How to harness the power of dynamic analysis to your benefit, even when it’s not a mandatory requirement of the seismic qualification process.

Seismic qualification by analysis of components can pose a significant challenge. The complexity of the simulation, the expertise required for the overall approach, the run time, and the hardware and software demands can substantially influence the efforts and costs of the entire task. While certain norms permit the use of the static coefficient method—an approach that falls among the simpler methods of analysis, producing results quickly—one might wonder: why even consider dynamic analysis?

Before diving into that, let’s briefly explore the static coefficient method. Static coefficient analysis is a static approach accounting for the response of flexible equipment and structures. Since the dynamic characteristics of the system are not defined, the analysis uses the maximum peak of the required seismic response spectra amplified by the static coefficient factor (usually 1.5) to account for modal superposition.
Dynamic analysis, in contrast, excites the system with the seismic response spectrum or the time-history enveloping the latter, without additional safety factors on the input motion. This is usually computationally expensive and demands expertise in defining boundaries and post-processing results.

Now, let’s examine the reasons why dynamic analysis can be beneficial:

• In static coefficient analysis, the results always correspond to the worst-case input motion scenario, i.e., equipment in full resonance. In most cases, this is a conservative consideration. Dynamic Analysis, on the other hand, provides a more accurate capture of the system’s response, applying seismic accelerations more precisely and thereby reducing the stresses and overall conservatism. This can yield significant benefits for the overall design and associated costs of the equipment and support.

• The conservatism in static analysis extends to anchorage design. Dynamic analysis can produce more precise anchor loads, potentially reducing anchorage requirements.

• Response spectrum or Time-history analysis offers a more accurate representation of deformations and displacements, crucial for understanding dynamic behavior. This could be proven vital for equipment interconnection or in determining electrical clearances.

• Sometimes static analysis is just unrealistic, particularly in applications of seismic isolation solutions with energy dissipation mechanisms or assessing the displacements of free hanging equipment such as Thyristor Valves suspended only at the top.

While static analysis is the go-to method for many engineering applications including seismic qualification of equipment, dynamic analysis can produce significantly more realistic results, reduce conservatism and overdimensioning of equipment, support structures and their anchorage. It exposes equipment and its support to real-world dynamic conditions, ensuring safety and reliability.