build bridges

Project work for statics 12-212

Constraints:

Structure must span 1 foot
The structure needs a place to hang a hook for load test at mid‐span
Each team is allowed to use only the provided material, including 18 of 6’’ by 1/8’’ by ¼’’ balsa wood, dental floss, and wood glue. You may use less material, but may not use more. You may use any tools.
The structure must be free‐standing and cannot be attached to the support.

Design:

Highlights:

Overlapping Design:

Inherently weaker at joints
- Strength depends heavily on strength of glue, which we are unable to calculate or control, rather than the strength of the material (which we have already measured)
Overlap ensures that at every vertical line of the beam, there is not a dependence on the joints for support

Simple Design:

Balsa is not strong (test capacity of 1.3lb downforce)
Reduces risk of human error in trying to achieve symmetry
Prevents uneven weight distribution, which can lead to more stress on certain areas and higher chance of breakage
Almost 2D design reduces need for consideration of forces, moments, and design symmetry along z-axis.

Predictions:

Testing Strength of Sticks:

Summing Fy,

N1+N2-1.3=0

N1=N2=1.3/2=0.65lbs

Internal Bending Moment at breakage:

0.65lb*3in=1.95lbin

Maximum Load Capacity:

Using 12 sticks, at the point where the string sits,

Strength= 1.95*6=11.7 lb in
T1Y + T2Y = W
T1Y = T2Y = W/2 lb

Summing Fy = N3 + N4 - T1Y - T2Y = 0
N3 = N4 = W/2 lb

At point A, Internal Bending Moment
= N3 * 1’
= W/2 lb in

W max / 2 = 11.7
W max = 23.4 lb

Failure Locations:

The Side: If angles aren’t the same, one side will have more load, and the structure is likely to break on that side
Along the X-axis: If it is unequal, Fx of the two strings would not be equal and there might be an resultant force along the x axis. Though the ends of the beam are not pinned/bolted to the table, there is no roller and the friction between the beam and the table is likely to be large enough such that resultant Fx is negligible.

Results:

The structure was a huge success! We exceeded our expected load, and only stopped adding weight to the load in fear that the bucket would break. The structure was not splintering, and the only signs of wear were the floss digging into the balsa wood at the top.

constraints:

structure must span at least 2 feet from the support wood piece (cantilevered)
structure must be attached to the given mounting block (to be later attached to a vise)
structure must have a place to hang a hook for the load test at the end
structure must be free-standing when fixed with the vise. The structure needs to be firmly attached to the provided support wood piece and should not interfere with the vise.
must use only the given materials:
- 36 pieces of 6" x 1/8" balsa wood
- wood glue
- floss
failure is defined as when the structure can no longer support the load at the 2 ft distance (e.g., too much sagging to span 2 ft horizontally) or the load bucket touches the ground.

Design:

The idea for this design is something that is relatively simple in shape, and a structure that favours compression forces into the mounting block as opposed to tension. The floss this time was rationed out, so I only used it to bolster the loading piece at the very end.

The physical building process for this project was a bit nit-picky, as I only had two hands to hold things together and the clamps I had on hand were either too heavy, or too small. Slow and steady with this structure seemed to be the key to clean and even joints.

Even still, I soon realized that each of the given pieces of balsa wood were not exactly the same length. Some even differed by up to 1/4"! Thankfully, balsa wood is also not the most rigid of materials so it worked out.

Given the strength of the balsa wood discovered in the previous project, I estimated that my structure would be able to carry the 5lb load maximum. It did not. However, coming in at 2.42lb it was the strongest structure in the history of the class.

How it broke:

My design was to focused on the structure breaking at the joints, I did not foresee the structure breaking within the beam itself.

During the build process, very near completion, I accidentally dropped the structure while getting clamps. This split a few of the pieces near the base, however I glued them back together and all was good. Because of this incident, I believed that the point of failure would be at one of the points of breakage from the fall. However the strength of the glue overpowered the strength of the wood!

results:

In conclusion, while I did not make the 5lb goal, I am very happy with the results and consider this project a success. I'd like to see how the structure would perform without the middle bar. I believe that I found a relatively simple and elegant solution to this problem, and would love to see how much further it can go.