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.

Post 3: The Vertical Lift Bridge

Hello again bridge enthusiasts!

I'm sorry that it has taken so long for me to post. I thought that my plan for a post on bascule bridge drive systems would be best served by a few pictures I haven't yet taken of some drive gears on display, so I thought instead I would move on to a different type of bridge altogether, (vertical lift) and post about bascule drive some other day.

I am having difficulty opening this overview on the type. The Vertical Lift bridge is one in which the deck lifts vertically upward away from the river, increasing the clearance beneath it. Of the three basic types of movable bridge, it is unique in that it never allows infinite clearance vertically for vessel traffic. Pioneered in the early 20th Century by Waddel, let's have a quick look at a few examples of the type:

 

We'll use this railroad bridge over the Clearwater River in Idaho as our typical vertical lift bridge. 

Immediately one ought to notice the towers, which serve as a support and guide-way for the lift span when the bridge is being raised. The bridge itself is pulled by a combination of drive ropes and  counterweight ropes  which are steel cables (chains are also possible) running over counterweight sheaves (pulleys) located in the tops of the towers. When actuated, the drive ropes pull on the lift span at each of the four corners and raise it straight up. As the deck goes up, the counterweight goes down, and the ropes pass over the sheaves, shifting the balance-point of the bridge from span heavy to weight-heavy. (These are two very important concepts in and of themselves and will be discussed in a later post). To close, the process is reversed and the bridge is lowered back down. As with bascule bridges, there is usually some sort of mechanical lock to prevent the possibility of the bridge coming open inadvertently. 

Tower Bridge; Sacramento, California

The vertical lift bridge is in my opinion a very interesting type. I like them. Their structure is on full display. It is notable that the Coast Guard loves these things. Their mechanism is not housed in some counterweight pit or pivot pier, and they are usually the largest examples of movable bridge. This is because at all times, the bridge is being supported at the four corners and never changes orientation or support. Bascule bridges must of course rotate to vertical, and swing bridges swap compression and tension between their top and bottom chords. (Or do they? Correct me on this if you know better). There are two drive types for these bridges, span drive, where the motors and gearing are located in a house on the lift span, and these pull up both sides of the bridge, and also tower drive, which use a pair of motors and gear drives in each tower to pull up each end of the bridge. Tower drive bridges have the problem of synchronizing both ends, span drive bridges have to keep track of long drive ropes as they twist around the towers and span. I will at some point include a diagram of this.

There are a few distinct drawbacks to the vertical lift span, however. As alluded to above, the channel is never fully unrestricted; there is still a maximum height on the vessels that may pass through it. The towers can be ugly or block sight-lines, and are often prone to being wiggled out of alignment (My Grandfather relates stories of the Hawthorne Bridge in Portland, Oregon). Additionally, the amount of bridge that has to be built to have utility is quite great. Speaking solely of western US navigable waterways, a vertical lift bridge sometimes does not make sense, simply because the complexities of building towers and adding counterweights to still limit maximum channel clearance does not make sense. Vertical lift bridges are in my opinion simply uneconomical for small sloughs and waterways where traffic is usually sailboats and not motor vessels. Europeans see things a little differently, but their vertical lift bridges are different too. (I don't have photos at this time, however).

Perhaps I just like vertical lift bridges because Seattle is the domain of the Bascule Bridge and I just need some variety. Regardless, I think the next post will complete the triumvirate of movable bridges, with the swing-bridge, and I hope you will enjoy it. Im sorry that this post wasn't as technically detailed as the last one, but fun nonetheless. See you soon!

Snake River Bridge, US 12 in Idaho