Denise Goldberg's blog

Monday, July 01, 2013

a balancing act

I'm continuing to keep an eye on the Memorial Bridge project web site both to watch for the bridge opening date and to satisfy my curiosity about the remaining work.

I find it simply amazing that the weight of the center span is balanced by that of the counterweights, especially given the weight of the span at 2,500,000 pounds. The numbers are "boggle the mind" big!

balancing, center span against counterweights

With all three spans now in place, the Memorial Bridge looks virtually complete. And if it were a fixed bridge, that would be true. But the final phase, making the myriad of interrelated adjustments needed for the lift span to run smoothly, is highly complex.

On Sunday, June 30, as reported earlier, the center lift span will stay in the down position beginning at 9:30 pm until 2:00 pm on Tuesday. This is necessary for the sheaves (pulleys) to be adjusted to their final position and bolted into place, allowing the bridge to move up and down smoothly.
At the same time, AWC is in the process of balancing the weight of the 2,500,000 lb center span with the weight of the counterweights. Stephen DelGrosso, AWC Project Manager, says, “The tolerances required for the lift mechanism are very tight, so multiple adjustments need to be made on all systems. This functional testing is an important part of the work remaining.” He explained that even after a lift bridge is opened to vehicle traffic and is on a regular lift schedule, standard procedure is to continue to monitor and adjust the lift mechanisms as needed.

Information courtesy of Memorial Bridge PR, from the News section of the Memorial Bridge site.

As I look at the screen shot from the Memorial Bridge webcam I see that the center span appears to sit a little higher than the fixed spans. I wonder if that is the final resting spot of the span; I guess I should ask!

Memorial Bridge, Portsmouth Kittery
(screen shot) from the Memorial Bridge webcam

It's so nice to get quick responses to questions posted on the Memorial Bridge website!

response to my question about the "down" position of the bridge

My question: As I look at the bridge via the Portsmouth webcams (with the center span down) it appears that the center span is slightly higher than the fixed spans. Is it, or is that an optical illusion? If it is, is that the expected final down position?

response from Memorial Bridge PR:
The center span is currently resting on temporary bearings that are 5.75 feet higher than the full down position. So, your eyes are not playing tricks on you! This current position allows the workers to have easier access to the control rooms and allows for more clearance for recreational and fishing vessels that need to pass under the bridge.