Steel Structure Guide WITH DETAIL FOR BEGINNERS

Most of Steel construction is done with a type of steel called mild steel. Mild steel is a material that is immensely strong.  

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:

1. ·       18 Honda City Cars
2. ·       2 and a half African Elephants
3. ·       1 and a half London City Route master Double-Decker Buses
4. ·       We urge you to try this at home unless you are married.

This immense strength is of great advantage 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 characteristic 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 the 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 the spray-on material called fire protection.

USE OF 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 labor-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 story 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 story structures - in multistory structures, the difference will be less, as the floors in multistory 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 prefabbed 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 shapes and clad with any type of material
• ·       A wide range of joining methods is available, such as bolting, welding, and riveting

DISADVANTAGES OF STEEL STRUCTURE:

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.

CONCRETE FRAME STRUCTURE GUIDE WITH DETAIL

Concrete frame structures are the most common type of modern building used nowadays internationally.  As the name conveys, this type of building consists of a frame or skeleton of reinforced concrete.

The structure is actually a connected frame of members, each of which is firmly connected to each other. In engineering phrase, 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.

Horizontal members of this frame are called beams, and vertical members are called columns.  Humans walk on flat planes of concrete called slabs. In frame structure building the column is the most important, as they are the primary load-carrying element of the building. If we disturb* a beam or slab in a building, this will affect only one floor, but the disturbance to a column could bring down the entire building. |*| Damage/Break

When we say concrete in the construction industry, we actually mean reinforced concrete.  Its full name is reinforced cement concrete or RCC.  RCC is concrete contains steel bars these are called reinforcement bars, or rebars.  That unification works very well, as concrete is very strong in compression, easy to produce and inexpensive, and steel is very strong in tension.  To make reinforced concrete, one first makes a mold, called formwork, that will contain the liquid concrete PCC (Plain Cement Concrete) & give it the form & shape we need.  Then according to the structural engineer's drawings, the steel reinforcement bars are placed & tied them in place using steel wire.  The tied steel is called a reinforcement cage because it is shaped like one.

Once the steel is in place, we can prepare the concrete, by mixing cement, sand, stone chips with the nominal ratio in a range of sizes of aggregate, and water in a cement mixer, and pouring the liquid concrete into the formwork till exactly the right consistency is reached.  The concrete will become hard in a matter of hours but takes a month to reach its full strength. 

(Check out: The Initial & Final Timing Of Mixed Concrete ). 

Therefore it is usually propped up until that period.  During this time the concrete must be cured or supplied with water on its surface, which it needs for the chemical reactions within to proceed properly.

(Check out : Best Method for Curing Of Concrete )

Working out the exact 'Ratio', or proportions of each ingredient is a technique 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 has more workability 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 1:2:4 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 newton 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.

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

Some other important components of concrete frame structures are:

Shear Wall:

Shear walls are important structural elements in aerial buildings. Shear walls are essentially very large columns - they could easily measure between 400mm thick by 3m long making them expose like walls rather than columns. Their behavior in a building is to help take care of horizontal forces on buildings like wind and earthquake loads. Normally, buildings are subject to vertical loads. Shear walls also carry vertical loads. It is important to understand that they only work for horizontal loads in one direction. These are usually not required in low-rise structures.

  

Elevator Shafts:

Elevator shafts are vertical boxes in which the elevators move up and down normally each elevator is enclosed in its own concrete box. These shafts are also very good structural elements, helping to resist horizontal loads, and also carrying vertical loads.

WALLS IN CONCRETE FRAME BUILDINGS

Concrete frame structures are strong and economical. Hence almost any walling materials can be used with them. The heavier options include masonry walls of brick, concrete block, or stone. The lighter options include drywall partitions made of light steel or wood studs covered with sheeting boards. The former is used when strong, secure, and sound-proof enclosures are required, and the latter when quick, flexible lightweight partitions are needed.

When brick blocks or concrete blocks are used, it is common to plaster the entire surface brick and concrete with a cement plaster to form a hard, longlasting finish.

CLADDING OF CONCRETE FRAME STRUCTURES

Concrete frame buildings can be clad with any kind of cladding material. Common cladding materials are glass, aluminum panels, stone sheets, and ceramic facades. Since these structures can be designed for heavy loading, one could even clad them in solid masonry walls of brick or stone.

Concrete Frame structure system is extremely economic, it requires no painting, finishing or fire protection. It is a suitable structural form for industrial buildings, warehousing, sports halls, community halls etc.