Engineering guide
This module calculates, to DIN EN 1998-1 (Eurocode 8), the support reactions that arise from earthquake action on structures and process equipment. The basis is the response spectrum method of Eurocode 8: from the reference ground acceleration at the site, the ground type and subsoil class, and the importance class of the structure, the design spectrum is established, from which the equivalent horizontal force on the structure follows. This equivalent force determines the reaction forces at the supports and anchorages.
In equipment and plant engineering, the seismic verification is relevant for vessels, columns, and their supports: the seismic horizontal force generates shear forces and overturning moments that must be resisted by support skirts, support brackets, saddles, and anchor bolts. The ground type and subsoil class capture the amplification of the ground motion by soft soil layers; the importance class increases the action for plants with a particular hazard potential.
Anyone who wants to calculate the earthquake load to Eurocode 8 obtains with this module the support reactions as input quantities for the subsequent verifications — for example of the anchorage, the support skirt, or the local load introduction into the vessel shell. For sites in Germany, the provisions of the National Annex (seismic zones, spectral parameters) must additionally be observed.
Typical workflow
Determine the site parameters
The seismic zone map yields the reference ground acceleration at the site; the ground type and subsoil class describe the geological conditions and define the soil parameter and the corner periods of the spectrum.
Define the importance class
The importance class of the structure determines the importance factor by which the ground acceleration is scaled — plants with an increased risk to people or the environment receive higher factors.
Establish the design spectrum
From the ground acceleration, the soil parameter, and the behavior factor q, which captures the energy dissipation of the structure, the design spectrum of horizontal acceleration versus natural period is established.
Determine the natural period and spectral value
For the governing natural vibration mode of the structure (e.g. the bending vibration of a column on its support), the fundamental period is determined and the corresponding spectral value of the design acceleration is read off.
Calculate the equivalent force and support reactions
From the spectral acceleration and the effective mass follows the total horizontal seismic force; its distribution over the height yields the shear force and overturning moment at the base, and from these the reaction forces at supports, anchorages, and anchor bolts.
Frequently asked questions
What distinguishes the ground type from the subsoil class?
In the German application context, the ground types describe the near-surface soil layers (rock, medium-dense or loosely packed soils), while the subsoil classes describe the deeper geological formation. The combination of both classes defines how strongly the soil amplifies the seismic waves, and thereby determines the shape and magnitude of the response spectrum to be applied.
What role does the behavior factor q play for process equipment?
The behavior factor reduces the elastic spectral acceleration, because ductile structures dissipate seismic energy through plastic deformation. Vessels, columns, and their anchorages are generally not very ductile, so only small q-values are justified — an overestimated behavior factor is a typical and dangerous source of error.
Must the vessel contents be included in the seismic mass?
Yes. The vibrating mass comprises the self-weight, internals, insulation, and the contents present in the design case. For liquid-filled vessels, hydrodynamic effects (impulsive and sloshing liquid fractions) can additionally become relevant; these are covered by the dedicated part DIN EN 1998-4 for silos, tanks, and pipelines.
Is the verification to DIN EN 1998-1 sufficient for pressure vessels?
DIN EN 1998-1 delivers the actions and support reactions. The load transfer into the vessel — support skirt, support brackets, anchorage, local shell loading — must then be verified with the applicable pressure vessel codes, for example DIN EN 13445-3 clause 16 or the S-series of the German AD 2000-Merkblätter.
Worked example
For a piece of equipment with an effective vibrating mass of 50 t, the total horizontal seismic force is to be estimated using the simplified response spectrum method of DIN EN 1998-1 — a worked example of an earthquake load calculation. The fundamental period of the structure lies in the plateau region of the spectrum.
Given
| Design ground acceleration ag = γI · agR | 0.8 m/s² |
|---|---|
| Soil parameter S (ground type B, Type 1 spectrum) | 1.2 |
| Behavior factor q | 1.5 |
| Effective mass m | 50,000 kg |
| Correction factor λ | 1.0 |
Solution
Design spectral value in the plateau region
For TB ≤ T ≤ TC, DIN EN 1998-1 gives:
Sd(T) = ag · S · 2.5 / q = 0.8 m/s² · 1.2 · 2.5 / 1.5 = 1.6 m/s²
The lower bound β · ag = 0.2 · 0.8 m/s² = 0.16 m/s² is clearly exceeded by Sd = 1.6 m/s² and is therefore not governing.
Total seismic force (simplified lateral force method)
Fb = Sd(T1) · m · λ = 1.6 m/s² · 50,000 kg · 1.0 = 80,000 N = 80 kN
This horizontal force is distributed over the structure; at the base it yields the shear force and overturning moment for the verification of the supports and anchorage.
Result
| Design acceleration Sd(T1) | 1.6 m/s² |
|---|---|
| Total horizontal seismic force Fb | 80 kN |
Technical content source: LV-Solved-Problems-Content 628d313247e7