Weston Middle School

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Introductory Video: Pushing to Failure

• Working in partners, We will be building prototype beam bridges using straws and pins, and measuring their efficiency
• Efficiency= Live Load(Capacity)/Dead Load
• Bridges will be 20” long, 4” wide
• Top surface should be long beams
• Label Foam Boards- will be used as jigs

For a given number of lengthwise beams, why does the height of the beam (web) matter?

Demo hinged I-beam model

Do diagonal struts within the plane of the deck affect the vertical stiffness?

Demo measuring efficiency

• Do not become too committed to bridge design- it will be changed at least 3 times!
• What matters is that you are trying different designs, and analyzing how they fail
• The design cycle- pick a solution- build-test-redesign
• Measure Efficiency of Beam Bridge using water bottles as weights( 1 8 oz bottle = 280 gm)- see testing form
• How is it failing? How Can it be strengthened?
• # of beams; height of beams
• Review failures- should fail in bending, not in connections
• Keeping straws in one plane important
• Complete testing sheet

  Mass Production of Beam Bridges
  
  

   

  Highway Beam Bridge

  ( MassPike)- Weston

   

   

   

   

   

   

   

   

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  Practice building a truss using straws-

Building Procedure:

• Pin Pairs of long beams to plans
• Insert vertical members, pin in place
• Insert diagonal members, pin in place
• Trim extra straws
• Carefully remove truss from board, bend over pins with pliers
• Repeat Procedure for 2nd truss

• Lesson 2:

  • Introduce Truss styles- see Truss Diagrams
  • Trusses are beams made of triangles, such that all elements are always in compression or tension ( not bending or shear)
  • The greatest bending moment( load x distance from support) is in the middle
  • Types of Trusses :
    • • King Post/Queen Post
      • Warren Truss:

      

      

      • Pratt Truss/Howe Truss:

      

      • Bowstring Bridge:

      

      • Lenticular Truss:

      

    • Trusses will pin on to sides of deck

    

    Sketch 3 possible designs- choose 1 to build

    • Design Trusses on graph paper- make two copies
    • Make using foam board jig- try different designs- show on board

    

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  • Truss showing gussets

   

   

   

   

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  Building Procedure:

  • Pin Pairs of long beams to plans
  • Insert vertical members, pin in place
  • Insert diagonal members, pin in place
  • Trim extra straws
  • Carefully remove truss from board, bend over pins with pliers
  • Repeat Procedure for 2nd truss

Test Trusses- Where does it bend?

Lesson 3: Connect trusses with cross pieces. Add

• top bracing
• bottom bracing
• portal bracing- (there are three dimensional planes).

• Why is a truss efficient?

Lesson 3-4: Load Test to close to failure- analyze weak points- reinforce where failing

Point loads will be applied close to center, so that the moment (=force x length ) varies directly with the load.



 

Why do bridges have trusses in 3 different planes?

• Test for efficiency= Live Load( capacity)/ Dead Load( weight of bridge)
• Weigh bridge to determine dead load.
• Test strength using water bottles(280 gm) , calculate efficiency.

Complete Truss Bridge Analysis Worksheet


• HOW DOES IT SPECIFICALLY FAIL?



If done- improve bridge; tape joints, assist other students; make small arch bridge or suspension bridge

Alternative: Suspension Bridge- Man Who Would be King







• Nova Science Now: Smart Bridges
  Can we engineer bridges that tell us what's wrong with them before it's too late?
  



 

Truss Bridge Analysis Worksheet

Truss Types

Sagamore Arch Truss Bridge

Straw Bridge Testing March 6th

Physics of Dynamic Loads(Walter Lewin)