Pressure Hazards Packet

This packet is intended for use in a mechanical engineering Statics course. An example problem involving pressure loading, caused by wind on a pickup truck, is presented. The importance of modeling is emphasized. The packet assumes the presentation is during a review session or late in the course, when assignment of a homework problem would not be appropriate. Under different conditions, the problem discussed might be given as a homework assignment.

Time for presentation is estimated as 15-20 minutes.

Objectives:

1. To gain an increased understanding of safety requirements involved in design.

2. To further recognize engineering responsibilities when modifying a design.

3. To relate basic engineering concepts of pressure to a real-life situation involving wind.

This packet includes the following items:

• Lecture material for the instructor
• Overheads for use during the lecture
• Handouts for the students

Download the Pressure Hazards Packet in printable Adobe Acrobat Format (pdf).  This includes overheads in a ready to use format.

Homework problem solutions, exam problems, and exam solutions are available to qualified recipients. Send an email with request information to Dr. Donald Bloswick.

Pressure Hazards Lecture Outline

1. Importance of Modeling: Pressure Loading (OVERHEAD 1)
1. Pressure loading, such as that occurring with wind or water, can be modeled in many circumstances as a two-dimensional distributed normal load.
2. The distributed normal load force can then be modeled as a point force acting at the centroid of the object. Various quantities can be calculated from this type of modeling.
3. Fundamental modeling skills can be used in a variety of engineering situations, and typically produce reasonable results.
2. Pressure Hazards Modeling Example (OVERHEAD 2)
1. The following example illustrates the effect of adding a camper shell to pickup truck in terms of wind loading moments.
1. Figure 1 shows a representative weight and dimensions for a small pickup truck. The designers of such vehicles must consider many factors to ensure safety.
2. A subset of these factors, which is itself very complex, deals with factors that would tend to cause the truck to tip over on its side. In this problem we will consider only one of these factors which is side wind loading.
3. (OVERHEAD 3) Assume you have a pickup truck such as that in Figure 1 and that you want to build a shell to put on the back which is 5 feet high and the length of the bed. The resulting vehicle would be as shown in Figure 2.
4. American Society of Civil Engineers (ASCE) Standard 7 deals with minimum design loads for buildings and other structures. It requires consideration of winds that generate at least 10 pounds force per square foot loading.
5. (OVERHEAD 4) Using this value and the weights and dimensions in Figures 1 and 2, determine the following:
1. What is the built-in tendency (moment in ft-lb) of the vehicles not to tip over on their sides?
2. What is the wind loading moment that would tend to tip the basic truck (as in Figure 1) over on its side?
3. What is the wind loading moment that would tend to tip the truck with shell (as in Figure 2) over on its side?
4. What does all this mean to you?
2. Example Solution: (OVERHEAD 5)
1. Part (a): The moment due to the weight of a vehicle about the line of contact of its tires is what tends to prevent it from tipping over on its side. On level ground as shown in the figure, this moment is 9,500 ft-lb (2.5 ft x 3800 lb) for the truck and 10,000 ft-lb for the truck with the shell.
2. Part (b): We need the moment about the line of contact of the tires (road surface) due to the wind. Because the truck can be considered as a composite of several standard figures we can find the force and centroid above the ground as follows:

Force = (30 sq. ft) x (10 lb per sq. ft) = 300 lb

Centroid location = 96/30 = 3.2 feet above the ground.

Moment = distance x force = 3.2 ft x 300 lb = 960 ft-lb

3. Part (c): (OVERHEAD 6) Again we need the moment about the line of contact of the tires due to the wind. Because the truck with shell can be considered as a composite of several standard figures we can find the force and centroid above the ground as follows:

Figure Area (sq. ft) Centroid (above surface - ft) Centroid x area

Centroid location = 291/60 = 4.85 feet above the ground.

Moment = distance x force = 4.85 ft x 600 lb = 2910 ft-lb

4. Part (d): Although the tendency not to tip over due to the added weight would increase by about 5%, the tendency to tip over due to the wind would more than triple. This is just one factor, which would need to be considered along with other potentially major factors such as the effects of uneven ground and the centripetal force due to turning.

Lecture reference material:

American Society of Civil Engineers. ASCE 7-95: Minimum Design Loads for Buildings and Other Structures. New York: ASCE, 1996.

Pytel, Andrew, and Jaan Kiusalaas. Engineering Mechanics: Statics. New York: HarperCollins College Publishers, 1994.

OVERHEADS

Importance of Modeling: Pressure Loading Pressure loading, such as that occurring with wind or water, can be modeled in many circumstances as a two dimensional distributed normal load. The distributed normal load force can then bemodeled as a point force acting at the centroid of the object. Various quantities can be calculated from this type of modeling.Fundamental modeling skills can be used in a variety of engineering situations, and typically produce reasonable results

Figure 1 shows a representative weight and dimensions for a small pickup truck. The designers of such vehicles must consider many factors to ensure safety. A subset of these factors, which is itself verycomplex, deals with factors that would tend to cause the truck to tip over on its side. In this problem we will consider only one of these factors which is side wind loading.

