CONCRETE FRAME STRUCTURES
Concrete frame structures are a very common – or perhaps the most common- type of modern building internationally. As the name suggests, this type of building consists of a frame or skeleton of concrete. Horizontal members of this frame are called beams, and vertical members are called columns. Humans walk on flat planes of concrete called slabs (see figure 2 at the bottom of the page for an illustration of each of the major parts of a frame structure). Of these, the column is the most important, as it is the primary load-carrying element of the building . If you damage a beam or slab in a building, this will affect only one floor, but damage to a column could bring down the entire building.
Working out the exact ‘recipe’, or proportions of each ingredient is a science in itself. It is called concrete mix design. A good mix designer will start with the properties that are desired in the mix, then take many factors into account, and work out a detailed mix design. A site engineer will often order a different type of mix for a different purpose. For example, if he is casting a thin concrete wall in a hard-to-reach area, he will ask for a mix that is more flowable than stiff. This will allow the liquid concrete to flow by gravity into every corner of the formwork. For most construction applications, however, a standard mix is used.
Common examples of standard mixes are M20, M30, M40 concrete, where the number refers to the strength of the concrete in n/mm2 or newtons per square millimeter. Therefore M30 concrete will have a compressive strength of 30 n/mm2. A standard mix may also specify the maximum aggregate size. Aggregates are the stone chips used in concrete. If an engineer specifies M30 / 20 concrete, he wants M30 concrete with a maximum aggregate size of 20mm. He does NOT want concrete with a strength of between 20-30 n/mm2, which is a common misinterpretation in some parts of the world.
So the structure is actually a connected frame of members, each of which are firmly connected to each other. In engineering parlance, these connections are calledmoment connections, which means that the two members are firmly connected to each other. There are other types of connections, including hinged connections, which are used in steel structures, but concrete frame structures have moment connections in 99.9% of cases. This frame becomes very strong, and must resist the various loads that act on a building during its life.
