Wind Load

Lesson: Basic Wind Loads ASCE 7-10 Page 1 of 21


Basic Wind Loads ASCE 7-10

Developed by J ohn Herrick

Course Description

If you design buildings you have to understand wind forces and how to prepare for them.
One of your tools in designing for wind loads on structures, including roofs, walls, and
windows, is the ASCE 7 Manual, Chapter 28, Envelope Procedure (formerly low-rise
buildings in Method 2).

This interactive online course gives you the 2010 updates to Chapter 28. You get
information, step-by-step instructions, and examples to help you in making your calculations
We’ll cover how to get started as well as the calculations for wind loads on the ends and
sides of a structure.

Author’s Note: All the information needed to complete this course is included in the course.
Most of the tables are included, but if you are going to be involved in the design of wind loaded
structures, I highly recommend that you get a copy of ASCE 7-10 for your own use.

Performance Objectives

At the conclusion of this course, you will be able to:
• Recognize updates to Chapter 28 of the ASCE 7 (2010 version)
• Discuss forces on structures
• Describe steps to calculate wind forces on structures
• Explain using ASCE 7 manual tables

Preface

Below is a picture by Daniel Bramer. It is of a hurricane (a tropical cyclone) coming onshore with
thousands of homes, businesses, and structures in its path. What is to become of them?

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The answer to that question lies mainly in your hands. Can you build your structure to
withstand the storm? What does that entail?

Scary, isn't it? The picture of devastation, coming your way. Look at that pressure wave
ahead of it!

The main thing in designing for wind loads is to determine the pressure the wind will bring to
bear. Secondly, what will the wind bring with it? Gravel from a lower roof? Branches from
nearby trees? Patio furniture from the neighbor's yard?

Let’s get started!

Getting Started

The Question Was: Can You Build Your Structure to Withstand the Storm?

The answer is yes, you can! J ust follow the steps outlined below.

First, what is the wind speed we are talking about? Some hurricanes have wind gusts of over
200 miles an hour in isolated spots, but the probability of that happening in the life of the
building is quite remote, so we design for the more reasonable wind speeds. Below is the 2010
ASCE table for the southeast showing wind speeds to be used (for example Miami needs to
design for 170 mile per hour winds or winds traveling at 76 meters per second):

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Note that in the previous version of this map (the 2007 version), the wind speed in Miami
was 145 mph, not 170! This 2010 chart has not been adopted yet by the State of Florida,
so, for now the 2007 chart will rule.

STEP 1: Determine your Basic Wind Speed. Plot your structure on the above map. If you are in
mid-western Florida, for example, you may lie between the 140 and 150 mph lines.

STEP 2: If you wish, you can extrapolate between the lines, or use the higher line. We could
use the higher 150 mph as our Basic Wind Speed (the simpler option), but for this example we
will use halfway between. We will then have to subtract all the pressures resultant from the 150
mph design storm from that of the 140 mph storm; take 50 percent of this subtraction amount,
and add it to the 140 mph storm pressures. If you are 15 percent of the way from the lower
speed to the higher you would add 15 percent, and so on. For our example using the halfway
mark, we will use the average.

STEP 3: Determine if your building's Main Wind Force-Resisting System (MWFRS) is
appropriate for the SIMPLIFIED Design Wind Pressure in accordance with ASCE 7 – 10,
Chapter 28, Part 2 (Envelope Procedure).

1. The building is a simple diaphragm (four walls and a roof).
2. The building is low-rise (not more than 60 feet tall and as wide as tall or more).
3. The building is an enclosed building. The definition of an enclosed building is:

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The criteria of openings less than 4 square feet is quite restrictive, but openings
can be limited if covers are to be placed over the windows, or they are of impact
resistant glass.

The difference between an enclosed structure and one that has a door blown in
(allowing an opening of more than 4 square feet) was dramatically shown in an
actual test. The two buildings built withstood hurricane force winds until the door
was opened. The unfortified structure failed:



(see http://www.msnbc.msn.com/id/39749066/ns/weather/)

4. The building is regular shaped (four walls and a roof, no frills).
5. The building is not flexible (not 4 times as tall as wide or more).
6. The building is not built with porches or other flimsy items which may be affected by
the wind, nor is it situated in an area where winds will be funneled onto it.
7. The building structure has no expansion joints or separations.
8. The building is not on a cliff or other high-wind location.
9. The building is approximately symmetrical with a flat roof, a gable roof, or hip roof of
less than or equal to 45 degrees.

