Wind and snow

Wind and snow on a building are a geometry problem as much as a load problem: which faces are walls and which are roof, which zone each part of a face falls in, which pressure coefficient applies at which pitch, and how many arrangements the code asks for. The generators read that geometry from the model and write the load cases. You supply the site; they supply the zoning, the coefficients and the cases.

Open them from Loads › Generators › Wind or Snow. Both open the Climate loads dialog, which lists the project's generators on the left; New wind and New snow add another. The generators are on every plan.

What they load

The generators put pressure on load panels and plates. A load panel hands the pressure to the beams around it; a plate carries it itself. Draw the building's envelope, its walls and roof, as one or the other, and the generator recognises it from the geometry. Nothing is declared by hand. Each surface is classed as Wall, Roof or Left out, with the reason for leaving it out:

The dialog draws the envelope in axonometry: pressure warm and suction cool for wind, the shape coefficient μ for snow, with a zone table beside it. While a generated case is in view, the Wind & snow zones display option shades every wall and roof panel with its zones and coefficients.

Roof shapes

* EN 1991 has no table or clause for this case. The generator applies common practice and says so in a note on the dialog, and in the report. Wherever the generator departs from the letter of the code, it writes a note.

A roof at 60° or steeper carries no snow, since μ₁ = 0. A flat roof at two heights, and roofs with several flat levels or pitched parts that do not fit one profile, are refused, and those panels are left unloaded.

Wind to EN 1991-1-4

Site

From those and the building height $h$, measured from the lowest node to the top of the roof, the dialog shows $v_b$, $q_b$ and the peak velocity pressure $q_p(h)$.

Cases to generate

Each ticked direction makes one case per $c_{pe}$ variant, where a table gives a pressure and a suction value, and per $c_{pi}$. A case name says which: Wind +X · roof − · cpi +0.2. Wall zones are A–E; roof zones F–J, and K–N on a hipped roof.

Snow to EN 1991-1-3

μ is computed from the pitch, not typed. Snow acts along gravity per square metre of plan, so a sloping panel carries $s\cos \alpha$ per square metre of its own area.

National annexes

The values built in are the Czech annex's: wind zones I–V and snow zones I–VII of ČSN EN 1991-1-4/NA and ČSN EN 1991-1-3/NA. Picking a zone fills in the value, which stays editable. Everywhere else the generators use the CEN recommended values, and the site values are yours to enter from your annex's maps. There is no map lookup by location. See National annexes.

What they produce

Load cases, all in the generator's own load group, which is exclusive: at most one of its cases acts in a combination, which is what the code intends for the wind directions and the snow arrangements. The group is classed as wind or snow, so the combination generator reads the right ψ factors.

When you create the cases, the dialog offers to add them to existing combinations, at γ_Q (1.5 at ULS, 1.0 at SLS), editable afterwards. Generating EN 1990 combinations from scratch is the combination generator's job, on Pro.

The cases stay live. A generator stores its inputs and the panels it loads, never the loads. They are worked out from the current geometry, so moving a node re-zones the walls and re-loads the roof. Geometry never adds or removes a case by itself. If a change means the envelope would produce different cases, the dialog says so and Update cases applies it. Editing a generator collects changes until you Apply them, with a preview of the cases added, reused and deleted. Case ids are reused, so combinations and pinned results keep pointing where they did.

Generated loads cannot be edited one by one; change the generator. Delete removes it with its cases.

In the report

The report's Wind & snow section sets out the site and the $q_p$ derivation, $c_{pe}$ and $w_e$ per zone and direction, or μ and $s$ per arrangement, the envelope as recognised, every note, and the cases generated, so a checker can follow each pressure back to its clause. It is in the free summary preset too.

Where they stop

• Orthogonal wind directions and axis-aligned rectangular plans only.
• No friction, no parapets, canopies, free-standing walls or open buildings. A building more than 30 % open on two faces gets a note, not the open-building coefficients.
• No drift against obstacles or parapets (§6.2), no drift from a taller adjacent building, and no exceptional snow (Annexes A and B).
• A dominant opening in the roof is noted; check it by hand.
• Site inputs ($v_{b,0}$, altitude, opening areas) are entered in metric units whatever the project's units. Pressures and forces follow the project.

The generators were checked against published worked examples, duopitch, hipped and multi-span halls and a portal frame, before they shipped. They are still a convenience, not an authority. Read the zone table and the notes before you trust the cases, the way you would check anyone else's wind calculation.