IS 875 (Part 3) Wind Wizard

This topic discusses the wind wizard when using the IS 875 (Part 3) code.

To access this configuration of the Wind Wizard... the Wind Loading Code has to be set to IS 875 (Part 3).

Once the wall and roof panels are in place, you use the Wind Wizard... to define sufficient site information to calculate the effective wind speeds and dynamic pressures for the required wind directions and heights around the building.

The wizard permits you to create wind load for any wind direction, it is up to you to create those loads for the directions most appropriate to your structure.

Unless explicitly stated all calculations in the IS 875 (Part 3) Wind Modeller are in accordance with the relevant sections of IS:875 (Part 3) - 1987 Second Revision, Reaffirmed 2003 and including Amendments 1, 2 and 3.

Wind Options 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/Buttons	Description
Property
Data Source	The wind velocity calculations are automated, the data source for the calculations is either: Input directly for the worst case, Input directly for each direction.
Consider Topography	When this box is checked, you will be asked to enter your own topographic data later in the Wizard.
Consider Cyclonic Effect	Choose this option to have Cyclonic Effect considered.
Clad frame - correlated pressure and suction	Choose this option for correlated pressure and suction in clad frames.
Apply Open Structure Wind Load	With this box checked, 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
Buttons
Next	Click Next to go to the Basic Data page.

Basic Data page

Property/Buttons

Description

Property

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. This field also gives the lowest level to which wind load is applied in the structure.

Orientation

IS 875 defines the zones assuming the longest principal axis of the building is in the Tekla Structural Designer global Y direction. Due to the potential complexity of the model, Tekla Structural Designer will not attempt to determine the correct angle for the axes, (i.e. default is zero) but you are able to enter a more appropriate value.

Note: The convention for Wind Directions is: wind along global +X is 0 degrees; along global +Y is 90 ; along global -X is 180 ; along global -Y is 270.

Building Height Exposed to Wind

Calculated from difference between Ground Level and max level of roof and wind wall panels in the model, with 'override calculated' option to permit user editing.

Building Width and Building Length

The smallest bounding rectangle is determined (considered over all roof and walls only), relative to the given orientation. The building length defaults to the maximum and the width to the minimum of the two dimensions. The 'override calculated' options permit user editing.

Building Type

The following options are available:

Temporary
Low hazard to life
All general buildings
Important buildings

Building Class

This is determined as follows:

A if max(Building Height Exposed to Wind, Building Width and Building Length) < 20m
B if max(Building Height Exposed to Wind, Building Width and Building Length) 50m
C if max(Building Height Exposed to Wind, Building Width and Building Length) > 50m

Basic Wind Speed

You need to enter the basic wind speed at the site.

Probability factor

The value of the probability factor is determined from the following table (Table 1):


	Basic Wind Speed
Building Type	33	39	44	47	50	55
Temporary	0.82	0.76	0.73	0.71	0.70	0.67
Low hazard	0.94	0.92	0.91	0.90	0.90	0.89
All general	1.00	1.00	1.00	1.00	1.00	1.00
Important	1.05	1.06	1.07	1.07	1.08	1.08

Buttons

Click Next to go to the Terrain page.

Terrain page

The next page of the Wizard allows you to enter the terrain data.


Property/Buttons	Description
Property
Terrain Category	Category 1 = Exposed open terrain few obstructions (Default) Category 2 = Open terrain scattered obstructions Category 3 = Terrain with numerous closely spaced obstructions Category 4 = Terrain with numerous large high closely spaced obstructions
Fetch a distance	The velocity profile for a given terrain category does not develop to full height immediately and in this release Tekla Structural Designer only deals with a single terrain profile so you are warned if the velocity profile has not developed over the fill height of the building for the defined fetch.
Buttons
Next	Click Next to go to the Topography page.

Topography page

If Consider Topography was checked, then the next page of the Wizard allows you to enter the topography data.


Property/Buttons	Description
Property	None - no feature (default) 2D Escarp - Cliffs and Escarpments 3D Hill - Hills and Ridges
Feature Height, Z	Effective height of the feature.
Site dist from crest, X	Distance upwind or downwind from the crest to the building site (-ve = upwind).
Site height above mean ground level, H	Height above mean ground level.
Upwind Slope Length, L	Actual length of the upwind slope in the wind direction.
Downwind Slope Length	Actual length of the downwind slope in the wind direction.
Buttons
Next	Click Next to go to the Results page.

Results page

The final page of the Wizard is a summary of the results.

V_{z max} = V_b x k₁ x k₂ x k₃ (m/s)

p_z max = 0.6 x V_z max2 (N/m²)

The above is for the highest point in the building only.


Property/Buttons	Description
Buttons
Finish	When you click Finish, the Wind 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 issues have been encountered.