If your structure fails any of these tests, you will have to design for Partially
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Enclosed (or open) building criteria, which is covered in another course. This
course, however, will give you many of the tools you need to follow up with the
next course.

STEP 4: Determine your roof angle, for our case it will be 20º. Angles over 45º are not allowed it
the Simplified procedure.

STEP 5: Determine your Exposure Category from ASCE 7 – 10, Chapter 26, Section 26.7.3
There used to be a Category A, but that has been discontinued. Now there are
Categories B, C, and D. Category B is the category most protected by other buildings (and other
structures) whereas Category D has no such protection. We will select B – exposed for our
example.


STEP 6: Determine your Building Risk Category from ASCE Table 1.5 -1 (we will use a single
family home Category II):
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Examples of “Category I” were previously given as:

•Agricultural facilities
•Certain temporary facilities
•Minor storage facilities

Step 7: Find your Topographic Factor (Kzt). In our case, we will assume that there is no
hill or escarpment, and so our Kzt is 1.0. If you do have a house on a ridge or hill the
calculations are done as follows:
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STEP 8: Determine your 'gamma' from the following table using your building height
and
your Exposure Category from STEP 5 (note that your adjustment factor will be 1.0 for
buildings 30 feet high or less - like ours - in sheltered areas Exposure B):

Summary:

Steps 1 & 2:
Select your Basic Wind Speed (140, adjusted to 145 mph)
This is where you use your location to determine your wind speeds.

Steps 3 & 4:
Determine if your building complies with the SIMPLIFIED PROCEDURE requirements.

If your building fits the Simplified Procedure, the pressures are relatively easily determined;
otherwise, you need to take many more factors into consideration.

Step 5:
Select your Exposure Category, B, C, or D (B for our example)
Determine if your building is sheltered or not.

Step 6:
Select your Building Category, I, II, III, or IV (risk) (II for our example)

Determine if your building is important relative to the number of occupants or being a
shelter or not.

Step 7:
Select your Topographic Factor (ours is 1.0)
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If your building is on a hill and in higher wind speed areas you need to design for higher wind
loads.

Step 8:
Select your 'gamma' due to building height and exposure (ours is 1.0)

If your building is taller and not sheltered you need to design for higher wind loads.

Now, armed with these factors, you are ready to go on to the next scenario where we
determine Pressures.


Wind Loads on the Structure

In the first section, we went through the items needed to begin any wind load design. For this
section, let’s assume that the SIMPLIFIED PROCEDURE conditions have been met and we
have a single family home in mind.

In designing the structure (Main Wind Force Resisting System), determine the direction of the
eight forces on the gable structure, four with the wind on one end of the building, then four on
the side, use Figure 28.4-1 of the ASCE 7 manual.

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Now that you have the direction, the areas the forces work on are as follows (A through
H, end and side winds):




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Notes:

1.All corners must be evaluated separately for the design wind loads.

2. The split between zones G and H (and E and F) is half of the building.

3. The dimension "2a" represents:
a. 20 percent of the width of the building or
80 percent of the roof height, whichever is smaller
c. but not less than 8 percent of the width or 6 feet

Our building is 30 feet wide and 30 feet tall, thus, "2a" equals:
a. The smaller of 20 percent of 30 (6 feet) or
80 percent of 30 (24 feet)---the smaller one is 6 feet;
c. Checked against 8 percent of 30 (2.4 feet) it is larger so okay, and
Checked against a minimum of 6 feet, okay

So 6 feet it is.

Next we go to the table in Figure 28.6-1 to select the pressures that will be experienced in all of
these zones.
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You may see that the table is for Exposure B at height of 30 feet and with Importance (I) of 1.0.
This matches our example, but you only have to multiply by the appropriate factors as
discussed in the first scenario for other conditions.

Because we will start by designing for wind on the end of the building, the angle of the roof
has no effect and we use 0 degrees.

Our wind speed is 145 mph.

ZONE A: Solving for pressure in psf for zone A (the end of the wall toward the corner) amounts
to taking the pressure from the table for 150 mph of 35.7 psf and the pressure for 140 mph of
31.1 psf and averaging them: (66.8/2), or 33.4 psf.

ZONE C: Solving for the pressure in zone C (the remainder of the end) is solved by
averaging the 150 and 140 mph forces. The horizontal force (C) at 150 mph is 23.7 psf and
at 140 mph of 20.6 psf for an average of (44.3/2), or 22.2 psf.

