The portion above the ground level and below the ground floor level is known as plinth. The portion above the ground floor level is known as super structure. It includes walls, columns, beams, floors, roofs, doors, windows, lintels, staircases etc. In this article, types of super structures based on the method of load transfer is first presented and then the various components are discussed giving their functions and types.
Types of Super Structures Based on Method of Load Transfer
On this basis there are two types:
1. Load Bearing Structures: In this type of structure the load on the structure is transferred vertically downward through walls. Loads from roof and floors gets transferred to wall and then wall has to transfer these loads as well as self weight.
Such constructions are used in residential buildings where dimension of rooms is less. Residential buildings up to ground + 2 floors can be built economically with such structures.
2. Framed Structures: In this type of structures a frame work of columns, beams and floors are built first. Then walls are built to partion the living area. The walls are subjected to selfweight only. This type of super structures are required when number of stories in a building is more and also when larger areas are to be covered free from walls. Table 1 shows the comparison between R.C.C. framed structures and load bearing structures.
Walls are built to partition living area into different parts. They impart privacy and protection against temperature, rain and theft. Walls may be classified as:
1. Load Bearing Walls: If beams and columns are not used, load from roof and floors are transferred to foundation by walls. Such walls are called load bearing walls. They are to be designed to transfer the load safely. The critical portion of the walls are near the openings of doors and windows and the positions where concrete beams rest. Minimum wall thickness used is 200 mm.
It is also recommended that the slenderness ratio of wall defined as ratio of effective length or effective height to thickness should not be more than 27. The effective height and effective length of a wall may be taken as shown in tables 2 and 3 respectively.
2. Partition Walls: In framed structures partition walls are built to divide floor area for different utilities. They rest on floors. They do not carry loads from floor and roof. They have to carry only self-weight. Hence normally partition walls are thin.
Table 4 shows the differences between load bearing walls and partition walls.
Depending upon the requirement these walls may be brick partition, clay block partition, glass partition, wood partition, and aluminium and glass partition.
Stone and Brick Masonry
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Applying mortar coats on the surfaces of walls, columns, ceiling etc. to get smooth finish is termed as plastering. Mortar used for plastering may be lime mortar, cement mortar or lime-cement mortar.
Lime mortar used shall have fat lime to sand ratio of 1:3 or 1:4. If hydraulic lime is used mix proportion (lime: sand) is 1:2. Cement mortar of 1:4 or 1:6 mix is very commonly used for plastering, richer mix being used for outer walls.
To combine the cost effectiveness of lime mortar and good quality of cement mortar many use lime-cement mortar of proportion (cement : lime : sand) of 1:1:6 or 1:1:8 or 1:2:8.
The objectives of plastering are:
- to conceal defective workmanship
- to give smooth surface to avoid catching of dust.
- to give good look.
- to protect the wall from rain water and other atmospheric agencies.
- to protect surfaces against vermit.
Requirement of good plaster are:
- It should adhere to the background easily.
- It should be hard and durable.
- It should prevent penetration by moisture
- It should be cheap.
Lime mortar is usually applied in 3 coats while cement mortar is applied in two or three coats for the stone and brick masonry. For concrete surfaces cement mortar may be applied in two or three coats. For concrete building blocks many times only one coat of cement mortar is applied. The first coat provides means of getting level surface. The final coat provides smooth surface.
If three coats are used second coat is known as floating coat. The average thickness of first coat is 10 to 15 mm. Middle coat thickness is 6–8 mm. The final coat is just 2 to 3 mm thick. If single coat is used its thickness is kept between 6 to 12 mm. Such coats are used on concrete surfaces not exposed to rain.
Instead of plastering entire surface of the masonry, special mortar finishing work is done to the exposed joints. This is called pointing. It consists of raking the joints to a depth of 10 mm to 20 mm and filling it with richer mortar mixes. In case of lime mortar pointing mix used is 1:2 and in case of cement mortar pointing mix used is 1:3.
Pointing is ideally suited for stone masonry because stones are having attractive colours and good resistance to penetration by water. Pointing gives perfection to weaker part of masonry (i.e. to joints) and it adds to aesthetic view of the masonry.