Seismic Loads: Explained in 1-minute

What are Seismic Loads?

Seismic Loading is caused by earthquake ground motion underneath buildings. These depend on the buildings mass and periods of vibration.

Seismic Loads are characterised as non-stationary random vibrations, and are one type of dynamic load which acts on structures.

Horizontal seismic ground motions are the main focus for seismic loading. However, note that vertical ground motions also play a very significant role.

※ What are a Building Periods of Vibration? Explained in 1-Minute

When to Consider Seismic Loads?

In most parts of the world, seismic loading is not a significant concern if the project site is very far away from the boundary of a tectonic plate or fault. For example, in the United Kingdom seismic loads are generally ignored in building design.

Key locations of exceptionally large Seismic Loading include the US eastern seaboard (California), Japan, Taiwan, Turkey, etc. Areas of high seismic risk are shown in the natural hazard map below (produced by Munich Re, 2010).

In such high-risk areas, seismic loading can be as high as 100% of the building weight acting in the horizontal direction and therefore a critical design consideration.

How to Calculate

For Wind Loading, the basic parameter for calculating the wind force is the Wind Speed (usually at 10 m height). For seismic loading, the basic parameter for calculations is the peak ground acceleration (PGA). This is the maximum expected acceleration of the ground based on earthquake records/simulations and is typically defined in national building standards.

※ What is the Peak Ground Acceleration? Explained in 1-Minute

A very basic way to calculate the design base shear acting on the building is by multiplying this PGA by the building mass (F = ma). In other words...

In practice, designing the building for this ground acceleration is impractical since the forces generated are too large and generate unreasonable member sizes. Therefore, this Acceleration is modified into a design acceleration by accounting for the buildings natural vibration periods, inherent structural damping and (very importantly) the ductility of the structure (normally represented by a behaviour factor such as q in the Eurocodes). The adjusted acceleration is called the spectral acceleration.

Note that the ductility of the structure has to be guaranteed by appropriate structural seismic detailing. In general, special moment-resisting frames provide the greatest ductility (and therefore the largest reduction to the design seismic force / acceleration).

※ What is Structural Ductility? Relationship to Seismic Loading.

※ Structural Detailing Requirements for Seismic Loading. Summarised

After obtaining the base shear, the base shear can be distributed as effective static loads acting at each storey along the building height using established techniques.

※ How are Seismic Loads Distributed? Explained in 1-Minute

General Calculation Methods

Seismic loads are generally obtained using three different methods:

• Effective Static Load Prodecure
• Response Spectrum Analysis
• Time-History Analysis

Of the methods presented above, Time-History Analysis is the most comprehensive but may be overkill for the majority of building projects. For example, in Japan, Time-History Analysis is generally only undertaken for buildings above 60 m in height. 

Things to Note

Note that seismic loads are considered to be purely inertial loads (they depend on the mass of the building). Since inertial forces are proportional to mass, a larger building mass means higher seismic loads. Therefore, structural engineers may occasionally try to avoid concentrating too much weight on the upper storeys of buildings in seismic regions.

※ What are Inertial Forces? Relationship to D'Alemberts Principle. Explained in 1-Minute

This article primarily focuses on horizontal seismic loading, but vertical seismic loading can be also be a significant concern (as much as 90% of the horizontal seismic loads).

※ What are Vertical Seismic Loadings? How are they Calculated?

Further Reading

We have provided some more reading topics below on this matter which might be of interest.

※ What is the difference between wind and seismic loading?

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