawings. As the two structures approach one another, the crews adjust the forms (or framework, if not cast concrete) to gently meld the structures so they meet perfectly.

Because of the quality of modern instruments, unless the engineer's drawings were faulty, the surveyor's layouts were faulty, or the instruments used by the construction crews were faulty, there should be very little error in construction. Good inspectors will catch mistakes by the workers before things get out of hand. When built properly, the adjustments needed to meet perfectly will be minor.

That said, it seems like so many public works projects go astray that it makes a person wonder!

 

[AMProducts]

There are a few things here... yes, our instruments are very good, as well as our ability to survey, measure, and create a set of realistic expectations of how a structure will handle it's load. I have to admit I have yet to see the bridge or understand the type of construction they are doing, but bridges are dynamic structures, while it may feel rock solid, they generally give a bit and are a massive interplay of tension and compression to move the forces where they need to support the structure. Generally speaking, when they have a direct line of sight (like on a bridge) and the structure is dynamic, it's pretty easy to just add some more tension and get everything to line up. Where it gets really complex is when you're trying to get two tunnels to line up.

Here's something that's difficult: http://www.romanaqueducts.info/aquasite/samosarchaic/

 

[Mark W.]

Now if you want to see the other side of the spectrum run down to Hwy 34 and check out the Eddyville bypass that the State of Oregon has been throwing money at for years. It was estimated to be the most expensive hwy job in Oregons history at 110 million. When I helped haul Thousands of truck and pup loads of crushed granite rock in to fill in the three valleys where they blew up the BRAND NEW BRIDGES they built because they didn't take into account the head lands movement. They were estimating it was going to run to 450 million to complete. I'll be surprised if that 7 miles of road is finished for 550 million and within 10 years of starting.

http://www.corvallisadvocate.com/2013/0117-oregons-highway-20/

http://www.oregonlive.com/pacific-n...f/2012/09/ill-fated_us_20_bridge_project.html

 

[Jamie6.5]

I was an instrument technician for many years. We once had an engineer submit drawings for a calibration jig, wherein we were supposed to machine the parts to 1/10,000th of an inch....AND THEN WELD THEM TOGETHER!!!!????

HaHaHa! Great!
Gotta love engineers don'tcha?

Someone told me about how the Air Force wanted stuff built. He said it went something like this:
Measure it with a mic(rometer)
Mark it with chalk.
Cut it with an axe.

My dad was an aircraft/aerospace QC guy most of his adult life. He said he'd have been a lot more productive if he hadn't had to spend so much time trying to explain why something didn't meet spec, to either the people that designed it, or the people who do the actual building of it.

 

[Certaindeaf]

Hopefully they built it high enough so that the next flood won't wash that strung wire contraption down the creek.

 

[44mag2ndamend]

Quite a Utopian picturesque kind a thing!

 

[Certaindeaf]

But think a the bums!

 

[redmud]

I hate it they put right in one of my fishing spots.

 

[3MTA3]

What's the difference between a crossing and a bridge?
Does it have bike paths and tofu dispensers?

 

[Certaindeaf]

I guess too many people hung themselves upon/from the steel bridge.. look at me! Choochoo train travelers probably got distracted from their angry birding or something.

 

[Certaindeaf]

Meanwhile the Sellwood is slated to be finished in 2020 and the 130 million spent on "studying" the I-5 bridge is money under the bridge/wasted. oh what fun

 

[PDXSparky]

Meanwhile the Sellwood is slated to be finished in 2020 and the 130 million spent on "studying" the I-5 bridge is money under the bridge/wasted. oh what fun

Actually it was more like $200 million spent on the Columbia River Crossing study. Like they really needed to study that the interstate bridge is totally inadequate for the volume of traffic it handles on a daily basis or that having to do a bridge lift causes a complete traffic F-up.

If you want to talk about making big things fit together, consider the Freemont bridge. The center suspension section was lifted in one piece and joined to the sections on each side of the river. That was a pretty impressive lift.