Before designing any structure or the different elements such as beams and columns, one has to first determine the various natural and man-made loads acting on them.These loads on a structure may be due to the following:
- Mass and gravitational effect, examples are dead loads, imposed loads, snow, ice and earth loads and hydraulic pressure.
- Mass and its acceleration effect, examples of such loads are earthquake, wind, impact and blast loads.
- Environmental difference, settlement, and shrinkage. These are also termed as indirect loads.
In India, the basic data on loads and their combinations is given by IS 875 Parts 1-5. Data regarding earthquake loads is specified by IS 1893.
The load that is fixed in magnitude and position is called the dead load. Determination of the dead load of a structure requires the estimation of the weight of the structure. Thus, one needs to calculate the weight of the slabs, beams, walls, columns, partition walls, false ceilings, facades, claddings, water tanks, stairs, brick fillings, plaster finishes, and other services.Dead weights of different materials are provided in IS 875 Part 1.
Also referred to as live loads are gravity loads other than dead loads and include items such as occupancy by people, movable equipment and furniture within the buildings, stored materials such as books, machinery and snow.They often vary in time and space. The code provides UDLs and Point Loads for various occupational categories. These are found in IS 875 Part 2.
Impact due to vertical crane, moving machinery and so on is converted empirically into equivalent static loads through an impact factor, which is normally a percentage (20% to 100%) of the machinery load. See clause 6.0 of IS 875 Part 2.
Snow and Ice Loads:
Snow and ice loads are to be considered in the mountainous regions in the northern parts of India. Thus, the roofs in these regions should be designed for the actual load due to snow or for the imposed loads specified in IS 875 Part 2 which ever is more severe. Freshly fallen snow weighs upto 96kg/m³ and packed snow 160kg/m³. Details on Snow load should be referred to IS 875 Part 4.
Winds are produced by the differences in atmospheric pressures, which are primarily due to the differences in temperature. Wind flow manifests itself into gales, cyclones, hurricanes, typhoons, tornadoes, thunderstorms and localized storms.The wind speeds of cyclones can reach upto 30 to 36m/s and in the case of severe cyclones up to 90m/s. Cyclones in India far exceed the design winds speed given in IS 875 Part 3. Horizontal wind flow exerts lateral pressure on the building envelope and hence has to be considered during design.
The wind pressure or load acting on the structural system and the structural or non-structural component being considered depends on the following:
- Velocity and density of air.
- Height above ground level.
- Shape and aspect ratio of the building.
- Topography of the surrounding ground surface.
- Angle of wind attack.
- Solidity ratio or openings in the wind structure.
- Susceptibility of the structural system under consideration to stead and time-dependent effects induced by the wind load.
The crest of the earth is made of large plates, and when these plates collide with each other, earthquakes occur. Though mose earthquakes have occurred near seismic belts, it seen that some earthquakes do hit non-active parts. Hence, it is necessary to incorporate some measures of the earthquake resistance into the design of all structures, since failure of structures due to earthquakes are catastrophic. Moreover, tall buildings may be at greater risk than single storey structures.
Earthquakes cause the ground to shake violently in all directions lasting for a few seconds in a moderate earthquake or even few minutes in very large earthquakes. The intensity of earthquakes reduces with distance from the epicenter of the earthquake.The magnitude and intensity are of interest to the structural engineer. Magnitude remains constant for an earthquake, where as the intensity reduces with increase in distance.
In addition to the peak ground acceleration of an earthquake, the following factors also influence the seismic damage;
- duration and frequency of ground vibration.
- distance from epicentre
- geographical conditions between the epicentre and the site.
- soil properties at the site and foundation type
- building type and characteristics.
Soil liquefaction, is another effect caused by earthquakes which produces a quicksand type condition, resulting in the loss of the bearing capacity of the soil.
Earthquake loads are dynamic and produce different degrees of response in different structures. When the ground under a structure having a mass suddenly moves, the inertia of the mass tends to resist the movement and creates forces called inertial forces, which are equal to the product of the mass of the structure and the acceleration.The mass is equal to the weight divided by the acceleration due to gravity.
Refer IS 1893 Part 1 code.