Do you ever wonder how parking garages can hold all the weight of the cars? For me it’s one of those things – like open heart surgery or jet engine maintenance – that has science and skill behind it but is in that large set of things I will never understand. I know I just have to trust that the people who do this work do it right. As you carefully drive from one end to the other of a parking deck, making tight turns at the ends to get to the next level to eventually find your spot, do you wonder how those concrete columns are connected to entire floors of concrete, and how the floors of concrete hold up vehicle after vehicle in neat rows?
I don’t wonder about parking garages every day, but today I watched the ongoing construction of one in Kirkland, Washington, just outside Seattle and I am less foggy about them. I watched with great interest not only because I saw various stages of the process, but also because my son Bradley, who built the cottage on my property six years ago, is managing this project. Here he is with his family at the job site. Beth is holding two-week-old Zoe and Brad has two-year-old Piper, who was not as fascinated with the rodbusting as I was.
Eventually this site will become a six-story, mixed use building, meaning in this case retail on the ground level and apartments above. Its architectural rendering is posted behind the fence.
Right now it’s a hardhat area at the beginning stage of construction.
This was my first view of the job site.
The white concrete columns – see them? – are standing on the lower level of what will be the underground parking deck of the new building. (The people who will live in the apartments above will need a place to park their cars below.) If you look a little closer at the columns (next photo), you see rods sticking up from them. The rods stick up much taller than the concrete of the column because…
…once you put the floor of the second level in, the rods from the first level need to stick up through the floor, like this. You’ll see why shortly.
Those rods are strong steel called rebar that’s caged in by more strong steel. Before the concrete encases it and forms the strong concrete column, the assembled steel looks like this …
The guys in the photo below, who cage the rebar, assembling the strong innards of each concrete column, are the “rodbusters.” They are using a kind of cable tie to connect the rebar to the steel caging pieces around it.
When they finish one, the crane comes along, picks it up and brings it to its mate, i.e. to the rebar that’s sticking up from the level below. The workers in the next photo are helping the crane operator to guide the caged rebar to the exact spot, and…
…Ah, there it goes.
Each assembled steel column is then surrounded by a wooden concrete form. See the rows of wooden forms below? The height of the form will be the height of the concrete column.
A gigantic cement truck comes along next with its very long arm and pours concrete into each form. When the concrete is dry, the workers remove the forms and set them aside for the next use. The steel inside adds a lot of strength to the column. Notice again how the rods stick up much taller than the concrete of the column. The part that’s sticking up becomes the base for the next level.
Now for the floor. You know what a concrete floor looks like. But under the concrete are cables, very strong steel cables. They are red in the picture below. Just as the steel rebar in the columns makes the columns much stronger, the steel of the cable makes the floor much stronger – in fact strong enough to support all the weight of the cars.
After the concrete is poured onto the wooden platform, making the floor, the cables are pulled from all sides and secured (sealed) at a specific tightness. This provides the support so the floor doesn’t collapse. After the concrete dries, the no-longer-necessary wooden platform (now underneath) is removed.
Rebar is great for adding strength to the columns, but cables are preferable to rebar for the floors. To gain the same strength/support using rebar, you would have to make the floors much thicker, which uses more concrete and makes the building overall taller. The curve of the cables plus the tension gained from the pulling means the floor can be thinner (less concrete is used and the building is not as tall overall), which is a more efficient use of materials and space. You have to know your math and your science to make this all work of course, and you have to have the right machinery and good materials and the project manager and the inspectors and all the workers, including the rodbusters. Without the rodbusters, forget it.
If you didn’t know all this before, as I didn’t, don’t you feel better now about parking garages? I do!
I also feel very proud of my son.