Snow load

Contents

• Concept
• Define climate load panel
• Define edge object
• Edge object properties
• Panel properties
• Generate snow load

Concept

Snow macro (added in FEM-Design 21) allows for automatic snow load generation according to EN1991-1-3 and is located in the Load tab > Macro panel.

Define climate load panel

The first step of automatic load generation is definition of the climate load panels by selection of one of the predefined roof types and their associated structures. The roof type definition should be done with the Climate load panel command.

Define edge object

Edge objects can be defined on roof edges or on parts, in two ways: manually or automatically. Following edge objects can be defined:

• connected roofs which models multi-span roofs, 
• attic walls, 
• tall building (parts), 
• snow fences, 
• snow overhangs. 
  
   

Manual edge object creation

In Snow load dialog, select Define edge object and Draw edge object, then start Default settings and select the edge object type at Snow load panel edge object dialog. Afterwards, select the the corresponding edge of a climate load panel, and finally define the segment (whole or part of the edge). 

Automatic edge object creation

In Snow load dialog, select Generate edge object(s) automatically and check (or modify) the settings in the Snow load panel edge objects wizard dialog.

The wizard can automatically recognize the following types of edge objects

• multi-span roofs, 
• attic walls, 
• tall building parts next to a roof, and 
• bottom, horizontal roof edges to generate snow fences or snow overhangs, if we request for them.
  
   

Note:

• By default, edge object is set to None for all edges.
• Edge objects are mutually exclusive to each other. 
• Edge objects can be assigned only to straight edges.
• Obstacles can be only at the edge of the roofs.
• By making splits in a roof region (that was drawn, not associated to a structural element), obstacles (practically attic walls) can be placed inside the roof, too. 
  
   

Edge object properties

The properties of the defined edge objects can be modified with the Edge object properties tool.

• If two edge objects next to each other have exactly the same attributes, they are merged automatically. 
• Edge objects cannot be deleted, instead they must be set to None. 
  
   

Panel properties

Roof’s thermal property influences the snow load intensity. We can specify thermal coefficient values for each climate load panel in the Panel properties dialog.

For some National Annexes, an additional dialog is available that offers help determining Ct. 

Generate snow load

In the Snow load generation dialog, first the location dependent characteristic snow load on the ground and the exposure coefficient need to be defined.

Depending on the National Annex, additional dialogs may be available to offer help for determining these values.

Furthermore, we can choose whether we want drifted and exceptional snow load cases to be generated. Psi values for the generated load group(s) also can be set. 

By pressing OK, snow loads are generated and assigned to one of the following load cases:

• Snow load, undrifted
• Snow load, drifted, global¹, wind direction X+
• Snow load, drifted, global¹, wind direction X-
• Snow load, drifted, global¹, wind direction Y+
• Snow load, drifted, global¹, wind direction Y-
• Snow load, drifted, local², wind direction X+
• Snow load, drifted, local², wind direction X-
• Snow load, drifted, local², wind direction Y+

• Snow load, drifted, local², wind direction Y-
   

¹Global loads act on the whole roof
²Local loads act on a part of the roof, eg. at attic walls and tall buildings. These load cases contain only that part of the load defined by Eurocode that is above the global load of the roof.

 

Note: Some of the above mentioned load cases may not be generated if they are not necessary.

The above mentioned load cases are assigned to a load group with the following Custom relationship between them:

• Undrifted load case always acts alone.
• Drifted global load cases can act alone or together with an other global load case that is not parallel with them.
• Drifted local load cases always act together with the global load case in the same direction, but they can also be combined with any other local load case(s).
  
  
   

If  ‘Simplified relationships between snow load cases’ option was checked, only relationships which contain all the local load cases are generated.