snapshot of the page taken as our search engine crawled the Web.
The web site itself may have changed. You can check the current page or check for previous versions at the Internet Archive. Yahoo! is not affiliated with the authors of this page or responsible for its content.
ASCE 7 Wind Load Generator


IES VisualAnalysis 6.0
1

ASCE 7 Wind Load Generator
Overview

Starting with VisualAnalysis Version 6.0, IES has introduced a tool to help with creating wind loads on
structures. The tool is based on the most current version of the ASCE-7 document, Chapter 6, Method 2
analytical provisions (ASCE 7-05 at the time of this writing). It is important to dispel some myths
regarding wind loads and the use of the tool.
Myth 1: Wind loads on structures are well understood.
This statement could not be farther from the truth. While the theory of compressible fluid flow (wind)
has been well established and has been automated through numerical analysis by the aircraft and
aerospace industries, its application to ground based structures is not a simple task. The random nature
of the environment around the structure and the randomness of the wind source in general do not lead
to simple aerodynamic analysis by software.
Myth 2: ASCE 7 provides a complete and clear method for handling wind loads.
Again this statement is false! If you have ever attended a short course by an expert on the subject of
wind loads you quickly learn that all but the simplest questions asked by the audience will receive a
response of the form youll need to use your judgment on that one The code provisions are an
attempt to handle the basics of wind load generation through assumptions. These assumptions are
quickly violated by most real world structures.
Myth 3: IES makes wind load generation simple and automates the p rocess.
Obviously at IES wed love to make this a true statement but it is not. You still need a thorough
understanding of wind and aerodynamic behavior along with SCE 7s provisions to use these in IES
software. We will attempt to help you in this document by looking at the software process for wind
loads.
Assumptions
In this document when pages, sections, tables, or figures are mentioned, you can assume they
are found in ASCE-7 unless otherwise noted.
VisualAnalysis wind load calculations are based on ASCE 7 Analytical Method 2 for wind loads
described in section 6.5
VisualAnalysis does not check to guarantee that the limitations of Method 2 are met. When
using the wind load pressures generated in VisualAnalysis you have made the decisions that
the Method 2 limitations are met.

IES VisualAnalysis 6.0
2

Basic Parameters
At the start of a new project in VisualAnalysis you would be best served by setting up some basic project
parameters These can be found in the project manager under the General and Wind categories
which are displayed when nothing is selected in the current active view. Here is a view of those
parameters:

The Occupancy Use parameter is used to set the wind importance factor I per Table 6-1. 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 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 Load Case button

Service load case properties can also be modified through the Load | Load Case Manager menu, the
Service Cases tab.


IES VisualAnalysis 6.0
3

Obviously 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 are described in section 6.2 and will directly affect wind
pressures. Projected Width and Side Length specify the dimensions B and L respectively as described in
section 6.3. These dimensions are also shown in Figure 6-6. Internal pressure specifies the direction of
the internal pressure term used in equation 6-17 which is the basic equation for wind pressure in
Method 2. This entry affects the sign on the q
i
(GC
pi
) 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, i.e. 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 C
p

coefficients in Figure 6-6.
The Roof Pressure setting specifies which one of the two
roof pressure C
p
coefficients should be used in Figure 6-6.
Not all but most roof wind pressure coefficients C
p
have
two entries in the table so the proper one needs to be
used to generate the pressure per Equation 6-17. 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 C
p
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.
Welcome to the world of wind loads per ASCE 7.
The final entry in the Service case dialog which might be
useful is the Advanced Settings button. If this button is
selected the dialog at right will appear.
As you can see, this dialog allows you to refine entries for
the directionality coefficient Kd as defined in Section 6.3
and Table 6-4. You are also allowed to specify
topographic effects per section 6.5.7, volume reduction
factor per 6.6.11.1.1, and an air density adjustment per
commentary section C6.5.10. Note that as you enter
different Air Densities, the constant in the qz equation
will change to the new velocity pressure coefficient.



IES VisualAnalysis 6.0
4


Creating Areas

Wind loads 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 loadings you will want
areas generated for all exterior walls and the entire roof. As noted above, ASCE 7 Method 2, equation 6-
17 makes use of internal and external pressures. This equation requires that internal pressures should
cancel each other 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 and the direction of internal pressure will have a direct effect on the total building uplift
force.
Loading Areas
Once wind load parameters are specified and areas have been created for all exterior walls and the
building roof 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 rea Loads menu item
or use the right mouse button and select pply rea Load from the drop down menu The following
dialog will then appear:

IES VisualAnalysis 6.0
5


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 this area is along
with the outside direction. The possibilities are:

Note that you must select the type for each area to be loaded. Here is a place where engineering
judgment will certainly come into play. If your structure is different from a simple flat box building and
walls for instance are not at 90 degrees from each other youll need to make a decision as to how to
classify the wall or roof. The direct effect of this classification is that it will indicate which C
p
coefficients
in Figure 6-6 will be used. Please take some time studying Figure 6-6 as it is unquestionably at the core
of ASCE 7 Method 2 wind loads.

IES VisualAnalysis 6.0
6

When assigning a wind load to an area note also that the dialog requires you to answer the question:

Selecting the proper answer is critical to the direction of the load generated by equation 6-17. For many
buildings with re-entrant corners VisualAnalysis cannot determine the outside direction of a wall and
therefore you must supply the answer here The default answer will be set up based on Visual nalysiss
best guess as to the outside direction. If you create your areas manually, you can control this normal