M. To add an IS-875 (Part 3): 2015 wind load

To add a wind load definition and load cases for the Indian IS-875 (Part 3): 2015 code, use the following procedure.

This procedure is used to generate a series of wind intensity vs. height values based on the procedure of the IS-875 (Part 3): 2015 code. The wind intensity will either be the net pressure coefficient or the force coefficient multiplied by the design wind pressure, based on the user selection.

Note: This procedure may be used for Rectangular Clad Buildings and Unclad Frame Buildings. The Load & Definition dialog can be used to also create definitions for Signs or Lattice Tower structures.

Notes: The following limitations pertain to generation of IS 875 (Part 3) : 2015 wind loads:

The wind profile for the loading is calculated based on Y ordinate set to 0 rather than lowest point of the building. Thus, the topographical factor (k3) may not be relevant and should be applied with caution.
Only Y-up models may be used (that is, the SET Z UP command may not be used).
The resulting wind load intensity vs. height values generated for IS 875 (Part 3) : 2015 are in metric, regardless of the current input units.
The dynamic effect of wind load is not considered.
Cl. 6.3.2.4 and Annex B for changes in terrain categories (Table 3 Fetch and Developed Height Relationships) are not considered.
For rectangular buildings, the frictional drag per Cl. 7.4.1 is not considered.
For free standing walls and hoarding type structures, the check for oblique wind loads per Cl. 7.4.2.3 is not considered.
For lattice tower structures, the corner wind effect on square lattice towers with flat-sided members per Cl. 7.4.3.5(b) is not considered.

On the Loading ribbon tab, select Wind Load Generator > IS 875 (Part 3) : 2015 in the Load Generation group.

The Wind Load Generation - IS 875 (Part 3) : 2015 dialog opens.
Specify the General Information for the wind load:
1. Select the Type of Structure from the drop-down list.
  - Rectangular Clad Building
  - Unclad Frame Building
  The parameters on the Wind Load Data tab update based on this selection.
2. (Optional) Type the Structure Height and Ground Level Elevation values. The default values are taken as the maximum Y coordinate of the structure and zero, respectively.
3. Type the Height Interval for Intensity to use for developing the height vs. intensity values for the wind load definition. The default value is 5 m.
4. (Optional) Check the Custom option for the Starting Loadcase No and then type a load case number for the first generated wind load case. The default is the next load cases number after the highest load cased in the model.
Specify the Wind Parameters for the wind load:
1. Specify the Basic Wind Speed (Vb) by either:
  
  selecting a city from the drop-down list
  
  or
  checking the Custom option and typing the basic wind speed in m/s
2. Specify the Risk Factor Coefficient (k1) by either:
  
  selecting the class of structure the drop-down list
  
  or
  selecting Custom from the drop-down list and typing the value for k1
3. Type the Topography Factor (k3) to use.
4. Specify the Importance Factor (k4) by either:
  
  selecting the importance category from the drop-down list
  
  or
  selecting Custom from the drop-down list and typing the value for k4
5. Specify the Aerodynamic Roughness Height (z0i) by either:
  
  selecting the Terrain Category from the drop-down list
  
  or
  selecting Custom from the drop-down list and typing the value for z0i. The values of the Height Factor (k2) may also be entered for each height interval in the wind intensity data table for the custom option.
6. Type the Wind Directionality Factor (Kd).
7. Type the Area Averaging Factor (Ka).
8. Type the Combination Facotr (Kc).
The wind intensity table and graph update with the resulting values.
Select the Wind Load Data tab.

Enter the wind load data based on the selected structure type

Rectangular Clad Buildings

Setting

Description

Wind Direction

Select one of the global directions for the wind: +X, -X, +Z, or -Z.

Building Dimensions

By default, the dimensions are calculated based on the minimum and maximum global coordinates of the structure along the X, Z, and Y global axes, respectively. You may also type the Length Along X (Lx), Length Along Z (Lz), and Height (H) dimension values.

Use Coefficient Factor

Select the option for evaluating the coefficient factor:

None - no coefficient factor is multiplied with wind intensity value.
Pressure Coefficient - select this option to specify parameters to calculate the pressure coefficients on building face per Cl. 7.3.2 and 7.3.3.
Force Coefficient - select this option to directly specify the Force Coefficient Factor (Cf) on the building per Cl. 7.4.2.

