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What
different parts of the bridge do.
(Note:
You might want to review "Queen
Post Bridge Terms" before reading this section). The
picture below shows what the sides of our bridge will look like after
they are built. The
bridge side looks a lot like the “bow” of a bow and arrow set. The
top parts of the bow (top gusset, top chord and end post) are under
compression and support the main load. The bottom parts of the bow
(bottom gusset and bottom chord) are like a bow string. They are
under tension and keep the compressive parts of the bow from spreading
out. The gusset plates also keep the joints together and prevent
buckling. The vertical hips keep the structure from twisting.
Tools
used to design the bridge
To
design the bridge you will need a sheet of paper about 40 inches by 10
inches, tape (to tape down the piece of paper), a pencil, a ruler, a
"T" square, and the materials you bought at the hobby shop.
Why the
numbers 36, 18 and 8 are important.
Before we
start, it is important to remember three numbers: 36, 18 and 8.
"36" is important because
the longest piece of wood we can buy without "special ordering" is
just 36 inches long.
"18" is
important because the
length of
the tester's pressure plate is 18 inches. Remember also
from our beam bridge that when the
pressure plate pushes down on the top part of a bridge, it applies
pressure at two points, 18 inches apart. We will wind up placing
our vertical hips 18 inches apart, 9 inches from either end of the
bridge.
"8" is
important because the
bridge cannot be any higher than 8 inches. However,
in order to give ourselves some leeway, we will make the bridge 7 1/2
inches tall.
Drawing
the Bridge
The main
reason we draw the bridge is to know what it will look like. It
is easier to correct misunderstandings with paper and pencil than it
is with a saw and glue! Also, the drawing helps us create a
pattern for the end post.
We start by
tracing the piece of wood we will use for the bottom chord directly
onto the paper. Make sure it is parallel to the bottom edge of
the paper.
Next we mark
where the vertical hips will go, 9 inches from each end of the bottom
chord.
We then use
the "T" Square to draw the outside edges of the vertical hips.
Next, we use
the piece of wood we will be using for the vertical hip to trace the
other edge of the vertical hip onto the paper.
The vertical
hips will be 7 1/2" tall, so mark a point 7 1/2" from the bottom of
the bottom chord on each side of the vertical hips (4 points total).
These 4
points show where the top of the top chord will go. So use the
piece of wood you will be using for the top chord to trace in the top
chord.
The top edge
of the right end post goes from the top right hand corner of the
bottom chord to the rightmost point you drew in step 5. So use
your end post pieces to trace in the right and left end posts.
You can also
trace in where the gusset plates will go, 1 1/2 " from the top of the
top chord and 1 1/2" from the bottom of the bottom chord.
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