Boom Construction Competition
The experimental objective of this lab is to design and construct a boom following the specifications provided. The boom will be entered in a competition against other booms in the section, and will be judged by a ratio using the following criteria: boom weight and length, weight held, and anchor time. The group with the highest competition ratio wins.
A boom is used to lift and move heavy objects, often objects that are much heavier than the boom itself. Distributing the weight of the object, or the load, being lifted over the length of the boom is the main problem in boom design. The design must consider the maximum load the boom will be required to lift, how high the load will be lifted, and whether the boom will be moved or remain stationary while loaded.
Examples of Booms
Certain types of bridges use booms. A cantilever bridge uses two booms extending from a common base. One type of cantilever bridge is a cable-stayed bridge (Figure 1).
The Ed Koch Queensboro Bridge is a double cantilever bridge (Figure 2). It has two bases with two booms extending from each base and the cantilevers placed end to end.
The Grand Bridge over Newtown Creek is a swing bridge, also known as a rotating bridge (Figure 3). This bridge has two booms mounted on a base that rotates.
Figure 4 shows a bascule bridge, more commonly known as a drawbridge, where it is clear that the bridge uses a big, very flat boom.
Not all bridges are booms. Suspension bridges use a deck that is supported by steel cables, not booms. Examples of suspension bridges are the Brooklyn Bridge, Manhattan Bridge, Verrazano-Narrows Bridge, and the George Washington Bridge.
Cranes are the most common example of booms. The crane pictured in Figure 5 is a tower crane. These cranes are a fixture on construction sites around the world. A tower crane can lift a 40,000-pound load. It is attached to the ground by anchor bolts driven through a 400,000-pound concrete pad poured a few weeks before the crane is erected (Howstuffworks.com, 2003).
Stress and Strain
The design of a boom must consider the properties of the materials used to build the boom. The mechanical properties and deformation of solids are explained by stress and strain. When an external force is applied to a material, it changes shape (e.g. changes length and cross-section perpendicular to the length). Understanding how deformation will affect materials is a critical consideration in boom design.
According to Serway and Beichner in “Physics for Scientists and Engineers,” stress is the external force acting on an object per unit cross sectional area. Strain is the measure of deformation resulting from an applied stress (Figure 6).
The expression (1) for tensile stress shows the relationship between an applied force and the cross-sectional area.
In (1), σ is the stress, F is the applied force, and A is the cross-sectional area of the object perpendicular to the force. The resulting strain (2) is calculated by dividing the change in length of the object by the original length.
In (2), ΔL is the change in length and L0 is the object's original length.
There are three basic types of stresses; tensile (pulling or stretching), compressive (squeezing or squashing), and shear (bending or cleaving). Consider a straight metal beam. If a tensile stress is applied to both ends, its length will increase in both directions of the force, while its cross-sectional area perpendicular to the force applied will decrease. Under compressive stress, the opposite will occur. If the beam is subjected to shear stress, it will bend towards the direction of the applied force, and both the length and cross-sectional area of the beam will become distorted. Figure 7 depicts a graphic representation of the three common forms of stress.
Strain is proportional to stress for material dependent values of strain. If the material is known, it is possible to derive strain from measured stress, and vice-versa, up to a certain level of stress. This unitless mathematical constant is referred to as the elastic modulus, or Young’s modulus. The moduli of different materials is an important factor to consider when designing or building any form of structure meant to encounter stresses.
A stress-strain curve graphically shows the relationship between the stress and strain of a material under load. Figure 8 shows the stress-strain curve of a common metallic building material. In the elastic region, the material will regain its original shape once the stress or load is removed. Note that the elastic region in Figure 8 is fairly linear. The slope of this linear portion of the stress-strain curve is the elastic modulus, as defined above.
The elastic limit for a material is the maximum strain it can sustain before it becomes permanently deformed (i.e. if the stress is decreased, the object no longer returns to its original size and shape). In the plastic region, the material loses its elasticity and is permanently deformed. A linear approximation with the elastic modulus is no longer accurate. The ultimate tensile strength is the maximum stress a material can undergo. The fracture stress is the point at which the material breaks. Fracture stress is lower than the ultimate tensile strength of a material as by the time a material reaches that level of stress, it has already begun to rip apart. Thus, the cross-sectional area is constantly decreasing until the material finally breaks.
