A practical wind-load workflow for low-rise frames (EN 1991-1-4)

Wind is the action that most often controls low-rise steel frames - portal sheds, industrial units, light commercial buildings - yet it is also where the most arithmetic slips happen. The EN 1991-1-4 procedure is not difficult, but it is a chain, and an error early on propagates all the way to the member forces. This is the workflow we use to keep it honest.

Step 1 - Basic wind velocity

Start from the fundamental basic wind velocity $v_{b,0}$ for the site, taken from the wind map. Apply the directional and seasonal factors to get the basic wind velocity $v_b$. For most permanent buildings the directional and seasonal factors are taken at their conservative default, but they exist for a reason and can be used where the orientation or construction season genuinely justifies it.

Step 2 - Peak velocity pressure

The heart of the calculation is the peak velocity pressure $q_p(z)$, which combines the mean wind with the turbulence (gust) at the reference height. Here the terrain category does the heavy lifting: it sets the roughness length and therefore both the mean wind profile with height and the gust factor. Town terrain shelters the building and lowers $q_p$; open country or coastal exposure raises it. Choosing the terrain category correctly is the single most influential decision in the whole workflow.

Step 3 - Pressure coefficients

With $q_p$ established, you read the external pressure coefficients $c_{pe}$ for the building's surfaces. For a duopitch roof these are tabulated by zone - the edge and ridge zones (F, G) see much higher suctions than the central zones (H, I, J), because that is where the wind separates and accelerates. Walls have their own zones.

Then add the internal pressure coefficient $c_{pi}$, governed by the openings: a dominant opening on the windward face pushes the internal pressure up and can dramatically worsen the net load on the roof and leeward wall. You must check the worst realistic opening case, including the accidental loss of a door or panel.

The net pressure on any surface is then:

$w = q_p \,(c_{pe} - c_{pi})$

Where the slips happen

The arithmetic is simple; the judgement is not. The recurring mistakes are:

• Wrong terrain category, which scales the whole pressure field.
• Forgetting the edge and corner zones, where the worst suctions live and where cladding and fixings are most at risk.
• Mishandling internal pressure, especially the dominant-opening case that turns a manageable roof load into an uplift problem.
• Losing track of signs - pressure versus suction - when combining external and internal coefficients.

Turning it into design

The net pressures feed straight into the frame analysis and, critically, into the uplift and overturning checks that often govern light structures - and into the cladding and fixing design, which sees the peak local suctions. To put real numbers on $q_p$ for your site and terrain without hand-tracing the chain, use the wind load calculator, which works the EN 1991-1-4 procedure step by step. The related load-combinations article covers how the wind case combines with dead, imposed and snow to find the governing member forces.