Previous essays on the new Tacoma Narrows Bridge construction:
During its construction, I was given an extraordinary opportunity to tour the construction of the new suspension bridge, up close and personal — a once in a lifetime experience, courtesy of a patient who supervises the concrete work on the bridge.
The tour began at the west anchor site, where the concrete was mixed and distributed.
We are all familiar with concrete — a structural material consisting of a hard, chemically inert particulate substance, known as aggregate (usually sand and gravel), that is bonded together by cement and water.
The strength of the concrete is determined by the ratios of fine and coarse aggregate, water and cement — higher fine aggregate ratios with lower water content produces concrete of greater strength and endurance. Of course, the addition of steel rods (rebar) is nearly universal in structural development.
The components of the plant were assembled into a complete plant in a few days time, transported to the west anchor site, and shortly thereafter began producing concrete at a rate of 150 to 200 yards a day (the average concrete truck carries 9-10 yards, so 15-20 truckloads a day). Bridge construction requires some serious quantities of concrete: each of the towers alone requires 8000 yards (1000 truckloads, as trucks were filled to 8 yards to match lift capacity), and the caissons and shore anchors required substantially more.
One question which naturally arises: since the existing bridge is a steel structure, why not build the new bridge from steel girder to compliment the appearance of the old? The answer is simple: cost. Steel is extremely expensive in this day and age. The reasons for its high cost are many: huge increases in Chinese consumption (China’s structural steel utilization went from 100 million tons in 1997 to 260 million tons in 2003, and continues to rise rapidly); the scarcity and expense of raw materials, scrap iron and iron ore; and the high cost of refined coal (coke) to fuel blast furnaces due to environmental restrictions. So concrete wins hands down on a cost basis. It is also far lower in costs of long-term maintenance, not requiring regular painting and rust prevention — a big problem in the salty air of Puget Sound.
Now, concrete trucks make lousy amphibious vehicles, so how do you get the concrete out to the caissons (bridge piers) and the towers? Barges are too slow, and the steep, high banks of the Narrows prevents nearby water access. This problem is solved by pumping it out, through surprisingly small pipes.
The concrete is delivered to the construction site by pumping it through a piping system, which runs from the pump on the west anchor through 6 inch lines running under the existing bridge, then down to the caisson.
The pump is manufactured by a German company, the equipment is called the Putzmeister, which translated means “pump master” — although different translations have been entertained …
The pipes — about 6-8 inches in diameter — run along a walkway constructed under the existing bridge (the yellow pipe adjacent to the walkway, left), which also allows walking access to the caissons and towers. At the tower, they descend about 150 feet, to the caisson of the existing span, then across a catwalk to the new caisson. Here the concrete is loaded into a bucket, then lifted by construction crane to the top of the towers. Total distance from pump to bucket is about 1/3 mile.
At the end of each pour, the pipes must be cleaned. The solution is ingenious: Nerf balls!. A section of pipe is removed near the pump, and several large Nerf balls are placed inside. Water pressure is used to push these through the pipes, forcing the residual concrete ahead. When they arrive at the caisson, they are then returned through the now-empty pipe with high-pressure air.
One non-official pastime of the crew is firing the returning Nerf balls from the hose near the pump; they sometimes travel 3-400 yards after exiting. One Nerf ball thus launched ended up on the grill of a Mack truck driving over the bridge; it’s location is unknown, presumably at a Wal-Mart in southern California… 
Having toured the West anchorage, Mike now leads us to the access catwalk which will take us to the caissons and out to the tower. But first we make a brief stop at the anchor and suspension cable for the existing bridge, shown on the left. While the cable appears to be fixed at first glance, it is actually capable of motion — a great deal of motion, in fact.
The steel ring surrounding the cable where it enters the anchorage is part of a rocker mechanism, which allows the cable to stretch as weight on the bridge varies. The cable actually moves back and forth through the ring — sometimes as much as 8-12 inches during heavy load periods, as the cable stretches and relaxes. Watching this motion is more than a little unnerving — especially if you have ever strung a guitar, and waited for the high E string to snap as you bring it to pitch. Not that this would ever happen to the bridge cable — at least one would hope.
At this point, several members of our tour group gracefully bow out — this is an interesting tour, to be sure, but walking out on a narrow catwalk 200 feet above the water — much less heading up the tower — has made several of the folks a bit squeamish.
The catwalk has been built underneath the existing roadway, on the bottom of the structural frame of the bridge above which supports it. It has a rather solid feel — as it rests on steel girders — but there is a 36 inch railing made of construction webbing and 2 by 4’s separating you from a horrible death, screaming in terror and clawing the air, as you fall for seemingly endless seconds to a crushing impact in the frigid waters and wild currents of the Sound, where octopuses dine on your bloated flesh and rescue boats circle hopelessly above …
OK, get a grip…
The yellow pipe seen in the picture on the the left (looking back towards the shore) is the slick line, which transports concrete out to the tower.
Of course, now that you’re at the West tower of the existing bridge, there is one small problem: you need to get down to the caisson, at water level. At this point you are over 200 feet above the water, so it’s time to hike down the stairs.
The stairs??!!
Construction guys (and gals) have a certain mojo, a macho not found in ordinary mortals: they have races up and down these stairs. Mike tells us the record time coming up is 2 minutes 12 seconds. The best time down: 5 seconds — “but that was a jumper” — referring sadly to a favorite Tacoma Narrows Bridge pastime: suicide.
Black humor is not found solely among physicians, it appears.
Near the top of the stairs — out of view, unfortunately — there is a peregrine falcon nest. This magnificent raptor is endangered — but making a comeback in many parts of the country — and is the fastest bird on earth. It has been clocked at nearly 200 MPH in vertical dives, feasting on pigeons, gulls, and other birds, which it hits in mid-air. Mike relates that the “pop” of a falcon hitting its prey is very impressive, and the mother often swoops workers near the stairs to protect the young in her nest.
No falcons are present today. Fortunately…
The stairs are surprisingly stable, and before long we are at the water level, on the caisson of the existing bridge. Left is a view looking up at the stairs and tower, with the roadway overhead and the stairway seen on the left.
The caisson is the original one, built for the first Narrows Bridge (“Galloping Gertie”), and currently supports the existing bridge tower, and although showing its age, is still very sturdy in appearance. And, it turns out, we’ve also arrived at the offices of the construction crew.
To provide a local workplace out on the bridge piers, container offices were brought out by barge and located between the legs of the existing bridge towers. Far from the bustle of the roadway above, they would appear to be quiet and safe — except for one problem: falling objects.
Objects falling or thrown from the bridge above are moving at over 100 MPH when they reach the water. In one incident, a short piece of PVC pipe was knocked off the catwalk above, and pierced the roof of the offices, the seat of a chair, and impaled itself in the floor. Fortunately, no one was in the chair at the time; the roof was reinforced with steel plating shortly afterwards.
The pier is loaded with equipment: electrical generators, cranes, air compressors (used for the flotation tanks when the new caissons were being constructed), and lots of pipes and wires. Yet everything is orderly: there is an obsessiveness about orderliness and cleanup to minimize the risk of injury.
The view from the caisson, under the bridge, is rather spectacular. It gives one an appreciation of the extraordinary feat of engineering involved in building a modern suspension bridge. Even the current bridge — built half a century ago — inspires a kind of awe when seen from this perspective.
So, that’s all for this part of the bridge construction tour.
Next, we’ll be putting your acrophobia to the acid test: riding the elevator to the tower strut.
