BRIDGE
C O N S T R U C T I O N
INTRODUCTION
The following bridge folio has been written in a format that would be good for
showing grade 10 students. An integration of hand sketching, word-processing
skills, CAD drawings, internet and book researching, material selection, safety,
sequencing and Engineering mechanics and materials are all used in writing
this brief. When running this type of program in the school the video (Brunel’s –
‘The great divide’) would be shown to inspire students in the bridge design process.
Resource books and pamphlets on bridges and ply would be provided, yet, it would
be expected that students research and write up their folios as homework. An
excursion to Brisbane city to see her bridges or to a plywood manufacturer would
also be good additional ways of enriching the activity for the students depending
on time limits and budget restrictions. Alternatively, the video on plywood
manufacture (‘The timber industry’) would be shown to students.
ACKNOWLEDGMENTS
A thanks goes to the following people:
Stuart McKenzie (MISTER PLY & WOOD, Maroochydore) for supplying
pamphlets and technical data on plywood.
Tamara Fitzgerald (BRIMS WOOD PANELS, Brisbane) for supplying
pamphlets, technical data and veneer samples of plywood.
Tony Smith (Dakabin State High School) for assisting in obtaining the Timber
industry video.
Bob Watson (Redcliffe State High School) for assisting in obtaining the Brunel
science series videos with bridge construction information.
Richard Challenor (Starkey & Christoe cabinetmakers) for assisting with
technical data.
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BRIDGE
C O N S T R U C T I O N
BRIEF
Manufacturing Technology 2
Bridge Building Competition
You are to construct a bridge using plywood as described below. The bridge can be
modelled on any natural or built structure and must meet the following
specifications:
1.
The mass of the bridge (and the base material) is to be as small as
possible.
2.
The internal width of the bridge is to be not less than 100 mm.
3.
The bridge is to consist of a single span of one metre.
4.
The bridge is to have the highest structural efficiency (E).
E = LOAD (supported in grams) / MASS (of the bridge)
E = L/M
The maximum Load is to be 25kg.
Materials - Any plywood may be used from the following list:
9.5 mm, 6 mm, 4 mm, 3mm,
2 mm bending ply
1 mm veneer
PVA wood adhesive or hot melt glue may be used.
Other materials may be used provided justification is given to the lecturer prior to
commencement of the project.
Process:
Any process, that will suit the properties of the material, may be used in the
construction of the bridge.
Testing:
The bridge is to be tested with a load of 25 kilograms placed at the centre of the
span (without failure).
Provision must be made for a steel plate to be placed at the centre of the structure to
apply the LOAD.
Time allocation
Week 1
Design and parameters.
Week 2 & 3
Construction.
Week 4
Testing and evaluation.
Assessment criteria
Criterion
6 5 4 3 2
1
Quality of production
2
Test efficiency (E)
3
Design and engineering calculations
And sketches
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BRIDGE
C O N S T R U C T I O N
4
Data & Properties of plywood
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BRIDGE
C O N S T R U C T I O N
RESEARCHING PLYWOOD CHARACTERISTICS
Face grain direction.
Plywood is made from slicing thin
veneers of timber into sheets. These
sheets are glued together with the
grain at 90o to each other. This
method produces a sheet of board that
is engineered to have the same
strength properties in both plane
directions in a sheet. Most sheets
(except bendy ply) would have just
less than 50% of timber fibres
following the direction of the face
grain, whilst having just more than
50% of timber fibres following the
face grain.
Five laminates prior to gluing and pressing.
Natural timber has proven, through testing,
to be strong in both tension and compressive forces.
Compression
Tension
Face grain.
Plywood sheets have the natural tension
and compressive strengths in both
directions in the plane of a sheet. This
makes ply good for membranes, nail
gussets in trusses, webs in box beams and
‘I” beams, as well as bracing in buildings.
Final engineered plywood sheet.
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BRIDGE
C O N S T R U C T I O N
RESEARCHING PLYWOOD CHARACTERISTICS
Plywood is stronger in tension when a
force is applied parallel to the grain rather
than perpendicular to the face grain. This
is because generally ply contains odd
numbers of laminations that are
predominantly parallel to the face grain.
When force is applied at 450 to natural
fibres less strength is evident as joins in
sheet are placed in shear to each other.
Reducing its slenderness can increase
capacity of slender plywood panels.
Stiffeners are used (membranes, webs, etc),
and are more effective if attached parallel to
the force, this assists in gaining more
stability.
