Timber Bridge

About 

Strategy:

• We want to avoid zero force trusses→usually put into bridges as safety mechanisms and to satisfy stability requirements and avoid other trusses buckling

• Avoiding them in our design to decrease bridge weight to holding weight ratio

• when only 2 members form a joint and they dont lie on the same line and there is no external force, they are both zero force

  • This is why we avoided pieces on either end of the bridge that make the ends square instead of trapezoidal–they are always zero-force unless a load is placed directly on one of the end joints (which it won’t be)

• when 3 members form a joint and 2 out of 3 are on the same line, the 3rd will always be zero-force unless there is an external force acting on it

  • This means we want the weight on our bridge to be as widely distributed as possible–as in have the cinder block resting on as many truss joints as possible–to avoid having parts of the bridge not pulling their weight

• warren truss bridge will be lighter because there are less supports and will be probably less costly

• Warren trusses with middle vertical supports will be heavier because there are extra supports, but the force will be more spread out among joints even though it will be more costly

• Our bridge can combine these two designs–having the warren trusses in the outer parts of the bridge at joints that will not have loads placed directly on them to reduce weight, then having a type of vertical truss near the middle at locations where loads will be placed to more widely distribute the forces.

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Ratio:

Weight: 14.6lbs

Tolerance: between Tyler and Dr. Sanchez

Strength-to-weight ratio: (14.6lbs + 190lbs) / (14.6lbs) = 14

Interpretation: Our bridge could withstand 14 times its own weight. In other words, for every pound of material used on the bridge we were able to support on average 14 pounds.

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Factor of Safety:

Calculate the factor of safety for the bridge, assuming a design stress of 50 lbs. Provide the calculations as well as an interpretation of these results.

Factor of Safety = Failure strength / design stress =>190/50 = 3.8

Interpretation: If our bridge was designed to hold a load of 50lbs, it was able to hold 3.8 times the expected load.

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Strengths:
While our bridge bent a lot more than most groups, we think this heavily contributed to the bridge’s success, as it prevented the bridge from simply snapping under a heavy load. The trusses were extremely helpful in the stabilization of the structure, particularly the vertical ones that lie directly underneath the loading weight. While MDF was likely not the most ideal material to connect the trusses, it was flexible enough to suffice and fit within our budget constraints. Our half inch plywood reinforced half lap joints on the bottom of the frame also held very well, except for the one which we made a mistake on the orientation. Our last piece that contributed to the success of our bridge was a solid piece of half inch plywood in the middle of the bridge, which helped stabilize the area experiencing the most force.

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Weaknesses:
Our bridge very clearly failed at one point, which was one of the half lap joints on the base of the structure. The reason for this failure was the orientation of the joint. Our intention was to orient the half lap joints such that the screws combining the two pieces would face horizontally, which would greatly increase the structural stability of the joint as the majority of the stress would not be placed on the thin interlocking pieces. Instead, the stress would be applied to the thicker portion of the strong pine lumber. However, a mistake was made during construction that resulted in this piece being oriented the wrong way in the final construction. Although the correctly built joint also failed, as seen in the picture, we think this was due to the other side of the bridge failing first, and that it would have held if the bridge had been constructed correctly. Furthermore, this mistake caused the quarter inch plywood supports of the joints on this piece to be on the left and right side, rather than the top and bottom. This made the supports useless, as the force experience here was purely vertical. We also believe we would have seen better results if we were able to do a full lumber frame, but we had to economize as we got towards finishing the project.