Engineering guide
Module ASEQ determines the seismic base shear for nonbuilding structures — such as vertical vessels, columns, silos, stacks or support structures — to the American codes IBC 2012 and ASCE 7-10. The base shear is the resulting equivalent horizontal load at the base (fixity or support), used to size the anchorage, foundation and load-bearing structure for the earthquake load case.
The calculation follows the equivalent lateral force procedure based on the response spectrum: from the site (spectral accelerations SS and S1 per the ASCE 7-10 maps), the Site Class, the occupancy category with its Importance Factor, and the structure type with its response modification coefficient R, the seismic response coefficient Cs is determined. Multiplied by the effective seismic weight W, this gives the base shear V.
In practice, the module is needed above all for plant components delivered to the USA or to countries with IBC-based building codes — typically vessels and equipment whose stability verification to ASCE 7 is required. For European sites, Eurocode 8 (DIN EN 1998) applies instead.
Typical workflow
Define structure type and occupancy category
First, the Structure Type (Nonbuilding Structure not similar to buildings, e.g. vessel, stack) and the Occupancy Category per IBC 2012 Table 1604.5 are selected. From these follows the Importance Factor per ASCE 7-10 Table 1.5-2, which raises the safety level where failure would have increased consequences.
Determine the site parameters
Via the site location (zip code) and the maps of ASCE 7-10 (Figures 22-1 through 22-11), the spectral accelerations for short periods (SS) and for the 1-second period (S1) are read off; the Soil Site Class per Table 20.3-1 provides the site amplification factors. From these follow the design spectral values SDS and SD1.
Set the natural period of the structure
The governing natural period is either taken from an independent dynamic analysis (Actual Calc Period) or approximated from height and stiffness. It decides whether the structure lies on the plateau or in the descending branch of the response spectrum.
Calculate the seismic response coefficient Cs
Cs follows from SDS divided by the ratio R/I (response modification coefficient of the structure type over the Importance Factor). For longer periods, Cs is reduced via SD1/(T·R/I); on the lower side, the ASCE 7-10 minimum values for nonbuilding structures apply, plus an S1-dependent lower bound in regions of strong seismicity.
Form the base shear and use it downstream
With the total seismic weight W (dead weight plus applicable contents and attachments per ASCE 7-10 Section 13.3.1), the base shear V = Cs·W follows. It is distributed over the height and yields the overturning moment, anchor forces and section forces for the support skirt, legs or foundation.
Frequently asked questions
What distinguishes 'Nonbuilding Structures not similar to buildings' from buildings?
Vessels, stacks, silos or process columns have no building-type lateral force resisting systems (frames, shear walls) and usually a mass that is concentrated or continuously distributed over the height. ASCE 7-10 Chapter 15 therefore assigns them their own, predominantly lower R values — they can dissipate less energy plastically than ductile buildings. Using building R values for vessels leads to unsafe, too small seismic loads.
What role does the Site Class play?
Soft soils amplify the ground acceleration considerably compared with rock. Site Classes A through F per ASCE 7-10 Table 20.3-1 control the amplification factors Fa and Fv used to convert the mapped values SS and S1 into the design values SDS and SD1. Without a geotechnical report, Site Class D is often assumed conservatively; for Site Class F a site-specific investigation is mandatory.
What is included in the seismic weight W?
In addition to the dead weight of the structure, it includes the operating medium (for vessels usually the governing contents), insulation, refractory lining, internals, piping shares and permanently attached appurtenances. A common mistake is calculating with the empty weight only — the operating condition usually governs; for liquid-filled tanks, hydrodynamic effects (impulsive and convective components) must also be considered.
Does the calculation also apply to sites outside the USA?
The procedure is tied to the hazard maps and classifications of ASCE 7-10/IBC 2012. For sites in Europe, Eurocode 8 with the national hazard maps must be applied; however, many international projects (Middle East, Asia) contractually specify IBC/ASCE, in which case the spectral values of the site country are converted into the ASCE format.
Worked example
For a vertical process vessel (Nonbuilding Structure not similar to buildings) at a US site, the design spectral values have already been determined. The vessel is short-period (natural period on the plateau of the spectrum). This worked example calculates the seismic base shear to ASCE 7-10.
Given
| Design spectral value SDS | 1.0 g |
|---|---|
| Response modification coefficient R | 3 |
| Importance Factor Ie | 1.25 |
| Seismic weight W | 400 kN (operating condition) |
| Natural period T | short-period (plateau range) |
Solution
Seismic response coefficient
Cs = SDS / (R/Ie) = 1.0 / (3/1.25) ≈ 0.417
Check the minimum value
For nonbuilding structures, the lower bound is Cs,min = 0.044·SDS·Ie ≥ 0.03:
Cs,min = 0.044 · 1.0 · 1.25 = 0.055 < 0.417 → the calculated value governs.
Base shear
V = Cs · W = 0.417 · 400 ≈ 166.7 kN
This horizontal force, together with the associated overturning moment, must be applied to the anchorage and foundation design.
Result
| Response coefficient Cs | ≈ 0.417 |
|---|---|
| Base shear V | ≈ 166.7 kN (≈ 42 % of the weight) |
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