Plywood has good compressive strength in
both directions, parallel to face and
perpendicular to face, yet it is strength is
limited with compressive forces that are 450
to the face grain.
Plywood can be susceptible to shear forces
with edgewise bending also.
As = Shear area.
t = Full thickness of plywood sheet.
d = Depth of sheet carrying shear.
For example:
As = 2/3 d t.
As = 2/3 x 30 x 2.
As = 59.9.
The thicker the sheet
or the deeper the load
area the less chance of
shearing of plywood
sheet, thus causing
bridge failure.
In plane bending is where the laminations
of the sheets of plywood are put under shear
forces to become ‘un-stuck’, this can lead to
bridge failure also.
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BRIDGE
C O N S T R U C T I O N
RESEARCHING STRONG SHAPES
First goal for building a bridge is
to try and limit the amount of
deflection that it has. Deflection
places additional stresses on the
bridge structure. Below are some of
the basic strong shapes that were
noticed in researching the design
of my bridge.
Deflection
Honeycomb structure is
a strong set of shapes that
bee’s use in their hives.
‘I’ section
Castle beam uses the
honeycomb idea.
‘T’ section
Circles (tubes) make strong shapes.
Box sections
Triangles also make
for strong structures.
Arched shapes are also strong.
‘U’ section
Combination of 2 ‘U’
sections joined together.
Triangles joined together
Orthotropic beam
Box girder section uses half
honeycomb shape well.
Pre-tensioned
concrete beam
6
Triangle shape with paddle pop
sticks and string, the string
goes into tension when force is
applied on top of the triangle.
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BRIDGE
C O N S T R U C T I O N
RESEARCHING BRIDGE STRUCTURE DESIGNS
The problem we have as students is traverse a
chasm like the one shown left – with the lightest
possible amount of bridge weight. There are five
basic types of bridges that are used by bridge
engineers.
1 – Balanced or Cantilever Bridge: Uses a main
pillar with a load balanced from one or both sides.
Would require a large amount of materials to
construct this type of bridge in this given
competition.
Pros – Can span distance with compressive and
tensile strength resistance.
Cons – This design works best with a central
support – this is not allowed in this competition.
2 – Beam Bridge: Uses a pre tensioned concrete
form or strong triangular shapes to get its
strengths. They are often built with a slight arch
across their length.
Pros – Would work well in ply using the
orthotropic beam shape to get strength.
Cons – Could require a large amount of ply to be
efficient thus become too heavy.
3 – Suspension bridge: Cables are used to hold
the road surface up. They are quite often arched in
appearance and can span great distances.
Pros – using light fishing line or string would
assist in keeping the weight down.
Cons - Bridge needs to have great tensile strength
and must be firmly anchored to the ground at the
ends – this is not allowed in the competition.
4 – Arch Bridge: uses the natural strength of
curves to deflect the weight.
Pros – very strong and could be made of plywood
sections with membranes and webs.
Cons – Needs to have strong support at point A and
B – are the benches used in testing strong enough?
5 – Truss Bridge: often used with bream bridge
base it utilizes the triangle frame structure.
Pros – good way of strengthening a beam bridge.
Cons – requires a lot of extra materials therefore
extra weight.
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BRIDGE
C O N S T R U C T I O N
TYPES OF BRIDGES
The following bridges are examples of current bridges in use around the world.
Cantilever
Beam
Suspension
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BRIDGE
Arch
C O N S T R U C T I O N
Truss
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BRIDGE
C O N S T R U C T I O N
DESIGN IDEATIONS
Gusset plates used to
assist with strength.
Concept 1 – Beam Bridge
Tops of ‘I’ beams
cut back to allow
for solid joining
Slight arch to assist
with deflection.
Top & bottom of beam
would have a 1 mm veneer
sheet glued on to limit the
amount of twisting.
Arched shape template
would be used to construct
Beam Bridge to ensure
consistent arch.
‘I’ beam construction used with 2 mm
ply veneer glued together with P.V.A.
Castle beam
construction to
limit bridge
weight.
Ply ribs joined with
half checks before
top & bottoms of ‘I’
beam are glued on.
Pros
-
Cons
-
-
Instead of honeycomb,
holes could be drilled
to save time.
Finished bridge would have good slimline finish and look quite good.
Bridge construction utilises the strength of triangles and excess weight is easily
removed.
Doesn’t utilise the vertical shoulders of the gap being traversed.
Requires large amounts of plywood for the construction process – too heavy?
Doesn’t use large flat surfaces of plywood – so it is susceptible to shear forces because of
thin sections of plywood.
