Johns Hopkins University
What is Engineering?
M. Karweit
Project: Build a bridge
OVERVIEW:
Bridge design includes many parameters: cost, materials, span, expected loadings,
method of construction, footing. Further, since public safety is at stake, material
imperfections, unanticipated loadings, modeling approximations, etc., must be recognized
and incorporated as safety factors. Bridge design is part art, part science, and part
engineering.
Overall design will depend on the materials to be used in construction. For
example, concrete is very strong in compression and very weak in tension. So concrete
elements of a bridge should be used where compression loading exists. Once the design
has been formulated, loads on all components must be calculated. Whether a structural
member will be in tension or compression or whether it will experience torsion or
bending will determine how that member will have to be constructed. With a knowledge
of the properties of the construction materials and an understanding of how to calculate
loads within a structure, an engineer has the basic tools for designing a bridge.
By the time you begin the project outlined below, you will be suitably equipped to
design a bridge out of an atypical building material: spaghetti. Your “arsenal” of
expertise will consist of the following:
1)
2)
3)
4)
5)
empirical testing of bending, tensile, and compression strengths of spaghetti.
relationship of length and diameter to these strengths.
theory of simple trusses under load.
design and construction of a loaded triangular truss.
practical experience with joint construction (i.e., how to use epoxy).
Project Description:
Each group is to build a bridge made from spaghetti and glue/epoxy. The object
is to construct a bridge that will carry the heaviest load while still meeting specifications.
Bridges will be loaded until they fail.
03/07/16
Build A Bridge
1
Johns Hopkins University
What is Engineering?
M. Karweit
Rules:
1. The bridge is to be built from spaghetti and glue, epoxy, or resin.
2. The bridge shall be free-standing and must span two level surfaces which are one
meter apart.
3. The support for the bridge shall be from the top of the level surfaces. The edges of the
level surfaces cannot be used in any way for support.
4. The bridge must include a decking of spaghetti to provide a suitable road surface at
least 5 cm wide across the full span of the bridge. Three conditions must be met:
a) gaps in the bridge deck are not to exceed 2 mm
b) a block of wood (5 cm x 5 cm x 10 cm) representing a car must be able
to move along the length of the decking unobstructed from end to end
c) the deck of the bridge must not be more than 5 cm above or below the
ends of the bridge at any point along its length.
5. You will be provided with a “loading platform” consisting of a piece of plywood
(.7 cm x 5 cm x 10 cm) and a U-bolt. This platform is to be attached at the center of the
bridge such that the bottom of the U-bolt is no more than 5 cm from the top of the bridge
decking. All loads will be suspended from this U-bolt, and there must be a clear space
directly below it to allow loads to be attached. Loads will be attached using an S-hook,
and, if necessary, a 10 mm diameter metal rod extension. If during loading, the bridge
twists in such a way as to cause the bridge to touch the rod at any point other than the Ubolt, thus lending additional support, the bridge will be disqualified.
6. The maximum vertical depth of the bridge, from the highest point in its structure to the
lowest cannot exceed 50 cm.
7. The maximum weight of the bridge including the loading platform must not exceed
0.75 kilograms.
03/07/16
Build A Bridge
2
Johns Hopkins University
What is Engineering?
M. Karweit
Bridge Schematic:
> 5cm.
< 2mm.
Bridge Decking
Loading
Platform
and U-bolt
< 5cm
< 50 cm
Load
100 cm
Total Weight < 750 gms.
03/07/16
Build A Bridge
3