Denise Goldberg's blog

Monday, June 24, 2013

a bridge, counterweight balance

One of the first things that caught my eye when I saw the Memorial Bridge last Monday were the very large chains that are attached to the counterweights and to the side of the towers of the bridge. The chains are massive and while the look is interesting I suspected there was a (non-decorative) purpose to them.

I was standing near a bridge worker at one point so I took advantage of his presence to ask about the chains. It was good to have an answer but it was clear I should have been taking notes! I was happy when I was looking at the news items on the bridge web site to find that I wasn't the only person asking about the chains. If you're curious too, I've included the answer as copied from the Memorial Bridge web site below.

center span raised, counterweight chains bearing weight on the side of the tower

This photo was taken with the center span in a raised position, the counterweights down. You can see the chains at the attachment point on the tower of the bridge.

balance <> lift bridges

What do the chains do?

The bridge operates on the simple principle of balance. These are “counterweight balancing chains” used to balance out the weight of the 64 cables that lift the bridge center span. The chains attach to the concrete counterweights which are used to help raise and lower the center span on the bridge. The center span is connected to the concrete counterweights, one on each end, via 64 cables which are draped over 4 very large sheave (pronounced shiv) wheels (pulleys at the top of the towers).

The cables themselves weigh a lot. Just as the counterweights counterbalance the center lift span, something needs to counterbalance the 64 cables. So, as the bridge raises and the counterweights descend, an increasing amount of the chain length (and chain weight) hangs from the steel towers rather than from the counterweights. So as the length and weight of cable on the bridge side of the sheave wheels decreases, the length and weight of chain supported by the counterweight also decreases, maintaining the balance that is so critical to proper operation.

Information courtesy of Memorial Bridge PR, in answers to questions in the News section of the Memorial Bridge site.