LOADS ON BUILDINGS
LOAD TYPES
The determination of the loads acting on a structure is a complex problem. The nature of the loads varies essentially with the architectural design, the materials, and the location of the structure. Loading conditions on the same structure may change from time to time, or may change rapidly with time.
Loads are usually classified into two broad groups: dead loads and live loads. Dead loads (DL) are essentially constant during the life of the structure and normally consist of the weight of the structural elements. On the other hand, live loads (LL) usually vary greatly. The weight of occupants, snow and vehicles, and the forces induced by wind or earthquakes are examples of live loads. The magnitudes of these loads are not known with great accuracy and the design values must depend on the intended use of the structure.
In structural analysis three kinds of loads are generally used:
The structure first of all carries the dead load, which includes its own weight, the weight of any permanent non-structural partitions, built-in cupboards, floor surfacing materials and other finishes. It can be worked out precisely from the known weights of the materials and the dimensions on the working drawings. Although the dead load can be accurately determined, it is wise to make a conservative estimate to allow for changes in occupancy; for example, the next owner might wish to demolish some of the fixed partitions and erect others elsewhere.
Live Loads (LL)
All the movable objects in a building such as people, desks, cupboards and filing cabinets produce an imposed load on the structure. This loading may come and go with the result that its intensity will vary considerably. At one moment a room may be empty, yet at another packed with people. Imagine the `extra' live load at a lively party!
Wind Load (WL)
Wind has become a very important load in recent years due to the extensive use of lighter materials and more efficient building techniques. A building built with heavy masonry, timber tiled roof may not be affected by the wind load, but on the other hand the structural design of a modern light gauge steel framed building is dominated by the wind load, which will affect its strength, stability and serviceability. The wind acts both on the main structure and on the individual cladding units. The structure has to be braced to resist the horizontal load and anchored to the ground to prevent the whole building from being blown away, if the dead weight of the building is not sufficient to hold it down. The cladding has to be securely fixed to prevent the wind from ripping it away from the structure.
Snow Load (SL)
The magnitude of the snow load will depend upon the latitude and altitude of the site. In the lower latitudes no snow would be expected while in the high latitudes snow could last for six months or more. In such locations buildings have to be designed to withstand the appropriate amount of snow. The shape of the roof also plays an important part in the magnitude of the snow load. The steeper the pitch, the smaller the load. The snow falling on a flat roof will continue to build up and the load will continue to increase, but on a pitched roof a point is reached when the snow will slide off.
Earthquake Load
Earthquake loads affect the design of structures in areas of great seismic activity, such as north and south American west coast, New Zealand, Japan, and several Mediterranean countries. Only minor disturbances have been recorded in east Asia and Australia.
Thermal Loads
All building materials expand or contract with temperature change. Long continuous buildings will expand, and it is necessary to consider the expansion stresses. It is usual to divide a reinforced concrete framed building into lengths not exceeding 30 m and to divide a brick wall into lengths not exceeding 10 m. Expansion joints are provided at these points so that the structure is physically separated and can expand without causing structural damage.
Settlement Loads
If one part of a building settles more than another part, then stresses are set up in the structures. If the structure is flexible then the stresses will be small, but if the structure is stiff the stresses will be severe unless the two parts of the building are physically separated.
Dynamic Loads
Dynamic loads, which include impact and aero dynamic loads, are complex. In essence, the magnitude of a load can be greatly increased by its dynamic effect.
LOAD TYPES
The determination of the loads acting on a structure is a complex problem. The nature of the loads varies essentially with the architectural design, the materials, and the location of the structure. Loading conditions on the same structure may change from time to time, or may change rapidly with time.
Loads are usually classified into two broad groups: dead loads and live loads. Dead loads (DL) are essentially constant during the life of the structure and normally consist of the weight of the structural elements. On the other hand, live loads (LL) usually vary greatly. The weight of occupants, snow and vehicles, and the forces induced by wind or earthquakes are examples of live loads. The magnitudes of these loads are not known with great accuracy and the design values must depend on the intended use of the structure.
In structural analysis three kinds of loads are generally used:
- Concentrated loads that are single forces acting over a relatively small area, for example vehicle wheel loads, column loads, or the force exerted by a beam on another perpendicular beam.
- Line loads that act along a line, for example the weight of a partition resting on a floor, calculated in units of force per unit length.
- Distributed (or surface) loads that act over a surface area. Most loads are distributed or are treated as such, for example wind or soil pressure, and the weight of floors and roofing materials.
The structure first of all carries the dead load, which includes its own weight, the weight of any permanent non-structural partitions, built-in cupboards, floor surfacing materials and other finishes. It can be worked out precisely from the known weights of the materials and the dimensions on the working drawings. Although the dead load can be accurately determined, it is wise to make a conservative estimate to allow for changes in occupancy; for example, the next owner might wish to demolish some of the fixed partitions and erect others elsewhere.
Live Loads (LL)
All the movable objects in a building such as people, desks, cupboards and filing cabinets produce an imposed load on the structure. This loading may come and go with the result that its intensity will vary considerably. At one moment a room may be empty, yet at another packed with people. Imagine the `extra' live load at a lively party!
Wind Load (WL)
Wind has become a very important load in recent years due to the extensive use of lighter materials and more efficient building techniques. A building built with heavy masonry, timber tiled roof may not be affected by the wind load, but on the other hand the structural design of a modern light gauge steel framed building is dominated by the wind load, which will affect its strength, stability and serviceability. The wind acts both on the main structure and on the individual cladding units. The structure has to be braced to resist the horizontal load and anchored to the ground to prevent the whole building from being blown away, if the dead weight of the building is not sufficient to hold it down. The cladding has to be securely fixed to prevent the wind from ripping it away from the structure.
Snow Load (SL)
The magnitude of the snow load will depend upon the latitude and altitude of the site. In the lower latitudes no snow would be expected while in the high latitudes snow could last for six months or more. In such locations buildings have to be designed to withstand the appropriate amount of snow. The shape of the roof also plays an important part in the magnitude of the snow load. The steeper the pitch, the smaller the load. The snow falling on a flat roof will continue to build up and the load will continue to increase, but on a pitched roof a point is reached when the snow will slide off.
Earthquake Load
Earthquake loads affect the design of structures in areas of great seismic activity, such as north and south American west coast, New Zealand, Japan, and several Mediterranean countries. Only minor disturbances have been recorded in east Asia and Australia.
Thermal Loads
All building materials expand or contract with temperature change. Long continuous buildings will expand, and it is necessary to consider the expansion stresses. It is usual to divide a reinforced concrete framed building into lengths not exceeding 30 m and to divide a brick wall into lengths not exceeding 10 m. Expansion joints are provided at these points so that the structure is physically separated and can expand without causing structural damage.
Settlement Loads
If one part of a building settles more than another part, then stresses are set up in the structures. If the structure is flexible then the stresses will be small, but if the structure is stiff the stresses will be severe unless the two parts of the building are physically separated.
Dynamic Loads
Dynamic loads, which include impact and aero dynamic loads, are complex. In essence, the magnitude of a load can be greatly increased by its dynamic effect.