In addition to these intrinsic materials factors, the behavior of materials as they age and are used in service must be considered in boom design. These factors do not relate directly to the boom design in this lab, but they must be considered when deciding what material to use for an actual design. The loss of desirable properties through use, called fatigue, is important. Non-static loads, repeated loading and unloading, or loads that include vibrations or oscillations will eventually lead to failure in service. Special care must be taken with live loads and situations where small motions may be magnified by design features.
There are many factors to consider in any design project. When designing and constructing the boom for this competition, remember to consider the materials being used and what might cause those materials to fail under a load.
An extremely important tool in an engineer’s skillset is the use of simulation tools. Many times, complex designs require quick analysis that cannot be done by calculation or prototyping, so simulation comes in handy and as it saves both time and money. Specifically, Finite Element Analysis (FEA) is a process that a computer software undergoes to simulate a desired scenario. The software calculates mathematical formulas at hundreds to thousands of points on a design, referred to as a mesh, and computes them simultaneously to get a complete look at what would happen. This simulation only works for isotropic materials, meaning that the materials properties (tensile strengths, modulus, etc.) are the same for all sides of the material. This may not be true for certain materials, and an engineer must take this into account. While members of the group are designing and building a boom, others will simulate a basic design in order to understand the effect of the forced displacement, and use that information to improve the initial design.
A brief digest of the tools and functions of the Fusion 360 Design workspace is presented below. This digest is specific to this virtual lab and many of the CAD functions can be performed differently in other situations.
- The Orbit tool allows rotating the current view around a pivot point in three dimensions. This can also be done by grabbing the orthographic cube in the top right corner and rotating it. To make the view normal to a face, edge, or corner, click the respective part of the orthographic cube.
- The Zoom tool allows for the magnifying or minimizing of an object.
- If the component is lost in the view, either use the Pan tool to move the current view in 2D until it is found, or click Zoom Window > Fit. These tools will help navigate the workspace while putting together the boom.
- Once a component is placed into the workspace, Fusion 360 will remember the component in that space. To move it again, use the Move/Copy tool after right clicking the component (Figure 10).
- Once the component is moved to the desired position, use the Capture Position tool to set the component in that place (Position > Capture Position). If it is ever moved unintentionally, use the Revert button (Position > Revert) to move the component back to its original position. These tools will only show up when the model is an unsaved position (Figure 11).
- Deleting parts requires an extra step. Clicking on an object automatically selects the face of a body and not the whole body, and Fusion 360 will not be able to delete a face. To get past this, in the toolbar go to Select > Selection Priority > Select Body Priority (Figure 12).
- A part can only be deleted by selecting its body. Make sure to use the same steps to change the selection priority back to face, or to edge/component if necessary. Another way to delete is to right click on the component in the Browser tab and click Delete.
- The Align tool is found under the Modify tab of the toolbar. This is useful when attempting to place cylinders in a position defined by an opening. Simply select the outside face of the cylinder and the inside face of the hollow cylinder to align the two parts. Make sure to check the Capture Position Box or use the Capture Position tool to save this setup of the model (Figure 13).
Changing Boom Length
- To cut a part of the boom, first open the boom file by double clicking on it from the Data Panel.
- Click on the circular face of the boom. In the toolbar, go to Modify > Press Pull. Use the blue arrow in the current view to change the length of the boom, or type in the desired change in Distance.
- Use Save as to make a new component with a different name. Only the total length supplied by the materials provided can be used (do not increase the length of the full dowels or skewers).
The competition rules must be followed at all times during the competition. Violation of any of these rules will result in the disqualification of the design.
- The boom is to be secured (i.e. anchored) to the white plastic anchorage provided at the front of the lab
- The boom must extend at least 1.5 meters horizontally from the front edge of the anchorage
- The boom must be anchored in two minutes or less
- The boom may not touch anything but the anchorage
- The basic weight ratio (3) for the competition uses the weight supported divided by the boom weight
- The winning design will be determined by the weighted design ratio (4), which uses the weight ratio, anchor time, and boom length
- Which aspects of the competition ratio are most advantageous?
- How can the boom be built and/or reinforced to prevent as much deflection as possible?
