Using the BS6399-2 Wind wizard
Method page
This page allows you to specify the method that you want to use to calculate the wind loading on the building, and the source of the wind data.
| Property | Description |
|---|---|
| Wind code | There are two calculation methods available:
|
| Data Source | There are three options for the source of the wind
data:
|
BREVe location page
This page allows you to define the location of the site using the BREVe database, and to define various options to be considered in the wind analysis. Once you have retrieved the data for a site from the BREVe database you can edit these to take account of your local knowledge of the site.
| Property | Description |
|---|---|
| Grid Ref. | This shows the grid reference of the site which you have picked through BREVe, irrespective of the method you use to define the site location. |
| Site Altitude, A | You are able to override the altitude determined by BREVe by entering a value directly here. |
| Air Density | You need to enter air density at the site. |
| Ground Level in model | If for some reason, the level 0.0m in the Tekla Structural Designer model does not correspond to the ground level, for example you have used a site datum rather than a building datum, then this field allows you to set the appropriate value so that the reference heights for the wind can be calculated correctly. |
| Orientation of building known | If you know the orientation of the building with
respect to North, then you can define this information by checking
this box. You can then define a value which relates the building
direction axes of your Tekla Structural Designer model to geographic
north. If you want to use the Hybrid method, then you must know and define the building orientation. For the Standard method, the orientation is not essential. If you don't define the building's orientation then North is not shown in graphics views and all the Sd values are set to 1.0. |
| Orientation of North |
The orientation of North is defined using the same convention as is applied to the orientation of the Building Direction Arrows. This can best be understood by reference to a couple of examples: In the first example the building axes are aligned in the default directions (Dir 1 = 0° = Global X), and the orientation of North has been set to 315°. The resulting relation between the building axes and North is as shown below: 
In the second example the building direction has been input with Dir 1 = 30° and the orientation of North has been set to 250° In this case the building axes are related to North as shown below: 
|
| Consider Topography | If you select this checkbox, then BREVe uses the
topographic data it recovers for the site and determines the
Altitude Factor Sa in accordance with Clause 2.2.2.2.3.
Otherwise the topographic data is ignored and Sa is
calculated in accordance with Clause 2.2.2.2.2. Note: In theory the
topography could be significant for some directions and not for
others.
|
| Consider Obstructions | With this box checked, BREVe uses the obstruction data it recovers for the site and determines the Effective Height He as defined in Clause 1.7.3.3. Otherwise the obstructions are ignored and He is taken as Hr - see Clause 1.7.3.2. |
| Using BREVe, there are 2 methods available for you to define the site location: | |
| Site By Ref... | You can define the grid reference of the site. You define this either as a national grid reference, or by specifying the Easting and Northing information for the site. There are several Internet based tools available which allow you to determine the Ordnance Survey grid reference from a postcode or given location, for example www.streetmap.co.uk or www.multimap.co.uk |
| Site By Map... |
You can pick the site from a Land / Town Map, • You can pick the site from a Orography Map. • You can pick the site from a ground roughness Category Map, The site data is analysed fully by BREVe. Parameters are either set automatically but conservatively (Safe parameters within a 1 km square). |
Other location page
This page allows you to define the site details when information is not available from the BREVe database, for instance if it is located outside of the UK.
| Property | Description |
|---|---|
| Site Altitude | You are able to override the altitude determined by BREVe by entering a value directly here. |
| Air Density | You need to enter air density at the site. |
| Ground Level in model | If for some reason, the level 0.0m in the Tekla Structural Designer model does not correspond to the ground level, for example you have used a site datum rather than a building datum, then this field allows you to set the appropriate value so that the reference heights for the wind can be calculated correctly. |
Other Standard Wind data page
If you select the Standard Method and Other Data Source, then the next page of the Wizard allows you to enter the wind data yourself.
| Property | Description |
|---|---|
| Basic Wind Speed | You need to enter the basic wind speed at the site. |
| Ground Roughness | The following settings are available:
|
| Consider Topography / Altitude Factor, Sa | When this box is checked, you need to use your own topographic data and determine the Altitude Factor Sa in accordance with Clause 2.2.2.2.3. Otherwise Sa is calculated in accordance with Clause 2.2.2.2.2 and you are not able to override it. |
| Season factor | You need to enter the season factor (default 1.0). |
| Probability factor | You need to enter the probability factor (default 1.0). |
Other Hydrid Wind data page
If you select the Hybrid Method and Other Data Source then the next page of the Wizard allows you to enter the data for ground roughness and obstructions yourself. However, most of the data is then dependent on the wind direction, so you must also make your choice of wind directions on this page.
