Buildings are frequently generalized into two main categories based on their primary structural material, concrete or steel. In reality, most buildings are more of a composite construction – using concrete and steel both, where their characteristics can best be put to use. For example, almost all buildings have concrete foundations, owing to the economy of using concrete in compressive conditions. Likewise, almost all structural concrete is reinforced with steel bar, typically no less than 0.15% by volume.
Reinforced concrete (R/C) is also readily used to form columns, beams, walls and slabs. There are many advantages to using reinforced concrete as a total building system.
- Concrete aggregate is almost universally available, leading to reduced material costs in remote locations.
- Concrete is also fairly easy to work with.
- R/C slabs require much less depth than a steel floor system. For a typical span of around 20 ft. a R/C slab would be about 8” thick; an equivalent steel floor would be about double the depth. This allows designers to add more floors while maintaining same total building height – a major advantage in cities with height restrictions like Washington, DC, where nothing may be taller than the Washington Monument.
- R/C slabs can be supported by monolithically cast columns. This frees designers from having to place columns on a rectangular grid. This is especially applicable to residential developments with units of varied layouts. Designers may locate the columns in mechanical chases and other nooks and crannies hidden within interior partition walls.
- In super-tall buildings, occupant comfort is often the limiting factor. High winds can cause a building to sway uncomfortably. Owing to the shear volume of concrete used in solid structural walls, concrete buildings tend to be stiffer and therefore less prone to uncomfortable motion.
In March of 2008, I began work on a 6-story concrete apartment building in Chicago. The architectural floor plan incorporated several different unit layouts, and the architects wanted to hide the columns out of the sight of the residents. A concrete structure permitted the necessary freedom in column placement. Also, a vertically oriented mechanical system limited the need for large ducts to cut across the ceiling. Therefore, the architects could take advantage of the short floor-to-floor height and save façade material and construction costs.
I made a few visits to the site to inspect the reinforcement layout. The structural drawings specify the size, length, quantity and location or rebar. Most slabs have a continuous bottom mat of regularly spaced rebar. A top layer of reinforcement is placed over columns and other areas of the slab that experience convex bending. Wire chairs are used to make sure that the reinforcement is placed at the correct depth. It’s important that all of the bars are properly supported and tied down, so that they’re not knocked out of place while crew places the concrete. This is a major concern when long tubes are used to convey the concrete to the pour location from the truck.
During my most recent trip to the site, I had the chance to watch the pouring of the concrete roof. A pump truck was used to lift the concrete up to the roof. The concrete then dropped straight down through a chute. An operator used a remote control to move the boom. Meanwhile, a team of laborers raked the concrete over the roof surface and into corners. There were a couple of workers with vibrators, which help the concrete consolidate into the forms. The biggest guys on the crew usually work the vibrators, which they carry around on their backs. Later, another set of skilled workers used screeds and fanning machines to level the surface of the concrete.
Building formwork and placing concrete is labor intensive. It can be backbreaking work, but only a few tasks require highly skilled workers. This makes cast-in-place concrete a very desirable building material for places with relatively low-cost labor. On the other hand, cast-in-place concrete is rarely used in countries, like Denmark, with very high labor costs. Fortunately, in Chicago, labor costs for steel and concrete construction are very competitive, so the building system is usually based on the architectural and engineering requirements of the project.