Transcript Document

Bridge Design
part 4
Note: have pdf file of bridge designs ready
for structural analysis
By Alan Pennington, materials taken from
and adapted West Point Bridge Design
Learning Objectives
• Calculate the structure of various
bridges.
• Determine the load strength of every
member in a truss.
• Evaluate a truss, to determine if it can
carry a given load safely.
• Using vectors we can make diagrams of the
different types of bridge designs and
calculate how the forces will be applied to
different parts of the bridge
Create the Structural Model
To model a Bridge, we must define
(1) the geometry of the structure,
(2) the loads,
(3) the supports and reactions.
We begin by drawing the three-dimensional bridge
structure as a pair of two-dimensional trusses.
Since these two trusses are identical, we only need
to analyze one of them.
The geometry of the truss is shown below. The
dimensions indicate the locations of the centerlines
of the members.
Determining Internal Member Forces
• Decide on the total amount of weight that the bridge
will carry. This is W. Note that the total load applied
to each truss is only half of the total weight (0.5W),
because the bridge is made of two main trusses, one
on each side
• Next to each member on the diagram, you will see a
decimal number. For each member, multiply the
decimal number by the value of W you determined.
• This product is the internal member force, expressed
in the same units you used for W. If the decimal
value is positive, the member is in tension. If it is
negative, the member is in compression.
For example, let’s assume that this truss has a total load W=10
newtons, divide by two because of two sides for 5
newtons.divide by 3 for the three members where the load is
applied. Then the bottom chord member on the left-hand side
of the truss has an internal force of
Use the supp_civil_strcanal.pdf file to make
worksheets and have students calculate
different parts of the different bridge designs