Lecture 1 Concrete Frame PDF

Mousl university2nd semester : Building Construction 2 Architecture departmentDate : xx / xx / xxxx Iraq CONCRETE FRAME STRUCTURES (SKELETON SYSTEM) Building Cons...

Mousl university2nd semester : Building Construction 2 Architecture departmentDate : xx / xx / xxxx Iraq CONCRETE FRAME STRUCTURES (SKELETON SYSTEM) Building Construction2 Assistant Teacher. : Adil Khalil  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, Vertical members are called columns. Humans walk on flat planes of concrete called slabs. 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. When we say concrete in the building trade, we actually mean reinforced concrete. Its full name is reinforced cement concrete, or. RCC is concrete that contains steel bars, called reinforcement bars, or rebars. This combination works very well, as Concrete is very strong in compression, easy to produce at site, and inexpensive, Steel is very strong in tension. To make reinforced concrete, one first makes a mold, called formwork, that will contain the liquid concrete and give it the form and shape we need. Then one looks at the structural engineer's drawings and places in the steel reinforcement bars, and ties them in place using wire. The tied steel is called a reinforcement cage, because it is shaped like one. Once the steel is in place, One can start to prepare the concrete, by mixing cement, sand, stone chips in a range of sizes, and water in a cement mixer, and pouring in the liquid concrete into the formwork till exactly the right level is reached. The concrete will become hard in a matter of hours. but takes a month) 28 days) to reach its full strength. 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. 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 language, these connections are called moment connections,(rigged connection ) 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 windspeeds, 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.  Some other important components of concrete frame structures are: Shear Walls are important structural elements in high-rise buildings. Shear walls are essentially very large columns - they could easily measure 400mm thick by 3m long - making them appear like walls rather than columns. Their function 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 gravity. Shear walls also carry vertical loads. It is important to understand that they only work for horizontal loads in one direction - the axis of the long dimension of the wall. These are usually not required in low-rise structures. 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 are used when strong, secure, and sound-proof enclosures are required, and the latter when quick, flexible lightweight partitions are needed. When brick or concrete blocks are used, it is common to plaster the entire surface brick and concrete - with a cement plaster to form a hard, long-lasting 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. What are the advantages and disadvantages of reinforced concrete frame structure? Advantages 1- Low Cost (Than Steel Structures) 2- Good Safety (Compared to its Price) 3- High Compressive Strength (Best choice for lower earthquake zones) 4- Material Availability (Than Steel Structures) 5- Wide Worker Availability & Easy Workmanship/Operation 6- Easy Maintenance & Lower Maintenance Cost 7- Better resistance against fire 8. flexibility Disadvantages: 1- Lower Safety (Compared to Steel Structures) (Fire Protection issues are excluded) 2- Seasonally Operatable in some areas (Cold/Hot Weather areas) 3- Lower Tensile Strength (Not Recommended in High Earthquake zones) 4- Larger area is occupied. E.g. Larger columns, beams etc... (than Steel Structures) 5- Weak Architectural Design flexibility 6- Non Recycle-able 7- Heavy load of Structures STEEL FRAME STRUCTURES  What are the advantages and disadvantages of steel frame structure? This 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 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 spray-on material called fire protection. WHERE STEEL FRAME STRUCTURES ARE USED 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(store) 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 are 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 site is lifting the steel members into place (with cranes) and bolting, the work at 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. 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 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 site, as a lot of work can be pre-fabbed 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.