As already defined superstructure is the part of structure above ground level which is visible easily. In a building column, walls, beams, slabs, doors, window etc. form the superstructure while in bridges piers and deck form superstructure. The visible part of dams and water tank are superstructures of dam and water tank respectively. In this Article various loads acting on superstructure are discussed and types of construction discussed. To get safe structures at the same time without ignoring economy of the structure, it is necessary to estimate the various loads acting suitably. Indian standard code IS: 875–1987 specifies various design loads for buildings and structures. They have grouped various loads as under:
- Dead loads
- Imposed loads
- Wind loads
- Snow loads
- Earthquake loads
- Special loads
Details of earthquake load is covered in IS: 1893 – 1984 which should be considered along with other types of loads given in IS-875. The code also gives various load combinations to be considered in the design.
DEAD LOADS
The dead load in a building comprises the weight of roofs, floors, beams, columns, walls, partition walls etc. which form permanent part of the building. It is to be found by working out volume of each part and then multiplying with unit weight. Unit weight of various materials are listed in part-I of IS: 875. Unit weights of some of the common materials are presented in Table below.
Material | Weight |
Brick Masonry
Stone Masonry Plain cement concrete Reinforced cement concrete Timber Wooden floors hard wood 16 mm thick 28 mm thick Terrazo paving Country Tiles (single) including battens Mangalore Tiles with battens Mangalore tiles with flat tiles A.C. Sheets 6mm thick A.C. Sheet 5mm thick |
18.8 kN/m3
20.4 – 26.5 kN/m3 24.0 kN/m3 25.0 kN/m3 5 to 8 kN/m3
160 kN/m3 200 N/m3 240 N/m3 700 N/m3 650 N/m3 785 N/m3 170 N/m3 110 N/m3 |
IMPOSED LOADS (IL)
The loads which keep on changing from time to time are called as imposed loads. Common examples of such loads in a building are the weight of the persons, weights of movable partition, dust loads and weight of furniture. These loads were formerly known as live loads. These loads are to be suitably assumed by the designer. It is one of the major load in the design. The are to be suitably assumed by the designer. It is one of the major load in the design. The are to be suitably assumed by the designer. It is one of the major load in the design. The minimum values to be assumed are given in IS 875 (part 2)–1987. It depends upon the intended use of the building. These values are presented for square metre of floor area. The code gives the values of loads for the following occupancy classification:
(i) Residential buildings–dwelling houses, hotels, hostels, boiler rooms and plant rooms, garages.
(ii) Educational buildings
(iii) Institutional buildings
(iv) Assembly buildings
(v) Business and office buildings
(vi) Mercantine buildings
(vii) Industrial buildings, and
(viii) Storage rooms.
The code gives uniformly distributed load as well as concentrated loads. The floors are to be investigated for both uniformly distributed and worst position of concentrated loads. The one which gives worst effect is to be considered for the design but both should not be considered to act simultaneously.
In a particular building, imposed load may change from room to room. For example in a hotel or a hostel building the loads specified are,
UDL | Concentrated load | |
(a) Living rooms and bed rooms
(b) Kitchen (c) Dining rooms (d) Office rooms (e) Store rooms (f) Rooms for indoor games |
2 kN/m2
3 kN/m2 4 kN/m2 2.5 kN/m2 5 kN/m2 3 kN/m2 |
1.8 kN
4.5 kN 2.7 kN 2.7 kN 4.5 kN 1.8 kN |
WIND LOADS
The force exerted by the horizontal component of wind is to be considered in the design of buildings. It depends upon the velocity of wind and shape and size of the building. Complete details of calculating wind load on structures are given in IS-875 (Part 3) -1987. Brief idea of these provisions are given below:
(i) Using colour code, basic wind pressure ‘Vb’ is shown in a map of India. Designer can pickup the value of Vb depending upon the locality of the building.
(ii) To get the design wind velocity Vz the following expression shall be used:
Vz = k1 k2 k3 Vb
Where k1 = Risk coefficient
k2 = Coefficient based on terrain, height and structure size.
k3 = Topography factor
(iii) The design wind pressure is given by
pz = 0.6 V2
where pz is in N/m2 at height Z and Vz is in m/sec. Up to a height of 30 m, the wind pressure is considered to act uniformly. Above 30 m height, the wind pressure increases.
SNOW LOADS
IS 875 (part 4) – 1987 deals with snow loads on roofs of the building. For the building to be located in the regions wherever snow is likely to fall, this load is to be considered. The snow load acts vertically and may be expressed in kN/m2 or N/m2. The minimum snow load on a roof area or any other area above ground which is subjected to snow accumulation is obtained by the expression S = ì S0 Where S = Design snow load on plan area of roof.ì = Shape coefficient, and S0 = Ground snow load.