Limitations (EC1991-1-4 Wind wizard)
This page discusses the limitations to the wind wizard.
Throughout the development of the Wind wizard extensive reference has been made to the References and we consider it advisable that you are fully familiar with these before using the software.
In addition, because wind loading is complex and its application to general structures even more so, it is essential that you read and fully appreciate the following limitations in the software:
Geometry
Warning: You should seek specialist advice for building shapes that are not covered by the Standard - BS EN 1991-1-4:2005.
- EC1 1-4 does not treat downwind re-entrant corners as special cases - see BS6399 Clause 2.4.3.1 c). So, they are ignored in the software and no warnings are given.
- EC1 1-4 does not handle stepped profiles, or inset storeys - see BS6399 clauses 2.4.4.2 and 2.5.1.7. Hence the software does not handle them automatically, but does generate warnings if such cases are detected - so you can manually edit the zones according to your engineering judgement.
- Open sided buildings are beyond scope.
- Free standing walls and sign boards are not considered.
- Canopies are not considered.
- Exposed members are not considered, for example lattices, trusses......
- Barrel-vault roofs and domes are not considered
- Dominant Faces are not explicitly handled - Clause 7.2.9 (5). However, you can use Table 17 to calculate the necessary Cpi value or values and manually apply to a loadcase or individual zone loads.
Loaded areas
The difference between the loaded area of wall panels and roof panels defined at the centre-line rather than the sheeting dimension is ignored.
Wind direction
- All outward faces within 60 degs of being perpendicular to wind direction - loads applied as windward normal to face. All inside faces within 60 degs to wind direction - loads applied as leeward normal to face. All other faces considered as side.
- Orthogonal wind directions at the definition of the user.
Overall loads
- Lack of correlation of pressures between the windward and leeward sides. For Overall loadcases, the software automatically reduces the windward and leeward wall pressures only. EC1 1-4 and the UK NA both suggest that the reduction "may" be applied to roofs as well.
- Division by Parts rule for "slender" buildings -Clause 7.2.2 and Figure 7.4 - not applied.
- Friction Forces - Clause 5.3 (3), equation 5.7 and Clause 7.5.
During the "Update Zones" process, checks are performed to see if the effects can be disregarded, (Clause 5.3 (4)), and a 'Friction needed' warning is generated if not. When they cannot be disregarded you will need to manually model the friction forces as lateral loads in a separate loadcase and include them in your combinations.
Beneficial loads
- Asymmetric and Counteracting Pressures and Forces - Clause 7.1.2 and NA.2.23. Beneficial loads are not automatically removed - instead you are able to flag individual loads to be reduced to zero.
Singapore National Annex - Minimum horizontal loads
- The Foreword to the Singapore National Annex to EN 1991-1-4 Wind Actions has a minimum horizontal load requirement (1.5% characteristic dead weight). Therefore if this National Annex has been applied, it is the users responsibility to check that this requirement has been met (by ensuring that the horizontal component of the factored wind load is greater).
Finland National Annex
- We do not consider thermal inversions for buildings > 100m tall
Norway National Annex
- We do not consider the transition zones between changes in terrain category.
Wind loading on wall panels
Automatic zoning applies to all wall panels subject to the limitations described below:
- Vertical walls on rectangular buildings -Clause 7.2.2 - the assumption for wall wind forces is that the building is rectangular or close to being rectangular.
- Wall panels that are more than 15° from the vertical are outside the scope.
- Internal walls are not covered by EC1 1-4 and in any case are not automatically identified but you can manually edit the zones to apply the roof coefficient or otherwise as you see fit - see BS6399 Clause 2.4.3.2a.
- EC1 1-4 does not specify how to treat recesses in side walls - see BS6399 clauses 2.4.3.2 b) and 2.4.3.3 and 3.3.1.5. So, they are ignored but warnings are given.
Wind loading on roof panels
- Automatic zoning only applies to all triangular roof panels and quadrilateral roof panels that are not concave, i.e. all of the internal angles < 180°
- Special care should be taken for winds blowing on duopitch with slopes that differ by more than 5°. If the wind is blowing on the steeper slope (that is that the less steep slope is downwind of ridge), the downwind slope should be set to be a flat roof with mansard at eaves for this wind direction.
- Mansard and multipitch roofs are not detected automatically, although certain special cases can be handled if you set the appropriate type manually - see EC1991 1-4 Wind Zones.
- BS 6399 Table 8 curved and mansard eaves - zones start from edge of horizontal roof.
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Roof overhangs are not explicitly handled. It is suggested that you should define two separate roof panels - one forming the overhang and the other covering the inside of the building. For a small overhang, you can then manually define Cpi values to be the same as Cpe for the adjacent wall panel, (Clause 7.2.1 (3)). Reference 6, p45, implies that larger overhangs can be manually handled by using BS6399, Clauses 2.5.9.3 and 2.6.3, i.e. standard external coefficients for the top surface and Table 18 for the internal coefficients.
Note: The only slight issue here is that there are two sets of edge zones which will occupy a slightly larger area than strictly necessary.
Additional wind loads
There may be situations when you perceive a need to manually define loads that cannot be determined automatically. You can do this by defining additional wind loadcases to contain these loads and then include these with the relevant system generated loads in design combinations in the normal way.