Previous posts on the new Narrows Bridge:
- History of the Tacoma Narrows Bridges
- The Two Towers I: Intro
- The Two Towers II: Concrete Thinking
- The Two Towers III: Anchor Management Classes
- The Two Towers IV: Out & Down
- The Two Towers V: The Struts
- The Two Towers VI: To the Top
- The Two Towers VII: Stairway to Heaven
- The Two Towers VIII: Spinning Beginning
- The Two Towers IX: Wheels Over Water
- The New Bridge at Christmas
- The Two Towers X: Compacting the Cable
- The Two Towers XI: Cable Banding
- The Two Towers XII: The Cranes
- The Two Towers XIII: Life on the Bridge
- The Two Towers XIV: Heavy Lifting
- The Two Towers XV: The Flying Trapeze
For those who may be new to this series, I have been blogging the construction of the new Tacoma Narrows Bridge. See the above posts for more information on the Narrows Bridges, the engineering challenges, and a first-hand tour taken of the construction site.
As bridge sections continue to be added, either by direct lift or trapezing, the moment of truth must arrive: placing the final sections into position. With all sections in place save the last four, the remaining spaces are only 8 mm. wider than the sections which will bridge them.
Tough problem. Easy answer: make the gap wider.
The 44 sections of bridge are attached to one other in five larger segments. The center span and two short segments between the tower legs are separated from each other by a one-section gap. A second gap is present at each end between the long segment extending from the anchors and the short segment between the tower legs. The segment closest to the anchors is not yet secured directly to them, however, and therefore are able to be moved horizontally, somewhat like a porch swing.
Some porch swing.
The gaps which will hold the final sections lie on either side of the towers. The sections lying between the tower legs are the shortest, and will be moved first to place the 3rd and 4th sections, connecting the short tower segments with center span.
This is accomplished by something of a Rube Goldberg-style pulley system. Winches at the top of each tower (seen on the right) pull cables running down the tower, which then turn toward the center span end of the section to be moved. When the cables are winched, the ends of these short sections are pulled about 6 1/2 feet away from the center span, providing additional space to position the two sections connecting the tower bridge deck section to the center span. They are then eased back, and the inserted sections joined with rivet plates and welding.
Two down, two to go.
The last two sections, on the shore-side of the towers, are a bit more challenging. The long sections between the anchors and the towers are far more massive than the tower sections, and require a different approach. Nine sections are joined — nearly a quarter-mile in length — between the anchors and the gap on the outside of the towers. Engineers fasten metal rods to the last section closest to the anchor, passing them through the concrete of the anchors, behind which are placed hydraulic jacks. These jacks provide the 414,000 pounds of force required to pull the joined sections toward the anchors, increasing the gap sufficiently to place the last section. This move is possible because the final piece joining these sections to the anchors — a large expansion joint — has not yet been placed.
Thus inserted, these last sections complete the 5400-foot deck, leaving only the welding and riveting of the new sections and the wrapping of the cables, before the project is completed. More on these aspects in my next post.