This document serves as tutorial to show the RAM Elements tool for wind loads generation on members (open structure).
Open the “45.00m-height electrical tower.retx” model. The structure is formed by A36 steel equal leg angles for all segments and elements in the tower. The configuration is typical for electrical transmission towers.
All support bases are assumed to be pinned for the four-legged tower.
Gravitational loads are assumed as follows:
Imposed load considered as dead loads:
The referenced model has been created with the previous loads. This example is focused on assigning the wind loads using the wind loads generation tool for members.
Lateral loads to apply:
The model already has several design and service load combinations based upon ASCE 7-16 code.
1. Select the load case for which the wind loads will be assigned.
2. Select the desired members to assign wind loads using the tool. For this example, select all the members in the tower model since the tool can assign wind loads at once for the whole selection for one wind direction.
3. Select the Members > Loads on members spreadsheet.
4. On the Spreadsheet ribbon tab, select the Assign wind loads tool in the Active spreadsheet tools group.
The Wind load generation for members dialog opens.
5. Enter the structure geometry in the Building geometry group of data, as follows:
Note. The tower dimensions are used to fill the geometry of the building. These field are defined in ASCE/SEI 7-16, Minimum Design Loads and Associated Criteria for Buildings and Other Structures, chapter 26 and chapter 27, Directional Procedure, Part 1.
· Length: The length value is always the dimension of the structure parallel to the wind direction as it is shown in the image above.
· Width: The width value is always the dimension of the structure perpendicular to the wind direction as it is shown in the image above.
· Mean roof height: The Mean Roof Height is defined as the average between the roof eave height and the elevation of the highest point on the roof surface. For roof angles of less than or equal to 10°, there is an exception and the mean roof height is permitted to be taken as the roof eave height.
· Ground level: Is the level or vertical coordinate (Y) that represents the ground level of the structure.
6. Enter the wind parameters in the Parameters group, as follows (refer to notes at the end of this section for an explanation of each field):
Note. The previous data is defined in ASCE/SEI 7-16, Minimum Design Loads and Associated Criteria for Buildings and Other Structures, chapter 26 and chapter 27, Directional Procedure, Part 1.
· Basic wind speed: The basic wind speed, is used to determine design wind loads on the structures.
· Gust factor: Gust-effect factor, which is a design value that represent or is defined as the ratio between a peak wind gust and mean wind speed over a period of time.
· Enclosure category:
§ Enclosed. Building that has the total area of openings in each wall, that receives positive external pressure, less than or equal to 4 sq. ft (0.37 m2) or 1% of the area of that wall, whichever is smaller.
§ Partially enclosed. Building that complies with the following conditions:
o The total area of openings in a wall that receives positive external pressure exceeds the sum of the areas of openings in the balance of the building envelope (walls and roof) by more than 10%.
o The total area of openings in a wall that receives positive external pressure exceeds 4 ft2 (0.37 m2) or 1% of the area of that wall, whichever is smaller, and the percentage of openings in the balance of the building envelope does not exceed 20%.
§ Partially open. Building that does not comply with the requirements for open, partially enclosed, or enclosed buildings.
§ Open. Building that has each wall at least 80% open. This condition is expressed for each wall by the equation A0 >= 0.80 x Ag.
o A0 = Total area of openings in a wall that receives positive external pressure.
o Ag = Gross area of the wall in which A0 is identified.
· Elevation above sea level: Design value to define the elevation or altitude above the sea level used by the application to calculate the ground elevation factor to adjust for air density.
· Exposure category: For each wind direction considered, the upwind exposure shall be based on ground surface roughness that is determined from natural topography, vegetation, and constructed facilities.
§ B. Urban and suburban areas, wooded areas, or other terrain with numerous, closely spaced obstructions that have the size of small family houses.
§ C. Open terrain with scattered obstructions that have heights generally less than 30 ft (9.1 m).
§ D. Flat, unobstructed areas and water surfaces. This category includes smooth mud flats, salt flats, and unbroken ice.
· Directionality factor: Design factor determined from Table 26.6-1 of the ASCE 7-16 code.
· Wind direction: Gives the user the capability to choose the wind direction (angle). It is measured from the global positive X axis towards the global positive Z axis of the model.
7. Enter the topographic parameters in the Topographic factor, Kzt group, as follows (refer to notes at the end of this section for an explanation of each field):
· User defined. This option allows to use a topographic factor (Kzt) value chosen by the user. If site conditions and locations of buildings and other structures do not meet all the conditions specified in the code, then Kzt may be set to 1.0.
· Use calculated. If the user desires the application to calculate the topographic factor it is necessary to enter the multipliers K1, K2 and K3 properly as it is recommended in the code.
§ K1. Factor to account for shape of topographic feature and maximum speed-up effect.
§ K2. Factor to account for reduction with distance upwind or downwind of crest.
§ K3. Factor to account for reduction speed-up with height above local terrain.
8. Enter data in the Aerodynamic shape factor group, as follows (refer to notes at the end of this section for an explanation of each field):
Note. The data used in this group is defined in the Australian Standard AS/NZS 1170.2-2011, Structural design actions, Part 2: Wind actions, section 5 and Appendix E for exposed structural members.
· User defined. This option allows to use parameters defined by the user to calculate the aerodynamic shape factor.
· Aspect ratio (Kar). Aspect ratio correction factor for individual members.
· Inclination factor (Ki). Factor to account for the angle of inclination of the axis of members to the wind direction.
· Force coefficient (CFx). Drag force coefficient for a structure or member in the direction of the member local axis X.
· Force coefficient (CFy). Drag force coefficient for a structure or member in the direction of the member local axis Y.
The wind load is calculated using the following expression:
Load = qz * G * Cfig * Width
qz is the velocity pressure evaluated using ASCE/SEI 7-16, Chapter 26.
G is the gust-effect factor determined from ASCE/SEI 7-16.
Cfig is the aerodynamic shape factor evaluated using the AS/NZS 1170.2-2011 criteria.
Width is the member face width against the wind.
9. To see a report of calculated wind loads for selected members before confirming the assigned values in the dialog press the View report button and a report window will be displayed.
10. Click OK to close the dialog. The wind loads are added for the current load case for all the selected members.