Engineering task and calculation objective
The UBC module calculates seismic loads on vertical cylindrical structures according to the Uniform Building Code (UBC) 1997 — for columns, vertical tanks, stacks, and similar self-supporting equipment. It determines the governing seismic internal forces: the shear force, the overturning moment at the base fixity, and the bending moment at any height X above the base.
For slender vertical equipment, earthquake loading is, alongside wind, frequently the design-governing load case for the shell wall thickness in the lower section, for the anchorage, and for the foundation. The module implements the equivalent static force method of the UBC: from the operating mass of the equipment, the height, and the diameter of the structure, the total seismic load is determined and distributed over the height; from this follow the shear force and bending moment distributions.
From the bending moment at the base and at the freely selectable section plane, together with the allowable vessel stress and the weld joint efficiency, the module calculates the required (corroded) wall thickness — at the base and at height X. The UBC 1997 has been superseded in the USA by the International Building Code with ASCE 7, but it is still used internationally, in older specifications, and for re-evaluations of existing plants.
Standard and calculation basis: Uniform Building Code UBC: 1997
Calculation workflow
- Record geometry and masses: The inputs are the outside diameter, the height, and the shell thickness at the base, plus the operating mass of the equipment (including contents, internals, and insulation). From mass and stiffness follows the dynamic behavior — the fundamental period of vibration of the equipment acting as a cantilever.
- Determine the seismic coefficients per UBC: According to UBC 1997, the seismic zone, soil profile type, importance factor, and response behavior of the structure are translated into the seismic coefficients from which, together with the period of vibration, the design base shear is obtained as a fraction of the total weight.
- Load distribution over the height: The total seismic load is distributed over the height according to the UBC approach, with a portion applied as a concentrated force at the top for slender structures. This yields the shear force at the top, the shear force distribution, and, by integration, the overturning moment at the base fixity as well as the moment at the selected section height X.
- Verify the required wall thickness: From the bending moment, the axial force (dead weight), and the internal pressure contribution, the longitudinal stress in the shell is formed and compared with the allowable vessel stress, taking the weld joint efficiency into account. The module reports the required corroded wall thickness at the base and at height X — the basis for stepping the shell wall thickness over the height of the equipment.
Input quantities
| Quantity | Symbol | Unit |
|---|---|---|
| Outside diameter | D = | ft |
| Outside diameter | D = | m |
| Height | H = | ft |
| Height | H = | m |
| Pressure factor (Table 16-H, for cylindrical buildings=0.8) | Cq | - |
| Importance factor (Table 16-K, standard=1, hospitals=1.15) | Iw (1, 1.15) | - |
| Velocity | v = | mph |
| Velocity | v = | m/s |
| Velocity | v = | km/h |
| Reference Pressure | qs = | psf |
| Reference Pressure | qs = | N/m² |
| Wind force | F = | N |
| Base thickness | t = | in |
| Condition | t = | mm |
| Weight, total, in operation | W = | lb |
| Weight, total, in operation | W = | kg |
| Weight per foot of height | w = | lb/ft |
| Weight per foot of height | w = | kg/m |
| Vibration period | T | s |
| Allowable vibration period (Ta>T) | Ta | s |
| Seismic force | V = | lbf |
| Seismic force | V = | N |
| (0.075..0.4) | Z (.075-.4) | - |
| Importance factor (1=standard, 1.25=hospital) | I (1, 1.25) | - |
Frequently asked questions
UBC 1997 has been withdrawn — when is it still used for calculations?
The Uniform Building Code was replaced in the USA from 2000 onward by the International Building Code (IBC) with the load provisions of ASCE 7. However, many international project specifications, existing plants, and older customer standards still refer to UBC 1997; it therefore remains practically relevant for re-calculations, modifications, and comparison with legacy documentation. For new construction, Eurocode 8 governs in Europe and ASCE 7 in the USA.
Which mass must be used in the seismic calculation?
The governing mass is the operating mass: empty weight plus internals, insulation, platforms, and the operating contents. For columns, the test condition (water-filled) must additionally be considered, because the mass is largest there — although it is often combined with reduced seismic requirements, since an earthquake and a pressure test are unlikely to occur simultaneously. The module calculates with the entered total operating mass.
Why is the moment also needed at an arbitrary height X?
The bending moment increases from top to base, and the required wall thickness accordingly. Tall columns are therefore fabricated with shell course thicknesses stepped over the height. With the moment at section plane X, the wall thickness required at each course boundary can be verified, saving material instead of carrying the base wall thickness over the full height.
Does the UBC calculation replace the stability and stress verification per the pressure vessel code?
No. The module delivers the seismic internal forces and the wall thickness required from the longitudinal stress consideration. The complete verification of the equipment additionally comprises internal/external pressure per the pressure vessel code (e.g. the German AD 2000 code or ASME VIII), the superposition with wind (not simultaneously with earthquake at full magnitude), buckling checks under axial compression and bending, and the anchorage and foundation verifications.