Struttin’ My Stuff: Up To The Tower Struts
Previous posts in the New Narrows Bridge Series:
A narrow walkway connects the old and new caissons, over which courses air pressure hoses, electrical wiring, the slick line (for pumping concrete) — leaving a fairly narrow bridge over the water to the new caisson. Seen here, the red-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.
Before long, like some ancient prophet, Mike tells us: “The Time Is At Hand” – it’s into the elevator. The elevator is a cozy 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.
So up we go, moving skyward from the caisson…
…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 Gertie,” 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, under which stress even reinforced concrete will fail. Longitudinal stabilization (lengthwise) is accomplished through the suspension cables and bridge deck, secured by the anchors on either shore. Lateral stability (side to side) is maintained by the struts.
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 on the left and center.
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 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 the “bird cages” later.
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.
The center of the strut is hollow, allowing space to work and later for tensioning rods (see below). 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 for me …)
The purpose of these tension 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 on the left.
Next: we’ll travel to the top, to the bird cages.
