Inputs
Comparison table
| Case | Wind speed | Area | Cd | G | Kz | Design pressure | Total force |
|---|---|---|---|---|---|---|---|
| Example A | 35 mps | 100 m2 | 1.2 | 1 | 1.05 | 0.99 kPa | 99 kN |
| Example B | 55 mps | 150 m2 | 1.3 | 1.1 | 1.18 | 2.56 kPa | 384 kN |
| Example C | 90 kmh | 180 m2 | 1.4 | 1 | 1.1 | 1.34 kPa | 241 kN |
Formula used
- q = 0.5 × ρ × V² (dynamic pressure)
- p = q × Kz × Kzt × Kd × G × Cd × I × Ks (design pressure)
- F = p × A (total wind force on projected area)
How to use this calculator
- Enter wind speed, projected area, and choose your units.
- Set coefficients and multipliers based on your design basis.
- Select auto Kz for a quick estimate, or enter Kz manually.
- Press Calculate wind load to view results above the form.
- Click Add result to comparison table to store scenarios.
- Use Download PDF or Download CSV for reports.
Wind actions on long-span structures
Bridges behave like tall, open-profile elements, so wind pressure often governs serviceability. The calculator starts with dynamic pressure q = 0.5ρV², where ρ defaults to 1.225 kg/m³ at sea level. Because pressure scales with V², increasing wind speed from 40 to 50 m/s raises q by about 56%. Use site-specific basic wind speeds when available.
Projected area and aerodynamic drag
Projected area is the silhouette normal to wind: deck depth × span for a simplified check, plus exposed barriers and truss members when appropriate. Drag coefficient Cd depends on shape and solidity; streamlined box girders may be near 1.0–1.3, while bluff sections and trusses can exceed 1.5. If parapets or utilities are present, include their added area and adjust Cd consistently.
Exposure, height, and gust effects
Wind profile increases with elevation and terrain roughness. In auto mode, Kz is approximated using a power law and limited between 0.7 and 2.2 to stay realistic. Exposure B represents urban or wooded terrain, C open terrain, and D flat, unobstructed coastal zones. Gust factor G commonly ranges 0.85–1.15 for simplified static design.
Design multipliers for reliability
Directionality Kd (often 0.85) accounts for wind direction frequency, while Kzt captures topographic speed-up near ridges or escarpments. Importance I raises loads for critical routes, and shielding Ks reduces loads when upstream obstructions are justified. The calculator combines these into design pressure p, then multiplies by area to get total force F for component checks.
Interpreting results and documenting cases
Use the comparison table to store alternatives such as different heights, exposures, or deck configurations. Export CSV for spreadsheets and PDF for submittals with traceable inputs and outputs. Always align factors with your governing standard, and confirm whether you need additional effects like vortex shedding, flutter stability, or along-wind versus cross-wind components. For sizing, check pressure units (Pa, kPa, psf) and force units (N, kN, lbf, kip) to avoid mistakes.
FAQs
1) Which wind speed should I enter?
Use the basic or reference wind speed specified for your site and return period. If you have multiple directions, use the governing speed. Remember the load scales with the square of speed, so small changes can dominate results.
2) How do I choose projected area?
Use the bridge silhouette normal to wind. For a deck-only check, multiply deck depth by the loaded span. Add parapets, barriers, utilities, truss members, and signage when they materially increase exposed area.
3) What if I already have Kz from a standard?
Switch Kz mode to Manual and enter the coefficient from your governing standard. This preserves your code-derived value while still applying the other multipliers and unit conversions consistently.
4) Does this include uplift, torsion, or vibration?
No. The calculator provides along-wind pressure and resultant force on projected area. Effects such as uplift, torsion, vortex shedding, flutter, and cross-wind response require separate aerodynamic or dynamic analysis.
5) Why might my code check give a different result?
Standards differ in definitions for gust, exposure, directionality, and load combinations. Align each factor with the same standard and confirm whether your wind speed is 3-second gust, mean hourly, or another reference.
6) When should I change air density?
Use a lower density for high elevations or hot conditions, and a higher density for cold air at low elevation. If you do not have site data, the default 1.225 kg/m³ is a reasonable starting point.