# Wind Load on Monopitch and Duopitch Roofs in Germany

## Technical Article on the Topic Structural Analysis Using Dlubal Software

### Technical Article

In Germany, DIN EN 1991-1-4 with the National Annex DIN EN 1991-1-4/NA regulates the wind loads. The standard applies to civil engineering works up to an altitude of 300 m.

Wind is naturally an action variable in time on a structure located outdoors. The wind load is classified as a variable, free action so that the loading can be combined with other actions (for example, imposed load or snow) in defined design situations according to the combination standard DIN EN 1990. Changes in the aerodynamic coefficients due to other actions (snow, traffic, or ice) and due to modifications of the structure have to be considered during construction. However, windows and doors are assumed as closed in case of wind loads. Windows and doors that are unavoidably open have to be considered as accidental design situations.

The dynamic wind load has to be displayed in a simplified way as equivalent wind pressure or wind force to the maximum action of turbulent wind. Wind acts on the outer surfaces in case of closed structures and additionally on the inner surfaces in case of permeable or open structures. The action has to be applied as perpendicular to the surfaces considered. In case of large surfaces under circulating wind, a friction component has to be considered additionally, parallel to the surface area.

The wind standard DIN EN 1991-1-4 with the National Annex of Germany specifies the wind load as a characteristic value. This value is determined by a basic wind velocity with an annual probability exceedance value of 2% and an average return period of 50 years.

The resulting wind load in case of sufficiently rigid buildings not susceptible to vibrations can be described as a static equivalent force that depends on the peak velocity. In contrast, for buildings susceptible to vibrations, the peak velocity is additionally modified with a structural factor to determine the static equivalent load [1], [2].

In simple terms, structures are not considered as susceptible to vibrations if the deformation under wind load caused by gusty wind resonance is not increased by more than 10%. This criterion applies to typical buildings with a height of up to 25 m that are not susceptible to vibrations. In all other cases, the following classification criterion can be used [3]:

Formula 1

$$xSh ≤ δhrefh · h bb 0,125 · hhref2$$

where
xS = is the head displacement in m due to self-weight applied in the wind direction
h = building height in m; href = 25 m
b = width of the building perpendicular to the wind direction in m
δ = logarithmic decrement of damping according to DIN EN 1991-1-4, Annex F

Type of StructureBuilding Damping δmin
Reinforced concrete structure0.1
Steel structure0.05
Mixed structure (steel and concrete)0.08

#### Height-Dependent Peak Velocity Pressure

The wind load on a building not susceptible to vibrations depends on the peak velocity pressure qp. This value results from the wind velocity of a gust of wind with a length of two to four seconds by taking into account the surrounding terrain conditions. To determine the load at a location, the National Annex of Germany contains a wind zone map with corresponding basic values of the basic wind velocities vb,0, the basic values of the basic wind velocity pressures qb,0, and a specification of various terrain types (categories I - IV) [1], [2], [3].

If the wind zone increases, the basic value of the basic wind velocity increases as well.

The terrain category increases with the roughness of the terrain.

TerrainDescription
Terrain category IOpen sea; lakes with at least 5 km open area in the wind direction; smooth, flat land with no obstacles
Terrain category IISite with hedges, individual farms, houses, or trees (for example, agricultural area)
Terrain category IIISuburbs, industrial or commercial areas; forests
Terrain category IVUrban areas where at least 15% of the area is covered with buildings of which the average height exceeds 15 m
Mixed profile coastTransition region between terrain categories I and II
Mixed profile inlandTransition region between terrain categories II and III

The peak velocity pressure vb,0 can be determined by defining the basic value of the basic wind velocity qp and the terrain type.

