Open the Parameters of permeability dialog by clicking Parameters (the Advanced option must be selected) on the Permeability tab for the French snow/wind codes.
At the top of the dialog is the Member field. Using the < and > buttons, you can select a member belonging to a structure envelope. (A member can also be highlighted directly in the diagram of the structure on the right side of the dialog.) The number of the selected member is displayed next to the Member field; whereas, in the diagram of a frame, the member is highlighted in red.
After the member belonging to the envelope is indicated, you must define segments for this member. Each segment is assigned a different permeability value. You can define the segments in the following ways:
- Relative - Values refer to the member's length that is 1.0. For example, specifying the following values : 0.25; 0.5; 0.75 in the Definition of segments field means that 4 segments are defined for a bar:
- 1st segment: From the beginning of the member to the point at 1/4 of its length
- 2nd segment: From the point at 1/4 of the member's length to the midpoint of its length
- 3rd segment: From the midpoint of the member's length to the point at 3/4 of its length
- 4th segment: From the point at 3/4 of the member's length to the end of the member
- Absolute - Values are the lengths of the member segments expressed in the units selected in the Job Preferences dialog. For example, the following values of 4; 8; 12; 16 means that 5 segments were defined for a member.
Next, you must specify the permeability values for member segments:
- Permanent permeability - Specifies the permeability value that is permanent for each segment of a member. For example, if the following values are specified for 4 member segments: 0; 50; 0; 0, it means that permeability is 50% in the second segment and 0 in the remaining ones. Note: The permeability of a completely open wall is100%; whereas, for a completely closed wall it is 0%
- Doors - Specifies the permeability values (doors). For example, if the following values are specified for 4 member segments: 0; 0; 0; 0, it means that for all the segments the permeability is 0.
When Disregard roof elements when calculating average permeability option is selected, the bars that belong to a structure envelope and are roof elements are not considered in calculating the average permeability for the left and right side of a structure.
Pressure generated on member elements is reduced by the value of the given permeability coefficient, according to the formula:
Lw x ( (100 - Pp) / 100)
Where:
- Lw - calculated wind load
- Pp - percent of permeability in an element
For example, a permeability value of Pp = 20, gives a calculated wind load of: Lw x ( (100-20) / 100 ) = Lw x 0.80.
Example
Members 1 and 2 of the structure in the following image had the following values specified:
Definition of segments - Relative:
25; 0.5; 0.75 (4 segments)
Values of permeability:
Bar 1: 0.0; 50.0; 0.0; 0.0
Bar 2: 0.0; 10.0; 0.0; 0.0
The distribution of wind pressure (such as, wind L/R permeability) that resulted, is shown in the following image.