| Property | Description |
|---|---|
| Direction |
Initially there are 4 orthogonal wind directions relative to the Tekla Structural Designer axes, (not geographical North), but you are able to update these using the Dir. buttons and / or changing the direction value as required. (Note: Minimum 1° difference between directions). At least one direction must be defined. Each row of the grid operates in a similar manner to the relevant fields of the Other Standard Wind data page |
| Consider Topography / Altitude Factor, Sa |
Reference 8 (section 4.10, page 26) essentially recommends using the Standard Method approach to topography even for the Hybrid Method. So, when calculating the Terrain and Building Factor, Sb, we ignore the effects of topography, that is we take Sh = 0. When the box is checked, you need to use your own topographic data and determine the Altitude Factors Sa as defined in Clause 2.2.2.2.3. Otherwise Sa is calculated as defined in Clause 2.2.2.2.2 and you are not able to override it. Note: In theory
the topography could be significant for some directions and
not for others.
|
| Season factor | You need to enter the season factor (default 1.0). |
| Probability factor | You need to enter the probability factor (default 1.0). |
Results
The final page of the wizard is a summary of the results - peak velocity pressure ranges.
BREVe Standard Method
Initially this method creates 4 orthogonal wind directions relative to the Tekla Structural Designer axes, (not geographical North). Except for the Hybrid Method with Other Data, you can update the wind directions either by using the 'Dir.' buttons or by changing the direction value as required.
Separately, for each relevant parameter of the Standard Method, BREVe determines the worst case over all its 30° sectors. If the orientation of the building is not known, then Sd is taken as 1.0 for all directions. Otherwise we determine the worst case Sd for each direction. You cannot override the system value in either case.
The worst case Sd is based on splitting the difference to the next direction, with a minimum of ±15° and maximum of ±45°. Within these ranges Sd is interpolated.
For each reference height in the model, we then calculate the site wind speed (Vs using equation 8) and thus the effective wind speed (Ve using equation 12) and the dynamic pressure (qs using equation 1) for each direction. When calculating actual loads on walls and roofs, we use the qs value for the relevant reference height, but the Results page only shows the maximum values for each direction.
The Vortex view shows the effective wind speed calculated for each reference height for each 30° sector. Since a single worst case value is used for each parameter, the speeds for different sectors only differ due to Sd provided that the orientation of the building is known.
BREVe Hybrid Method
In this case, BREVe uses the directional method to determine the parameters required to calculate Vs using equation 8, for each height in the building at 30° intervals, (0° to 330°) taking the diagonal dimension `a' as the default 5.0m. (The size effect factor is applied when determining individual loads). We then use equation 27 to determine Ve and equation 16 for qs.
For each required wind direction the worst case Ve is used for each height, based on splitting the difference to the next direction, with a maximum of ±45 degrees. Within these ranges Ve is not interpolated.
Theoretically, it is possible for a quadrant to use different 30° directions for each height, so the critical wind direction is not displayed in the summary.
The Vortex view shows the effective wind speed calculated for each reference height for each 30° sector.
Other Standard Method
The calculation of Ve and qs are very similar to the BREVe Standard Method, (see above), except that the worst case data has been entered by you, and this page allows you to enter your own values for Sd.
As there is no data for each 30° sector, the Vortex view only shows the effective wind speed calculated for each reference height for each direction.
Other Hybrid Method
The calculation of Ve and qs are be very similar to the BREVe Hybrid Method, (see above), except that the data has been entered by you for each direction only so a direct calculation can be performed instead of taking the worst case over a range of sectors. Also this page allows you to enter your own values for Sd.
As there is no data for each 30° sector, the Vortex view only shows the effective wind speed calculated for each reference height for each direction.
Finishing the Wind wizard
When you click Finish, the wizard generates the wind zones for the entire building for each of the specified wind directions.
Before moving on you should take a moment to inspect the Wind Model status on the Project Workspace> Status tab, in order to check that no Limitations have been encountered.