Previous posts in the New Narrows Bridge Series:
Types of Suspension Bridges
Suspension bridges are an ingenious application of engineering. They are surprisingly old–the oldest suspension bridge still operational in the world, the Wheeling Suspension Bridge across the Ohio River in West Virginia–was built in 1849.
Suspension bridges come in two different designs; the elongated “M” shape, and the “A” shaped design called a cable-stayed bridge. The engineering is quite different between them. The weight of the bridge deck on a suspension bridge is borne by two different types of forces: compressive–the downward pressure on the bridge towers, and tension–the transfer of weight horizontally along ropes, chains or cables. In the cable-stayed bridge, the weight of the deck is borne primarily by the tower (or towers), as each radiating cable bears the tension of supporting only a small section of the bridge platform. The recently-completed Millau Viaduct in southern France is a stunning example of such an A-shaped design.
Cable-stay suspension bridges require taller towers and greater number of towers as the length of the span increases, and therefore are better suited for shorter distances, over land or non-navigable water. The more conventional M-shaped suspension bridge, such as the Tacoma Narrows, the Verrazano Narrows, or the Golden Gate, can bridge much greater distances between towers–and therefore longer spans–by transferring much more of the weight to the shore as cable tension. In order to secure the cables and resist the enormous tension forces generated, the cables are embedded at either end of the span in anchorages, typically massive concrete structures or embedded rock.
Design Considerations for the Narrows
Although much of the Puget Sound area geology is composed of glacial till–densely compacted rocky soil of glacial deposits, compressed by the enormous glaciers which carved out the Sound–the banks of the Narrows, like most high-bank areas around the Sound, is comprised of a looser, sandy soil prone to slides and erosion, often with underground springs. So engineers needed to create a structure which would resist these forces. The answer was concrete en mass–23,000 yards each, nearly three times the amount of each tower and two-thirds the amount of each caisson (which is 25 stories from the surface to the bottom of the Narrows). And the bulk of this mass is below ground: a 65 foot hole was dug at each site, and filled with reinforced concrete to ground level. The front edge of this massive block was beveled, so that the horizontal tension of the cables would distribute the force on the anchors more evenly against the soil of the bank, exerting tractive force downward at an angle rather than directly horizontal.
Building the Anchors
On a massive concrete base is built what appears to be two thick walls–but which are really four walls, two on each side, enclosing a narrow space where the cables will be secured.
The base plate is a heavy steel plate, perforated in numerous places with steel pipe, reinforced with cross-bracing.
Once lowered into place between the two walls, the base plate is then encased in concrete, which leaves only the perforations visible. These will be used to secure the bundles of the cables (more on these later), distributing their force over a very large cross section.
At the front end of each of the side compartments is the anchor saddle, called a splay saddle. Seen here, before being placed at the top of the anchorage:
Below are examples from other suspension bridges of how this aligns the individual cable tendons to distribute the tension:
The roof of the anchorage will form the roadway leading to the bridge deck; the open space between the walls below is for potential expansion to a second deck in the future.
Well, that’s all for this part of our virtual tour of the new Tacoma Narrows Bridge. Next time, we’ll be packing our lunches and hiking out beneath the existing bridge to the caissons. See you there!