Materials and Equipment
- Two thick dowels (1.1 cm × 122 cm)
- Two thin dowels (0.8 cm × 122 cm)
- Six bamboo skewers (30.5 cm)
- 3D printed dowel connectors
- Cellophane tape
- Kevlar string
- Fusion 360 (Remote students)
Note: A saw is available to cut the dowels. Ask a TA for assistance, as only TAs may use the saw.
Note for Hybrid Session
In-person students are expected to complete the competition part of the procedure, indicated by the label (In-Person). Remote students are expected to complete the virtual boom analysis part of the procedure, indicated by the label (Remote). Any procedure labeled (Hybrid) is expected to be completed through a joint collaboration between remote and in-person students.
1. Boom Design and Construction (Hybrid)
- Assess the materials and consider the design options, keeping in mind the competition specifications. Preliminary sketches must be completed during this process
- Sketch the basic design in pencil using the lab notes paper provided by a TA or on the EG1003 website. Label the design clearly and discuss the pros and cons of the design with your teammates.
- Construct the boom based on the completed sketch and the available materials. A TA will provide the materials allowed for the design. If the design is modified during the construction phase, make sure to note the changes and describe the reasons for them.
- While in-person students begin to construct the boom, remote students will analyze a sample boom simulation on Fusion 360 to gain a better understanding of the forces that will affect the boom. After viewing the simulation, remote students will then move to construct their groups boom design in Fusion 360. This is highlighted in the section below.
- After the virtual students have completed the Virtual Boom Analysis procedure, and in-person students have completed step 5 of this procedure, ask a TA to begin load testing for your in-person boom.
- A TA will weigh the boom and record the weight in the competition spreadsheet for the section.
2. Virtual Boom Analysis (Remote)
The steps below will go through how to analyze and understand the boom simulation, and find the corresponding design ratio for it. The boom simulation was made using the parts with the same shape of the in-person boom parts, and an assumed material with the approximate inputted properties of wood was used (Research isotropic vs orthotropic materials). The data needed for the design ratio will be found and translated and the forces along the boom will be analyzed to improve the in-person boom design.
- The example of the boom model can be found in a ZIP folder here. The latest version of Fusion 360 must be downloaded for the parts to appear properly.
- Unzip the parts to a space on the computer.
- In Fusion 360, open the Data Panel (icon with 9 boxes at the top left) and click Upload. Select the unzipped part files to upload them to Fusion 360’s cloud memory. Fusion 360 can only import files uploaded to its cloud.
- Ensure that the Design workspace is open in Fusion 360. This is indicated at the top left.
- Double click on the Theoretical Boom file in the Data Panel to open it.
Mass of the Boom
- Ensure that the Design workspace is open in Fusion 360. This is indicated at the top left.
- Go to the Browser on the left-hand side of the workspace. Use the small, white arrows on the left of the part names to open their folders. Open the Bodies folder of the part, right click the body of interest, and select Properties (Figure 9).
- In the Properties dialog that appears, record the mass of the body.
- Repeat these steps for all the bodies in the boom design and add the masses to obtain the total mass of the boom. Record the total mass of the boom in your Lab Notes.
Length of the Boom
- Ensure that the Design workspace is open in Fusion 360. This is indicated at the top left.
- In the toolbar, go to Inspect > Measure.
- In a side view of the boom, select a point at the front of the anchor and then another point at the tip of the boom.
- In the Measure dialog, check the XYZ Delta box.
- From the Measure dialog, record the length in meters in your Lab Notes.
Understanding the Results
- Switch the workspace from Design to Simulation at the top left of the screen. Note: If you get an interference warning it can be ignored since seeing the contacts allows better visualization of the boom’s deflection.
- The results will now be calculated and obtained. In the Browser tab, right click Results > Solve. In the Solve dialog, click Solve 1 Study. Wait for the simulation to complete.
- After the simulation has been solved, the workspace will show a deflected boom similar to the one shown below in Figure 11.
- The scale on the right will be reading Safety Factor with a scale from 0-15. Make sure the minimum safety factor is above 1. The safety factor is the yield strength of the material divided by the maximum stress calculated at a point. If this is under 1, this is the simulation showing a broken part.
- Use the drop down menu of the other options, for example the drop down menu for Reaction Force (figure 11), to analyze the boom further. Simulation contacts describe the interaction between parts in an assembly. Take a look at where the contact force is and use this knowledge for the analysis of your team's boom. This simulation is being displaced 20 cm, just like your boom will be, so the general reactions should be relevant to the actual boom that will be built by the in-person students.
