high-rise apartment building

Concrete Framed Strucutres

 

Concrete frame structures are a very common - or perhaps the most common- type of modern building.  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.  Of these, the column is the most important, as it is the primary load-carrying element of the building (See figure 2 at the bottom of the page for an illustration of each of the major parts of a frame structure). If you damage a beam in a building, it will usually affect only one floor, but damage to a column could bring down the entire building.

The concrete frame structure is actually a connected frame of members, each of which are firmly connected to each other. In engineering parlance, these connections are called moment 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.These loads include:

• Dead Loads: the downwards force on the building coming from the weight of the building itself, including the structural elements, walls, facades, and the like.

• Live Loads: the downwards force on the building coming from the expected weight of the occupants and their possessions, including furniture, books, and so on. Normally these loads are specified in building codes and structural engineers must design buildings to carry these or greater loads. These loads will vary with the use of the space, for example, whether it is residential, office, industrial to name a few. It is common for codes to require live loads for residential to be a minimum of about 200 kg/m2, offices to be 250 kg/m2, and industrial to be 1000 kg/m2, which is the same as 1T/m2. These live loads are sometimes called imposed loads.

• Dynamic Loads: these occur commonly in bridges and similar infrastructure, and are the loads created by traffic, including braking and accelerating loads.

• Wind Loads: This is a very important design factor, especially for tall buildings, or buildings with large surface area. Buildings are designed not to resist the everyday wind conditions, but extreme conditions that may occur once every 100 years or so. These are called design wind speeds, and are specified in building codes. A building can commonly be required to resist a wind force of 150 kg/m2, which can be a very significant force when multiplied by the surface area of the building.

• Earthquake Loads: in an earthquake, the ground vigorously shakes the building both horizontally and vertically, rather like a bucking horse shakes a rider in the sport of rodeo. This can cause the building to fall apart. The heavier the building, the greater the force on it. Its important to note that both wind and earthquake impose horizontal forces on the building, unlike the gravity forces it normally resists, which are vertical in direction.

 

The concrete frame rests on foundations, which transfer the forces - from the building and on the building - to the ground.