Using the EC1991 1-4 Wind Wizard with BREVe data

Tekla Structural Designer
Modified: 17 May 2023
2024
Tekla Structural Designer

Using the EC1991 1-4 Wind Wizard with BREVe data

This topic runs through each page of the wizard if working to the UK or Ireland National Annex when using the BREVe data source.

Data Source (BREVe) page

 
Property Description
Data Source When working to either the UK or Ireland National Annex the following wind data sources are available:
The remainder of this section assumes one of the above BREVe options has been selected.
Consider Orography If you select this checkbox, then the orographic data, (either recovered by BREVe for the site or manually entered), is used to determine the Orography Factor co as clause A.3. When calculating calt, the altitude of the upwind base of the orographic feature is used for each wind direction considered. Otherwise the orographic data is ignored, co is 1.0 for all heights and calt is the same for all directions, using the Site Altitude.
Consider Tall Neighbouring Structures If the conditions in clause A.4 are met, then the wind loads need to be based on height zn, see equation (A.14). With this checkbox selected, you are able to enter sufficient data to check if this applies. If applicable, then zn will be used as the reference height for all wall panels and roof panels in the model.
Consider Obstructions With this checkbox selected, the obstruction data, (either defaulted by BREVe depending on the roughness category for the site or entered manually), is used to determine the Displacement Height, hdis as (A15) in clause A.5. Otherwise the obstructions are ignored and hdis is taken as zero.
Apply Open Structure Wind Load

With this checkbox selected, additional wind forces are applied to those members, ancillaries and equipment that have the Apply Open Structure Wind Load property selected in their properties.

Note: This option is only displayed if at least one entity has been selected to have open structure wind load applied.

For more information, see: Open structure wind loads

Assuming you have chosen to use BREVe Grid Ref data, clicking Next takes you to the BREVe location page.

BREVe location page

 
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.
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.

Note:

In the situation where the building orientation is not known, the peak wind velocity and peak velocity pressures are determined for all compass points around the building (full 360degs). The maximum velocity and pressure from all these directions are then applied to each required direction round the building.

In doing so the value of the direction factor, cdir used is that for a particular compass direction (as are the orography and roughness/obstructions factors).

When the location of a building is known but its orientation is not, the above factors are all known for each compass direction - the fact that the building orientation is not known relative to the compass then means it is sensible to look for the maximum around the full compass and apply the maximum conservatively to all building directions.

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:


For a more detailed example and explanation, see this support article.

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).

Click Next to move to the Basic Data (BREVe) page.

Basic Data (BREVe) page

This page allows you to review the site details taken from the BREVe database.

 
Property Description
Site Altitude, A The basic site altitude of your model's base.
Air Density Air density at the site.
Ground Level If for some reason, the level 0.0m in the Tekla Structural Designer model does not correspond to the ground level, for example a site datum may have been used rather than a building datum, then this field allows you to set the appropriate value so that the reference heights can be calculated correctly.
Fundamental Basic Wind Velocity (vb,map) The value required is defined as "the characteristic 10 minutes mean wind velocity, irrespective of wind direction and time of year, at 10m above ground level in open country terrain with low vegetation such as grass and isolated obstacles with separations of at least 20 obstacle heights", but is the value before the altitude correction is applied. (Valid range 1.0 to 1000 m/s). See Clause 4.2 and NA.2.4
Season Factor, cseason Valid range 0.01 to 10.0 - default 1.0.
Probability Factor, cprob Valid range 0.01 to 10.0 - default 1.0.
Default Height for Internal Pressure (zi) Clause 7.2.9 (7) implies that all internal pressures should be calculated using a single reference height, (zi), defaulting to the height of the structure. Leaving Use Building Height checked ensures that the value is automatically updated if the geometry of wall panels or roof panels changes.
Click Next to move to the Roughness and Obstructions (BREVe) page.

Roughness and Obstructions (BREVe) page

This page of the wizard automatically defaults the data for ground roughness and obstructions for you.

 
Property Description
Terrain Category Options available are:
  • Sea - this setting is for sites where the distance to sea is between 0 and 1 km, not for offshore sites. As the worst case must be for wind blowing across the sea, there is no need to specify data for upwind buildings or distance in town.
  • Country - the worst case must be for wind blowing across open ground, there is no need to specify data for upwind buildings or distance in town,
  • Town - for this category you need to specify data for upwind buildings and distance to the edge of the town, so the relevant fields are active. If you want to ignore obstructions, then you need to enter a zero value for have For this category, the Upwind distance from edge of town to site cannot be greater than the Upwind distance from sea to site.
Click Next to move to the Orography (BREVe) page.

Orography (BREVe) page

 

Note: When using Breve Data you should really leave it up to the system as to whether orography is significant or not. If you chose not to consider orography, the actual factor is not applied; however the site data is still displayed to allow you to check that your decision to ignore it is reasonable.
 
Property Description
Orographic Feature Options available are:
  • None - no feature, i.e. co = 1.0.
  • 2D Escarp - Cliffs and Escarpments,
  • 3D Hill - Hills and Ridges.
(See Clause A.3)
Base Altitude, A This value is used to calculate Calt instead of the Site Altitude because the Orography is significant.
Note: Calt will be calculated at ze for each wall and roof panel, not zs.
Effective Crest Height, H Effective crest height of the feature. (Figures A.2 & A.3)
Upwind Slope, Lu Actual length of the upwind slope in the wind direction. (Figures A.2 & A.3)
Downwind Slope, Ld Actual length of the downwind slope in the wind direction. (Figures A.2 & A.3)
Horizontal Distance to Crest, x Distance upwind or downwind from the crest to the building site. (Figures A.2 & A.3)

If on the Data Source page you chose to consider tall neighbouring structures, clicking Next takes you to the Tall Neighbouring Structure, otherwise it takes you to the Results (BREVe) page.

Tall Neighbouring Structure page

For all methods, if Consider Tall Neighbouring Structure was checked, the penultimate page allows you to determine if tall neighbouring structures affect the design of this structure. (Clause A.4)

Otherwise, the wizard will proceed directly to the Results page and the actual heights of wall and roof panels are used throughout.

The parameters, Height of Tall Neighbour, hhigh, Largest Horizontal Dimension of Tall Neighbour, dlarge and Distance to Tall Neighbour, x are all as described on Figure A.4 of the code.

 
Property Description
Average Height of Neighbours, have The default is calculated from the BREVe data or the values entered for the Roughness & Obstructions. If Override calculated dimension is cleared, then the value will be updated whenever the wizard is run, otherwise the user-value is used. (Figure A.4)
Height of this structure, hlow The field is for information only - difference between top of highest wall / roof panel and ground level in the model. (Figure A.4)

Click Next to got to the Results (BREVe) page.

Results (BREVe) page

The final page of the wizard is a summary of the results - peak velocity pressure ranges.

Initially there are 4 orthogonal wind directions relative to the Tekla Structural Designer axes, (not geographical North), but you are able to update this 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.

You are able to use the <<Details...>> button to obtain additional information, including the values of intermediate factors used in the calculations.

BREVe Data

BREVe determines the parameters required to calculate qp(z) for each height in the building at 30° intervals, (0° to 330°).

For each required wind direction the worst case qp is used for each height, based on splitting the difference to the next direction, with a maximum of ±45 degrees. Within these ranges qp 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 Peak Velocity Pressures calculated for each reference height for each 30° sector.

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.

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