Information provided by Peter Freitag:
There are two steps in to be followed when designing your balsa wood
bridge. The first step is to complete your design sketches. These sketches
help the engineer to develop different ideas about bridge designs. Your
design should have a super-structure and a sub-structure. This step is for
only getting a rough idea of what your bridge should be.
The second step is to do a three view sketch. You will complete a top, side,
and end view of the model bridge you plan to build. The placement of the
views is noted in the grid paper provided. Draw the views full-sized. The
wood used for the bridge will be 1/8-inch wide. There are also
specifications for the Technology Student Association national contest in
bridge building included on the last two pages of your bridge design
book. An exact span for the bridge will be provided before you begin
drawing your three view drawings. The roadway surface inside the
bridge must be open to allow a block of wood which is one inch thick and
two inches wide to pass through the entire length of the bridge roadway
surface. When this is done you are ready to start actually building your
bridge.
When
building your balsa wood bridge you should begin by using masking tape
to attach your full size three-view sketch or drawing to a piece of foam board or cardboard. Cover the three view drawing with wax paper.
Use masking tape to attach the wax paper. Cut strips of wood to fit the
outline of the bridge, which the three view sketch/drawing provides. Use
an X-Acto knife to cut the wood. In order to ensure a snug fit, cut the
pieces a little long on the first cut and then trim the ends. Pin each piece
into its place once the fit is attained. When all of the cut strips fit
smoothly, glue them in a place. Use pins to hold the pieces flat.
After the outline has been glued into place, fill in the wood strips which
make up the other members of the first side of the bridge. Make sure that
joints are completely dry before removing the pins. After the first side has
dried, carefully remove the pins and place the finished side on a flat
surface. Complete side two by following this procedure.
Once side two is completed build cross braces across the roadway of the
bridge. Build each of them exactly the same length. Assemble the cross
braces which support the top of the bridge. Build each of them exactly the
same length also. After the cross-braces have been built and dried, attach
them to the sides. A simple squaring device made of cardboard will help
to square the sides with the cross brace members.
Complete the model bridge by checking all joints to see that they are well
glued and that all members are tightly in place. The bridge you designed
and engineered has now been made! Now that your balsa wood bridge has been completed you will test it for
efficiency. The first step in this is to measure its weight. This is done using
an electronic scale which measures in grams. The goal is to find out how
much weight your bridge can hold without breaking. This is done by
suspending weights from its roadbed. Once you have put on too much
weight, and your bridge begins to break you take off the last weight you
put on, and that is the maximum load of your bridge. Using the equation
(Load*4.45)/Mass of structure=Efficiency, you can calculate the efficiency of your bridge. This is the final step of your balsa wood bridge project.
boat repair yards use a cold set two part epoxy
for installing wooden stringers in fiberglass boats. Check with Pacific Yachting Magazine
Vancouver B.C. Can.
Balsa is very light yet exceptionally strong. Although classified as a hardwood, balsa wood's density ranges from only four to
twenty pounds per cubic foot. ProBalsa's laminated blocks average 9.5 pounds per cubic foot.
Balsa wood gets its great strength from its vascular system, a network of tiny tubes which transport nutrients and water
throughout the tree. Similar to honeycomb, the vascular system is capable of carrying tremendous compressive and shear
stresses. ProBalsa's low density and exceptional strength make it an ideal composite core material.
Finished balsa wood, like you find in model airplane kits, varies widely in weight. Balsa is occasionally found weighing as little
as 4 lbs. per cu. ft. On the other hand, you can also find balsa which will weigh 24 lbs or more per cu. ft. However, the
general run of commercial balsa for model airplanes will weigh between 6 and 18 pounds per cu. ft. Eight to twelve pound
balsa is considered medium or average weight, and is the most plentiful. Six pound or less is considered "contest grade",
which is very rare and sometimes even impossible to obtain.
Most hobby shops have a large rack of balsa sheets, sticks, and blocks that you can choose from if you are going to build a
model airplane from scratch. Undoubtably, because of the nature of balsa, the actual weight of each piece of wood of the
same size can vary slightly. When you select the pieces you want to buy you should keep their final use in mind. Logically one
should select the lightest grades for the lightly stressed model parts (nose blocks, wingtip blocks, fill-ins, etc.) and the heavier
grades for important load bearing parts of the structure (spars, fuselage stringers, etc.). To a large extent, this selection is
already partly done for you. Here at SIG, we purposely cut up our lightest raw balsa into blocks, and our hardest raw balsa
into sticks. Sheets are cut in the entire wide range of density.
Paulownia elongata Carolinia is a true hardwood, it
is light in color and light in weight. It's appearance
closely resembles American ash. Physically, it
ranks between balsa and poplar. The wood is
extremely easy to machine and finish. It is
dimensionally stable to a fault, does not crack or
warp, and will take on and release moisture without
damage.
Paulownia is an extremely fast-growing hardwood
tree. In the Spring, it will produce leaves of
exceptional size, (up to 3' across!), as well as cascades of fragrant blossoms. Foliage will shed after
the first good frost.
