What are loads and how do they affect buildings?
As a civil engineer, I am always concerned with the study of loads and how a structure can best resist them. A load is basically a force that a building or structure must resist. Loads cause stresses and deformations in a structure, and it is our responsibility as designers to make sure that our design can withstand the normal service loads and the eventual loads (I will define them later). In most cases, loads act vertically or laterally on a structure.
Types of loads
Let us start with vertical loads. Vertical loads or gravity loads are the forces that act vertically on the roof or floor system. They are divided into two categories: permanent or dead loads (D) and live loads (L). Permanent or dead loads consist of the weight of the structural elements that make up the structure, as well as any coverings that make the structure comfortable and pleasing to the eye. We call these loads "permanent" because they never change. Live loads are additional loads that act on a building. They are caused by the people who use the building and what they put on it (furniture, equipment, etc.).
As a structural engineer, I consider all materials when calculating the permanent loads on a building. This includes, for example, the materials and components used to insulate the building from heat and sound, as well as partitions, cladding, flooring, paneling, etc. Individually, these elements may seem quite light, but when all the weights are added together, they can represent a significant amount of kilograms or tons acting on the structure. These loads are in addition to the dead weight of the structure, which may include the weight of the floor/roof, beams, bearing walls, columns, bracing, etc. Permanent loads are always present during the life of a structure, unlike service loads which are sometimes more and sometimes less.
Operational loads are more difficult to predict than permanent loads because it is complicated to determine exactly how many people are using a space at any given time or how furniture is distributed and materials are stored in a particular space. When it comes to live loads in Chilean territory, the standard NCh1537.Of2009 "Structural Design - Permanent Loads and Service Loads" is used to determine the magnitude of the load depending on the type of building.
The service loads used in the design of a structure can vary depending on the service spaces in a building. For example, a mechanical room in an office building has a higher live load than the offices themselves because these rooms often contain very heavy mechanical equipment, whereas normal offices only house a few people and some furniture.
Other vertical loads considered when designing a structure are those caused by external climatic factors such as snow, wind and rain loads. Snow, wind, or rain loads should not be ignored because the additional weight can be even greater than the weight of the supporting roof structure. For example, an exceptional snowfall in Santiago in 2017 caused the roofs of many warehouses and sheds to collapse.
Chile, a seismic country
Lateral loads acting on structures include wind loads, seismic loads, and soil pressures. These loads act in the direction perpendicular to the structural systems of walls, floors, and roofs of buildings. Lateral loads in a building are generally resisted by load-bearing walls and bracing. When we see large steel "X's" in buildings, we know that they play an essential role in controlling lateral displacements.
In addition, wind loads can act perpendicularly on the surface of a building/structure, but can also create a suction force at a certain distance from the building. This is referred to as positive and negative pressure. Wind loads on a building increase with the height of the building. In a high-rise building, the wind pressure at the top of the building is much higher than at the ground. Many of us have experienced firsthand the force that wind can exert during a storm. That is why it is so important that the structure of a building can withstand these loads.
Earthquakes are the cause of seismic accelerations (and the resulting forces) in a building. The seismic loads that are considered in building design vary depending on the location of the building. In a location such as Valparaiso (seismic zone 3), seismic loading is more important for building design than in another place like Lonquimay (seismic zone 1). On the other hand, the magnitude of seismic loading during an earthquake is directly related to the weight of the building. Buildings constructed with heavy materials such as reinforced concrete have a higher seismic weight than a light steel structure. Therefore, our building design must take into account all of these variables. Our Chilean standard NCh433 "Seismic Design of Buildings" is constantly evolving as studies on the seismic behavior of buildings emerge.
Ground pressure loads occur when the ground pushes against a wall and causes lateral earth pressure. These loads occur in subway walls, retaining walls, and tunnels. The magnitude of this lateral load depends on the type of soil and its depth. A building with a very deep subgrade will likely have very strong side walls that can withstand a large lateral earth load. Proper design should prevent cracks in the walls and provide adequate protection from lateral earth loads. The structural engineer should check to see if water can accumulate on a wall. If necessary, hydrostatic pressure loads should be calculated. Installing a paving system with drains is one way to prevent water from accumulating on basement walls.
We hope this blog post helps you better understand the types of loads and their effects on a structure. If you would like more information or need professional advice, please do not hesitate to contact us.
