A structure is designed to resist different types of loads that can act during its lifetime. Thus loads are the most important parameter while designing a structure and therefore, the nature and magnitude of the loads they may experience must be accurately defined.
There are different types of loads, and the question is which loads may occur simultaneously that needs to be addressed when specifying the design loading. Hence, knowing the different classification of loads are very important.
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Classification of Loads
Loads are classified based on
- The direction of the load
- Duration of the load
- Spatial distribution of the load, and
- Source of the loading
The direction of the Loading
Based on their direction in which they act on a structure, loads are classified into two major categories. They are
- Gravity (Vertical) Loads
- Lateral (Horizontal) Loads.
Gravity loads are the loads that act along with the gravity (Vertical direction) while Lateral loads act in the other two (horizontal) directions. This classification is important in determining the structural system that is to be used.
Duration of the Loading
With respect to the duration, loads are classified into
- Permanent Loads
- Temporary Loads.
Spatial Distribution of the Loading
Spatial distribution of the loads implies that the space in which the load acts. Based on this, loads are classified into
- Concentrated Loads
- Distributed Loads
- Varying Loads.
This is a very common classification that we often come across. There is nothing like purely concentrated load, the space taken by the load is so small that we consider it to be Concentrated.
Temporal Distribution
Temporal distribution is the rate of change in the magnitude of the load. Based on this, loads are classified into
- Static
- Dynamic
Static loads are independent of time and the magnitude of the load does not change with respect to time (at least we assume it that way). Self-weight of a structure is a static load.
Dynamic Loads are time-variant loads. It changes its magnitude with respect to time. The change is very noticeable and has an impact on the structure. Vehicles moving on a bridge deck or forces developed due to earthquake are some of the dynamic loads.
Source of the Loading
Based on the source, loads are classified into many types. Some of them are
- Earthquake (Seismic) loads
- Wind load
- Construction load
- Snow load
- Earth pressure
The above classification covers the major types of loads acting on a structure. Now let us see some of the common types of loads acting on the structure and these loads may fall under one or more of the above-listed categories of classification.
Types of Loads
These are the common types of loads that a structure can experience. All the loads listed here may not act on a structure all the time, as it depends on the function of the structure.
Dead load
Tags: Gravity Load, Permanent Load, Distributed Load, Static Load.
Any structure is made of different elements and these elements have Self-weight which cannot be altered. It never changes with time. Hence, it is called Dead Load. IS 875 (Part 1) is all about Dead Load.
Determination of Dead Load is quite simple. Unit Weight times the volume of the element gives Dead Load.
Dead Load = Unit Weight * Volume
The unit weight of different material can be found out experimentally or taken from IS 875 Part 1.
Super Dead load
Permanent loads on a structure other than the self-weight of the structure are the super dead load. This includes loads caused by permanent set up of machines or towers etc and also partition walls (which are permanent), false ceilings if any.
Imposed Load / Live Load
Tags: Gravity Load, Temporary Load.
This load varies according to the time. The minimum live loads per square meter area for different types of structures is given in IS 875 (Part 2).
Live Load changes with respect to the function of the structure. For example, People and furniture make up a major part of Live load for Residential buildings, Vehicular traffic makes up Live Load for Roads and Bridges.
However, imposed loads do not act simultaneously on all the floors, all the time, hence it can be reduced. This process is called Live Load Reduction. IS 875 Part 2 explains the procedure and conditions to reduce live load.
The imposed load can be either Distributed or Concentrated load. Generally, designers consider it as a Distributed load.
Wind Load
Tags: Lateral Load, Temporary Load, Distributed Load, Dynamic Load.
Indian IS 875 (Part 3) is all about Wind Load. Although wind acts as a load, it is considered as Pressure on the structure.
The pressure of wind depends on various factors like the height of the structure, terrain, topography and Wind speed. For details about wind design, refer IS 875 part 3 (2015)
As the height of the structure increases, wind load becomes critical. For buildings with less height (6 to 7 stories), wind load is not significant.
Sometimes, wind load also causes Uplift. If a structure cannot withstand this, additional permanent load needs to be imposed.
Snow Load
Tags: Gravity Load, Temporary Load, Distributed Load.
This is common at the places of Snow Fall. Snow load is imposed on the roof by snow during Snowfall. Indian IS 875 (Part 4) deals with Snow Loads.
It is not the result of a single action but rather it gets accumulated overtime during snowfall. This gets dangerous if not considered, as snow generally tends to move by wind causing unbalanced roof loads.
Seismic Load
Tags: Lateral Load, Temporary Load, Concentrated or Distributed, Dynamic Load.
Although we call it seismic load, it is an inertia force caused by the deformation (acceleration) of the ground due to heavy shaking by an Earthquake.
Bureau of Indian Standards has published seismic hazard map of India. It has divided India into 4 zones. Zone II being less effective to Earthquakes while Zone V being more risky and high chances of getting Earthquakes.
The design to this loading also depends on factors like Importance of the structure, Spectral acceleration and Response Reduction factor.
Seismic loads are generally taken as Concentrated load acting on each storey. It can also be considered as Distributed and Continuous load which makes it complex to analyse and design.
Either of the Seismic load or Wind load is effective, not both. Indian IS 1893 is dedicated to Earthquake and seismic loading.
Earth pressure
Tags: Lateral Load, Distributed Load, Static load
The load applied to the structure laterally due to the surrounding soil. The load due to earth pressure varies with its depth, any surcharge, the type of soil and its moisture content.
Earth pressure may be active, passive or at rest. Various earth pressure theories like Rankine theory are used to calculate the earth pressure values. Earth pressure is a major load in designing structures like Retaining walls, dams, bridge abutments and basements.
Special Loads
Apart from the listed, there are many more loads which depends on the function of the structure. Some of them are
- Temperature Load
- Hydrostatic Pressure
- Construction Load
- Blast load
The function of the structure
As already mentioned, it is highly unlikely that a structure experiences all the above loads at a particular time. Hence, it is very important to determine the type of loads that a structure can experience at a time. It depends on the function of the structure.
For example,
- Loads like Dead Load, Vehicular Traffic (Imposed Load), wind load acts on the Bridges and rarely Seismic loads.
- Normal buildings get loads like Dead Load, Imposed Load, Wind or Seismic Load. In areas where snowfall is common, snow load is considered.
- A retaining wall experiences, dead load, surcharge load (Imposed Load), Earth pressure and sometimes seismic loads.
- Tunnels experience Dead Load, Earth pressure and seismic loads.
- A huge structure like bridges where it is assembled in parts, construction loads are to be considered too. If it is not considered in the design, it might lead to early deflection in many cases.
- A blast load is caused by a bomb or any other similar kind. It is impossible to assess this load and design for it. The better way is to go for Progressive Collapse which makes a major part of structure safe.
- Temperature load creates secondary stresses which may or may not be considered depending on the function of the structure.
Load combinations are thus defined and specified by the building standards with load factors (weightings) for various types of loads in order to ensure the safety of the structure under different maximum expected loading scenarios.