ZONE E: Solving for pressure in psf for zone E (the edge of the roof part way back) amounts
to taking the suction pressure from the table for 150 mph of 42.9 psf and the suction for 140
mph of 37.3 psf and averaging them: (80.2/2), or 40.1 psf. NOTE: the overhang may be a
factor here; if so, use the "Eoh" factors.

ZONE F: Solving for the pressure in zone F (the remainder of edge of the roof) is solved by
averaging the 150 and 140 mph uplift forces. The vertical force for 150 mph is an uplift force of
24.4 psf. That for 140 mph is 21.2 psf. The average is (45.6/2) or 22.8 psf.

ZONE G: Solving for pressure in psf for zone G amounts to taking the pressure from the table
for 150 mph of a suction of 29.8 psf and the suction pressure for 140 mph of 26.0 psf and
averaging them: (55.8/2), or 27.9 psf suction. NOTE: the overhang may be a factor here also, if
so, use the "Goh" factors.

ZONE H: Solving for pressure in psf for zone H (the remainder of the lee side of the roof)
amounts to taking the pressure from the table for 150 mph of a suction of 18.9 psf and the
suction pressure for 140 mph of 16.4 psf and averaging them: (35.3/2), or 17.2 psf suction.

Summary

ZONE A: 33.4 psf
ZONE C: 22.2 psf
ZONE E: 40.1 psf suction
ZONE F: 21.2 psf suction
ZONE G: 27.9 psf suction
ZONE H: 17.2 psf suction

Design wind pressures are both pressure toward the structure and suction away from the
structure. Note that the suction force (uplift) in Zone E are the strongest. Each of the parts of
the structure needs to be designed to withstand these forces.

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Wind Loads From Wind on the Side of a Structure

In the first scenario, we went through the items needed to begin any wind load design. For this
scenario, let’s assume that the SIMPLIFIED PROCEDURE conditions have been met and we
have the single family home in mind.

MWFRS Design:

In designing the structure (Main Wind Force Resisting System), all gable structures with the
wind on the side of the building use Figure 28.6-1 Case A of the ASCE 7 manual:

Notes

1. Note that above the word Transverse is the phrase Reference Corner. This is the corner
that is being evaluated. All corners must be evaluated separately for the design wind loads.

2. The dimension "2a" represents:

a. 20 percent of the width of the building or
80 percent of the roof height, whichever is smaller
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b. but not less than 8 percent of the width, or 6 feet.

Example

Our building is 30 feet wide and 30 feet tall, thus "2a" equals:

a. The smaller of 20 percent of 30 (6 feet) or
and 80 percent of 30 (24 feet)---the smaller one is 6 feet;
c. Checked against 8 percent of 30 (2.4 feet) it is larger so okay, and
Checked against a minimum of 6 feet, okay.

So 6 feet it is.

Next we go to the table in Figure 28.6-1 to select the pressures that will be experienced in all of
these zones.
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You may see that the table is for Exposure B at height of 30 feet and with Importance (I) of 1.0.
This matches our example, but you only have to multiply by the appropriate factors we
discussed in the first scenario for other conditions.

Since we're designing for wind on the side of the building, the angle of the roof has a major
effect and we will use our roof angle of 20 degrees.

Our wind speed is 145 mph.

ZONE A: Solving for pressure in psf for zone A (the end of the wall) amounts to taking the
pressure from the table for 150 mph of 49.4 psf and the pressure for 140 mph of 43.0 psf and
averaging them: (92.4/2), or 46.2 psf.

ZONE B: Solving for the suction in zone B (the end of the wall under the roof) amounts to
taking the suction from the table for 150 mph of 13.0 psf and the suction for 140 mph of 11.4
psf and averaging them: (24.4/2), or 12.2 psf.

ZONE C: Solving for pressure in psf for zone C (the remainder of the lower wall) amounts to
taking the pressure from the table for 150 mph of 32.9 psf and the pressure for 140 mph of
28.7 psf and averaging them: (61.6/2), or 30.8 psf.

ZONE D: Solving for the suction in zone D (the remainder of the upper wall) amounts to taking
the suction from the table for 150 mph of 7.2 psf and the suction for 140 mph of 6.3 psf and
averaging them: (13.5/2), or 6.8 psf.

