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Design
a Virtual Bridge...
Win a $10,000 Scholarship!
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West
Point Bridge Designer Tutorial |
2.
Creating the Structural Model |
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A
structural model is a mathematical idealization of a real structure.
The model is a tool to help us predict how the real structure will
behave when it is loaded. |
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Before
we begin the truss design, let's take a moment to familiarize
with the Drawing Board--the screen area on which we
will create, test, and optimize our structural model. Note
that the screen already shows the river valley, roadway,
abutments, and bridge deck, as well as our Howe
Through Truss template, which is outlined in light gray
dotted lines.
At the
left and bottom edges of the Drawing Board are rulers, which
show the dimensions of the project site in meters. Above the
Drawing Board are several toolbars, and to the right is a
list of the steel truss members in our structural model. The
list is blank, of course, because we haven't created any members
yet.
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We'll
begin the structural model by drawing joints--the points at
which our truss members will be connected together. |
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Note that
six joints have already been created automatically. These
six joints, which cannot be deleted or moved, represent the
connecting points between the concrete bridge deck and the
steel structure.
There
are only four more joints to add. Their locations are indicated
with dotted circles on the template. To draw these joints,
move the mouse cursor (indicated by two perpendicular
red lines) directly over
each of the dotted circles and click the left mouse button.
After four clicks, the result should look like the picture
at right.
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If
you accidentally draw a joint in the wrong place, just click the Undo
button on the WPBD toolbar. |
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Now we're
ready to draw the steel structural members that will
form our truss. From the Design Tools palette in the
upper left-hand corner of the Drawing Board, select the Member
tool, as shown at right. The mouse cursor will change
to a pencil.
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Members
are drawn from joint to joint, and the dotted lines of the template
show us which joints to use as the endpoints for each member. |
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To begin,
move the cursor over any joint and press the left mouse button.
While holding this button down, drag the mouse to an adjacent
joint, using the template as a guide. When the cursor reaches
the second joint, release the mouse button, and a dark gray
member will be drawn between the two joints. Use this procedure
to draw all 18 members that make up the Howe Through Truss.
The result should look like the picture at right.
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Our
structural model is complete! Though it might seem that all we've
done is to draw a picture of a truss, we have actually completed a
preliminary design of its engineering properties as well. You've probably
already noticed that, on the lower toolbar, there are three drop-down
lists, which currently read Carbon Steel, Solid Bar,
and 120 mm. |
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These
are the Member Properties lists.
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The three lists
are used to define the material, cross-section, and size
of structural members. The values displayed in these lists are automatically
assigned to all newly created members. Thus all 18 members in our
structural model are 120 millimeter solid bars made of carbon steel.
Are these members
strong enough to carry the highway loads that our bridge will be
subjected to? The only way to find out is to test our design.
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