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Snow load

The calculations are performed in accordance with the standard EN 1991-1-3. These procedures are included:

Snow load

The snow load on the roof is calculated using formula (5.1):

where is:

μi

  • Snow load shape coefficient, values are described below

sk

  • Characteristic value of the snow load on the ground. It can be entered with the help of snow map or entered manually.

Ce

  • Exposure coefficient

Ct

  • Thermal coefficient

The exposure coefficient Ce is obtained according to the selected topography. The values are in accordance with the table 5.1 of EN 1991-1-3:

Topography

Coefficient Ce

Windswept

0.8

Normal

1.0

Sheltered

1.2

Mono-pitched roofs

The shape coefficient μ1 in accordance with figure 5.1 of EN 1991-1-3 is used for mono-pitched roofs. The value of this coefficient depends on the roof pitch:

Shape coefficients μ1 and μ2

The value of the coefficient μ1 is equal to 0.8 if the sliding of the snow is prevented (snow fences etc.).

Duo-pitched roofs

The shape coefficient μ1 in accordance with figure 5.1 of EN 1991-1-3 is used for duo-pitched roofs. The figure showing the dependency of the coefficient value and the roof pitch is shown in the chapter "Mono-pitched roofs". The value of the coefficient μ1 is equal to 0.8 if the sliding of the snow is prevented (snow fences etc.). These three load cases are created for duo-pitched roof in accordance with chapter 5.3.3:

Load cases for duo-pitched roofs

Multi-span roofs

The shape coefficients μ1 and μ2 in accordance with figure 5.1 of EN 1991-1-3 is used for multi-span roofs. The figure showing the dependency of the coefficients values and the roof pitch is shown in the chapter "Mono-pitched roofs". The value of the coefficient μ1 is equal to 0.8 if the sliding of the snow is prevented (snow fences etc.). These load cases for undrifted and drifted snow are created in accordance with chapter 5.4:

Load cases for multi-span roofs

Cylindrical roofs

The shape coefficient μ3 in accordance with figure 5.3.5(1) of EN 1991-1-3 is used for cylindrical roofs. Its value is depending on ratio h/b, where h is the height of cylindrical roof and b is the roof span. The coefficient values are shown in the following figure:

Coefficient μ3

The snow load is considered in the parts where the roof pitch is smaller than 60° according to the chapter 5.3.5. Following load cases are considered:

Load cases for cylindrical roofs

Following load case based on the standards CSN/STN 73 0035 is considered additionally for Czech and Slovak nation annexes :

Drifted snow in accordance with Czech and Slovak national annex

This load case is considered under these circumstances:

  • This scheme will be considered for all cylindrical roofs with ratio h/b greater than 1/8.
  • This scheme will be considered for all cylindrical roofs in snow areas IV and V.

Roofs abutting and close to taller construction works

The shape coefficients for these roofs are calculated according to the chapter 5.3.6 of EN 1991-1-3. These load cases are considered:

Load cases for abutting roofs

The shape coefficients μ1 and μ2 are calculated using following formulas:

Where is:

μs

  • Snow load shape coefficient due to sliding of snow from the upper roof

μw

  • Snow load shape coefficient due to wind

The value of the coefficient μs is equal to 0 for α ≤ 15°. The following formula is used for α > 15°:

Where is:

μ

  • The shape coefficient for the upper roof, the value is 0.8

bs

  • Horizontal distance of the roof ridge and fascia

ls

  • Length of drifted snow

The coefficient μw is calculated using following formula:

Where is:

b1

  • Span of upper roof

b1

  • Span of lower roof

h

  • Vertical distance between lower roof and fascia of upper roof

γ

  • The weight density 2kN/m3

The drift length ls is calculated using formula

where is:

h

  • Vertical distance between lower roof and fascia of upper roof

The drift length ls is limited by the interval <5m;15m>. The shape coefficient is calculated using lineat interpolation between μ1 and μ2 for buildings where ls is greater than span of lower roof.

Drifting at projections and obstructions

The following loading figure is created in accordance with the chapter 6.2 of EN 1991-1-3 for the roofs with obstructions:

Loading figure for obstructions

The coefficients μ1 and μ2 are calculated using formula (6.1):

where is:

γ

  • The weight density of snow 2kN/m3

h

  • The obstruction height

sk

  • The characteristic value of the snow load on the ground.

Snow overhanging the edge of a roof

Snow overhanging the edge of a roof is calculated in accordance with chapter 6.3. Following formula is used:

where is:

k

  • The coefficient to take account of the irregular shape of the snow

s

  • The most onerous undrifted load case appropriate for the roof

γ

  • The weight density of snow 3kN/m3

The coefficient k is calculated using formula:

But following expression has to be fulfilled:

where is:

d

  • The depth of the snow layer

Snow load on snowguards and other obstacles

Snow load on snowguards and other obstacles is calculated according to the chapter 6.4 of EN 1991-1-3. Following formula is used:

where is:

s

  • The snow load on the roof relative to the most onerous undrifted snow load case

b

  • The width on plan (horizontal) between the guards or obstacles

α

  • The roof pitch

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