A2 -Ye

The statement of design goal 
From the experience of designing the bridge through WPBD, a simple design rule was learned. The simple and classical bridge structure would be worked the best. In the design of the Knex Bridge, this design rule has been  applied from the beginning. The Knex Bridge was designed based on the classical Pratt design and  was slightly adjusted according to the limiting of the Knex Bridge model.

Design constraint:
   1. The angle between different chords should be 180 °, 90°or 45°.
   2. The length of the chords and the shape of the joints are limited. And further more, the combination of the chords and joints are limited.
  3. The cost of different chords and joints should be concerned.
  4. The span of the bridge should be longer than 2’.
  5. The width of the chord should be 4”.(The chord should be 3-3/8”)
  6. The top of the bridge should be flat to support the apparatus for testing

Over view of the designed bridge

Side View of the Bridge

Top View of the Bridge

"Truss Bill of Materials" spreadsheet image 

A statement of how your bridge changed during design

The bridge did not change a lot during the design process. A rough model of the design had been existed in designer's brain and just need be expressed using the chords and joints. From the life experience, the two ends of the bridge, which would be placed on the “bridge piers”, are going to be the most fragile part during the test. The design increases the “thickness” of the bridge for the purpose to decrease the pressure on the joints on the two ends. How the bridge is going to work is unknown right now. More modification are needed during the actual load test. The balance between the cost and the weight it could bear should be reached.

What you learned from designing this bridge.

In the process of drawing top view and side view of the bridge, an concept has been realized that the positions of the joints have actually decided the positions of the chords. The joints played multiple functions in the building of the bridge. The joints expanded the length of the bridge, connected the chords and beat the weight of the bridge. The optimal design of the joints would extremely benefit the design of the bridge.  Drawing of the bridge help to organize the ideas of the design. The process of the design Knex Bridge is much different compare to the WPBD design. There are more constraints in designing the Knex Bridge.