| Applies To | |||
| Product(s): | RAM Structural System | ||
| Version(s): | Any | ||
| Environment: | N/A | ||
| Area: | N/A | ||
| Subarea: | N/A | ||
| Original Author: | Bentley Technical Support Group | ||
How is the wind typically applied to the model?
The slab edge determines the exposure (Ram Frame – Loads – Exposure) and we assume some kind of vertically spanning cladding transfers the loads to the diaphragms. The program calculates a total force based on total exposure, considering one windward surface and one leeward surface per diaphragm. Where the wind may hit more area, user defined loads should be used.
How are IBC/ASCE7 wind pressures calculated?
ASCE 7-10 includes three different procedures for wind load calculation: directional, envelope, and wind tunnel procedure. Only the directional procedure is implemented into the program. Program generated wind loads calculated per ASCE7-10 are ultimate loads.
IBC 2006/2009 Section 1609.1.1 refers to Chapter 6 of ASCE 7-05 for Wind load requirements. It lists alternative methods and wind speed values. Only the requirements for ASCE 7-05 are implemented. ASCE 7-05 includes three methods for wind load calculation: Method 1- Simplified Procedure, Method 2- Analytical Procedure, and Method 3- Wind Tunnel Procedure. Method 2 is further composed of two different provisions: a method for Low-Rise Buildings and a method for Buildings of Any Height. Among these methods, only Method 2-Analytical Procedure for Buildings of Any Height is implemented. Program generated wind loads calculated per ASCE7-05 are service loads.
Section 1609 of IBC 2003 gives wind requirements specifically focused on a simplified procedure acceptable for enclosed, low-rise, simple diaphragm buildings having a height of less than 60 feet. For other types, it refers to Section 6 of ASCE 7-02 where three methods described above for ASCE 7-05 are mentioned. Again, only Method 2-Analytical Procedure for Buildings of Any Height is implemented. Program generated wind loads calculated per ASCE7-02 are service loads.
User defined story forces may be used to apply Method 1- Simplified Procedure, Method 2- Analytical Procedure for Low Rise Buildings, or Method 3- Wind Tunnel Procedure.
How are the wind pressures viewed?
You can see the total force using Process – Results – Applied Story Forces, or Report the “Loads and Applied Forces” to see the intermediate wind pressure calculations. The Report – “Exposure Boundaries” is useful for visualizing the exposed surfaces of the model.
Is wind uplift considered?
No, the program only applies the total horizontal force for each story or diaphragm. Uplift loads are not yet considered.
While there is no good work around for this limitation some users apply negative magnitude Live Loads in order to check the beams (or foundations) for uplift. It's important to note that the program still sees this as a Live load and factors it accordingly, so some adjustments in the magnitude of the applied load or customization of the load combinations might be required.
How does sloped framing affect the wind loads?
The program currently applies the wind loads based on the simple rectangular area of the structure defined by the story data. Adjustments in the column and wall elevations do not affect the total applied wind loads at this time.
Use the Report - "Exposure Boundaries" to see a visualization of the exposed wind surfaces whenever there is some concern over the height or tributary width being used to calculate total story forces.
Can wind loads on open structures be generated?
No, when the program generates wind loads it always assumes the structure is fully clad all the way down to the ground level with one complete windward and leeward surface.
For any other condition and where rigid diaphragms exist, "User defined story forces" should be used. User defined story forces are defined in RAM Frame under Loads - Load Cases by selecting "User defined story forces" from the drop down menu to the right of the toggle for wind loads. When there is no rigid diaphragm use "Nodal loads." Nodal lateral loads are defined in the Modeler in the Elevation view.
The program generated wind loads are not completely accurate for any of the situations below.
- open structures,
- partially exposed structures,
- structures with multiple windward surfaces (e.g. a "U" shaped plan),
- structures with a stepping foundation.
- upper portion of a structure modeled in a two-stage lateral analysis with lower portion omitted
Is cantilever or prying action from pressure on a parapet considered?
No, the force to the top level diaphragm is equal to the pressure on the parapet times the parapet area plus the pressure on the top half of the story below. The additional reaction to the roof the results from cantilevering the parapet wall up from the level below is not accounted for. The Report - Exposure boundaries helps to clarify exactly what areas are considered in the wind load calculations. .
See Also
Structural Product TechNotes And FAQs
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