Use ASME/ASCE when the project is governed by US codes, international standards, or when USGS spectral hazard maps are available for the site. Use COVENIN when the project is located in Venezuela and must comply with local regulatory requirements, which use state-level seismic hazard maps and a distinct soil classification system.
- ASME / ASCE
- COVENIN
The ASME/ASCE seismic method follows ASCE 7 Chapter 11–15 for equivalent lateral force (ELF) analysis. You need three site-characterization inputs and the structural response modification factor.
Site Class
The site class describes the mechanical properties of the soil or rock at the site to a depth of 30 m (100 ft). It is used to determine site amplification factors (Fa and Fv) that scale the mapped spectral accelerations to design values.| Class | Description | Shear Wave Velocity vs (m/s) |
|---|---|---|
| A | Hard rock | > 1500 |
| B | Rock | 760 – 1500 |
| C | Very dense soil and soft rock | 360 – 760 |
| D | Stiff soil | 180 – 360 |
| E | Soft clay soil | < 180 |
| F | Special soils requiring site-specific evaluation | — |
Site Class D is the default assumption under ASCE 7 when no geotechnical investigation has been performed. Class F always requires a site-specific ground motion study and cannot use the standard amplification factors.
Spectral Accelerations
Ss — Mapped Short-Period Spectral Acceleration (g)The mapped maximum considered earthquake (MCE) spectral response acceleration at short periods (0.2 s), expressed as a fraction of gravitational acceleration (g). Obtain this value from USGS Unified Hazard Tool or ASCE 7 Figures 22-1 through 22-6 for the project coordinates.Typical range: 0.0 g (low seismicity) to 3.0 g or higher (near active faults).
S1 — Mapped 1-Second Period Spectral Acceleration (g)The mapped MCE spectral response acceleration at a 1-second period, expressed as a fraction of g. Also obtained from USGS or ASCE 7 maps. S1 characterizes the long-period content of the ground motion, which is particularly important for tall vertical vessels and skirt-supported columns.Typical range: 0.0 g to 1.5 g or higher in high-seismicity zones.
Response Modification Factor (R)
R — Response Modification CoefficientThe R factor accounts for the ductility, overstrength, and energy dissipation capacity of the support structure. Higher R values reduce the seismic design force, reflecting the system’s ability to deform inelastically without collapse.R values per ASCE 7 Table 15.4-2 depend on the support type:
Enter the R value appropriate for the support configuration selected in the Support card.
| Support Type | Typical R Range |
|---|---|
| Skirt support (steel) | 2.0 – 3.0 |
| Leg support (braced) | 2.5 – 3.5 |
| Saddle support | 2.0 – 3.0 |
| Lug support | 2.0 – 3.0 |
How R Factor Varies by Support Type
The response modification factor is not a material property — it is a property of the structural system and its lateral force-resisting mechanism. The same vessel on different support types will have different R values and therefore different seismic design forces. General guidance:- Skirt supports on vertical columns act as cantilever shell structures and typically use lower R values (less ductile behavior).
- Leg supports with diagonal bracing have more defined load paths and may qualify for slightly higher R.
- Saddle and lug supports on horizontal vessels generally follow the R value of the connected structure (grade slab, frame, or pipe rack).
PSI accepts the R factor as a user input rather than computing it automatically, because the appropriate value depends on structural details, connection type, and project-specific engineering judgment. Confirm the R value with the structural engineer of record before finalizing the calculation.