Module Overview

The seismic module in TankCode 650 implements API 650 Appendix E (14th Edition) in full — including the two-mass impulsive/convective model, SRSS base shear combination, overturning moment, J-ratio anchorage check, and sloshing freeboard. All calculations reference ASCE 7-16 site coefficients internally so you only enter the mapped spectral values.

If you haven't read the underlying theory, the companion article on seismic design for API 650 tanks covers the two-mass model, why tanks behave differently from buildings, and how the J-ratio governs whether anchors are required.

Step 1 — Seismic Inputs

Navigate to Modules → Seismic. The input panel has two groups: site hazard parameters and structural parameters.

Site Hazard Parameters

Enter the mapped spectral accelerations from ASCE 7-16 Figures 22-1 and 22-2 for your project location:

  • Ss — 0.2-second mapped spectral acceleration (g)
  • S1 — 1.0-second mapped spectral acceleration (g)
  • Site Class — A through F per ASCE 7-16 Table 20.3-1

TankCode 650 internally applies the site coefficients Fa and Fv to compute SMS, SM1, SDS, and SD1. You do not enter these intermediate values manually.

Structural Parameters

  • I (Importance Factor) — 1.0, 1.25, or 1.5 per API 650 Table E-2
  • Rwi / Rwc — Response modification factors. Default: Rwi = 4, Rwc = 2 for anchored tanks. Unanchored tanks use Rwi = 3, Rwc = 2.
  • Tl — Long-period transition period (seconds), from ASCE 7-16 Figure 22-14 for your region. Typical US values: 6–16 s.

Tip: The importance factor I = 1.5 applies to tanks storing hazardous contents (flammable or toxic) or those classified as SUG III. For standard non-hazardous product tanks, I = 1.0 is typical. Using I = 1.5 unnecessarily will over-design the anchorage.

Step 2 — Geometry Is Auto-Populated

Tank geometry (diameter D, shell height Hs, liquid specific gravity G) is pulled automatically from the Shell module. You do not re-enter dimensions in the seismic panel. This ensures consistency and prevents the most common error in seismic design — using a different liquid height for seismic than for shell thickness.

If you haven't run the Shell module yet, the seismic panel will prompt you to complete it first. See the shell module walkthrough for how to set up geometry inputs.

Step 3 — Reading the Results

After clicking Run Seismic Analysis, the results panel shows three groups of outputs:

Mass and Period

MASS & PERIOD Wi — impulsive 847 kN Wc — convective 312 kN Ti (impulsive period) 0.18 s Tc (convective period) 3.84 s FORCES Vi (impulsive shear) 254 kN Vc (convective shear) 62 kN V (SRSS base shear) 261 kN Mrw (overturning moment) 2,840 kN·m ANCHORAGE & FREEBOARD J-Ratio 0.923 Anchor verdict Required δs (sloshing wave) 0.42 m Freeboard check PASS ✓
Three result groups: mass/period, forces, and anchorage/freeboard checks.
  • Wi — impulsive liquid mass (participates with tank wall, high frequency)
  • Wc — convective (sloshing) mass (low frequency, long period)
  • Ti — impulsive natural period. Most tanks are short-period (Ti < 0.5 s)
  • Tc — convective period. Typically 2–8 s for large-diameter tanks

Forces and Moments

Base shear is combined using SRSS (square-root-sum-of-squares): V = √(Vi² + Vc²). Do not add Vi and Vc directly — that overestimates the total shear because the two modes are not in phase.

The overturning moment Mrw is used in the J-ratio anchorage calculation and in the foundation load module. Both values are passed automatically to those modules.

J-Ratio Interpretation

The J-ratio determines whether anchors are needed and what design equation applies:

  • J < 0.785 — self-anchored; no hold-down anchors required
  • 0.785 ≤ J ≤ 1.54 — mechanically anchored; size bolts per API 650 E.6.2.1
  • J > 1.54 — shell compression governs; check per E.6.2.2 and increase shell thickness or provide additional dead load

Common mistake: Treating J > 1.54 as a simple bolt-sizing problem. When J exceeds 1.54, the compressive stress in the shell bottom course must be checked separately against the allowable per E.6.2.2. TankCode 650 flags this condition explicitly and shows the shell compression stress alongside the allowable.

Step 4 — Freeboard Verification

The sloshing wave height δs (per API 650 E.4.5) is calculated from the convective spectral acceleration and tank diameter. TankCode 650 compares δs against the available freeboard (distance from the maximum fill level to the tank rim).

If freeboard is insufficient, the app highlights the deficit in red and suggests either reducing the maximum fill level or designing a fixed roof to resist liquid impact. For tanks in SUG III, freeboard equal to δs must be provided — it is not optional.

Step 5 — Seismic in the PDF Report

The seismic section of the generated PDF includes: site classification summary, spectral values table, mass participation table (Wi, Wc, Hi, Hc), force calculations with intermediate steps, J-ratio working, and freeboard check. The report references clause numbers from API 650 14th Edition Appendix E throughout. See the PDF report export guide for how to generate and customise the report.

See the Seismic Module in Action

Request a demo and walk through a live seismic analysis with your own tank geometry.

Request Demo