Traditional Japanese use includes ceremonial furniture, musical instruments, containers shoes and
more. Two reasons that support these uses are the wood's dimensional stability in general, and its
ability to remain stable in relation to moisture. Although the wood is light, it is extremely strong and
will not crack or split, even when spikes or nails are driven through. The lumber makes excellent
furniture, decorative moldings, siding and laminated beams.
STRENGTH OF BALSA WOOD COMPARED TO OTHER WOODS
| Species | Weight Lbs./Cu. Ft. | Stiffness Strength | Bending Strength | Compression Strength |
Balsa |
8 |
72 |
70 |
75 |
Balsa |
10 |
100 |
100 |
100 |
Balsa |
14 |
156 |
161 |
149 |
Spruce |
28 |
230 |
260 |
289 |
Yellow Pine |
28 |
222 |
277 |
288 |
Douglas Fir |
30 |
241 |
291 |
341 |
Hickory |
50 |
379 |
638 |
514 |
Oak |
48 |
295 |
430 |
366 |
Basswood |
26 |
261 |
288 |
288 |
Black Walnut |
37 |
301 |
506 |
512 |
BALSA STICKS WEIGHT IN OUNCES
SIZE 36" x |
# pieces |
STOCK DENSITY in pounds per cubic foot | ||||||
6 |
8 |
10 |
12 |
14 |
16 | |||
1/16" x |
1/16" |
16 |
.125 |
.167 |
.208 |
.250 |
.292 |
.333 |
3/32" |
16 |
.188 |
.250 |
.312 |
.375 |
.438 |
.500 | |
1/8" |
8 |
.125 |
.167 |
.208 |
.250 |
.292 |
.333 | |
3/16" |
8 |
.188 |
.250 |
.312 |
.375 |
.438 |
.500 | |
1/4" |
4 |
.125 |
.167 |
.208 |
.250 |
.292 |
.333 | |
3/8" |
4 |
.188 |
.250 |
.312 |
.375 |
.438 |
.500 | |
1/2" |
2 |
.125 |
.167 |
.208 |
.250 |
.292 |
.333 | |
3/32" x |
3/32" |
8 |
.141 |
.188 |
.234 |
.281 |
.326 |
.375 |
1/8" |
8 |
.188 |
.250 |
.312 |
.375 |
.438 |
.500 | |
3/16" |
4 |
.141 |
.188 |
.234 |
.281 |
.326 |
.375 | |
1/4" |
4 |
.188 |
.250 |
.312 |
.375 |
.438 |
.500 | |
3/8" |
2 |
.141 |
.188 |
.234 |
.281 |
.326 |
.375 | |
1/2" |
2 |
.188 |
.250 |
.312 |
.375 |
.438 |
.500 | |
1/8" x |
1/8" |
4 |
.125 |
.167 |
.208 |
.250 |
.292 |
.333 |
3/16" |
4 |
.188 |
.250 |
.312 |
.375 |
.438 |
.500 | |
1/4" |
4 |
.250 |
.333 |
.416 |
.500 |
.538 |
.667 | |
3/8" |
2 |
.188 |
.250 |
.312 |
.375 |
.438 |
.500 | |
1/2" |
1 |
.125 |
.167 |
.208 |
.250 |
.292 |
.333 | |
3/16" x |
3/16" |
2 |
.141 |
.188 |
.234 |
.281 |
.326 |
.375 |
1/4" |
2 |
.188 |
.250 |
.312 |
.375 |
.438 |
.500 | |
3/8" |
2 |
.281 |
.375 |
.469 |
.563 |
.656 |
.750 | |
1/2" |
1 |
.188 |
.250 |
.312 |
.375 |
.438 |
.500 | |
3/4" |
1 |
.281 |
.375 |
.469 |
.563 |
.656 |
.750 | |
1" |
1 |
.375 |
.500 |
.625 |
.750 |
.876 |
1.00 | |
1/4" x |
1/4" |
1 |
.125 |
.167 |
.208 |
.250 |
.292 |
.333 |
3/8" |
1 |
.188 |
.250 |
.312 |
.375 |
.438 |
.500 | |
1/2" |
1 |
.250 |
.333 |
.416 |
.500 |
.583 |
.667 | |
3/4" |
1 |
.375 |
.500 |
.625 |
.750 |
.876 |
1.00 | |
1" |
1 |
.500 |
.667 |
.832 |
1.00 |
1.166 |
1.333 | |
3/8" x |
3/8" |
1 |
.281 |
.375 |
.469 |
.563 |
.656 |
.750 |
1/2" |
1 |
.375 |
.500 |
.625 |
.750 |
.876 |
1.00 | |
1/2" x |
1/2" |
1 |
.500 |
.667 |
.832 |
1.00 |
1.166 |
.1333 |
1" |
1 |
1.00 |
1.333 |
1.666 |
2.00 |
2.333 |
2.667 | |
3/4" x |
3/4" |
1 |
1.125 |
1.500 |
1.875 |
2.250 |
2.625 |
3.00 |