Finding the Reaction Force
- The reaction force will be determined. In the Results toolbar, go to Inspect > Reactions and click the bottom face of the boom anchor as the Entity. By Newton's Second Law, the static displaced structure should have a reaction force on the fixed face equal to an imaginary weight placed when the beam was deflected. There is a force on the bottom face of the boom anchor that is a resultant of the 20 cm displacement at the end of the boom. This is the reaction force.
- From the Reactions dialog, record the axial force on the same axis that the displacement was directed on (Figure 12). It should be the largest of the three axial forces.
- Using the Reaction Force found, calculate the weight supported (5) as well as the basic weight ratio (3) and weighted design ratio (4) for the given virtual boom. This is done by following the math below derived from Newton’s Second Law. Assume Anchor time as 30 seconds.
3. Competition (In-Person)
Note: Attaching the boom to the anchorage is a critical phase of the competition. Anchoring will be timed. Making sure there is a plan to anchor the boom quickly will improve its standing in the competition. Practice anchoring before the trial begins. The boom will be disqualified if anchoring the boom takes more than two minutes.
- When the TA says "go," attach the boom to the anchorage and shout "done" when the boom is anchored. The TA will only stop the timer once there are no more hands touching the boom. The TA will give the anchoring time. This value will be used to compute the boom's design ratio.
- A TA will measure the boom and record the length in the competition spreadsheet for the section
- A TA will attach a basket to the end of the boom and add weights until the boom deflects (bends) 0.2 m vertically. The load supported will be weighed on the lab scale and recorded in the competition spreadsheet for the section (Figure 10)
- The design ratio for the in-person boom design will be used to decide the winner of the competition.
A TA has prepared an Excel file with the section's results. It can be accessed in the Lab Documents section of the EG1003 website. This chart must be included in the PowerPoint presentation and in the Data/Observations section of the lab report. The lab work is now complete. Please clean up the workstation. Return all unused materials to a TA. Refer to the Assignment section for the instructions to prepare the lab report.
Individual Lab Report
Follow the lab report guidelines laid out in the EG1004 Writing Style Guide in the Technical Writing section of the manual. The following points should be addressed in the appropriate section of the lab report.
- Describe the rules of the competition in the Introduction. What consequences did the rules have for any design decisions? Use the appropriate equations in the answer.
- What factors were considered in designing the boom? Was any of the background information used?
- What was the basic weight ratio and design ratio for the design?
- Describe how the components chosen functioned in the design, and describe its height/length/shape
- Describe the advantages and disadvantages of the boom design
- Discuss design improvements. How can the design be optimized (i.e. improve the ratio) based on experience?
- Which elements of the boom were stressed by the load, in what directions, and contributed to the failure?
- How can theoretical models help to predict experimental models?
- Does the simulation tell us everything about the system? If not, what sources of error are important to mention?
- Include the spreadsheet with every boom's results. Describe the results and talk about other designs in the class.
Remember: Lab notes must be taken. Experimental details are easily forgotten unless written down. EG1004 Lab Notes paper can be downloaded and printed from the EG1004 Website. Use the lab notes to write the Procedure section of the lab report. At the end of each lab, a TA will scan the lab notes and upload them to the Lab Documents section of the EG1004 Website. One point of extra credit is awarded if the lab notes are attached at the end of the lab report. Keeping careful notes is an essential component of all scientific practice.
Team PowerPoint Presentation
Follow the presentation guidelines laid out in the EG1004 Lab Presentation Format in the Technical Presentations section of the manual. When preparing the presentation, consider the following points.
- How can the boom design be improved?
- Other than the examples given in this lab, what are other examples of booms?
- Which elements of the boom (e.g., wooden dowels, 3D printed dowel connectors, Kevlar string, etc.,) were stressed by the load, in what directions, and contributed to the failure?
How Stuff Works website. 2003. SHW Media Network. Retrieved July 28, 2003. http://science.howstuffworks.com/tower-crane3.htm
Jennings, James. 2015. “Up, UP in a Crane: What Life is Like as a Tower Crane Operator.” Philadelphia. Accessed 14 January 2020 from www.phillymag.com
Serway, R., Beichner, R., Physics for Scientists and Engineers with Modern Physics, 5th Edition. Fort Worth, TX: Saunders College Publishing, 2000