Shade Sail & Awning Wind Load: Pressure, Force & Load per Fixing

The formula

This is the expression the tool evaluates. Every term is named underneath, with the unit it must be supplied in.

How the result is derived

Step by step, from the values you type to the figure on screen.

1. The 6 inputs are read from the form on every keystroke: Design Wind Speed, Air Density, Force Coefficient, Sail Area, Fixing Points and Share of the Load on the Worst Fixing.
2. Each value is checked against the accepted range in the input table below. A value outside its range stops the calculation rather than producing a misleading figure — the results blank out and a message appears.
3. The validated values are substituted into the expression above, which resolves Wind Force on the Sail together with every supporting figure in one pass — no value is carried over from a previous entry.
4. The supporting outputs — Dynamic Pressure, Dynamic Pressure, Wind Force, Equal Share per Fixing, Load on the Worst Fixing, Load on the Worst Fixing, Design Wind Speed and Force at 10% More Wind — come from the same pass, so they always describe the same case as the headline figure.
5. Results are rounded for display only. The full-precision value is used throughout the chain, so reading a rounded intermediate figure back into the tool by hand can shift the last digit.

What each input means

Where to read each value on the floor, the unit it must be in, and the range the tool accepts.

What the tool returns

The headline figure and every supporting value it is built from.

Worked example

Substituting

A 450 kgf pull on one corner is a post and footing problem, not a fabric problem, which is why the engineer's share figure matters more than the sail's weight.

How to use it

1. Work through the input groups in order — Wind and Sail & Fixings. The defaults are a realistic case, so you can change one value at a time and watch what moves.
2. There is no calculate button. Every figure recalculates as you type or drag, which is what makes this usable for a what-if sweep rather than a single answer.
3. Read Wind Force on the Sail in the dark results panel — that is the headline figure, expressed in kN.
4. Check the supporting rows underneath (Dynamic Pressure, Dynamic Pressure, Wind Force, Equal Share per Fixing, Load on the Worst Fixing, Load on the Worst Fixing, Design Wind Speed and Force at 10% More Wind) before acting on the headline — they are where an implausible input usually shows itself first.
5. Reset to defaults returns every field to the reference case, which is the quickest way to check whether a surprising result came from the tool or from an input you had changed earlier.

Where this is used

• Process planning — establishing Wind Force on the Sail before a trial is booked, so machine time and material in Agrotextiles & Environmental Shielding are committed against a calculated figure rather than an estimate.
• Costing and quotation — Wind Force on the Sail is an input to the cost sheet, and quoting from a worked number rather than a remembered one is what keeps a margin intact.
• Troubleshooting — when the floor result drifts from plan, entering the measured values (starting with Design Wind Speed) shows how much of the gap in Wind Force on the Sail each variable explains.
• Teaching and study — the accepted ranges bracket normal Agrotextiles & Environmental Shielding practice, so moving one variable at a time shows the shape of the relationship rather than a single answer.

Reading the result

Typical bands and what each one is telling you.

Assumptions and limits

• The physics is the dynamic pressure of moving air, half the density times the speed squared, multiplied by a force coefficient and the area. Everything that makes a real canopy differ from a flat plate, its pitch, curvature, edge shape, the blockage behind it and the gust factor, sits in the force coefficient and the design speed, and both belong to the wind code for the site (EN 1991-1-4, ASCE 7, AS/NZS 1170.2 or the local equivalent) and to the engineer who applies it. Knitted shade cloth sheds part of the load through its open area; the maker's reduction, if any, applies to the force shown here, and a coated or solid fabric takes it all. Fabric pretension adds to the corner loads on top of the wind and is not included. **This is the wind-load arithmetic for a quotation or a sanity check; the fixings, posts and footings are the engineer's design.**
• Every input is bounded to the range normal practice occupies (Design Wind Speed 1 to 80 m/s, Air Density 0.9 to 1.4 kg/m³ and Force Coefficient 0.1 to 3 x, and so on for the rest). Those bounds are guard rails against typing errors, not a claim that the formula fails one unit outside them.
• The calculation is deterministic: the same inputs always give the same result. It carries no allowance for machine condition, operator skill, ambient conditions or lot-to-lot material variation unless an input above explicitly represents one.
• Nothing is sent anywhere. The maths runs in your browser, so the numbers you type never leave the page.

Standards and further reading

• EN 1991-1-4, Eurocode 1, actions on structures, wind actions: design speed, peak pressure and force coefficients, including free-standing canopies.
• ASCE 7 (USA) and AS/NZS 1170.2 (Australia and New Zealand): the equivalent wind-load codes most shade-sail markets design to.
• EN 13561, External blinds and awnings, performance requirements: the wind resistance classes a retractable awning is rated by.

Questions people ask