Previous posts on the new Narrows Bridge:
For those who may be new to this series, I am blogging a tour of the new Tacoma Narrows Bridge construction taken recently. See the above posts for more information on the Narrows Bridges, the engineering challenges, and the first parts of the tour.
Having survived the journey over the catwalk, and the descent into Hell–sorry, the 250 steps down to the old bridge caisson–the time has come to explore the west tower of the new Narrows Bridge. A narrow catwalk connects the new and old caissons, over which course air pressure hoses, electrical wiring, the slick line (for pumping concrete-the yellow pipe above)–leaving a fairly narrow walkway over the water to the new caisson. Seen above, the orange elevator by which we will ascend into the heavenlies, leaving behind family, friends, hope, and all that we cherish, lies waiting to escort its terrified captors to certain death. But like a proverbial last meal of Chicken McNuggets, we get to savor some of the mechanics of work on the new caisson first–specifically, the area where the slick line transfers its concrete to the yawning bucket for transport by construction crane to the top of the towers. Large droplets of water fall on our heads–although it is not raining: the irrigation systems used to keep the setting concrete wet drip down from high on the towers.
But before long, like some ancient prophet, Mike tells us “The Time Is At Hand:” it’s into the elevator. It’s a tight steel cage, designed to hold maybe 6 or 8 occupants, max. On the right sits a lever which controls its ascent, and Mike moves beside it as the cage door clanks closed behind us, sealing our doom. He turns to me, instructing me on how it works–“Sometimes the elevator jams, so in case I need to get out to fix it, you can operate it.”
We are most decidedly not amused. But evidently this does happen on occasion. For my next exciting adventure, I think I’ll try the petting zoo…
So up we go, moving upward from the caisson…
…above the level of the existing bridge deck…
…and up to our first stop: “lingerie, cosmetics, personal items, bridge struts…”
Bridge struts?? I wonder on which floor the panic disorder medication is sold…
Large suspension bridges need to be engineered to handle enormous stresses–something that the engineers on the first bridge, Galloping Gerty, failed to take fully into account, to their eternal shame. The new Narrows bridge has been designed to withstand a Richter 9 earthquake–which, if true, means it will be the only structure still left standing in Puget Sound. There is enormous torque on concrete towers 550 feet tall, which under stress even reinforced concrete cannot handle. Longitudinal stabilization is accomplished through the suspension cables and bridge deck, secured by the anchors on either shore. Lateral stability is maintained by the struts.
On the existing bridge, the struts are cross-braced steel beams between the two arms of the tower. On the new bridge, they are hollow concrete boxes.
Building these struts–suspended in mid-air between the towers–is an act of engineering wizardry. The lowermost strut is the easiest, since it can be supported from below by scaffolding on the caisson, as shown below.
The middle and top struts do not have this option: they must be built essentially in mid-air. This is accomplished by building what is known as a “dance floor.” Steel rods are placed in pre-cast openings in the inside face of the tower concrete, and steel I-beams are laid across them. Additional I-beams bridge the distance between the towers, allowing a wood floor to be built to support the forms and provide a working area. This can be best seen from below on the caisson view looking upward, and from above on a photo of the top strut construction during removal of one of the “bird cages” from the towers (photo taken by the construction crew. More on “bird cages” later).
Wood forms are then built to mold the poured concrete, and after the forms are in place, prefashioned rebar cages are lifted by crane to the strut platform, where they are positioned prior to the concrete pours which will encase them.
The center of the strut is hollow, allowing space to work and later for tensioning rods. This view–taken by the construction crew–looks down inside the forms where workman secure the rebar (notice the ever-present safety harnesses on the crew, which they seem to have forgotten on my tour…).
Once the concrete box is poured and completed, tensioning rods are passed through the center of the box between the towers. The purpose of these rods is to significantly increase the strength of the concrete, in a process called post-tensioning. Basically, concrete handles compressive forces extremely well (think: the towers moving towards one another), but is weak under tension (the towers moving away from each other). Steel, in contrast, is much stronger under tension than compression. Rebar increases strength under tension considerably, but the addition of steel rods pulling the towers together nearly quadruples the tensile strength of the strut. The rods are tightened by hydraulic jack, then capped in concrete, as seen below.
When the rods were tensioned, the towers moved nearly two inches closer to each other.
So we exit the elevator at the level of the middle strut (the upper one was not yet built at the time of the tour). This may be the first and last time I step on a dance floor–at least while sober. We are, ummm, seriously far up in the air, standing on the dance floor for the middle strut, which is nearly completed. But it is a secure environment: the fencing around the dance floor is substantial. The mass of the strut seen up close is genuinely impressive. The cross bracing seen on the outside, below, is for cosmetic purposes only, to echo the design of the struts on the existing bridge.
After touring the strut and the tensioning bars, it’s time to move even higher: to the bird cages at the very top of the towers. The dance floor upon which we are standing will be removed in a few weeks; the place I am standing will be no more. It’s not exactly a Star-Trekian “To boldly go where no man has gone before”, but rather “To fearfully stand where no man will stand again.” Not half-bad, really–kinda cool, in a way. Here’s a picture of your intrepid reporter, strutting his stuff in full construction gear, braving great heights with a timid heart, risking his life to bring you the latest news on the New Narrows Bridge:
Well, that’s all for this session–I think I need to go lie down for a while. Next time we’ll head to the bird cages at the very top.