Post 4: The Swing Bridge

The Swing Bridge: this is WA-529NB over Steamboat Slough

The last of the three primary movable bridge types is the Swing Bridge, which opens on a pivot to clear the channel. The pivot can be either in the center of the movable span, or offset from it with a counterweight. In common form, this pivot usually takes the form of a large concrete cylinder utilizing a rack and pinion arrangement for gearing the bridge open. This cylinder sits in the channel and when the bridge is open must necessarily take the weight of the entire span. (Again we see a trend with movable bridges, when open all of their weight is usually concentrated on a small area: the swing bridge pivot, the bascule trunnion and the lift bridge counterweight ropes.) The drive motors are located to my knowledge either in the pivot mechanism or in a machinery house above the bridge. When opening, mechanical wedges first retract to allow the bridge to pivot, and then the motors drive a shaft down through the road deck to the rack and pinion and bearing. There are two different primary bearing types, ring and center, but I do not recall any particulars of the two. I think it may be more appropriate to break into a couple pictures to round out this post, and better demonstrate what I am talking about.

The drive mechanism of a swing bridge

Included to show an elevation of the joint with the fixed span

Swing bridges are something of an older type and many movable bridge authorities would tell you that they are obsolete. The biggest drawback of the swing bridge? It basically halves the channel for vessels with the big concrete obstacle in the way:

Swing Bridge in the open position

 In my personal experience, they are also often painfully slow. At the same time I think they are also some of the prettier of the movable bridges; they actually look like a normal bridge for the most part (Wait for heel-trunnion bridges to see what I mean) and are getting harder to find in the modern day. Lots of swing bridges have entered what I call "Once Upon a Swing Bridge" status, permanently closed or open and rusting away.  Often times, the swing span is even replaced by something else, like a vertical lift span as seen here:

The silver swing span is in my opinion more elegant, but the brutal functionality of the lift span has won out in this case. Having a channel which is twice as wide certainly doesn't hurt either, and since the days of commercial sail are long behind us, a wider channel with a limited maximum clearance is far more valuable than a narrow unlimited one. Barges tend to be wide and flat, not narrow and tall, perfectly unsuited to the traditional swing bridge. That's not to say they don't have their use and place, they'll continue to silently revolve.