EGR 1301: Introduction to Engineering
Laboratory # 4: Predicting Strength of Truss Bridges
Part 1: Materials Testing & Truss Testing
1. Watch the first 3 slides of the PowerPoint presentation that was posted on Blackboard under Course Documents / Labs.
2. Watch the video of the material properties testing and other teams' bridge tests that was posted on Blackboard under Course Documents / Labs.
3.
The table below contains actual test data from Dr. Skurla’s lab, held on Tuesday, Sep 12. Calculate the strength of each of the specimens using the formulas provided in the PowerPoint presentation.
4. From what you have learned, which of the super glue brands should be purchased for you to build your truss bridges from (circle one)?
a. Ace Hardware Super Glue Gel
b. Scotch Super Glue Gel
c. Krazy Glue Gel
5. Predict the maximum load that a bass wood element with cross-sectional dimensions of 1/4 in. x 1/4 in. will be able to support.
6. Predict the maximum load that a bass wood element with cross-sectional dimensions of 3/16 in. x 3/16 in. will be able to support.
7. Predict the maximum load that a bass wood element with cross-sectional dimensions of 1/8 in. x 1/8 in. will be able to support.
Part 2: Use of ModelSmart Software to Predict Load-Bearing Capacity of Truss Bridges
1. Watch the remainder of the PowerPoint presentation for instruction on how to use the ModelSmart software.
2. Truss #1 - Design a truss bridge that is 10” long across the bottom and 4” long across the top with the top centered. The top truss member should be 5” higher than the base. Other requirements for Truss #1 include:
a. Use the highest grade of balsa wood and the largest cross-sectional area available (i.e., 1/4” Balsa – BAL8D3). NOTE: The “8” in the wood ID stands for 8/32 inches, which simplifies to 1/4 inch.
b. Make a sketch in the space provided below and identify the truss member that fails with an “X” over the truss member.
c. Determine the ratio of breaking load (lbs) to structure weight (grams).
d. Next to each truss member that does not fail, write the efficiency ratio of that member. Indicate with a “C” if the member is loaded in compression and with a “T” if the member is loaded in tension.
3. Truss #2 – Continue to use the same truss design and grade (i.e., D3) of wood as in Truss #1, but change the elements to the smallest element dimensions (i.e., 1/8” Balsa – BAL4D3)
a. Make a sketch in the space provided below and identify the truss member that fails with an “X” over the truss member.
b. Determine the ratio of breaking load (lbs) to structure weight (grams).
c. Next to each truss member that does not fail, write the efficiency ratio of that member. Indicate with a “C” if the member is loaded in compression and with a “T” if the member is loaded in tension.
4. Truss #3 – Continue to use the same truss design as in Truss #1, but change the elements to the poorest grade and the largest cross-sectional area (i.e., 1/4" Balsa – BAL8D1).
a. Make a sketch in the space provided below and identify the truss member that fails with an “X” over the truss member.
b. Determine the ratio of breaking load (lbs) to structure weight (grams).
c. Next to each truss member that does not fail, write the efficiency ratio of that member. Indicate with a “C” if the member is loaded in compression and with a “T” if the member is loaded in tension.
5. Truss #4 – Continue to use the same truss design as in Truss #1, but you may use any possible combination of truss members grade/size to get the best possible ratio of breaking load to structure weight. You may use double members in this design.
a. HINT: In order to maximize the performance of your truss, try to get the efficiency ratios of each truss member to 0.5 or greater (i.e., except in those cases where you are already using the smallest and weakest members).
b. Print out your truss and your results report.
c. Calculate the load/weight ratio and write it on the printout of your truss.
6. Truss #5 – Continue to use the same 10” width for the base of your truss. Otherwise, you may change the design in any way that you would like. Try to keep the load/weight ratio as high as possible.
a. Print out your truss and your results report.
b. Calculate the load/weight ratio and write it on the printout of your truss.