-
Research actual bridge
designs
-
Design a model bridge based on
research
-
Submit construction drawings for
approval
-
Build a bridge with toothpicks that can be tested for
structural strength
-
Submit
final report on structure (includes a detailed torque, force, and collapse
analysis)
- Regular round or flat
wooden toothpicks ( or, you might want to look at Hobby Lobby's "craft grade" non-tapered toothpicks with
squared off ends... but they are hard to find and are not always in
stock)
- Elmer's white or
brown glue (no hot glues)
- Paper for the roadbed
(optional)
- String for suspension designs only; 3 meters max
(nylon offers high resistance to breakage under tension)
- RESEARCH AND
SUBMITTAL OF APPROVAL DRAWINGS
Each student will research bridge designs from literature or
actual structures noting bracing points and reinforcements and how each design
takes into account gravitational and load forces and the materials used. Each
student will then create a bridge design and submit a drawing for approval
(drawings should include dimensions (m) and 3 aspects of the bridge... a side view,
a top view, and end view). Approved drawings will
be returned and used to construct the bridge.
Drawings can be actual size or scaled down; if you scale down you must
include a drawing scale.
- DESIGN
AND DRAWING CHANGES
As you begin to construct your bridge, you will undoubtedly
want to make improvements (changes) to your design. Design changes are allowed.
However, you must revise your drawings showing the changes you intend to make and
submit them for approval prior to proceeding with construction. Once change
approval has been secured from Mr. P., construction can continue (be aware that I may make
minor adjustments/improvements to your requested changes).
Your target
maximum weight of the finished bridge is 80
gm. (a total weight more than 80 gms. will be allowed but carries a grade
reduction penalty...the greater the overage, the greater the penalty). Any
design can be used as long as the roadbed is flat and unobstructed to allow a
matchbox car to travel its length. The roadbed does not actually have to support
the car; the car is used only to check clearance (i.e. the roadbed can be made
of paper to save on weight). The bridge must be free standing and allow for a 2
cm. high x 30 cm. wide board to pass under the bridge while the bridge rests on a flat surface.
Only the materials listed may be used to build the bridge. Excessive amounts of
glue may not be used as part of the structure, i.e. the bridge or bridge joints may not be
completely covered with glue.
Each bridge will be placed on two support structures
(see photo below). An 8"
long, 1/2" diameter wooden dowel
will be set across the center of the bridge on top of
the road surface (set perpendicular to and on top of the road). Heavy-duty
wire is looped over each
end of the dowel which supports hanging weights below the bridge. 1 kg weights
are added until
all the weights are used up (total of 11 kg. / 25 lbs.) or the bridge
structurally fails (breaks).
A statics analysis and
discussion must be included (however, this will actually be done in class...
there is nothing to prepare ahead of time). This analysis includes diagramming the actual CW and CCW torques, FN's,
and Fg's (which includes the hanging weight and the weight
of the bridge). View a sample calculation:
Example.
The statics analysis is done only after your bridge has been weight tested.
Bridges that
collapse under the weight of 11 kg must include a collapse analysis. This
analysis includes a discussion on how the bridge failed (i.e. rotational
forces, weight forces, torques, etc.). Your discussion should include a
"remedy-of-design" which basically describes what you would have done
differently (i.e. recommend design changes) to overcome the collapse.
Each bridge will be graded using a ratio of supported load weight to total bridge
weight i.e. heavier bridges must support more weight to achieve an equal grade.
Further, a collapse analysis must be submitted for bridges that structurally
fail.
Check your individual class calendars for due dates;
NOTE: there are two due dates for this project... design drawings are
due first, then actual bridges).
-
Lea's bridge supporting 25 lbs.!
1.
BEER AND JOHNSTON:
SIMPLE TRUSSES SECTION REVIEW
2.
BEER AND JOHNSTON: SIMPLE TRUSSES REFERENCE FIGURES
Beer and Johnston, McGraw
Hill, Engineering Vector Mechanics; analysis of simple trusses.
BRIDGE DESIGNS
....one of the BEST SITES for getting ideas for designs!
BRIDGE
CONSTRUCTION
....research
real bridge designs!
TYPES OF
BRIDGES
...also
search the web using "toothpick bridges" for numerous reference sites
THE CATENARY TOOTHPICK BRIDGE
...some
project construction
examples i.e. how to go about building your model bridge
MODEL BRIDGE DESIGN
...good
advise from a student with a lot of examples