Pressure Coefficient on Face

Use these options when the Use Coefficient Factor option selected is Pressure Coefficient:

Setting	Description
Wind on Face	Select the face of the structure (as labeled in the diagram in the dialog) to which this wind definition is applicable.
External Pressure Coefficient (Cpe)	Select to use either the Code Calculated or Custom (manually typed) value of Cpe. If the maximum range of l/w from Tabe 5 of IS 875 (Part 3): 2015 is exceeded, then the upper values of Cpe for 3/2 < l/w < 4 are still used. You may also enter custom Cpe values.
Internal Pressure Coefficient (Cpi)	Check this option to include the Internal Pressure Coefficient (Cpi).
Building Permeability	Select the range of openings to overall building exterior from the drop-down list. This is used to select the Cpi and Cpnet values per Clause 7.3.2.1 of IS-875. Alternatively, select Custom from the drop-down list to type a value to use for Cpi
Use (-) Cpi	Check this option to use negative Internal Pressure Coefficient (Cpi) value to calculate Net Pressure Coefficient (Cpnet)

Force Coefficient on Building

Type the Force Coefficient (Cf) value as per clause 7.4.2 of IS-875 when the Use Coefficient Factor option selected is Force Coefficient.

Unclad Frame Building

Setting	Description
Frame Type	Select if the structure contains a Single frame or Multiple frames spaced along the wind direction. For either frame type, you must specify the Solidity Ratio (Φ) at each height interval of the structure in the frame information table. For multiple frames, you must also specify the Effective Solidity Ratio (Φ_e) at each height interval. Providing the Solidity Ratio (Φ) for a single frame at the given height interval, the force coefficient (Cf) will be selected according to the user provided solidity ratio values from Table 31 of IS 875 (Part 3) : 2015 by interpolation. Providing the Effective Solidity Ratio (Φ_e) for multiple frames at the given height interval, the shielding factor (H) will selected according to the user provided effective solidity ratio values from Table 32 of IS 875 (Part 3) : 2015 by interpolation.
Member Profile Cross Section	Select if the cross section profile of the frame(s) is Flat Sided or Rounded. For rounded members, you must also specify the Diameter of the members used at each height interval in the frame information table.
Frame Spacing Ratio	For Multiple frames, type the frame spacing ratio as indicated in Table 32 (Clause 7.4.3.4) of IS-875.
Force Coefficient	The intermediate values along with the calculated values are displayed in the table. If the Custom option is selected, then you must specify the Force Coefficient (Cf) values at each height interval. For multiple frames in the Custom option, you must also specify the Shielding Factor at each height interval in the table.

Select the method of assigning the wind loads to the model:

Type of Structure	Assignment method options
Rectangular Clad Building	There are four method of assignments for the generated load cases: Automatic - The program will select faces A, B, C, and D. The list of members in this selection are displayed in the read-only Member List cell. Group - An existing group name is used to define the members which make up the building wind faces. Member List - Each face is defined by typing a list of members. Range - Type the minimum and maximum values for the global coordinates (in the units shown) for each face. Check the Exclude option for an row to exclude assigning wind loads to this building face.
Unclad Frame Building	There are three method of assignments for the generated load cases: Group - An existing group name is used to define the members which make up the frames. Member List - Each frame is defined by typing a list of members. Range - Type the minimum and maximum values for the global coordinates (in the units shown) for each frame. Check the Exclude option for an row to exclude assigning wind loads to this frame.

Type of Structure

Assignment method options

Rectangular Clad Building

There are four method of assignments for the generated load cases:

Automatic - The program will select faces A, B, C, and D. The list of members in this selection are displayed in the read-only Member List cell.
Group - An existing group name is used to define the members which make up the building wind faces.
Member List - Each face is defined by typing a list of members.
Range - Type the minimum and maximum values for the global coordinates (in the units shown) for each face.

Check the Exclude option for an row to exclude assigning wind loads to this building face.

Unclad Frame Building

There are three method of assignments for the generated load cases:

Group - An existing group name is used to define the members which make up the frames.
Member List - Each frame is defined by typing a list of members.
Range - Type the minimum and maximum values for the global coordinates (in the units shown) for each frame.

Check the Exclude option for an row to exclude assigning wind loads to this frame.

Click Generate.

For a rectangular clad building type, a series of four load definitions and eight load cases are generated, each with four load items for each of the four wind directions. The definitions contain the load vs design wind pressure values.

For an unclad frame type, the definitions include height vs intensity. For this case, the intensity generated is the force coefficient (Cf or Cf') multiplied by the design wind pressure (Pd).

The wind load generation data is stored within the STAAD input file.

You may edit the height vs design wind pressure values generated by clicking Edit in the Load & Definition dialog when one of the wind definition intensities is selected. After editing and regenerating the wind loads, all previous wind definitions and associated loads are deleted.