There are different types of loads on structures and building components such as slabs, beams, columns, and walls. These structural loads are dead loads, imposed loads, wind loads, snow loads, earthquake loads, and special loads. The loads on structures act as vertical loads, horizontal loads, and longitudinal loads.
The vertical loads consist of dead load, live load, or the imposed load, and impact load. The horizontal loads are comprised of wind load and earthquake load. The longitudinal loads such as tractive and braking loads are considered in the special cases of the design of bridges, gantry girders, etc.
These types of loads on structures and building components are considered as per the standard practices prescribed by the standard codes of practice and are used for structural analysis and design.
Structural analysis and design are done to find out the required dimensions of each member component, such as width, height, length, depth, and thickness, to determine the strength compressive strength of concrete to be used, the type of reinforcement, and its quantity to be used. These also give us the required dimensions of each component to calculate cement, sand, and aggregate quantity in the case of a concrete structure.
Types of Loads on Structures and Buildings
Safety and economy are the two major requirements for the construction of a building or any structure. This can be achieved by doing the structural analysis using the right loads and their combinations. If the loads considered are higher, then the structure will be safe, but then the construction cost will be very high affecting the economy.
The safety of the structures is compromised when the loads are considered less than the standard practices, but it will be economical. So the estimation of various loads acting on any structural needs to be based on the standard practices and their combination shall be selected as per the code.
American Standard Code ASCE 7: Minimum Design Loads for Buildings and Other Structures specify various design loads for buildings and structures.
Types of Loads on Structures are:
- Dead loads
- Imposed loads or live loads
- Wind loads
- Snow loads
- Earthquake loads
- Special loads
1. Dead Loads (DL)
Dead load or the self-weight of the structure or building components is the first load considered during the analysis and design of structures. Dead loads are permanent or stationary loads and it is the self-load of each component that is transferred to the structure throughout the life span. It is a vertical load.
Dead load is primarily due to the self-weight of structural members, permanent partition walls, fixed permanent equipment, fittings, and the weight of other different materials. It majorly consists of the weight of roofs, beams, walls, columns, etc. which are the permanent parts of the building.
The calculation of dead loads or the self-weight of each structure is done by multiplying the volume of each member section with the unit weight of the material of that member.
Table: Unit weights of some of the common materials used in buildings and structures
| Sl. No | Material | Weight |
| 1 | Plain Cement Concrete (PCC) | 24 kN/m3 |
| 2 | Reinforced Cement Concrete (RCC) | 25 kN/m3 |
| 3 | Brick Masonry | 18.8 kN/m3 |
| 4 | Stone Masonry | 20.4-26.5 kN/m3 |
| 5 | Cement Plaster | 20 kN/m3 |
| 6 | Floor Finishes | 0.6-1.2 kN/m3 |
| 7 | Timber / Wood | 5-8 kN/m3 |
| 8 | Steel | 78.5 kN/m3 |
For Example, the column is made of Reinforced cement concrete and has a length, width, and depth as shown in the figure:
Then the dead load of the column is calculated as”
The total volume of concrete = 3 x 0.4 x 0.4 = 0.48m3
The dead load of the column = 0.48m3 x 25KN/m3 = 12KN
2. Imposed Loads or Live Loads (IL or LL)
Imposed load or the live load is the second vertical load that acts on the structures and building components. These loads are either moving loads or come from movable objects which do not cause any acceleration or impact on the structure.
Imposed loads or the live loads are acts on the structure during the intended use or occupancy of the building including weights of movable partitions or furniture, fixtures, other equipment, etc.
Live loads keep on changing from time to time. These loads are specified by the standard codes of practice for different types of building structures based on the intended use, such as commercial buildings, residential buildings, industrial buildings, and various others.
Live load is one of the major loads in structural analysis and design. The minimum values of live loads to be assumed depend upon the intended use of the building.
The standard codes give the values of live loads for the following types of occupancy:
- Residential buildings–dwelling houses, hotels, hostels, boiler rooms, plant rooms, garages
- Educational buildings
- Institutional buildings
- Assembly buildings
- Business and office buildings
- Mercantile buildings
- Industrial buildings, and
- Storage rooms.
