IS-875 (Part 3): Wind Load dialog

Used to enter code parameters for generating wind intensity values based on the IS-875 (Part 3): Wind Load code

Opens when Generate is clicked in the Add New Wind Definitions dialog with the IS-875 (Part 3): Wind Load type selected.

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.

Common tab

Setting

Description

Year of Publication

Select the edition of the IS-875 code from the drop-down list.

2015

Structure Type

Select the type of structure from the drop-down list:

Rectangular Clad Building
Unclad Building
Free Wall and Hoardings
Lattice Tower

Maximum Height

By default, this dimension is taken as the maximum Y coordinate of the structure. You may also type a Height value.

Ground Level Elevation

(Read-only) Always assumed as zero.

Height Interval for Intensity

The vertical interval (along the global Y axis) for calculating the wind intensity.

Basic Wind Speed (Vb)

Setting	Description
Cities	Select the name of the location from the drop-down list to use the mapped wind speed per Annex A of IS 875 (Part 3) : 2015.
Use Custom	Check this option to type the wind speed directly in m/s.

Wind Parameters

Based on the selected or typed values, the Design Wind Speed is automatically calculated using the formula ( Vb × k1 × k2 × k3 × k4 ) per clause 6.3 of IS 875 (Part 3) : 2015 at given height interval. A Height vs. Velocity graph is plotted for the value of height and design wind speed (Vz).

Setting	Description
Class of Structure	Select class of structure from drop-down option and corresponding Risk Coefficient Factor (k1) value will be automatically selected as per Table 1 of IS 875 (Part 3) : 2015. Alternatively, select Custom from the drop-down list to directly type the k1 value for use.
Terrain Category	Select terrain category from drop-down option and corresponding height factor (k2) will be automatically calculated as per Table 2 of IS 875 (Part 3) : 2015 and corresponding Aerodynamic Roughness Height (z0i) value will be automatically selected as per clause 6.3.2.1 of IS 875 (Part 3) : 2015. The Aerodynamic Roughness Height is used for calculating hourly mean wind speed factor (k2,i) and determined mean hourly wind speed at different height as per clause 6.4 of IS 875 (Part 3) : 2015. Alternatively, select Custom from the drop-down list to directly type the k2,i and z0i values for use.
Importance of Structure	Select the importance category of the structure from drop-down option and corresponding Importance Factor (k4) value will be selected as per Clause 6.3.4 of IS 875 (Part -3 ) : 2015.
Topography Factor (k3)	Type the topographic factor to use. The value should be between 1 to 1.36 as per clause 6.3.3.1 of IS 875 (Part3) : 2015.

<structure type> tab

Note: The title and parameters on this tab vary based on the Structure Type selection made on the Common tab.

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.

Free Wall and Hoardings

Setting

Description

Walls or Hoarding Parameters

Setting	Description
Horizontal Dimension (b)	Type the horizontal dimension of the wall or hoarding component.
Vertical Dimension (h)	Type the vertical dimension of the wall or hoarding component.

Setting	Description
Structure Height (H)	This is calculated as the difference between the maximum and minimum Y coordinates in the model. You may also type a value here.

Height Above Ground

Select either the Automatic (calculated) or Custom option. For the automatic option, the height above ground (h') value used is taken as the structure height, H, minus the vertical dimension, h. The custom option lets you type a different value.

Force Coefficient

The Force coefficients factor (Cf) will be calculated automatically based on the width to height ratio (horizontal dimension, b, divided by vertical dimension, h) and whether the free-standing wall or hoarding are above ground or on ground as given in Table 26 of IS 875 Part (3) : 2015. No interpolation of Table 26 is assumed. So if a width to heigh ratio, b/h, is between to tabulated values, then the next greater value of Cf is used.

If the Custom option is selected, then you must specify the Force Coefficient (Cf) value.

Lattice Tower

Setting

Description

Tower Information

Setting	Description
Structure Cross Section	Select if the tower structure cross section is Square or Equilateral Triangle.
Member Profile Cross Section	Select if the cross section profile of the tower is Flat Sided or Rounded. For rounded members, you must also specify the Diameter of the members used at each height interval in the solidity ratio table.

Solidity Ratio

For either tower type, you must specify the Solidity Ratio of the front face (Φ) at each height interval of the structure in the solidity ratio table.

Force Coefficient

The Force coefficients factor (Cf) will be calculated at each height interval automatically based solidity ratio and the tower type as per the following table (with linear interpolation between tabulated values). The force coefficient table also displays the intermediate calculated values.

Tower Type	Member Cross Section	Code Reference
Square	Flat Sided	Table 33 / Clause 7.4.3.5(a)
Square	Rounded	Table 34 / Clause 7.4.3.5(d)
Equilateral Triangle	Flat Sided	Table 33 / Clause 7.4.3.5(a)
Equilateral Triangle	Rounded	Table 35 / Clause 7.4.3.5(e)

If the Custom option is selected, then you must specify the Force Coefficient (Cf) value.

Design Wind Pressure tab

This tab displays the resulting height vs. design wind pressure (pd) chart and graph per Cl. 7.2 of IS 875 (Part 3). The program includes a check for the minimum design wind pressure of 0.7×pz. The intensity values generated are the net pressure coefficient or the force coefficient times the design wind pressure (pd).

Setting	Description
Wind Directionality Factor (Kd)	Defaults to 1.0.
Area Averaging Factor (Ka)	Defaults to 1.0.
Combination Factor (Kc)	Defaults to 1.0.