A load is a force that a building or structure needs to be able to resist. Loads cause stresses and deformations to a structure. The structure or part of the structure does not fail when these loads are applied. Loads can be applied vertically or laterally on a structure.Determining the total load acting on a structure is very important and complex.
There are two different types of loads acting on a structure:
1)Vertical loads
2) Horizontal loads
Vertical loads
Vertical loads, or gravity loads, are those forces that are applied perpendicular to the roof or floor system. These are separated into two categories: Dead Loads and Live Loads.
Dead Loads
These loads are permanent loads which are carried to the structure throughout their lifespan. Dead loads are also called as stationary loads. These loads occur mainly due to the self-weight of the structural members, fittings, fixed partitions, fixed equipment, etc. Dead loads are always present throughout the lifetime of a structure, compared to live loads which can come and go.
Live Loads
Live load refers to loads that do, or can, change over time, such as people walking around a building (occupancy) or movable objects such as furniture. Live loads are variable as they depend on usage and capacity. Typical live loads may include; people, the action of wind on an elevation, furniture, vehicles, the weight of the books in a library and so on. A live load can be expressed either as a uniformly distributed load (UDL) or as one acting on a concentrated area (point load). Live loads are harder to predict than dead loads
Snow loads
Snow load is the downward force on a building's roof by the weight of accumulated snow and ice. The roof or the entire structure can fail if the snow load exceeds the weight the building was designed to shoulder. The amount of snow load depends on the height of building, size & shape of the roof, the location of building whether it’s on the slope or not, the frequency of snow etc.
Horizontal Loads
Lateral loads are live loads that are applied parallel to the ground; that is, they are horizontal forces acting on a structure. This includes wind, seismic and earth loads. Lateral loads on a building are usually resisted by walls and bracing.
Wind loads
Wind loads can be applied towards a surface of a building/structure but it can also be applied away from the building causing a suction force. These are called positive and negative pressures. Wind loads on a structure get greater the higher they are applied to a structure. On a high rise building, the wind pressures are significantly higher at the peak of the structure compared to at the ground level. If the height of the building is more than 15m then Windloads are considered in design. As an increase in using lighter materials in the construction, wind load for a building should be considered. The structure should be strong enough with the heavy dead weights and anchored to the ground to resist this wind load. If not, the building may blow away. Wind load acts horizontally towards roofs and walls.
Seismic load
Earthquakes are what cause seismic loading on a structure. Seismic loads used in designing structures vary depending on where the structure is relative to seismic zones and the potential for earthquakes. Additional structural elements required to resist these loads. The magnitude of seismic loads when an earthquake occurs is directly related to the weight of the building. Buildings with heavy materials such as concrete will have to be designed for greater seismic loading compared to a light framed steel structure.
Three mutually perpendicular forces act on the structure during an earthquake, two horizontal forces which acts in opposite direction and one vertical force due to the weight of the structure. As vertical force doesn’t affect much during earthquake whereas two opposite horizontal forces results in movement of the building during an earthquake. These two horizontal direction forces are considered in the design.
Earth loads
Earth loads occur when soil is built up against a wall causing lateral earth pressures. These loads can be seen on basement foundation walls, retaining walls and tunnels. The magnitude of this lateral load is dependant on the type of soil built up against the structure and the depth of the soil. A house with a very high basement would likely have foundations walls that would have to resist high lateral loading from the soil built up against it if the basement was fully underground. This can be one of the causes of cracking seen in basement walls if the wall was not built strong enough to resist these lateral loads. If water is allowed to build up against a wall, lateral loads from hydrostatic pressure would need to be designed for. Installing a weeping tile system is a way to prevent water from building up against a basement wall.
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