Table: Live Loads for Different Types of Buildings and Structures
| Types of floors | Minimum Live load KN/m2 |
|---|---|
| Floors in houses, hospital wards, dormitories | 2.0 |
| Office floors other than entrance halls | 2.5 & 4.0 2.5 when a separate storage facility is provided, otherwise 4.0 |
| Shops, educational buildings, assembly buildings, restaurants | 4.0 |
| Banking halls, office entrance halls | 3.0 |
| office floors for storage, assembly floor space without seating, public rooms in hotels, dance halls, waiting halls | 5.0 |
| Warehouses, workshops, factories | Light wt loads- 5.0 Medium – 7.5 Heavy – 10.0 |
| Garages | 4.0-7.5 |
| Stairs, landings, balconies, and corridors for floors but not liable to overcrowding | 3.0 |
| Stairs, landings, balconies, and corridors for floors liable to overcrowding | 5.0 |
| Flat slabs, sloped roofs | Access provided – 1.5 Access not provided – 0.75 |
However, in multistoried buildings chances of full imposed loads acting simultaneously on all floors is very rare. Hence the code makes provision for the reduction of loads in designing columns, load-bearing walls, their supports, and foundations as shown in the table below.
| Number of floors (including the roof) to be carried by the member under consideration | Reduction in Total Distributed Imposed Loads in % |
| 1 | 0 |
| 2 | 10 |
| 3 | 20 |
| 4 | 30 |
| 5-10 | 40 |
| Over 10 | 50 |
3. Wind Loads
Wind load is a type of horizontal load on a structure primarily caused by the movement of air relative to earth. Wind load is required to be considered in structural design especially when the height of the building exceeds two times the dimensions transverse to the exposed wind surface.
These types of loads on structures are considered in design if the height of the building is more than 15m. For low-rise buildings say up to four to five stories, the wind load is not critical because the moment of resistance provided by the continuity of floor system to column connection and walls provided between columns is sufficient to accommodate the effect of these forces.
Calculation of Wind Loads on a Structure:
Wind loads are considered in design if the height of the building is more than 15m. The intensity of wind load depends upon the velocity of wind, size, and height of the building.
To calculate the design wind pressure or the total load of wind on a building the following expression is used
Pz = 0.6 Vz2
Where Pz Is in N/m2 at height Z and Vz is in m/sec.
Up to the height of 30m, the wind pressure is considered to act uniformly. Above 30m the wind pressure may increase.
In order to calculate the Vz, the following expression is used
Vz = k1k2k3Vb
Where
k1 = Risk coefficient
k2 = Coefficient based on terrain, height, and structure size
k3 = Topography factor
4. Snow Loads (SL)
Snow loads are a type of load on structures and buildings which are considered only at the places where snowfall occurs. This load constitutes the vertical loads in the building. 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
Where S = Design snow load on plan area of the roof.
= Shape coefficient, and S0 = Ground snow load.
5. Earthquake Loads (EL)
Earthquake loads constitute both vertical and horizontal loads on the building. These type of loads causes movement of the foundation of structures. Earthquake forces are internal forces that developed on the structure because of ground movements.
The total vibration caused by an earthquake may be resolved into three mutually perpendicular directions, usually taken as vertical and two horizontal directions.
The movement in the vertical direction does not cause forces in the superstructure to any significant extent. But the horizontal movement of the building at the time of the earthquake is to be considered while designing.
Earthquake load acting on a structure is calculated by one of the following methods.
1. Response spectrum method
2. Seismic coefficient method.
6. Other Loads and Effects acting on Structures
In addition to the above loads, the following are the loads that affect the buildings and structure safety and serviceability:
- Foundation movement
- Settlement loads
- Thermal loads
- Elastic axial shortening
- Soil and fluid pressure
- Vibration
- Fatigue
- Impact
- Erection loads
- Stress concentration effect due to point load and the like.
