Anatomy of a Steel Dam (p2)
In its simplest form, the Redridge dam is a steel wall built across a ravine - 464 feet wide and 74 feet high. This wall is built from a series of steel I-beams set on 8 foot centers that support a layer of 3/8″ thick concave plates. Near the bottom of the dam these plates rest directly against the foundation itself, while further up they are supported by a line of 57 steel trusses which are anchored to the foundation with a combination of bolts and rods. The entire wall sits at a steep 55 degrees to the water, angled back downstream.
Its this support structure in combination with the concrete foundation that makes the Redridge dam an engineering marvel. A conventional dam of this size would of been made from concrete or masonry. The 10,000 yards of cement used here would of been dwarfed by the 100,000 or more required in such a structure. Such a dam would be immense in comparison, and take several years to build. The Redridge dam performed the same function, but using only 500 tons of steel beams and plates.
At the wings of the dam were not required to hold back a large amount of water, and are fairly simple. The steel plating rests at a 55 degree angle, anchored to the concrete directly at the bottom. The top end of the plating is held up by steel beams, placed at every 8 feet and anchored into the foundation. To keep the beams from kicking out they were further anchored by two rods an inch and a half in diameter threaded several feet into the concrete. The center of the plating is supported by another set of beams, riveted to the main beam at two points. Lets take a look at some details.
A close up look at the steel plating on the water-side of the dam. Each plate was 8×16 feet, and concave away from the water, giving the dam it’s distinctive scalloped appearance. This concavity was to allow the plates to contract and expand without putting pressure on the watertight joints between them. These plates were called boiler plates, because they were assembled in the same watertight manner as boilers. This pitting on the surface is the result of the constant contact with the water.
Here are the same plates, but seen from the inside at the point the plating meets the foundation. The plating looks as if it was originally painted black (probably with a rust proof coating of some type), and here protected from the elements the paint is till intact. While regular maintenance would require the dam to be painted regularly, the Redridge was re-painted at best only every 20 years or so.
This is a close shot on the truss as it joins the beam supporting the plating. (this is the middle joint in the diagram, marked as B) We discovered at some points along the dam, like here, there were additional cross members welded between the beams supporting the plates (seen on the right of the picture). We have no idea if there were original or added later. This point is further along the wing section, and the paint has more open to the elements resulting in its condition here.
Here is a look at one of the main support beams at its connection to the foundation. The concrete foundation here is covered in dirt and leaves, making it appear as if the beam goes straight into the ground. It does not, but instead sits on its own raised footing. The support beams were grouped by twos, with cross beams connecting them together. One of those cross beams is shown here, with a sharp bend. We’re not sure if this damage is due to some structural failure, or someone just banged the heck out of it. Various other cross beams along the wing section had similar damage however.
Now a close look at where the support beam meets the foundation. It looks as if the bottom of the beam is encased in concrete, then placed in a metal box. At the bottom of the box is a flange, which connects the box to the concrete with a number of rivits. The large nuts seen on the front of the box are connected to the metal rods which sun back from here and down into the concrete at an angle. These rods keep the beam from kicking out to the left of the picture.
After the wing sections the foundation drops down a few levels to the center section of the dam. This is the most impressive part of the whole structure, and we’ll take a closer look tomorrow.
Information found in this series comes primarily from “A Narrow Window of Opportunity: The Rise and Fall of the Fixed Steel Dam” by Terry S. Reynolds published in The Journal of the Society for Industrial Archeology (Volume 15 Number 1) in 1989. Other information and photos were taken from the Redridge Dam HAER report at the Library of Congress and a contribution from reader Dave Freeze.