Previous posts in the New Narrows Bridge Series:

  1. The Two Towers I: Introduction
  2. The Two Towers II: History of the Narrows Bridges
  3. The Two Towers III: The Caissons
  4. The Two Towers IV: Anchor Management Class
  5. The Two Towers V: The Tour Begins

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.
Elevator on caisson

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 to the heavenlies, leaving behind family, friends, hope, and all that we cherish, lies waiting to escort its terrified captors to certain death.

Tower elevator

Before long, like some ancient prophet, Mike tells us: “The Time Is At Hand”: it’s into the elevator. The elevator is 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 skyward from the caisson…

View from elevator 1

…above the level of the existing bridge deck…

View from elevator 2

…and up to our first stop: “Lingerie, cosmetics, personal items, bridge struts…”

Bridge struts?? I wonder which floor sells the anti-anxiety medication…

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 disgrace. 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, under which stress even reinforced concrete will fail. 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 reinforced 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.

Towers under construction
Strut support
Dance floor

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 (upper photos), and from above on the 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).

Once the dance floor is in place, wood forms are built to mold the poured concrete for the sides, 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.

Cross struts

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.

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…).

Strut construction
Tower rebar

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.

Tensioning rods

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.

Dr Bob on strut dance floor

Here’s a picture of your intrepid reporter, strutting his stuff in full construction gear, braving great heights with a timid heart, risking life and limb to bring you the latest news on the New Narrows Bridge. See you soon!