CONCRETE FRAMED STRUCTURES
A concrete frame is a common form of structure, comprising a network of columns and connecting beams that form the structural ‘skeleton’ of a building. This grid of beams and columns is typically constructed on a concrete foundation and is used to support the building’s floors, roof, walls, cladding and so on.
Beams are the horizontal load-bearing members of the frame. They are classified as either:
Main beams: Transmitting floor and secondary beam loads to the columns; or
Secondary beams: Transmitting floor loads to the main beams.
Columns are the vertical members of the frame and are the building’s primary load-bearing element. They transmit the beam loads down to the foundations.
Structural analysis is a very important part of a design of buildings and other built assets such as bridges and tunnels, as structural loads can cause stress, deformation and displacement that may result in structural problems or even failure.
The building regulations require that structures must be designed and built to be able to withstand all load types that they are likely to face during their life-cycle.
These loads include:
The downwards forces on the building coming from the weight of the building itself, including the structural elements, walls, facades, and the like.
They are also known as permanent or static loads, they are predominantly associated with the weight of the structure itself
The downward force on the building coming from the expected weight of the occupants and their possessions, including furniture
these occur commonly in bridges and similar infrastructure and are the loads created by traffic, including braking and accelerating loads.
Environmental loads may act on a structure as a result of topographic and weather conditions.
- Wind loads– This is a very important design factor, especially for tall buildings, or buildings with a large surface area
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Snow loads – The shape of a roof is a particularly important factor in the magnitude of the snow load. Snow falling on a flat roof is likely to accumulate, whereas snow is more likely to fall of a steeper the roof pitch
- Thermal loads – All materials expand or contract with temperature change and this can exert significant loadson a structure. Expansion joints can be provided at points on long sections of structures such as walls and floors so that elements of the structure are physically separated and can expand without causing structural damage.
Cladding – components that are attached to the concrete frame of a building to form non-structural, external surface. This is as opposed to buildings in which the external surfaces are formed by structural elements, such as masonry walls, or applied surfaces such as render.
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Whilst cladding is generally attached to the structure of the building, it typically does not contribute to its stability. However, cladding does play a structural role, transferring wind loads, impact loads, snow loads and its own self-weight back to the structural framework.
Since concrete has little tensile strength, it generally needs to be reinforced. Rebar, also known as reinforcement steel (or reinforcing steel), is a steel bar or mesh of steel wires used to strengthen and hold the concrete in tension. To improve the quality of the bond with the concrete, the surface of the rebar is often patterned.
Concrete frames can be precast (manufactured off-site) or cast on site.
Precast concrete is a form of concrete that is prepared, cast and cured off-site, usually in a controlled factory environment, using reusable moulds. Precast concrete elements can be joined to other elements to form a complete structure. It is typically used for structural components such as; wall panels, beams, columns, floors, staircases, pipes, tunnels, and so on.
Precast concrete frames are typically used for single-storey and low-rise structures. The concrete members transported to the site where a crane then lifts and places them into position to construct the frame
Concrete members can be formed on site with the use of formwork. This is a temporary mould into which concrete is poured. Traditional formwork is fabricated using timber, but it can also be constructed from steel, glass fibre reinforced plastics and other materials. Shuttering is perhaps the most popular type of formwork and is normally constructed on site using timber and plywood.
Slip form is a method of construction in which concrete is poured into the top of a continuously moving formwork. As the concrete is poured, the formwork is raised vertically at a speed which allows the concrete to harden before it is free from the formwork at the bottom..
Slip form is most economical for structures over 7 storeys high such as bridges and towers, as it is the fastest method of construction for vertical reinforced concrete structures, but it can also be used for horizontal structures such as roadways.