Bridge Design Analysis

Introduction

The purpose of this memo is to report the bridge prototype results and present our analysis in order to come up with the best bridge design possible.  We will take three bridge ideas and predict their weaknesses, cost them, estimate their strength and, finally, choose our favorite design.  The analysis will be presented in the results; our recommendation will be stated in the conclusion.

Results

 

Question 1 - Weaknesses

Refer to Figure 1, Figure 2 and Figure 3 to see our three design alternatives.

Bridge Design #1 appears very along the horizontal base.  Due to the base being a single layer of tongue depressors, it has little y-axis or z-axis strength.  This results from a lack of compression.  Another weak spot is where the arch meets up with the sides of the 2x4’s.  Only the edge of the arch rest on the 2x4 so I would expect the weakly supported edge to pop off from lack of compression.

Bridge Design #2 is very weak in the center of the base.  Although this base uses triangles for strength, the middle of the base is not supported by anything else.  There are no supports that triangle up from the side of the wall.  Also, there is not anything kind of support from above.  The middle is alone to fend for itself without and tension from above or compression from below.

Bridge Design #3 weak point exists in its length between the tongue depressor joints.  The 4-5 inches of tongue depressor floating between two joints provides excellent opportunities for the tongue depressors to fold over due to the lack of compression on the sides of each tongue depressor.

Question 2 – Cost

Bridge design  #1
Tongue depressors (non-standard)       188       x        $1.15     =       $216.2 Bolt/nut combination                            132      x         $0.50     =      $66
Washer                                                  264       x        $0.10     =      $26.4

String                                               90inches    x        $0.05     =       $4.50

Total                                                                                                    $313.10

 

 

Bridge design #2
Tongue depressors (non-standard)       232       x        $1.15     =       $266.80 Bolt/nut combination                            238      x         $0.50     =      $169

Washer                                                  576       x        $0.10     =      $57.60

Total                                                                                                    $493.40

Bridge design #3
Tongue depressors (non-standard)        95       x        $1.15     =       $109.25 Bolt/nut combination                             91      x         $0.50     =      $45.50

Washer                                                  182       x        $0.10     =      $18.20

Total                                                                                                    $172.95

Question 3- Rank

 

Bridge Design #1 could hold an estimate of 300 pounds according to our tension testing of the string.  If the base was strong enough, we believe the string both from above the base and below the base could cause the bridge to hold way over 300 pounds.

Bridge Design #2 could hold an estimate of 175 pounds.  The short tongue depressors would make the base strong, yet because there is no extra support for the middle of the base it would probably snap early.  Furthermore, the arch will help very little positioned how it is.

Bridge Design #3 could hold an estimate of 250 pounds.  Although the lack of materials would cause to believe this to be the weakest bridge, due to its simplicity we think it can hold the most.  It uses strong triangular shapes and lives little room for error along the x and y-axis.  The weakness would be along the z-axis.

Conclusion

Question 4

 

We believe Bridge design #3 to be our best design.  Not only is it simple to build and strong, it costs the least too.  The pound to cost ratio for each are:  Bridge Design #1---0.96 lbs/$, Bridge Design #2---0.35 lbs/$ and Bridge Design #3 1.45 lbs/$.  The third design is about 50% more efficient than the first and about 400% more efficient than the second.