Why is earthquake acceleration used instead of earthquake velocity in structural analysis?

Why is Earthquake Acceleration Used in Structural Analysis?

Indonesia is one of the most earthquake-prone countries in the world due to its location along the Pacific Ring of Fire. As a result, designing earthquake-resistant buildings is essential to ensure public safety and minimize material losses. In structural analysis for earthquake resistance, engineers often use earthquake acceleration as a primary parameter instead of ground velocity or displacement. Why is acceleration the preferred choice? Here’s a detailed and structured explanation.

1. Acceleration is Directly Related to Forces Acting on Structures

When an earthquake occurs, ground motion causes buildings to experience acceleration. According to Newton’s Second Law of Motion:

F=ma

Where F is the inertial force, m is the mass of the structure, and a is the ground acceleration. This inertial force is the primary cause of damage to structural elements during an earthquake. Since acceleration directly determines the magnitude of these forces, it becomes a critical parameter for calculating seismic loads on buildings.

On the other hand, ground velocity and displacement do not have a direct relationship with force. Velocity represents the kinetic energy of ground motion, while displacement describes the amplitude of vibration. While these parameters are useful for understanding overall earthquake characteristics, acceleration remains the key factor in evaluating how structures respond to dynamic loads.

2. Acceleration is Easier to Measure Accurately

In practice, earthquake acceleration can be measured directly using instruments called accelerometers. These devices record ground acceleration during seismic events with high precision. In contrast, velocity and displacement are typically derived from acceleration data through integration processes. However, this integration can lead to cumulative errors (drift error), especially if the acceleration data contains noise or inaccuracies.

Therefore, using acceleration as the primary parameter ensures more accurate and reliable results in structural analysis compared to velocity or displacement.

3. Response Spectra are Based on Acceleration

In earthquake-resistant design, engineers use a tool called a response spectrum to predict how structures will respond to ground motion across different frequencies. The response spectrum graph shows the maximum response (such as displacement, velocity, or acceleration) of a structure subjected to seismic vibrations at various natural frequencies.

The response spectrum is fundamentally based on Peak Ground Acceleration (PGA), which serves as the primary parameter for defining seismic intensity. By utilizing this spectrum, engineers can estimate the maximum forces or displacements that a structure might experience without needing to analyze every detail of ground motion.

4. Correlation Between Acceleration and Structural Damage

Structural damage caused by earthquakes often correlates more closely with ground acceleration than with velocity or displacement. This is because acceleration reflects the intensity of dynamic forces acting on both structural and non-structural elements of a building.

For example:

• Structural elements like columns and beams are more likely to fail due to shear forces induced by high accelerations.
• Non-structural components such as partition walls or interior equipment are also vulnerable to sudden jolts caused by rapid accelerations.

Understanding these damage patterns allows engineers to design structural elements that can better withstand dynamic forces generated by earthquake accelerations.

5. Earthquake Design Standards are Based on Acceleration

Most international and national seismic design standards rely on acceleration as the primary parameter for calculating seismic loads. In Indonesia, for example, the Indonesian National Standard (SNI 1726) for earthquake-resistant design uses Peak Ground Acceleration (PGA) as a key input for determining equivalent static forces or response spectra in structural analysis.

In methods like equivalent static analysis outlined in SNI 1726, seismic loads are calculated based on PGA values at specific locations combined with other factors such as building mass and dynamic characteristics. Additionally, response spectra are used in dynamic analysis to determine maximum structural responses at various frequencies.

The reliance on acceleration in these standards highlights its importance in ensuring structural safety against earthquakes.

6. Velocity and Displacement Are Less Relevant for Structural Design

Although ground velocity and displacement are part of an earthquake’s motion characteristics, they are less relevant for direct use in structural design:

• Velocity primarily indicates the kinetic energy of ground motion and is more applicable in analyzing certain infrastructure like long-span bridges or underground pipelines.
• Displacement is more useful for understanding permanent ground deformation effects (e.g., surface cracks or fault displacements) caused by active faults.

For typical building design, however, the focus remains on how dynamic forces from acceleration impact structural elements.

Conclusion

Earthquake acceleration is used in structural analysis because it directly relates to inertial forces acting on buildings during seismic events. It is easier to measure accurately compared to velocity or displacement and forms the basis for response spectra and seismic design standards like SNI 1726. By using acceleration as the primary parameter, engineers can design safer buildings that are better equipped to withstand dynamic forces generated by earthquakes.

References

1. Newton’s Second Law: F=ma
2. SNI 1726:2023 – Procedures for Earthquake Resistance Design for Building Structures.
3. Chopra, Anil K., Dynamics of Structures: Theory and Applications to Earthquake Engineering, Prentice Hall.
4. Bozorgnia & Bertero (2004), Earthquake Engineering: From Engineering Seismology to Performance-Based Design.