Anatomy of A Bridge
Anatomy of A Bridge
Atin Sinha, Ph.D., P.E.
Albany State University
Atin Sinha, Ph.D., P.E.
Albany State University
Different Types of Bridges
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Truss Bridge
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Suspension Bridge
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Cable Stayed Bridge
Truss Bridge
Truss Bridge
Suspension Bridge
Suspension Bridge
Cable Stayed Bridge
Cable Stayed Bridge
Loads Experienced by A Bridge
 Static
(Structural)
 Variable
(Vehicular Traffic, Pedestrian)
 Environmental
(Wind, Earthquake)
Forces that Act on Bridges
 Tension
 Compression
 Torsion
 Shear
Suspension vs. Cable Stayed
Common Trusses Used in
Engineering:
Warren
Pratt
Howe
K Truss
Red represents tension, blue represents compression,
and green is no load
Now examine the same Warren truss,
but with added vertical members
Model Bridge Building Steps
 Learn
the Rules of the Competition
 Design the Bridge
 Get the Supplies
 Build the Bridge
 Test the Bridge
 Analyze the Failure
 Improve Your Design
Southwest Georgia Regional Model Bridge
Building Contest
Design Specifications for the Model Bridge
Span
22 in.
Maximum Allowable Weight
28.3 g
Maximum Allowable Height (Overall)
10 in
Max Superstructure Height
6 in
Max Substructure Height
4 in
Minimum vertical clearance through the bridge
2 in
Minimum horizontal clearance through the bridge 2 in
No height restriction for Middle Schools
Design the Bridge

Center the design around the loading point
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Provide extra bracing around the loading point
Choose a truss Warren, Pratt or design your own
Draw the bridge to scale on graph paper
Create a list of all the pieces
Cut the pieces to exact size
Label and weight every piece
Allow 10% weight for glue
If you exceed the weight limit, modify the design
Preparation for Building the Bridge

Get the wood
 Hobby stores
 Specialized Balsa.com
 Pick out the “good” wood
 Tools
 Glue
 Saw/Exacto knife
 Gram scale
 Clamps/clothespins
 Foam Board (optional)
 Workspace
 Good lighting
 Good ventilation
Build the Bridge
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Tape the top and bottom beams to the design on
the graph paper
Glue the remaining pieces and cure overnight
holding firmly using pins.
After both trusses are completed, tape them to
two fixed boxes spaced correctly apart
Glue bracing on top and bottom
Make sure the bridge is not leaning on one side
Weigh the bridge and record including the specs
Test the Bridge

Test before competition if:
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Testing equipment
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You have time to build another bridge
You are only testing your bridge to a certain point
Bathroom scale (top loaded only)
Bucket and sand (top or bottom)
Testing procedure
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Pour the sand slowly but steadily
Watch for deflection ( 2” is considered failure)
Evaluation of the Bridge
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Efficiency = mass that bridge held divided by mass
of bridge
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What failed (Broken member / joints unglued) ?
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In order to win typically score need to be well above 250
Video taping
Careful examination of broken pieces
Improving

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Design (Add a piece where it failed)
Construction (add notched joint / gusset plate)
Winners From the Past Years
Year
Bridge
Weight (gm)
Breaking
Load (gm)
Structural
Efficiency
2003
2004
2005
28.1
25.6
22.8
10276
5789
10197
365.7
226.1
447
2006
17.5
9488
542
2007
23.7
13253
559
2008
28.1
11114
396
2009
22.7
5930
261