Requires large amounts of fiddly cutting out of half checks etc.
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BRIDGE
C O N S T R U C T I O N
DESIGN IDEATIONS
Concept 2 – Truss Bridge
Whole structure could be wrapped
in 1mm thick veneer to even
further stiffen the bridge loading
capacity.
2 mm bendy plywood
bridge surface
Tubes make ideal
structure members on
account of their stiffness
and low weight.
Arched shape template
would be used to construct
Beam Bridge to ensure
consistent arch.
‘Frame lashed together with
fishing line and epoxy glue.
Gusset plates could
also be used to
strengthen the
joints
1 mm veneer used for
tubular sections
Veneer wrapped around
solid 12 mm bar of steel
for gluing.
Joint taped and
glued with P.V.A.
Pros
-
Very strong usage of tubular sections.
Would be a very good looking bridge, if tubes were well constructed.
Very light weight - because tubes of veneer are hollow.
Cons
-
Construction of veneer tubes would be tedious and time consuming.
Design of bridge isn’t using plywood so therefore isn’t really testing the strength of
plywood as the brief specify’s.
-
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BRIDGE
C O N S T R U C T I O N
DESIGN IDEATIONS
Concept 3 – Cantilever Bridge
2 mm 5 ply used as top to
assist in reducing twist.
Top of bridge has a convex curve
to assist in deflecting the weight.
Bridge uses both vertical
shoulders A & B of chasm
to hold weight.
‘Curved shape on the bottom
for aesthetics.
Membranes used to limit the
twisting of bridge under
weight.
Rounded holes cut out of
trusses and membranes
to lighten weight.
Half checks used for
joining of membranes
to cantilever trusses.
10 mm flush cut
dowel for joining
the two trusses
together.
Pros
-
Lightweight construction due to only one central truss section.
Most force would be successfully transferred to points A & B.
Use of dowels to join the trusses together allow for flexing and torsion – limits shear.
Cons
-
Not the prettiest looking bridge
Lots of hole cutting to make bridge low in weight.
-
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BRIDGE
-
Cantilever trusses
C toOgoNforSweight
T R Utesting.
C T I O N
is no room for the bolt
are central so there
DESIGN IDEATIONS ~ CHOSEN DESIGN
Concept 4 – Arch Bridge
Middle is narrower than the ends to
reduce weight and aid with deflection
of weight to points A & B.
1 mm veneer sheet 100 mm
wide to assist in reducing twist.
Holes drilled to reduce weight.
Membranes used to reduce
twist in the ply.
Extra bracing to assist
in limiting twisting
near weight test hole.
2mm 5 ply is to be used.
Doubly braced walls of
beam to limit shearing
of ply in the narrowest
section.
Shape of the base of the
orthotropic beam is curved
to assist in deflecting
weight to points A & B.
Orthotropic beam
shaped membranes.
Pros
-
Lightweight construction by utilising a box section type of beam.
Usage of curves like arch bridges to deflect weight to shoulders of the chasm.
Easy to construct design that looks good.
Usage of 2mm 5 ply will make bridge very strong in the resistance to shear forces.
Cons
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BRIDGE
-
-
Middle of bridge
is very narrow so
is therefore susceptible
shearing.
C toOfailure
N S Tthrough
R U C
T I O N
Requires drilling waste out to reduce weight – thus loosing possible strength.
JUSTIFICATION OF CHOSEN DESIGN
Concept 4 – Arch Bridge
This bridge was deemed to be the best for the competition because of the following
points.
~
~
~
~
~
~
~
~
~
The arch design directs the forces towards the vertical faces of the
chasm.
The orthotropic beam is a strong shape for resisting the central forces
that will be applied.
Using 5 mm ply will assist in keeping the strength in the narrow
sections of the bridge.
The bridge design of an arch is an aesthetically pleasing design;
this should fulfil the design aspect of the criteria.
The design should be easy to construct and this increases the
chances that the bridge will have good quality of production.
The design should be light enough to be under 200 grams which is
the approximate weight that previous years did well at.
The design uses current engineering principles from bridge
construction.
Properties of plywood were considered when designing the bridge,
issues like twist reduction with membranes and finding a 2 mm 5
ply that has very strong qualities.
The bridge design is very interesting and aesthetically pleasing to
me personally and I am very interested to see how well that it will
do!!!
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BRIDGE
C O N S T R U C T I O N
WORKING DRAWINGS
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BRIDGE
C O N S T R U C T I O N
SEQUENCE SKETCHING
Stage 1. Cut out basic size of board
needed for a template on the Bench
Saw.
