Effect of Lateral Loads on Buildings

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Introduction

For a structural engineer, the number one priority, when designing a building, is safety. To achieve this, vertical and lateral loads must be considered in the early stage of a building’s design. Like most loads, lateral loads can be divided into static and dynamic loads. Unlike vertical loads, the effect of lateral loads on buildings varies in intensities depending on the geographic location, structural material, height shape and size of the building. For example; For a high-rise building, lateral load intensity would be very high compared to a low-rise building.

What are LATERAL LOADS?

Lateral loads are live loads that act, on a structure/building, parallel to the ground. In other words, lateral loads on a building are forces acting in the horizontal direction on a building. These loads can cause a structure to shear or bend along the direction of the forces.

There are different types of lateral loads, the most common ones being Wind and Earthquake loads.

Types of Lateral Loads and Their Effect on Buildings

WIND LOAD

Wind load is a very complex type of lateral load as how to determine all the forces caused by wind load may not be fully understood. However, it must be accounted for when designing a building. Lateral loads affect a building by exerting different forces on the building. The forces exerted by wind loads are shear, twisting and bending moments, and deflection forces.

However, it is pertinent to note that, the intensities of these forces on a building depend on the velocity of the wind as well as the shape and height of the building. In other words, wind loads may not have grave effects on low rise buildings if neglected during the building’s design. Sadly, the same cannot be said for tall buildings. For a structural engineer, the major forces acting on a building are static and dynamic forces.

Static loads create elastic bending and twisting on the building, while dynamic loads create fluctuating forces all over the structure. Thus, creating motions, most commonly oscillations (thus dynamic). Taller, slender structures are impacted more significantly by dynamic wind load.

Wind torsional moment twists a high-rise on a vertical axis while bending moment “curls” the building at two ends on two parallel horizontal axes at both ends. Shearing wind force pulls the building on two countering directions on two parallel horizontal axes. Wind pressure is exerted uniformly on all the faces on the building, with an exception for the windward side which is affected differently based on the height.

EARTHQUAKE LOAD

Earthquake load is the most dangerous type of lateral load. Therefore, ignoring Earthquake load when designing a building, especially in earthquake zones, is a very costly mistake.

Earthquakes result in the movement of underground structures. these movements are categorized as a shake, rattle, and roll. Every building, in areas prone to earthquakes, must be able to withstand these three earthquakes loading intensities. Earthquake loads affect buildings by generating inertia forces, resulting in the building’s inertia mass trying to keep up with the ground. If the building is not connected to the ground, (for example, sitting on rollers), the building would not move with the ground and therefore no earthquake load effect, but most buildings are connected to the ground.

These inertia forces are responsible for the destruction of buildings as a result of horizontal and vertical shaking. However, the intensity of these forces depends on the mass, height, and stiffness of the building, as well as the proximity of the building to the epicenter. When designing tall buildings, only horizontal shaking conditions are considered critical, as it is assumed that design for vertical live loads covers the aspect of vertical shaking caused by earthquake loads.

Conclusion

The resisting systems for lateral loads (earthquake load and wind load) are similar. Designing these systems requires considering both as though they are horizontally applied to the building system. Lateral loads also exert constant and instantaneous forces, the wind load falls under the former while the earthquake load falls under the later. As earlier mentioned in this article, the magnitude of wind load depends on the height of the building, the shape of the building and the velocity of the wind. And the magnitude of the earthquake load depends on the mass of the building, and the proximity of the building to the epicenter. It is therefore fair to demand all structures design be done with possible lateral loads in mind.