Transcript Bridge Notes

Bridge Building Notes
Physics
Bridge Building Tips
Keep your hands clean! Oils and grease
from your skin can ruin your joints.
2. Go easy with the glue bottle. As a general
rule of thumb, if you can see it then you are
using to much.
3. Keep your bridge from twisting by using
lateral bracing.
4. It’s still true, measure twice and cut once.
5. Keep a log of every bridge you build.
Record notes, you won’t remember later
on.
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10.
Try to videotape testing your bridge.
You may get a clue on what failed first.
Always keep safety in mind when using
sharp tools. Most mistakes are made
when you aren’t paying attention.
Always test your bridge before taking it
to competition, but leave enough time to
build another.
Draw out your bridge on graph paper to
make sure that it is symmetrical.
Different trusses have different ways of
spreading out the load.
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15.
Wood has about the same strength in
tension, no matter how long it is.
Balsa wood sands very easily if needed.
You can mix glue with water to cut down
on weight. Doing this also helps the
glue to seep into the wood, creating a
stronger joint.
Remember to close your glue bottle
when you are done.
By cutting a piece in half, you more than
double its strength compression.
16.
Use lap joints whenever possible to get
the best strength.
Bridge Joints
Lap Joint: The lap joint is one of the strongest,
and you should use it whenever you can. It
strengthens compression members because it
adds stiffness.
 The lap joint has one draw back. The joint as
only as strong as the face of the wood. The face
of the balsa wood is typically not strong, and
tears easily. So make your lap joints have plenty
of surface area for the glue.
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End Joint: The end joint is not very
strong. In tension, the two pieces of
wood will just pull right away from each
other. In compression, this this will allow
the piece to bend in a perfect arc. The lap
joint holds the piece stiff, which does help
it to hold more.

Notched Joint: The notched joint gives
more strength than end joint but less than
the lap joint. It is more difficult to build,
so it is not very common.
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Gussets: If you have to use as end joint,
it is a good idea to add a gusset to make
it stronger. The gusset creates a lap joint,
which is strong in both tension and
compression. Try to make each part of
the gusset the same length. If there is
more glued to one piece, the one with less
surface are for the glue will pull away first.
Gusset
(plate)
Forces that Act on Bridges
Compression:
Compression is a pushing (compressing)
force. You can take the middle of a straw
and squeeze it between two fingers, the
straw flattens. However, is you hold each
end of a straw and push together it is
harder to make the straw compress or
flatten. The shorter the piece of wood is,
the more compression it can hold. The
longer the piece of wood is, the less
compression it can hold.
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Tension:
Tension is pulling. It would be hard to
break a straw if you held both ends and
pulled apart. Wood also has the ability to
resist tension. Tension may be applied
parallel to the grain of the wood, but
should avoid perpendicular to the grain.
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Torsion:
Torsion is twisting. When you wring out a
cloth, you are applying torsion to the
cloth. Different materials respond to
tension differently.

Shear:
Shear happens when two opposing forces
are acting on the same point. If you hold
a piece of wood with both hands next to
each other, and push up with one hand
and down with the other, you are applying
shear. Shear usually occurs horizontally,
and vertically.
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Trusses
Why is a truss not a type of bridge?
http://www.brantacan.co.uk/bxtruss.htm
 Why use triangles in trusses?
Most people know the answer; because a
triangle is the only rigid shape that can be
made with pinned joints - so there is only
one set of angles that will go with a given
set of sides.
http://www.brantacan.co.uk/truss.htm
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Pictures of trussing not in bridges:
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Pictures of trussing in gates:
Common Truss Designs
WARREN TRUSS - A Warren truss,
patented by James Warren and
Willoughby Monzoni of Great Britain in
1848.
 Can be identified by the presence of many
equilateral or isosceles triangles formed by
the web members which connect the top
and bottom chords. These triangles may
also be further subdivided.
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Warren truss may also be found in
covered bridge designs.
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PRATT TRUSS - The Pratt truss is a very common
type, but has many variations. Originally designed
by Thomas and Caleb Pratt in 1844, the Pratt truss
successfully made the transition from wood designs
to metal.
The basic identifying features are the diagonal web
members which form a V-shape. The center section
commonly has crossing diagonal members.
Additional counter braces may be used and can
make identification more difficult, however the Pratt
and its variations are the most common type of all
trusses.
The vertical members are in compression
while the diagonal members are in tension,
exactly opposite the structure of a Howe
truss.
HOWE TRUSS - A Howe truss at first appears
similar to a Pratt truss, but the Howe diagonal
web members are inclined toward the center of
the span to form A-shapes.
 The vertical members are in tension while
the diagonal members are in compression,
exactly opposite the structure of a Pratt
truss.
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Patented in 1840 by William Howe, this design
was common on early railroads. The three
drawings show various levels of detail. The
thicker lines represent wood braces; the thinner
lines are iron tension rods. The Howe truss was
patented as an improvement to the Long truss
which is discussed with covered bridge types.
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K TRUSS –
Red represents tension, blue represents compression,
and green is no load. The following figures are shown
under a load. The numbers represent percentage, where
the total load = 100. The numbers are rounded to the
nearest 5.
On this Warren truss, each of the down arrows represents 50%
of the load. Notice how the two middle pieces have no load.
Notice the load is quite a bit less on the ends of the top and
bottom chord than in the middle. When you build for efficiency,
keep in mind that for a bridge loaded in the center, the ends
can be smaller than the middle.
Now examine the same Warren truss, but with added
vertical members:
The added vertical members serve to break the top chord
into smaller segments, making it stronger. However, the
percentage of the load has increased both on top and
bottom. Also notice where the green members are now.
Notice the difference between the Pratt and Howe trusses
under a load. The Pratt has bigger numbers on both the top
and bottom chord, but its middle compression members are
shorter, and hold less than those on the Howe. But the load is
still concentrated in the middle, and gets less further to the
ends on both.
It is a trade off between the Pratt and Howe. For the Pratt,
you’d have to use bigger top and bottom chords, while on the
Howe you’d have to use bigger compression members.
How to Get Started
#1 – Know The Rules
#2 – Design the Bridge
#3 – Get Supplies
#4 – Build the Bridge
#5 – Test the Bridge
#6 – Build the Bridge Again
#7 – Write Lab Report
Bridge Designer
West Point Bridge Contest
http://bridgecontest.usma.edu/
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Model Bridge Designer
http://www.garrettsbridges.com/design/brid
gedesigner/
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Bridge PALS Rules
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http://www.bradley.edu/academics/eng/Ci
vil/Html/Bridge-Pal.htm
Click on Competition Rules on the right side
of the screen.
Sources
http://pghbridges.com/basics.htm
 http://www.garrettsbridges.com/design/tr
ussdesign/
 http://www.brantacan.co.uk/starterpages.
htm
 http://www.bradley.edu/academics/eng/Ci
vil/Html/BPal-Docs/BPal15-2006PhotoGallery/FrameSet.htm
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