Figure 1. Weight and dimensions of a small pickup truck

Assume you have a pickup truck such as that in Figure 1 and that you want to build a shell to put on the back which is 5 feet high and the length of the bed. The resulting vehicle would be as shown in Figure 2.

Figure 2. Weight and dimensions of a small pickup truck with shell

a. What is the built-in tendency (moment in ft-lb) of the vehicles not to tip over on their sides?

b. What is the wind loading moment that would tend to tip the basic truck (as in Figure 1) over on its side?

c. What is the wind loading moment that would tend to tip the truck with shell (as in Figure 2) over on its side?

d. What does all this mean to you?

SOLUTION

a. The moment due to the weight of a vehicle about the line of contact of its tires is what tends to prevent it from tipping over on its side. This moment is 9,500 ft-lb (2.5 ft x 3800 lb) for the truck and 10,000 ft-lb for the truck with the shell.

b. We need the moment about the line of contact of the tires (road surface) due to the wind. Because the truck can be considered as a composite of several standard figures we can find the force and centroid above the ground as follows:

Figure Area (sq. ft) Centroid (above surface - ft) Centroid x area

Force = (30 sq. ft) x (10 lb per sq. ft) = 300 lb

Centroid location = 96/30 = 3.2 feet above the ground.

Moment = distance x force = 3.2 ft x 300 lb = 960 ft-lb

c. Again we need the moment about the line of contact of the tires due to the wind. Because the truck with shell can be considered as a composite of several standard figures we can find the force and centroid above the ground as follows:

Figure Area (sq. ft) Centroid (above surface - ft) Centroid x area

Force = (60 sq. ft) x (10 lb per sq. ft) = 600 lb

Centroid location = 291/60 = 4.85 feet above the ground.

Moment = distance x force = 4.85 ft x 600 lb = 2910 ft-lb

d. Although the tendency not to tip over due to the added weight would increase by about 5%, the tendency to tip over due to the wind would more than triple. What are some other factors that might cause the vehicle to tip?

Pressure Hazards Lecture Handout

Figure 1 shows a representative weight and dimensions for a small pickup truck. The designers of such vehicles must consider many factors to ensure safety. A subset of these factors, which is itself very complex, deals with factors that would tend to cause the truck to tip over on its side. In this problem we will consider only one of these factors which is side wind loading.

Figure 1. Weight and dimensions of a small pickup truck

Assume you have a pickup truck such as that in Figure 1 and that you want to build a shell to put on the back which is 5 feet high and the length of the bed. The resulting vehicle would be as shown in Figure 2.

Figure 2. Weight and dimensions of a small pickup truck with shell

American Society of Civil Engineers (ASCE) Standard 7 deals with minimum design loads for buildings and other structures. It requires consideration of winds that generate at least 10 pounds force per square foot loading. Using this value and the weights and dimensions in Figures 1 and 2, determine the following:

1. What is the built-in tendency (moment in ft-lb) of the vehicles not to tip over on their sides?

2. What is the wind loading moment that would tend to tip the basic truck (as in Figure 1) over on its side?

3. What is the wind loading moment that would tend to tip the truck with shell (as in Figure 2) over on its side?

4. What does all this mean to you?

SOLUTION:

1. The moment due to the weight of a vehicle about the line of contact of its tires is what tends to prevent it from tipping over on its side. On level ground as shown in the figure, this moment is 9,500 ft-lb (2.5 ft x 3800 lb) for the truck and 10,000 ft-lb for the truck with the shell.

2. We need the moment about the line of contact of the tires (road surface) due to the wind. Because the truck can be considered as a composite of several standard figures we can find the force and centroid above the ground as follows:

Force = (30 sq. ft) x (10 lb per sq. ft) = 300 lb

Centroid location = 96/30 = 3.2 feet above the ground.

Moment = distance x force = 3.2 ft x 300 lb = 960 ft-lb

3. Again we need the moment about the line of contact of the tires due to the wind. Because the truck with shell can be considered as a composite of several standard figures we can find the force and centroid above the ground as follows:

Force = (60 sq. ft) x (10 lb per sq. ft) = 600 lb

Centroid location = 291/60 = 4.85 feet above the ground.

Moment = distance x force = 4.85 ft x 600 lb = 2910 ft-lb

4. Although the tendency not to tip over due to the added weight would increase by about 5%, the tendency to tip over due to the wind would more than triple. This is just one factor, which would need to be considered along with other potentially major factors such as the effects of uneven ground and the centripetal force due to turning.