Bridge Girders
Adria Hogan
Benedictine High School
Richmond, VA
Developed with funding from the American Council of Engineering Companies
of Virginia and the MathScience Innovation Center.
Question
Where should the girders that support bridges be spliced in order to preserve their
strength?
Grade/Subject
Grade Six Science, Physical Science, Algebra 1
Virginia Standards of Learning: 2010 Science 6 (6.1); 2010 Physical Science
(PS.1, PS.10); 2009 Algebra I (A.7)
21st Century
Curriculum
Engineering: Design and Build (2.22, 2.23); Civil Engineering (4.41, 4.43, 4.45)
Background
Bridges bring people and vehicles safely across a span. Safety depends on
strength. Bridges are only as strong as the individual structures of which they are
made. How do engineers design the supports of large structures using single or
multiple supports?
Reducing costs is important to any engineer designing a project, and bridge
projects are no exception. A good way to reduce construction costs is to splice or
join large girders together to create single large steel girders needed to span
highways. The critical question—where to splice the girders together—must be
answered.
Loads are divided into 2 categories. Dead load is the weight of the material making
up the bridge. They do not change. Live load is the weight of the things on the
bridge. Live loads are dynamic (they change). Bridge designers must design with
the maximum live load in mind.
Moment is the force of an object on the supporting member. In the case of a
bridge, the further out the object is from the support, the greater the moment.
Therefore, the largest downward vertical moment is found at the middle of the
span and the largest upward vertical moment is found at the supporting members.
Safety
Remember to follow your regular classroom rules for labs and activities.
Materials
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Bridge Girders
TI-83 or another graphing calculator, paper, pencil
Popsicle Sticks
Masking Tape
http://mathinscience.info
©MathScience Innovationn Center, 2007
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Procedure
Pennies
Objects that will provide 5”-10” vertical height (chairs)
1. Tape together 3 popsicle sticks end to end.
2. Place the chairs 1 popsicle stick length apart and tape the sticks to the chairs at
the ends. You will have a stick on either side that is unused.
3. Load the stick with pennies in increments of 10 at the center of the stick and
record how many pennies are needed to break the tape at the end.
4. Place the chairs 2 popsicle-stick-lengths-apart and tape the sticks to the chairs
at the ends. You will have a stick on one side that is unused.
5. Load the sticks with pennies in increments of 10 at the center of the stick and
record how many pennies are needed to break the tape at the end or the middle.
6. Move the sticks and re-tape them to the chairs so that they are taped to the
chair at the middle of the sticks on both sides.
7. Load the sticks with pennies in increments of 10 at the center of the chair space
and record how many pennies are needed to break the tape at the end or the
middle. Be sure the push down on the end of the stick that is outside the chair
space as much as the moment on the inside of the chair space.
8. Move the sticks and re-tape them to the chairs so that they are taped to the
chair at the end of the sticks on both sides.
9. Load the sticks with pennies in increments of 10 at the center of the chair space
and record.
Data
Analysis/
Results
Bridge Girders
# Sticks
Arrangement of Sticks
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# of Pennies at Break Point
©MathScience Innovationn Center, 2007
The graph represents the moment of a bridge girder. The vertical supports are at 0 and
6.28 (0 and 2 ) on the x axis. As an object moves further away from the vertical
supports, the girders deflect.
The popsicle sticks in the activity represent the splices in the steel girders used to hold up
a bridge.
Conclusions/
Questions
Based on the table of data compiled, where is it best to splice girders for bridge
supports? In theory, it is best to splice them where the forces pushing up and
balanced by the forces pushing down. On the graph, that occurs where the curve
crosses the x axis.
References
http://www.pbs.org/wgbh/buildingbig/educator/act_columns_ei.html
http://www.asce.org/kids/
MathScience Innovation Center
Information on educational programs available to students, teachers and school
divisions and procedures for registering for programs.
http://msinnovation.info
Bridge Girders
http://mathinscience.info
©MathScience Innovationn Center, 2007