ZONE E: Solving for suction in psf for zone E (the end of the roof) amounts to taking the
suction from the table for 150 mph of a suction of 42.9 psf, and the suction pressure for 140
mph of 37.3 psf and averaging them (80.2/2), or 40.1 psf suction.

ZONE F: Solving for suction in psf for zone F (the end of far side of the roof) amounts to taking
the pressure from the table for 150 mph of a suction of 29.8 psf, and the suction for 140 mph of
26.0 psf, and averaging them: (55.8/2), or 27.9 psf suction.

ZONE G: Solving for suction in psf for zone G (the remainder of the windward side of the roof)
amounts to taking the pressure from the table for 150 mph of a suction of 29.8 psf ,and the
suction pressure for 140 mph of 26.0 psf and averaging them (55.8/2), or 27.9 psf suction.

ZONE H: Solving for suction in psf for zone H (the remainder of the lee side of the roof)
amounts to taking the pressure from the table for 150 mph of a suction of 22.6 psf, and the
suction pressure for 140 mph of 19.7 psf, and averaging them: (42.3/2), or 21.2 psf suction.

Overhang E: Solving for suction in psf for overhang E (the end of near side of the roof)
amounts to taking the pressure from the table for 150 mph of a suction of 60.0 psf ,and the
suction pressure for 140 mph of 52.3 psf and averaging them (112.3/2), or 56.4 psf suction.

Overhang G: Solving for pressure in psf for overhang G (the remainder of the roof) amounts to
taking the pressure from the table for 150 mph of a suction of 47.0 psf, and the suction pressure
for 140 mph of 40.9 psf, and averaging them: (87.9/2), or 44 psf suction.

Design wind pressures are both pressure toward the structure, and suction away from the
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structure. Each of the parts of the structure need to be designed to withstand these forces.

Summary

ZONE A: 46.2 psf
ZONE B: 12.2 psf suction
ZONE C: 30.8 psf
ZONE D: 6.8 psf suction
ZONE E: 40.1 psf suction
ZONE F: 27.9 psf suction
ZONE G: 27.9 psf suction
ZONE H: 21.2 psf suction
Overhang E: 56.4 psf suction
Overhang G: 44.0 psf suction

Design wind pressures are both pressure toward the structure (with Zone A being the
strongest), and suction away from the structure (with the end of the roof with overhang being
the strongest). Thus, special care needs to be taken with the corner of the building and the
end of the roof.

Exam

1. If the building is "Enclosed", what does that imply?
A. The building windows must be covered or impact resistant
B. There are minimal openings (4 sf.)
C. You might use the Simplified Procedure
D. All the above

2. To be low rise, the building must:
A. Have windows that are covered or impact resistant
B. Be taller than wide
C. Be more than 60 feet tall
D. None of the above

3. To be "regular" shaped, a structure must:
A. Have openings that are of normal glass
B. Have no unusual geometrical irregularity
C. Have no spatial function
D. None of the above

4. To be allowed to use the "Simplified Procedure", the building:
A. Must have a roof less than 37 degrees for gable or less than 30
degrees for hip
B. Must have a roof less than or equal to 45 degrees for gable
C. Must have a roof less than 25 degrees for gable or less than 37
degrees for hip
D. None of the above

5. The Topographic Factor is relative to the:
A. Warp speed and roof height
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B. Category and number of occupants
C. Escarpment and ridge
D. None of the above

6. Where is the highest pressure (in a positive direction-toward the structure) on the end of a
building with the wind from that direction?
A. Zone A at the corner
B. Zone B on the side
C. Zone C remainder of the end
D. Zone D remainder of the side

7. How does one relate "2a" to the width of the building?
A. 1 percent of the width
B. 4 percent of the width
C. 20 percent of the width
D. 80 percent of the width

8. How do you relate the "2a" dimension to the roof height?
A. It has no relation
B. 4 percent of the height
C. 80 percent of the height
D. 100 percent of the height

9. What is the minimum dimension for "2a"?
A. Larger of 8 percent of the width or 10 percent of the height
B. Smaller of 8 percent of the width or 3 feet
C. Smaller of 8 percent of the width or 6 feet
D. Larger of 8 percent of the width or 6 feet

10. Where is the strongest suction pressure on a structure (the wind on the side of the
structure)?
A. Zone A side of wall
B. Zone E end of roof
C. Zone C remainder of wall
D. Zone G remainder of roof

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