Stage 2. Drawing out to scale the
sizes on 6 mm thick Medium Density
Fibre Board (M.D.F.).
Stage 3. Cut out shapes on the
Bandsaw, keeping as close as possible
to the waste side of the line.
Stage 4. Clean up the convex edges on
the Disc Sander ensuring that work is
done on the down side stroke of the
sander.
Stage 5. Using the Bobin Sander,
clean up the concave edges to a nice
smooth finish.
Stage 6. Mark out positioning of the
holes for weight reduction. And using
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BRIDGE
the Drill Press remove the
saws and spade bits.
C O N S T R Uholes
C T with
I O N
hole
SEQUENCE SKETCHING CONTINUED
Stage 7. Transfer these shapes onto 2
mm 5 ply ready for cutting out. This
enables the least amount of ply to be
used.
Stage 8. Repeat stages 1, & 3 – 6 to
get the bridge cut out and to finished
sizes for weighing.
Stage 9. Weigh the bridge to test
overall weight, if unhappy with weight,
more can be removed by drilling or
sanding edges down to create a smaller
overall shape.
Stage 10. Both sides get small tacks
nailed into them along the bottom
edges, 2 mm away.
Stage 11. Glue with P.V. A., the sides
to the bottom using the tacks to help
with alignment. Masking tape is used
to clamp ply together.
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BRIDGE
C O N S T R U C T I O N
Stage 12. Fit in the
membranes. These might need to be
sanded on Disk Sander to get perfect
fits. Once fitted these two can be glued
in and clamped with masking tape.
SEQUENCE SKETCHING CONTINUED
Stage 13. Sand down the top until all
edges are flush, this can be done on the
Disk Sander.
Stage 14. Glue down a 1 mm veneer
top using a 4 mm template to limit the
veneer from distorting, again use
masking tape for clamping.
Stage 15. After bridge has dried for a
couple of hours it can have all the joints
sanded by hand to improve its finish.
Stage 16. After the glue has finished
drying, about 24 hours, the bridge can
be tested. The whole process could then
be repeated if time allowed and if the
bridge needed improving.
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BRIDGE
C O N S T R U C T I O N
MATERIALS LIST
The following materials are to be used in the construction of the bridge. See
working drawing for details of parts shapes.
Item No. Item
1
Sides
Length Width Thickness Quantity
1040
120
2
2
2
1040
35
2
1
700
100
2
1
1100
100
1
1
3
4
Material
A grade model
aeroplane ply
2mm 5 ply
Bottom
A grade model
aeroplane ply
2mm 5 ply
Membranes A grade model
aeroplane ply
2mm 5 ply
Veneer
Radiata
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BRIDGE
C O N S T R U C T I O N
MACHINE USAGE & SAFTEY
The following are the identified machines and their related safety issues
identified.
Machine Hazards Risks
Item
DRILL
PRESS
Electricity
Rotating
Spindle
BAND
SAWS (2)
Electricity
Rotating
Blades
DISK
SANDER
Electricity
Rotating
Spindle
BOBIN
SANDER
Electricity
Rotating
Spindle
BENCH
SAW
Electricity
Rotating
Competence Risk
Control
Medium, level
Risk – Be aware
that loose
clothing can get
caught in
spindle
Medium, level
Risk – Be aware
that students
less than 16 yrs
need direct
guidance.
Low to Medium
Risk – Work on
the down
side Traverse the
up side, Keep
moving the
timber, thin
material can get
caught.
Low Risk – Keep
the material
moving to ensure
limited burning
of material.
High Risk –
Needs guards,
HIGH
Use simple
rules to guide
the
students.
HIGH
Use simple
rules to guide
the
students.
HIGH
Reinforce
technically
correct
procedures in
sanding.
HIGH
Reinforce
technically
correct
procedures in
sanding.
Need to have
competent
HIGH
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BRIDGE
C O N S T R U C T I O N
Blades
Don’t use short
material,
Ensure
blades are
support
when cutting
large sheets
two
Students
should work
together.
sharp!!
SELF EVALUATION
After the whole process of building the bridge and testing it, the following
questions were thought about in response to the bridge building exercise.
1. How well did the final bridge do during testing, did it hold 25kg? How much
weight did it hold? What was it efficiency? What was the best efficiency in the
class?
