Learn: Wind surface pressure
A surface coefficient translates peak velocity pressure into a signed pressure on one surface. A positive coefficient represents pressure in the chosen convention; a negative coefficient represents suction.
What the formula is saying
Multiplying qp by cp changes the magnitude and preserves the coefficient’s sign. The coefficient summarizes how flow around the shape changes pressure at the surface zone being studied.
Read the symbols in plain language
- qp(z)
- Peak velocity pressure
Peak velocity pressure. Use the peak pressure at the reference height appropriate to this surface calculation.
PaOne pascal is one newton per square metre. 1 MPa = 10⁶ Pa.
- cp
- Pressure coefficient
Signed multiplier relating the specified velocity pressure to surface pressure; suction is negative in this convention.
ratio / no unitA dimensionless ratio has no physical unit; 0.01 as a ratio is 1% when the percent option is selected.
- w
- Result to find
Wind surface pressure. The coefficient controls whether this contribution acts as pressure or suction.
Pa
Sort out the units first
qp and the result are in Pa, while cp has no unit. Pressure is not force: calculating a force also requires an appropriate area and treatment of pressure variation.
Assumptions before calculating
All coefficients are supplied exercise data. The designer must choose them from the relevant adopted standard, design situation and National Annex; the calculator does not select a country or verify that selection.
This is a first-generation Eurocode teaching relationship or an explicitly simplified coefficient calculation. The numbers supplied here are exercise data, not a recommendation for any country. Check the adopted edition, relevant clause, National Annex, applicability conditions and all other limit states before any real design.
Let’s solve one together
Read the given values, follow each operation, then check what the result means.
Read the supplied values as one complete study case. Find w and explain the result in the stated output unit.
- qp(z) · Peak velocity pressure
- 800 Pa
- cp · Pressure coefficient
- 1.2
Identify peak velocity pressure
Use the peak pressure at the reference height appropriate to this surface calculation.
(800) = 800 PaApply the signed surface coefficient
The coefficient controls whether this contribution acts as pressure or suction.
(800) × (1.2) = 960 Pa
Does this worked answer make sense?
A zero surface coefficient gives zero contribution even when wind exists. Changing cp from +0.8 to −0.8 reverses the sign without changing the magnitude.
A second worked example — different values
A second case uses different data. Predict which way the answer will change, then calculate it without reusing the first answer.
- qp(z) · Peak velocity pressure
- 1000 Pa
- cp · Pressure coefficient
- -0.8
Identify peak velocity pressure
Use the peak pressure at the reference height appropriate to this surface calculation.
(1000) = 1000 PaApply the signed surface coefficient
The coefficient controls whether this contribution acts as pressure or suction.
(1000) × (-0.8) = -800 Pa
Now try your own values
Change a value or its unit. The same method will show your calculation, step by step.
Results update only when you calculate. The lesson example above stays unchanged.
Your turn — check your understanding
Solve this separate case yourself. Use only the values below; the two worked examples use different data. Give the requested result in the selected unit.
- qp(z) · Peak velocity pressure
- 750 Pa
- cp · Pressure coefficient
- -0.6
Find: Learn: Wind surface pressure
A hint, not the answer
Multiplying qp by cp changes the magnitude and preserves the coefficient’s sign. The coefficient summarizes how flow around the shape changes pressure at the surface zone being studied.
qp and the result are in Pa, while cp has no unit. Pressure is not force: calculating a force also requires an appropriate area and treatment of pressure variation.
Show the full practice solution
Compare the steps with your work; revealing a solution does not mark the lesson complete.
Identify peak velocity pressure
Use the peak pressure at the reference height appropriate to this surface calculation.
(750) = 750 PaApply the signed surface coefficient
The coefficient controls whether this contribution acts as pressure or suction.
(750) × (-0.6) = -450 Pa
Avoid the common trap
Do not discard a negative cp. Do not substitute basic pressure qb for qp unless justified, and do not use a coefficient from another roof or wall zone.
When this method applies — and when it does not
This is one external or internal surface contribution, not automatically net wall pressure. Net action normally requires compatible external and internal pressures, surface zones, reference heights, loaded areas and governing combinations.
For study and understanding, not approval of a real structure, site operation or design. Apply the correct standard, National Annex and professional review to actual engineering work.
One idea understood. Keep going.
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Sources & further reading
References open in a new tab and explain the underlying principles. The teaching text and examples here are SimpleFlick’s own; the source organisations have not endorsed this calculator.
Reading focus: Wind surface pressure. Action-specific reading: EN 1991-1-1 for self-weight, EN 1991-1-3 for snow, and EN 1991-1-4 for wind. National and site data are not generated by this lesson.
Lesson updated: · Both examples and the separate practice case are checked against an independent high-precision numerical implementation. This verifies arithmetic for the stated model, not engineering certification.