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VisualAnalysis Tutorials: ASCE 7-10 Wind Loading

Overview

VisualAnalysis provides assistance for calculating wind load pressures that are applied to Area objects. No automatic wind loads are available for members, plates, or any other model objects in the software, though you may manually apply loads to these objects.

Area wind load generation is performed according to ASCE 7-10. Generating wind pressures per ASCE 7-05 is not an option in version 8. You should have a thorough understanding of ASCE 7 wind loading provisions in order to use this feature in VisualAnalysis. VisualAnalysis wind load calculations are based on the Wind Loads on Buildings-MWFRS Directional Procedure (ASCE 7-10 Chapter 27). VisualAnalysis does not check to guarantee that the limitations for this procedure are met. When using the wind load pressures generated in VisualAnalysis you must decide if the limitations are met.

Basic Parameters

At the start of a new project in VisualAnalysis you will need to define some basic ASCE project parameters. These can be found in the project manager under the "Project" and "IBC Wind" categories which are displayed when nothing is selected in the current active view.

The IBC Risk parameter is used to set the wind importance factor I. Vertical Axis is used to set the measuring direction above the ground elevation (z direction by ASCE 7). Ground elevation establishes the ASCE 7 z=0 location. Mean Roof Height, Wind Speed, and the Gust Factor set ASCE 7 variables h, V, and G parameters which are needed for pressure calculations. These parameters apply to every wind load case created in VisualAnalysis.

A second set of basic parameters must also be set for each load case which will contain wind loads. These parameters are accessed though the service case dialog. The properties of the selected service load case are available through the Command bar "Edit Load Case" button:


Any Service Load Case can also be accessed through the Load Case Manager on the Service Cases tab.

The Wind Criteria (ASCE-7) group is what we are interested in for wind loads. Please note that this information will only be enabled if the Source of loads is wind, otherwise it will be disabled. The source of loads is set in the second line of the dialog.

Exposure Category and Enclosure Class will directly affect wind pressures (ASCE 7-10 26.7). Projected Width and Side Length specify the dimensions B and L respectively as defined in section 26.3. These dimensions are also shown in Figure 27.4-1. Internal Pressure specifies the direction of the internal pressure term used in equation 27.4-1. This entry affects the sign on the qi(GCpi) term. When the pressure is defined as Outward, this term will have a minus sign and if Inward, the sign will be positive. Note that this sign convention is consistent with ASCE 7 wind load pressures. Positive pressures act inward on the building walls and negative signs act outward (suction) on the building walls. These signs are also seen in the Cp coefficients in Figure 27.4-1.

The Roof Pressure setting specifies which one of the two roof pressure Cp coefficients should be used in Figure 27.4-1. Most (but not all) roof wind pressure coefficients Cp have two entries in the table so the proper one needs to be used to generate the pressure per Equation 27.4-1. Please note that because of the entries just mentioned, there are four total wind load possibilities which can be can be checked: two internal pressure directions, each one having two roof pressure Cp possibilities. IES makes no assumptions as to which might be more critical in your design. Which settings to use and subsequent load cases to generate and analyze is entirely your decision.

Selecting the Advanced Settings button will display the dialog shown on the right.

As you can see, this dialog allows you to refine entries for the directionality coefficient Kd as defined in Section 26.3 and Table 26.6-1. You are also allowed to specify topographic effects per section 26.8, volume reduction factor per 26.11.1.1, and an air density adjustment per the commentary section C27.3.2. Note that as you enter different air densities, the constant in the qz equation will change to the new velocity pressure coefficient.

Creating Areas

The Wind Loads Generator in VisualAnalysis can only be applied to areas. Areas need to be established by either sketching or by using the Model | Create Areas Automatically menu. See the VisualAnalysis Help for more information regarding areas and their manipulation. Note that for wind loading you will want areas generated for all exterior walls and the entire roof. As noted above, equation 27.4-1 makes use of internal and external pressures. This equation requires that internal pressures should cancel on opposing walls and as such do not affect total base shear. In summary, if you do not have all walls loaded, this cancellation will not occur. Note also that this comment does not apply to roof pressures. The direction of internal pressure will have a direct effect on the total building uplift force.

Loading Areas

Once wind load parameters are specified, areas have been created for all exterior surfaces as shown below, the individual walls can now be loaded. To create wind loads, select an area or group of areas and use the Loads | New Area Loads menu item or use Right Click and select Apply Area Load from the drop down menu.


The following dialog will then appear.

Note that the Type of Load | Wind (ASCE 7) button will not be available unless the service case currently in the active window has a source of Wind. You must now select the type of surface the area is along with the outside direction. The possibilities are shown on the right:

Note that you must select the type for each area to be loaded. Here is a place where engineering judgment is required. If your structure is different than a simple flat box and walls are not perpendicular to each other, you'll need to make a decision as to how to classify the wall or roof. This classification will indicate which Cp coefficients in Figure 27.4-1 will be used. Please take some time studying Figure 27.4-1 as it is unquestionably at the core of ASCE 7 method we are using.

When assigning a wind load to an area, also note that the dialog requires you to define the direction of the "Area Normal" as outward +Z, or vice versa. Selecting the proper case is critical to the direction of the load generated by equation 27.4-1. To verify which direction the Area normal is, select the Area , and look under the Information heading in the Project Manager (the load dialog must be closed to do so). Use the global axes to determine how the direction of the Area normal compares to the outward direction. VisualAnalysis cannot determine the outside direction of a wall for buildings with re-entrant corners. You should verify the direction of each Area normal prior to Analysis. The default setting for the normal will be set based on VisualAnalysis's best guess as to the outside direction. If you create your areas manually, you can control the normal direction to ensure that all your Areas have an outward normal.

Conclusion

Wind load calculation per ASCE 7 is complicated and requires that you understand chapters 26 & 27 in detail. VisualAnalysis attempts to simplify the process by performing many of the calculations for you. The general process can be summarized by the following steps:

1.       Set overall project settings in the Project Manager, Project Settings.

2.       Set load case specific information in each Service Load Case for wind loads.

3.       Create Areas for all exterior walls and the entire roof of your structure.

4.       Apply Area Loads to appropriate areas in wind load service cases.

In order to account for directionality of the internal pressure and the duality of the roof pressure coefficients it may be necessary to create up to 4 different load cases for each wind direction being considered. Figure 27.4-8 of ASCE 7 furthermore indicates that at least 4 cases of overall directionality need to be considered. This can increase the total number of cases substantially.