RAM Frame - Wind Loads [FAQ]

 Product(s):RAM Structural System
 Area: Analysis

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. 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. In version 15.04 a feature was added to make it easier to paste in forces from a wind tunnel test.

How are the wind pressures viewed?

You can see the total force using Process – Results – Applied Story Forces, or Report - 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 in a compressed Elevation View.

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.

Why is the total applied wind load on a semirigid diaphragm more or less than a rigid diaphragm?

The total length of windward and leeward edges may exceed the exposure length when the slab edge has a return (like a "U" shaped structure) and the diaphragm is semirigid. This leads to more total force being applied (when using program generated wind loads) since all these edges are loaded based on the pressure, story height and mesh size.

The Report - Loads and Applied Forces lists both values to be clear:

Furthermore, when wind loads are applied to a meshed semi-rigid diaphragm only the windward and leeward edge nodes are loaded. if the applied deck does not extend to the slab edge overhang, then the program will fail to find the edges to load and the total applied load will then be less than it should be. It's imperative that the deck extend to the slab edge when using semi rigid diaphragms.

Additionally, when the diaphragm is semirigid, this can affects the building natural frequency which, in turn, can affect the wind gust factors and pressures and total forces. It may be better to manually input the structure frequency when using semirigid diaphragms.

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. For more on the effect of having sloped diaphragms in RAM Frame, see RAM Frame - Criteria - Diaphragms.

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. The same exposure height is assumed on the windward and leeward surfaces.

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.

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 that results from cantilevering the parapet wall up from the level below (prying force) is not accounted for. The Report - Exposure Boundaries helps to clarify exactly what building surface areas are considered in the wind load calculations. 

Can the entire RAM SS model be elevated like a structure on top of a podium to get the correct wind pressures?

Starting in version 17.02, you can build an elevated first story to place the rest of the model upon. This level could be as simple as a set of lateral columns under the columns above, or a set of lateral walls to support everything above. Make the columns vertically very stiff. Then set this first level to the ground level for Analysis, while leaving the ground level for Wind Load Generation "At Base".

Note, in the wind load definition there is a property for Mean Roof Height which is taken as the top story height be default. The user can override this value which will alter the pressure, but when calculating the windward pressure, the original story heights above ground are still used. In other words, the program is not adjusting Kz windward based on the input mean roof height. 

If you actually raise the structure, by adding a blank first story with height = h1,  it will elevate the structure and raise the wind pressure correctly, but it will also assume that the exposed height of the lowest level diaphragm is larger by h1/2 giving you too much force on the lowest framed level. This can be confirmed using the Loads and Applied forces report and the Exposure Boundaries report.

Thus it's better to model something physical for the lower supporting structure. 

Alternatively, it may be easier to just apply "User defined story forces" for the wind.