Types of Loads that Can Act in a Structure
If you’re a construction or civil engineer, probably 80% of your day goes by juggling loads acting on a member and strengths of the member. You will need to know all the types of loads that can act in a structure for designing it proper and strong. So, today we will discuss the types of loads.
The loads acting on a building can be divided into 3 main categories depending on the direction of their application – vertical, horizontal, and longitudinal. Among these, vertical loads are the most common since this is the way gravity acts.
There are many kinds of things causing vertical – or, gravitational loads, like the dead weight of a member, the live weight of things resting on that member, and the load of something impacting from above.
The most common types of loads acting in a structure are as follows:
1. Dead load
2. Imposed load
3. Wind load
4. Snow loads
5. Seismic forces
6. Shrinkage, creep, and heat effects
7. Other types of loads
Let’s run through each of these types of structural loads one by one now.
All the weight of the building itself is considered as the dead load of the structure. When considering a member, the dead load of the member is its own weight. The dead load acting on it would be the weight of all the permanent construction resting on the member.
The unit weight of the material of the member, whatever it was required to construct the member, is multiplied by the volume of the member to find out the total expected weight of it, i.e. the dead load of the member.
Imposed loads or live loads on a member are those loads coming onto the member which may change from time to time. That means people and furniture and machinery, and so on and so forth. Any weight imposed on the structure without constructed into it is considered a live load.
Needless to say, these loads are hard to calculate. For example, the same room designed for the living of two people, can suddenly be packed with a hundred people for a party. The designer and engineers will have to make educated decisions (read, ‘guesses’) about the amount of live or imposed load a structure or a member will experience.
The expected loads are indicated in most building codes depending of the building usage types, such as:
1. Residential building
2. Education building
3. Educational building
4. Business and office building
5. Retail buildings
6. Assembly buildings
7. Industrial buildings
8. Storage facilities
The air, when flowing in volume and with speed, can exert significant forces – even in gentle-weather areas, storms can get strong enough to uproot big trees. You can imagine how it would affect a flat wall standing in its way. This effect is called the wind load on a structure, which technically is the horizontal component of air current.
Most building codes should have enough meteorological data to indicate how much wind pressure your building may expect in the location you are constructing it in. Keep in mind that in most cases an RCC frame building can withstand standard wind loads up to 30 meters without sweat. However, above that you will need to seriously consider the wind load or risk structural failure.
In colder regions, the ice accumulating on top of a building becomes quite heavy. It will vary with the snowfall in the region, and as before, most building codes will take meteorological data and show you what kind of seasonal snow can get on top of your house.
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: S=μ S0 where, S= Design snow load on plan, area of the roof, μ= Shape coefficient, and S0= Ground snow load.
Earthquakes are the worst enemy of buildings, and are still the number one reason behind structural destruction. The reason for this is because the force comes from a direction you would not expect most of the time (down), and it works in three directions – vertical and two horizontals.
To counter seismic forces, earthquake loads need to be calculated from the expected earthquake coefficient, the soil settlement figures, and the kind of seismic waves that are more likely to hit the building. The seismic acceleration of the design can be estimated from seismic coefficients, which is defined as the ratio of acceleration due to the earthquake and acceleration due to gravity.
In addition to the above forces, a building may also get subjected to the following types of forces, depending upon the scenario.
1. Foundation movement
2. Elastic axial shortening
3. Soil and fluid pressure
7. Erection loads
8. Stress concentration effect
The designer will need to consider each type of possible load and match the cause of these forces to the building scenario at hand. Some of these, oddly, can come from within the building rather than from outside.