Peak velocity pressure
qp in kN/m² [3]
Approach 1
Table NA-B.1
Approach 2
NA.B.3.3
Approach 3
NA.B.3.2
Influence of sea level
NNmod
Less than 800 m above sea level1.0
Between 800 m and 1,100 m above sea level0.2 + Hs/1,000
More than 1,100 m above sea levelSpecial considerations required
Wind zone12341234
Fundamental basic wind velocity
vb,0 in m/s
22.525.027.530.0----
Directional factor
cdir
1.0----
Season factor
cseason
1.0----
Basic velocity pressure
qb in kN/m²
0.320.390.470.56----
Terrain categoryBuilding heightqp in kN/m²
qp (z) in kN/m²
Terrain category IUp to 2 m1.90 ⋅ qb ⋅ NNmod-----
2 m to 300 m2.60 ⋅ qb ⋅ (z/10)0.19 ⋅ NNmod
Terrain category IIUp to 4 m1.70 ⋅ qb ⋅ NNmod-----
4 m to 300 m2.10 ⋅ qb ⋅ (z/10)0.24 ⋅ NNmod
Terrain category IIIUp to 8 m1.50 ⋅ qb ⋅ NNmod-----
8 m to 300 m1.60 ⋅ qb ⋅ (z/10)0.31 ⋅ NNmod
Terrain category IVUp to 16 m1.30 ⋅ qb ⋅ NNmod-----
16 m to 300 m1.10 ⋅ qb ⋅ (z/10)0.40 ⋅ NNmod
North Sea Islands IUp to 2 m-1.10 ⋅ NNmod----
2 m to 300 m1.50 ⋅ (z/10)0.19 ⋅ NNmod
Coastal Areas and Baltic Sea Islands I - IIUp to 4 m-1.80 ⋅ qb ⋅ NNmod----
4 m to 50 m2.30 ⋅ qb ⋅ (z/10)0.27 ⋅ NNmod
50 m to 300 m2.60 ⋅ qb ⋅ (z/10)0.19 ⋅ NNmod
Inland areas II - IIIUp to 7 m-1.50 ⋅ qb ⋅ NNmod----
7 m to 50 m1.70 ⋅ qb ⋅ (z/10)0.37 ⋅ NNmod
50 m to 300 m2.10 ⋅ qb ⋅ (z/10)0.24 ⋅ NNmod
Inland areasUp to 10 m--0.50 ⋅ NNmod0.65 ⋅ NNmod0.80 ⋅ NNmod0.95 ⋅ NNmod
10 m to 18 m0.65 ⋅ NNmod0.80 ⋅ NNmod0.95 ⋅ NNmod1.15 ⋅ NNmod
18 m up to 25 m0.75 ⋅ NNmod0.90 ⋅ NNmod1.10 ⋅ NNmod1.30 ⋅ NNmod
Coastal Area and Baltic Sea IslandsUp to 10 m---0.85 ⋅ NNmod1.05 ⋅ NNmod-
10 m to 18 m-1.00 ⋅ NNmod1.20 ⋅ NNmod-
18 m up to 25 m-1.10 ⋅ NNmod1.30 ⋅ NNmod-
North and Baltic Sea Coast and Baltic Sea IslandsUp to 10 m-----1.25 ⋅ NNmod
10 m to 18 m---1.40 ⋅ NNmod
18 m up to 25 m---1.55 ⋅ NNmod
North Sea IslandsUp to 10 m-----1.40 ⋅ NNmod
10 m to 18 m---According to approach 2
18 m up to 25 m---According to approach 2

#### Determining the Local Basic Wind Velocity Pressure with Dlubal Online Service

The Dlubal online service Snow Load Zones, Wind Zones, and Earthquake Zones combines the standard specifications with digital technologies. The service places the respective zone map over the Google Maps map, depending on the selected load type (snow, wind, earthquake) and the country-specific standard. Using the search, you can place a marker on the planned construction location by defining the address, geographical coordinates, or local conditions. The application determines the characteristic load or the acceleration at this location using the exact height above sea level and the given zone data. If the new construction location cannot be identified by a specific address, you can zoom in and shift the focus to the correct location. With the displacement of the marker, the calculation is adapted to the new altitude and displays the correct loads.

The online service is available on the Dlubal website at Solutions → Online Services.

By defining the parameters...

2. Standard = EN 1991-1-4
3. National Annex = Germany | DIN EN 1991-1-4
4. address = Zellweg 2, Tiefenbach

...the following results are obtained for the selected location:

5. wind zone
7. fundamental basic wind velocity vb,0
8. basic wind velocity pressure qb

If you select a position above 1,100 m, the online service displays at point 6 "No defined wind load above 1,100 m | NCI A.2 (3)". No load can be determined according to the existing rule, and special considerations are required for this location.

