Concrete Framed Structures

CONCRETE FRAMED STRUCTURES


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. 

LOADS


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:

DEAD LOADS 

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

LIVE LOADS

The downward force on the building coming from the expected weight of the occupants and their possessions, including furniture

DYNAMIC LOADS

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

ENVIRONMENTAL 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

    Burj Khalifa the tallest building on the planet is designed in such a way that it withstands wind loads 
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The roof pitch is designed to withstand snow loads

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 


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. 

Glass cladding

 

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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 


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 frameImage result for precast concrete

FORMWORK


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 FORMWORK 

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.

 

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Details on Steel framed structures

This period of July we zero in structures, from wooden framed structures to concrete framed structures pre-engineered buildings and load bearing structures so makes sure to come back for more 
Most steel assembly is done with a type of steel called mild steel. Mild steel is a material that is immensely secure.  Take a circular bar of steel 1 inch / 25mm in diameter. If you were to attach this bar securely to your ceiling you could hang from it 20,000 Kg (which is 20 tons), or any one of the following:

  • 2 and a half African Elephants
  • 1 and a half London City Routemaster Double-Decker Buses 
  • 18 Toyota Vitz  

We urge you to try this at home unless you are coupled.

This immense strength is of great benefit to buildings.  The other important feature of steel framing is its flexibility.  It can bend without cracking, which is another great advantage, as a steel building can flex when it is pushed to one side by say, wind, or an earthquake.  The third component of steel is its plasticity or ductility.  This means that when subjected to great force, it will not suddenly crack like glass, but slowly bend out of shape.  This property allows steel buildings to bend out of shape, or deform, thus giving warning to inhabitants to escape.  Failure in steel frames is not sudden – a steel structure rarely collapses.  Steel in most cases performs far better in an earthquake than most other materials because of these properties.

However, one important property of steel is that it quickly loses its strength in a fire. At 500 degrees Celsius (930 degrees F), mild steel can lose almost half its strength. This is what happened at the collapse of the World Trade Towers in 2001. Therefore, steel in buildings must be protected from fire or high temperature; this is usually done by wrapping it with boards or a spray-on material called fire protection.

 

 PRIME EXAMPLES STEEL FRAME STRUCTURES 

Steel construction is most often used in 

  • High rise buildings because of its strength, low weight, and speed of construction
  • Industrial buildings because of its ability to create large span spaces at low-cost
  • Warehouse buildings for the same reason
  • Residential buildings in a technique called light gauge steel construction
  • Temporary Structures as these are quick to set up and remove

TYPES OF STEEL BUILDING CONSTRUCTION

There are several types of steel building construction. Steel construction is also called steel  fabrication.

Conventional Steel Fabrication is when teams of steel fabricators cut members of steel to the correct lengths and then weld them together to make the final structure. This can be done entirely at the construction site, which is labour-intensive, or partially in a workshop, to provide better working conditions and reduce time.

Bolted Steel Construction occurs when steel fabricators produce finished and painted steel components, which are then shipped to the site and simply bolted in place. This is the preferred method of steel construction, as the bulk of the fabrication can be done in workshops, with the right machinery, lighting, and work conditions. The size of the components is governed by the size of the truck or trailer they are shipped in, usually with a max length of 6m (20ft) for normal trucks or 12m (40ft) for long trailers. Since the only work to be done at the site is lifting the steel members into place (with cranes) and bolting, the work at the site is tremendously fast. Pre-engineered buildings are an example of bolted steel construction that is designed, fabricated, shipped and erected by one company to the owner.

Light Gauge Steel Construction is a type of construction that is common for residential and small buildings in North America and parts of Europe. This is similar to wood framed construction, except that light gauge steel members are used in place of wood two-by-fours. Light gauge steel is steel that is in the form of thin (1-3mm) sheets of steel that have been bent into shape to form C-sections or Z-sections.

WEIGHT OF STEEL FRAME STRUCTURES

Consider a single storey building measuring 5 x 8m (16 x 26ft). Let us first construct this in concrete, with four columns at the corners, beams spanning between the columns, and a 150mm (6″) thick concrete slab at the top. Such a structure would weigh about 800 kg/m2, or 32 Tons (32,000 kg) in total. If we build this of steel instead, with a sloping roof covered with corrugated metal sheeting with insulation, this would weigh only about 65 kg/m2. The steel framed building will weigh only 2.6 Tons (2,600 kg). So the concrete building is over 12 times heavier! This is for single storey structures – in multi-storey structures, the difference will be less, as the floors in multi-storey steel buildings are built of concrete slabs for the economy – but the difference is still significant. 

This low weight of steel frame buildings means that they have to be firmly bolted to the foundations to resist wind forces, else they could be blown away like deck umbrellas!

ADVANTAGES OF STEEL STRUCTURES

Steel structures have the following advantages:

  • They are super-quick to build at the site, as a lot of work can be prefaced at the factory.
  • They are flexible, which makes them very good at resisting dynamic (changing) forces such as wind or earthquake forces.
  • A wide range of ready-made structural sections are available, such as I, C, and angle sections
  • They can be made to take any kind of shape and clad with any type of material
  • A wide range of joining methods is available, such as bolting, welding, and riveting

DISADVANTAGES OF STEEL STRUCTURES

Steel structures have the following disadvantages:

  • They lose strength at high temperatures and are susceptible to fire.
  • They are prone to corrosion in humid or marine environments.
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