OBJECTIVES
1.  Explain how the bridge components support weight.
2.  Identify the three basic types of bridges.
3.  Design a bridge using given specifications that will support weight.
4.  Using student made design, construct a bridge with given materials that will hold weight.

1. ROADBED: supports the weight. The weight includes:
 a. The entire bridge
 b. The load it is holding
2. LOAD: the weight that is to be held by the bridge.
3. PLATFORM: the area at each end of the bridge. The roadbed rests on each of them.
4. SUPERSTRUCTURE: the part of the bridge that is located above the roadbed. It gives extra strength to the roadbed.
5. SUBSTRUCTURE: the part of the bridge that is located below the roadbed.  It gives extra strength to the roadbed.
6. SPAN: the distance from one platform to the other.

TYPES OF BRIDGES

1.  TRUSS: uses triangles as the main element of design. All components are usually straight.

 
 

CONTEST RULES

1. The bridge must be constructed ONLY from the following materials:
         -21 ft. of 1/8" x 1/8" balsa wood
         -non-toxic glue
2. Equipment:
         -one cardboard work area
         -wax paper to cover the drawing during construction
         -straight pins (to hold wood in place while the glue dries)
         -one X-acto knife
3. Specifications:
 A. Over-all bridge size
      length: 12" maximum   (NOTE: the SPAN, the distance between the bridge supports, will be 10")
     width: 3" maximum   (NOTE: The TEST BLOCK, the weight holding device, will be 2 1/2" wide)
 B. Roadbed
      height: 1/4" maximum from support surface
     width: 2 1/2" minimum (so that test block will fit on roadbed)
     length: 12" maximum (same as over-all length)
 C. Superstructure (if any)
      height: 3/4" minimum (so test block will fit on roadbed)
      width: 3" maximum
 D. Substructure (if any)
     depth below roadbed: 3" maximum
      width: 3" maximum
      length: 10" maximum
4.  Plans for the bridge must be completed in two views:
     -side view
     -roadbed
5.  Limitations:
     A. Only balsawood and non-toxic glue will be used.
     B. Wood cannot be treated in any way to change its strength or appearance.
     C.  Only the amount and type of materials specified will be used in the bridge.
     D.  Wood pieces may be bonded together with glue ONLY at joints.
     E.  Splitting or laminating are not allowed.

A.  Tools and Materials
 1.  Completed bridge (allow overnight drying time)
 2.  Testing equipment (provided by Instructor)
      a.  Testing block
      b.  Testing hook
      c.  Length of chain
      d.  Weights (various sizes)
      e.  Calculator
      f.  Bridge support device

B.  Procedure  (NOTE: To qualify for testing, the bridge must meet all specifications given in previous hand-outs.)
 1.  Check bridge to see that it meets all specifications.
 2.  Compare bridge to drawings.
 3.  Measure weight of bridge and record.
 4.  Attach testing block and hook.
 5.  Place bridge between support system.
 6.  Suspend chain on hook.
 7.  Place weight on chain in even, increasing measurements by sliding them onto the chain, then carefully placing the other end of the chain onto the hook, and SLOWLY lowering the weight to rest on the chain.
 8.  Continue until the bridge fails (breaks).
 9.  Determine weight held (test block, hook, chain, weights)
 10. Calculate efficiency using the following equalizing formula:

11. Clean up test area.