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.
- 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.
- For unclad frame builds, this feature can be used to generate the wind load in one direction only (for example, +X, -X, +Z, or -Z). To generate wind load in multiple directions, the manual workflow must be used.
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On the Loading ribbon tab, select in the Load Generation group.
The Wind Load Generation - IS 875 (Part 3) : 2015 dialog opens. -
Specify the General Information for the wind load:
-
Select the Type of Structure from the drop-down
list.
- Rectangular Clad Building
- Unclad Frame Building
- (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.
- 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.
- (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.
-
Select the Type of Structure from the drop-down
list.
-
Specify the Wind Parameters for the wind load:
-
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
-
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
- Type the Topography Factor (k3) to use.
-
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
-
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.
- Type the Wind Directionality Factor (Kd).
- Type the Area Averaging Factor (Ka).
- Type the Combination Facotr (Kc).
-
Specify the Basic Wind Speed (Vb) by
either:
- Select the Wind Load Data tab.
-
Enter the wind load data based on the selected structure type
Rectangular Clad Buildings
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 (Φe) 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 (Φe) 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.
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.
- Click Generate.
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).
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.