University of Colorado Bridges to Prosperity
Cables
With both anchors the next order of progression is stringing the cables across. At this point cables are set in the fixed anchor and both anchors have gathered sufficient concrete strength to bear the cables’ self-weight. We were now tasked with lifting five 1 1/8” steel cables over a 40 meter deep cliff. This was no small task, especially considering steel has a density of 490 pounds per cubic foot. Luckily for us the municipality was working on a water project a few miles away that warranted the use of an excavator which we were able to use. With the excavator and a thinner steel cable we were able to tow the cables up one by one and tie them into the anchor.
With this we had cables strung across but not to the correct height. A manual winch was used to raise the cables to above their final sag height. Once raised high the cables were left for a few days so that they could settle by their own weight and self-tension. After elongating to their respective lengths they were lowered into their final place.
How to Mix Concrete...
With over 100 bags of cement used, each weighing 110 pounds, how did we mix concrete without a portable drum-mixer? In short, we hand-mixed using the volcano method. To start we gathered all of the required materials of sand, gravel, cement, and water. Having limited resources, our sand and gravel were delivered as a mixture, nevertheless we used approximately 3½ wheelbarrows full of sand/gravel mix per batch of concrete. On top of sand/gravel mix a single bag of cement was added and then dry mixed. Dry mixing was achieved by shoveling the pile of sand/gravel mix and cement into an adjacent pile, then repeating to move the pile back. When displacing piles the mixture becomes more and more uniform. Once the dry ingredients are mixed, they are molded into a wide volcano so that water can be added into the center. When water is added, the reaction at the surface of the volcano acts as a barrier so that water doesn’t penetrate through. If left unmixed, the water would slowly dissolve into the volcano’s crater and only the center of the volcano would have a full concrete mixture. Since we want all of the material to become a good mixture of concrete, dry material outside of the crater is added to the rim to build up the sides. As this is done the water levels rises from being displaced by added dry ingredients and this continues until the volcano is built high enough to absorb all the water. After this, mixing with shovels and pick-axes spreads water throughout the dry mix, adding water as necessary to get the proportion just right. The wetter concrete is the weaker it will be. On the contrary, the drier concrete is the harder it will be to work with. Our strategy was to add just enough water so that it was workable, checking slump to verify we had ascertained a solid mixture.
Decking
The final and most exciting phase of building is laying decking and fencing. Wooden planks attached to wooden nailers sitting atop steel c-channels made-up our decking construction. Planks were spaced wide enough to let water drain through but narrow enough to not allow anything wider than a screw to fall through. Crossbeams were c-channels with a wooden nailer on top. The c-channel transferred decking loads to cables and held handrail cables at a constant height above the decking via suspender rebar. The wooden nailer was bolted to the c-channel so that decking planks could be bolted to the crossbeam.
Installation of decking was also the most dangerous task of construction. Try to imagine lifting heavy crossbeams into place, bending rebar around beams and cables, and bolting down wooden decking all while standing on thin cables hovering 40 meters in the air and you’ll get a sense of what the job demanded. Fall protection was worn at all times, and extra caution was used when handling materials and tools. Even after completion of decking the bridge was still not safe. At this stage there was nothing in place to fill the gap between decking and handrails. A fence was put in to fill this gap and protect people and objects from falling through. It was nailed to the deck siding so that a misstep couldn’t penetrate through, and wrapped snugly around the handrail to tie it off up top.
