Wind — structural loads
Characteristic wind actions to ČSN EN 1991-1-4 ed. 2 including the Czech National Annex. After choosing the wind zone and the terrain category, select the type of structure — the calculator determines the peak velocity pressure qp(ze), the pressure or force coefficients of every zone and load case given by the standard and draws pressure and suction in a diagram.
1 — Wind at the site
Zone from the wind map of the Czech National Annex. A custom value of vb,0 (e.g. from ČHMÚ) takes precedence over the zone. For vb,0 > 30 m/s and for special locations (hilltops, narrow valleys) vb,0 is specified on the basis of a statement by ČHMÚ (NA.2.4).
Zone V (vb,0 > 30 m/s): the fundamental value of the basic wind velocity has to be specified for the particular site on the basis of a statement by ČHMÚ (NA.2.4) — enter the specific value of vb,0.
Calculate orography
Annex A.3 applies to isolated hills, ridges, cliffs and escarpments. x is measured horizontally from the crest: negative upwind, positive downwind. The height z above the ground is the reference height ze of the structure. In the Czech Republic co is determined to Annex A.3; outside the situations of A.3(3) co = 1.00 unless ČHMÚ recommends another value (NA.2.13).
More parameters — cdir, cseason, ρ, kI
The Czech National Annex prescribes cdir = cseason = 1.0 (NA.2.6, NA.2.7), ρ = 1.25 kg/m³ (NA.2.18) and kI = 1.0 (NA.2.16). Any other value is a custom (non-standard) choice — the calculator flags it. cdir and cseason only enter the basic wind velocity vb = cdir · cseason · vb,0 (4.2) — pressures and forces do not multiply them again.
Terrain categories explained (Table 4.1)
0 — sea or coastal area exposed to the open sea (z0 = 0.003 m).
I — lakes or flat and horizontal area with negligible vegetation and without obstacles (z0 = 0.01 m).
II — area with low vegetation such as grass and isolated obstacles (trees, buildings) with separations of at least 20 obstacle heights (z0 = 0.05 m).
III — area with regular cover of vegetation or buildings or with isolated obstacles with separations of maximum 20 obstacle heights — villages, suburban terrain, permanent forest (z0 = 0.3 m).
IV — area in which at least 15 % of the surface is covered with buildings whose average height exceeds 15 m (z0 = 1.0 m).
2 — Type of structure
3 — Input: vertical walls of a building
Building with a rectangular plan, wind normal to wall D. Reference height ze = h for all zones (on the safe side). For the perpendicular wind direction swap b and d.
A flat roof has a pitch −5° < α < 5°. Zones F, G, H, I to Fig. 7.6; e = min(b, 2h). With parapets the reference height is ze = h + hp (7.2.3(3)). Table 7.2 covers parapets with 0.025 ≤ hp/h ≤ 0.10, curved eaves with 0.05 ≤ r/h ≤ 0.20 and mansard eaves with α ≥ 30°. A parapet below its range is calculated with the sharp-edge coefficients and above it with the upper table limit (no extrapolation), and flagged; curved and mansard eaves outside these limits return no result. For the perpendicular wind direction swap b and d.
Pitch 5° to 75° (Tables 7.3a/b); a lower pitch is a flat roof. The calculator covers the wind directions θ = 0° (onto the lower edge), 180° (onto the higher edge) and 90° (along the edges).
Pitch −45° to −5° (troughed) or 5° to 75° (Tables 7.4a/b); between −5° and +5° the roof is flat. The calculator covers θ = 0° (normal to the ridge) and 90° (along the ridge).
Pitch 0° to 30° (Table 7.6). φ = 0 is an empty canopy, φ = 1 a canopy fully blocked to the downwind eave (not a closed building). Between φ = 0 and 1 the minimum is interpolated linearly.
Pitch −20° to +30° (Table 7.7), a negative pitch is troughed. φ = 0 is an empty canopy, φ = 1 fully blocked to the downwind eave. Between φ = 0 and 1 the minimum is interpolated linearly.
Solid wall φ = 1. Porous walls and fences with φ ≤ 0.8 (including a wall with 20 % openings) are treated as plane lattices to 7.4(1) and 7.11 — out of scope; the values of Table 7.9 for φ = 0.8 only serve the interpolation for 0.8 < φ < 1. A return corner shorter than h is interpolated linearly. Parapets on buildings (ze = h + hp, 7.4.1(2)) are not covered.
cf = 1.8 for signboards with the lower edge at zg ≥ h/4; for zg < h/4 only if b/h ≤ 1 — otherwise the signboard is treated as a free-standing wall (7.4.3). ze = zg + h/2, Aref = b · h, eccentricity e = ±0.25 · b.
More parameters — cs·cd
Default value 1.00. Whether cs·cd = 1.0 may be used has to be verified to ČSN EN 1991-1-4, clause 6.2 — otherwise cs·cd is determined to 6.3 (in the Czech Republic procedure 1, Annex B) or Annex D. The calculator does no dynamic analysis; cs·cd only enters the force Fw.
cf = cf,0 · ψλ (7.11); in the Czech Republic cf,0 = 2.0 (NA.2.35). ψλ is determined to 7.13 from Fig. 7.36 — from the slenderness λ (Table 7.16) and the solidity φ (solid element φ = 1); the calculator does not compute it, ψλ = 1.0 is always on the safe side. Aref = ℓ · b for wind in the x direction; for the y direction enter the dimension d instead of b (Aref,y = ℓ · d, Fig. 7.25). ze is the maximum height of the section above ground (7.7(3)).
More parameters — cs·cd
Default value 1.00. Whether cs·cd = 1.0 may be used has to be verified to ČSN EN 1991-1-4, clause 6.2 — otherwise cs·cd is determined to 6.3 (in the Czech Republic procedure 1, Annex B) or Annex D. The calculator does no dynamic analysis; cs·cd only enters the force Fw.
Load diagram
- The fundamental value of the basic wind velocity vb,0 follows the wind map of the National Annex; for zone V the value is set by ČHMÚ. The orography factor co is 1.00 by default — it can be entered or calculated to Annex A.3.
- For walls and roofs of buildings the results are external pressures we. Internal pressure is not included — the resulting pressure on the envelope of a closed building combines the external and the internal pressure.
- The calculator determines characteristic wind actions: the peak velocity pressure, external pressures with the coefficients cpe,10, net pressures on canopies and free-standing walls and forces on signboards and elements with open sections. It does not cover internal pressure, the coefficients cpe,1 for small areas, dynamic response, wind friction, other structural shapes or load combinations. The calculator does not calculate the structural factor cs·cd; the value is entered by the user and the default value is 1.00. This is not a substitute for a complete structural design.