Engineering task and calculation objective
Module ASWD performs a wind load analysis for stacks, chimneys, flares and vertical vessels to ASCE 7-05, the American load standard for buildings and other structures. It calculates the total loading at the base of the component — shear force and overturning moment due to wind — as needed for the anchorage, support skirt and foundation checks of tall, slender plant components.
The velocity pressure is formed from the basic wind speed (3-second gust), the exposure category with the height-dependent coefficient Kz, the topography (Kzt), the wind directionality (Kd) and the Importance Factor; together with the gust effect factor G and the force coefficient Cf, this yields the wind force on the projected area. Since the velocity pressure increases with height, the module divides the component into segments (12×5 ft, 8×10 ft, 4×20 ft or 8×50 ft) and lists the loading for each segment individually — so the load distribution and base section forces can be traced and compared with other calculations.
Typical applications are export projects with IBC/ASCE requirements: vertical columns and vessels, free-standing chimneys and flue gas stacks for which, in addition to the seismic load, the wind load must be calculated and documented. For European sites, DIN EN 1991-1-4 applies instead.
Standard and calculation basis: ASCE 7-05
Calculation workflow
- Define the site wind parameters: The basic wind speed V (3-second gust per the ASCE 7-05 maps), the exposure category (Exposure B, C or D), the topographic factor Kzt, the directionality factor Kd and the Importance Factor of the facility category are entered.
- Component geometry and segmentation: The height and diameter (or width) of the stack or vessel determine the projected area exposed to wind. The module divides the height into standard segments (5, 10, 20 or 50 ft) so that the height-dependent velocity pressure is captured realistically.
- Calculate the velocity pressure per segment: For each segment height, the velocity pressure qz is determined from 0.00256·Kz·Kzt·Kd·V²·I (in psf, V in mph); Kz increases with height above ground according to the chosen exposure category.
- Form the wind force per segment: The force on each segment follows from F = qz·G·Cf·Af with the gust effect factor G (or Gf for vibration-prone, flexible structures), the force coefficient Cf for round or angular cross-sections per the ASCE 7-05 tables, and the segment area Af.
- Sum up the base section forces: Segment forces and their lever arms are summed to the total shear force and overturning moment at the base. These values enter the checks of the support skirt, anchor bolts and foundation and can be reconciled segment by segment with comparison calculations.
Input quantities
| Quantity | Symbol | Unit |
|---|---|---|
| Wind Map, Figure 6-1 | V | mph |
| Occupancy Category from Table 1-1 | 1-1 | – |
| Exposure Category from Sect. 6.5.6 | 6.5.6 | – |
| Topographic Factor from Sect. 6.5.7 | Kzt | - |
| Height of Vessel itself | h | ft |
| Height of Vessel Base Above Ground | Hb | ft |
| Diameter or Width of Surface Normal to Wind | D | ft |
| Wall Thickness of Vessel | t | in |
| Vessel Material Unit Weight | γt | pcf |
| Type of Shape | Shape | – |
| Damping Ratio = 0.010-0.070 | β | - |
| Wind Directionality Factor, Table 6-4 | Kd | - |
| Force Coef. from Fig. 6-21 | Cf | - |
| Is location in a hurricane prone region? | region? | – |
| Wind Direction (applicable for square cross-section only) | only) | – |
| Depth of protruding elements such as ribs and spoilers | D' | ft |
| Ratio Depth of protruding elements to Diameter or Width of Surface Normal to Wind | D'/D | - |
| Type of Round Surface | Surface | – |
| Height to Diameter Ratio | h/D | - |
| Wind Velocity Pressure | D*√(qz) | - |
| Terrain Exposure Constant Table 6-2 | α | - |
| Terrain Exposure Constants Table 6-2 | zg | - |
| Inside diameter or clear inside dimension between flats. | di | ft |
| Importance Factor (Table 6-1) | I | - |
Calculation options
Occupancy Category from Table 1-1
I · II · III · IV
Exposure Category from Sect. 6.5.6
A · B · C · D
Type of Shape
Round · Hexagon · Octagon · Square
Is location in a hurricane prone region?
No · Yes
Wind Direction (applicable for square cross-section only)
Normal · Diagonal
Type of Round Surface
Moderately Smooth · Rough · Very Rough · All
Rigid or Flexible
Rigid · Flexible
Rectangular, stepped distribution or Trapezodial Pressure Distribution
Rectangular Distribution · Trapezodial Distribution
Worked example
For one segment of a free-standing stack, the wind force is to be determined to ASCE 7-05 in this worked example. The segment lies at a height for which the height coefficient Kz = 1.04 applies (Exposure C); the projected segment area is 200 ft². The force coefficient was read from ASCE 7-05 Fig. 6-21 for the moderately rough circular cylinder.
Given values
| Basic wind speed V | 110 mph (3-s gust) |
| Height coefficient Kz | 1.04 |
| Topographic factor Kzt | 1.0 |
| Directionality factor Kd | 0.95 |
| Importance Factor I | 1.15 |
| Gust effect factor G / force coefficient Cf | 0.85 / 0.7 |
| Projected area Af | 200 ft² |
Solution
Velocity pressure
qz = 0.00256 · Kz · Kzt · Kd · V² · I
qz = 0.00256 · 1.04 · 1.0 · 0.95 · 110² · 1.15 ≈ 35.2 psf ≈ 1.69 kN/m²
Wind force on the segment
F = qz · G · Cf · Af = 35.2 · 0.85 · 0.7 · 200 ≈ 4,190 lbf ≈ 18.6 kN
This segment force is summed with its lever arm to the base into the total shear force and overturning moment; the remaining segments are calculated analogously with their respective Kz.
Result
| Velocity pressure qz | ≈ 35.2 psf ≈ 1.69 kN/m² |
| Segment wind force F | ≈ 4,190 lbf ≈ 18.6 kN |
All values are illustrative. The applicable standard and project-specific boundary conditions remain authoritative.
Frequently asked questions
Why is the component divided into segments?
The velocity pressure grows with height above ground (Kz profile). A calculation with a single average value underestimates the overturning moment, because the large pressures at the top coincide with the largest lever arms. The segmentation into 5 to 50 ft sections approximates the pressure profile in steps and, in addition to the total load, provides the load distribution for the shell check at various heights.
What must be considered for slender stacks beyond the static wind load?
Flexible structures with a natural frequency below 1 Hz are considered vibration-prone according to ASCE 7; instead of the rigid gust effect factor G = 0.85, the dynamic gust effect factor Gf must then be calculated with the natural frequency and damping. Independently of this, slender circular cylinders must be checked for vortex-induced crosswind vibrations (vortex shedding) — this phenomenon is not covered by the static wind load calculation and can cause fatigue damage.
What does the force coefficient Cf depend on?
For round cross-sections, on the surface roughness (smooth, moderately rough, very rough), on the slenderness ratio h/D, and on whether the cylinder is in the supercritical Reynolds regime (criterion D·√qz in ASCE 7-05 Fig. 6-21). Smooth cylinders reach Cf ≈ 0.5–0.7; very rough ones or those fitted with attachments considerably more. Ladders, piping and platforms on the shell also enlarge the effective exposed area and are often covered by blanket allowances.
Can I compare results to ASCE 7-05 directly with Eurocode wind loads?
Only with caution. ASCE 7-05 uses the 3-second gust as the basic wind speed, the Eurocode the 10-minute mean — the same site wind climate therefore leads to apparently very different numerical values. The safety concept (load factors) and gust model also differ. A comparison should always be made at the level of the resulting design section forces with the respective associated load factors.