#### wind pressure on surfaces

The acting wind pressure on a surface is the product of governing peak velocity pressure multiplied by the aerodynamic coefficient [1], [2].

For external surfaces:
we =qp(ze) ⋅ cpe
where
qp(ze) = peak velocity pressure
ze = reference height for the external pressure
cpe = aerodynamic coefficient for the external pressure

For internal surfaces:
wi =qp(zi) ⋅ cpi
where
qp(zi) = peak velocity pressure
zi =  reference height for the internal pressure
cpi = aerodynamic coefficient for the internal pressure

The resulting loading of external and internal pressure is the net pressure loading on a surface. The pressure on a surface is considered as positive and the pressure (suction) away from the surface as negative.

Net pressure:
wnet = we + wi

#### Selected Aerodynamic Coefficients

Pressure and suction loads are applied on the surface of a structure that is in the wind flow. The size of the action on the external surfaces depends on their load application area. A load application area is the surface that absorbs the flat wind loading actively and transmits it in a concentrated way to the structural system below. For this type of analysis, the standard contains aerodynamic external pressure coefficients that depend on the load introduction surface [1], [2].

External Pressure Coefficient cpe
Description
< 1 m²cpe,1Design of small structural components and their anchorages (for example, encasement or roof elements)
1 m² to 10 m²cpe,1 - (cpe,1 - cpe,10) ⋅ log10(A)
> 10 m²cpe,10Design of the entire structure

#### Vertical Walls of Buildings with Rectangular Floor Plan

The wind velocity naturally increases nonlinearly with the height from the ground. The resulting peak velocity pressure distribution can be applied in a simplified and scaled manner by the height for the windward building surface (windward area D), depending on the ratio building height h to building width b [1], [2].

The wall suction loads of the remaining leeward building surfaces parallel to the wind (areas A, B, C, and E) depend on the aerodynamics of the building. The final aerodynamic coefficients for the external surfaces can be determined and applied in a scaled manner, depending on the ratio building height h to building depth d.

AreaABCDE
h/dcpe,10cpe,1cpe,10cpe,1cpe,10cpe,1cpe,10cpe,1cpe,10cpe,1
≥5-1.4-1.7-0.8-1.1-0.5-0.7+0.8+1.0-0.5-0.7
1-1.2-1.4-0.8-1.1-0.5+0.8+1.0-0.5
≤0.25-1.2-1.4-0.8-1.1-0.5+0.8+1.0-0.3-0.5
Larger suction forces may occur in the suction area for detached buildings located at open area sites.
Linear interpolation of the intermediate values is allowed.
For buildings with h/d> 5, the entire wind load has to be determined by means of the force values according to DIN EN 1991-1-4 plus National Annex of Germany Chapters 7.6 to 7.8 and 7.9.2.

#### Monopitch roof

Similar to the dimensions of the building, the shape of the roof also has an aerodynamic effect on the external roof surfaces. A roof with an inclination greater than 5° with distinctive high and low eaves is called a monopitch roof. Due to the aerodynamics, wind loads are acting on the load application surfaces, depending on the roof inclination [1], [2].

