The SR 520 bridge features a draw span that allows boats and barges to pass through the floating bridge. The draw span remains closed to vessels and open to vehicle traffic 5 a.m. to 9 pm. weekdays because of commuter traffic.

Image courtesy of WSDOT

Making the Critical Connections

Every geographical area is different, so engineers pick a bridge style best suited to each location. When they decide to go with a floating bridge, the pontoons are usually built on shore near the construction site and then towed into place by barges and anchored. Then the support elements and deck of the bridge are added to the top.

In many cases, engineers must also account for water traffic, such as barges or military ships. For these situations, the bridge may have an elevated section (called a high rise) under which boats may pass. Or there may be a drawbridge that can be raised, although in this configuration, bridge traffic must halt.

Engineers use various techniques to keep floating bridges stationary and stable. Most use a combination of anchors and mooring lines. The anchors weigh many tons and along with the mooring lines let the bridge flex under stress without breaking.

Those are the easy parts. When it comes to making usable, durable floating bridges, it's often the shorelines that cause the most headaches. That's because shores are rarely near the surface of the water level, and often, the shore is jagged and at a much higher elevation.

That means engineers must design approaches that aren't too steep and let vehicles descend safely to the bridge's surface. Often, they build up the shoreline slowly by adding soil or rock to create a gentler ascent. Or they drive supports into the soil beneath the water to support a deck that's angled downwards toward floating portions. Still other bridges use tunnels to approach the watery part of their journey.

As engineers have honed their knowledge base, their feats have become more and more impressive. And nowhere is there a more awe-inspiring floating bridge than in Seattle. On the next page, you'll read all about the biggest pontoon bridge around.