Yes, my bridge held 25 kg. I was very nervous because I felt that I had, in my
obsession in trying to lighten my bridge as much as possible, removed too much
structure and depth in my beams. To my surprise, it held 50kg whilst the weight of
the bridge was 142 grams. Therefore, efficiency was 344. To my delight and
surprise, this was the best efficiency in the class.
2. What improvements could I have made to my bridge to make it better?
I feel that to make my bridge better, I could have lowered the weight by not applying
a coat of polish. Which added 6 grams to the weight and I feel it added nothing to
the strength. In addition, I feel that I made the depth of the beam too small in the
middle, which subsequently caused a fair amount of distortion. I didn’t destroy the
bridge, therefore I don’t know where the weakest points were for failure, but when
referring to my two other prototypes, I could assume it would be the weakest in the
middle section.
3. How did the brief influence my type of design?
Greatly, plywood is a good choice of material, but I wonder if it would have been my
desired material if the brief had been more open. Balsa is also a very strong
material as well as very light and I would have like to use it, but the brief
determines the materials to use.
4. What changes did I make during the manufacturing process?
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The first bridge that I
manufactured was huge,
C
O
N
S
T
R
U
C
T
I
O
N
the second bridge was quite good in size and weight yet when tested, didn’t hold the
weight. That is why 5 ply 2mm was chosen to increase the bridges performance. To
keep the weight down my prototypes got lighter, thinner and higher. They couldn’t
hold as much weight but the efficiency ratio was greater.
5. How did my research influence my design of bridge?
Greatly, I discovered the strengths of ply; I also found strong shapes for my bridge
design. Yet, I wonder what type of bridge I might have designed if I hadn’t
researched before I did my concepts of my bridges?
6. What have I learned from this process of designing bridges?
Ply is a strong and a product that has various uses. Strong jointing techniques are
important and understanding how the forces can be resisted against load is allimportant. In addition, it’s great fun!!!
7. What other bridges did I like?
Scott Rickaby’s bridge took the cake for me… it was unusual design. I don’t know
if it would make a good bridge to walk or drive on yet it held the weight very
nicely. The best looking bridges were the suspension bridges; they looked great and
held the weight quite well too.
REFERENCES
Internet sites that are good for Bridges
http://encarta.msn.com/index/conciseindex/25/concise.asp?mod=1&ti=761561057
http://www.discovery.com/stories/technology/buildings/bridges.html
http://www.iit.edu/~hsbridge/database/search.cgi/:/public/international/current/interna
tional_rules
http://www.buildingtechnology.com/bcba/bridges/basics.htm
http://www.iit.edu/~hsbridge/database/search.cgi/:/public/index
http://members.tripod.com/mrlewisclassroom/bridgedesign.htm
http://pghbridges.com/
http://www.pbs.org/wgbh/nova/bridge/
http://www.ndrs.org/physicsonline/bb-menu.html
http://www.bardaglea.org.uk/bridges/welcome.html
http://bellnet.tamu.edu/res_grid/trussb/designs.htm
http://www.ktca.org/newtons/12/bridges.html
Internet sites that are good for Plywood
http://home.vicnet.net.au/~woodlink/woodlink.htm
http://oak.arch.utas.edu.au/tech/ply.html
Books about bridges.
Bull, R. (1989). Starting design and Technology – Structures. London, Bath Press.
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Burns, M. (1982). Math for
smarty-pants. Boston:
Little, Brown and Company. C O N S T R U C T I O N
Clarke, D. (Ed.) (1979). The encyclopaedia of how it's built. New York: A & W Publishers,
Inc.
Corbett, S. (1978). Bridges. New York: Four Winds Press.
Dixon, M. (1990). Structures. Turin, Canale.
Gaff, J. (1991). Tell me about building, bridges and tunnels. Spain, Kingfisher Books.
Lambert, M. (1991). Technology in action – Building technology. England, Wayland
Publishers.
Oxlade, C. (1996). Super Structures. London, Belitha Press.
Science 55 Videotape: The Brunel Experience 1: The great divide: Building a Bridge.
Spangenburg, R. (1991). The story of America's bridges. New York: Facts on File.
Stephens, J. (1976). Towers, bridges, and other structures. New York: Sterling Publishing
Company.
Stix, G. (1993, Apr). Concrete solutions. Scientific American, pp. 102-112.
TV Ontario videotape: Trussworthy. Landscape of Geometry series. TV Ontario: (800) 3319566.
Whitney, C. (1983). Bridges. New York: Greenwich House.
Wollomir, R. (1994, Jan). Inside the lab and out, concrete is more than it's cracked
up to be. Smithsonian, pp. 22-31.
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