AreaFGHI
Flow direction θ = 0°2)
Inclination angle α1)cpe,10cpe,1cpe,10cpe,1cpe,10cpe,1cpe,10cpe,1
-1.7-2.5-1.2-2.0-0.6-1.2--
+0.0+0.0+0.0
15°-0.9-2.0-0.8-1.5-0.3--
+0.2+0.2+0.2
30°-0.5-1.5-0.5-1.5-0.2--
+0.7+0.7+0.4
45°-0.0-0.0-0.0--
+0.7+0.7+0.6
60°+0.7+0.7+0.7--
75°+0.8+0.8+0.8--
Flow direction θ = 180°
-2.3-2.5-1.3-2.0-0.8-1.2--
15°-2.5-2.8-1.3-2.0-0.9-1.2--
30°-1.1-2.3-0.8-1.5-0.8--
45°-0.6-1.3-0.5-0.7--
60°-0.5-1.0-0.5-0.5--
75°-0.5-1.0-0.5-0.5--
Flow direction θ = 90°
FhighFlow
cpe,10cpe,1cpe,10cpe,1
-2.1-2.6-2.1-2.4-1.8-2.0-0.6-1.2-0.5
15°-2.4-2.9-1.6-2.4-1.9-2.5-0.8-1.2-0.7-1.2
30°-2.1-2.9-1.3-2.0-1.5-2.0-1.0-1.3-0.8-1.2
45°-1.5-2.4-1.3-2.0-1.4-2.0-1.0-1.3-0.9-1.2
60°-1.2-2.0-1.2-2.0-1.2-2.0-1.0-1.3-0.7-1.2
75°-1.2-2.0-1.2-2.0-1.2-2.0-1.0-1.3-0.5
1) Linear interpolation of the intermediate values is allowed, provided that the sign does not change. The value of 0.0 is given for the interpolation.
2) For the flow direction θ = 0° and the inclination angles α = +5° to +45°, the pressure changes very quickly between positive and negative values. Therefore, the positive as well as the negative external pressure coefficient is given for this area. For such roofs, both cases (pressure and suction) have to be considered separately by considering first, only the positive values (pressure), and second, only the negative values (suction).

#### Duopitch roof

A roof shape consisting of two roof surfaces inclined in opposite directions that intersect at the upper horizontal edge in the roof ridge is called a duopitch roof. This geometry has its own aerodynamic effects on the load application areas [1], [2].

AreaFGHIJ
Flow direction θ = 0°2)
Inclination angle α1)cpe,10cpe,1cpe,10cpe,1cpe,10cpe,1cpe,10cpe,1cpe,10cpe,1
-1.7-2.5-1.2-2.0-0.6-1.2-0.6+0.2
+0.0+0.0+0.0-0.6
15°-0.9-2.0-0.8-1.5-0.3-0.4-1.0-1.5
+0.2+0.2+0.2+0.0+0.0+0.0
30°-0.5-1.5-0.5-1.5-0.2-0.4-0.5
+0.7+0.7+0.4+0.0+0.0
45°-0.0-0.0-0.0-0.2-0.3
+0.7+0.7+0.6+0.0+0.0
60°+0.7+0.7+0.7-0.2-0.3
75°+0.8+0.8+0.8-0.2-0.3
Flow direction θ = 90°
-1.6-2.2-1.3-2.0-0.7-1.2-0.6--
15°-1.3-2.0-1.3-2.0-0.6-1.2-0.5--
30°-1.1-1.5-1.4-2.0-0.8-1.2-0.5--
45°-1.1-1.5-1.4-2.0-0.9-1.2-0.5--
60°-1.1-1.5-1.2-2.0-0.8-1.0-0.5--
75°-1.1-1.5-1.2-2.0-0.8-1.0-0.5--
1) For the flow direction θ = 0° and the inclination angles α = -5° to +45°, the pressure changes very quickly between positive and negative values. Therefore, the positive as well as the negative value is indicated. For such roofs, four cases have to be considered, where the smallest or largest value for areas F, G, and H is combined with the smallest or largest values for areas I and J. Positive and negative values cannot be mixed on a roof surface.
2) For roof inclinations between the indicated values, linear interpolation is allowed, provided that the sign of the pressure coefficients does not change. For inclinations between α = +5° and -5°, the values for flat roofs have to be used according to DIN EN 1991-1-4 plus Chapter 7.2.3. The value of zero is given for the interpolation.

#### Dipl.-Ing. (BA) Andreas Niemeier, M.Eng.

Mr. Niemeier is responsible for the development of RFEM, RSTAB, and the add-on modules for tensile membrane structures. Also, he is responsible for quality assurance and customer support.

#### Reference

 [1] Eurocode 1: Actions on structures - Part 1-4: General actions - Wind actions; German version EN 1991-1-4:2005 + A1:2010 + AC:2010 [2] National Annex - Nationally determined parameters - Eurocode 1: Actions on structures - Part 1-4: General actions - Wind actions; EN 1991-1-4/NA:2010-12 [3] Albert, A. (2018). Schneider - Bautabellen für Ingenieure mit Berechnungshinweisen und Beispielen (23rd ed.). Cologne: Bundesanzeiger.

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