Natrural loads
A structure's main function is to transfer loads.
1. Natural loads
Gravity load: self-weight
Wind: due to regular and continuous change in atmospheric pressure from place to place on earth's surface air flows across the surface of earth, that is wind. All structures built on the earth's surface have to resist the forces from wind.
If an obstruction is placed in the path of the wind it alters the pattern of the wind flow. which is why planes fly and boats sail. If the object is fixed to the earth's surface, like a building, the wind must flow around and over it. How the wind does it depends both on the wind speed and the shape of the object. Although the pattern of wind flow around buildings is complex the resulting loads from the alteration of wind flow are predominantly at right angles to the surface of the building.
Earth pressure: This because the natural surface has found a shape that is at rest ( not over geological time of course ) so, an alteration will cause forces. If dry sand is piled into a heap, there is a maximum slope for the sides. What is happening inside the heap is complex, and is further complicated bye the addition of water, ( which is why sand castle can be made ) if, however, a heap with vertical side is required, forces are needed to keep the heap in the unnatural shape. In buildings, this occurs when the building has a basement, or is built into a sloping site.
Water pressure: Under the surface of the earth, depending on the local geology and climate, there will be, at some level, water. The top level of this water is called the water table. this level may be at the surface or many metres down in desert. if the siting of the building interrupts the natural water table an unnatural water table is created around and under the building. Not only are the walls loaded by the water pressure but it also causes upward loads on the floor. The building is trying to float!
ground movement, earthquake, altering slightly in surface due to climate or geological change.
Temperature: As structure often exposed to the ambient climate their temperature may vary considerably, from a hot summer day to a cold winter night, and in some case this may cause loads by expand and contract at the joint.
A successful structure must be able to resist the effects of some or all of these natural loads for the whole of its useful life. On the whole these loads cannot be avoided and are an integral part of structure's existence.
2.Useful loads: Unlike natural loads. which cannot be avoided and so must be tolerated, useful loads are ones that are welcomed. These loads happen because the building and hence the structure have been constructed for a useful purpose. and unlike the natural loads, there is a choice for useful loads. they can be horizontally like to store sand and grain in basement or water in pool or rather like earthquakes, machinery housed in a building may tend to shake a building sideways like vibrating machine or crane. industrial proceses may increase or reduce the ambient temperature.
3. Accidental loads: The occurrence of it is inextricably bound up with concepts of safety.
