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1        Final Design

As established in the Design Analysis:

v  Barrel Pontoon Structure

v  Recycled Plastic Drum Material

v  Component fixing using (depending on material of to-fix structure):

  • Plastic structures, methods include clipping, bracketing, or bolting
  • Wooden structures, methods include dowelling, bolting, roping, or bracketing
  • Metal structures, methods include welding, riveting, or bolting

From the analyses of existing designs and available materials, a Final Design was created for the MFB that would be used to as civil infrastructure in the Tonle Sap area.

There are a number of key parts to the design, which include:

1)      Polyethylene (Plastic) Drums

2)      Steel Brackets

3)      Wooden 4”x6” Timbers

4)      Wooden Decking

1.1    Polyethylene (Plastic) Drums

Polyethylene drums are used as the bridge’s buoyancy devices.  The 220-liter drums are used as they provide suitable buoyancy levels for our MFB design (as discussed in 4.1.1.2.3) and are readily available in Cambodia for the reasonable cost of two dollars per unit (EWB Australia, 2009).  The drums are 220 liters in volume, which provide a buoyancy force capable of supporting approximately 220Kg when fully submersed in fresh water (Marine & Industrial Distribution Ltd, 2009).  The external dimensions of the drums are 580mm in diameter and 910mm in length, as shown in Figure 5.1.

One section of the bridge will have four drums attached to it, two on each end, which will result in a theoretical gross maximum load of 880 kg, reducing safety issues.  To extend product life, the polyethylene drums will ideally be manufactured with UV stabilizers to reduce the decomposition of the material when exposed to ultra-violet light for long periods (as discussed in 4.1.1.2.3).


Figure
5.1: Artist’s Visualization of a Polyethylene Drum

1.2    Steel Brackets

Steel brackets are manufactured to allow the secure attachment of the drums onto the bridge structure.  The steel brackets are comprised of two halves, one standard bracket and also a modified bracket that are bolted together when placed around one of the drums, as shown in Figure 5.2.


Figure
5.2: Artist’s Visualization of a Modified Bracket

A key feature of the modified brackets is the two hollow rectangular boxes that are attached by gusseting and welding to either side.  The attached boxes are used to locate the drums onto the cross timbers which locate the drums and provide the support for the bridge structure.  One section of the bridge will have eight of these brackets, two being around each of the plastic drums, as shown in Figure 5.3.


Figure
5.3: Artist’s Visualization of a Drum Assembly

1.3    4”x6” Timbers

A standard sized timber of cross sectional dimension, 4 inch by 6 inch (4”x 6”) is used throughout the design.  Standard sized timber was purposely used to simplify the design and construction and increase the flexibility of the final design.  The use of generic material throughout the design has large economic benefits, as discussed in 4.2.1.


Figure
5.4: Artist’s Visualization of a 4”x6” Timber

The timber used in this bridge design will be treated to ensure protection from material decay because of water exposure.  During the construction of the bridge, the timbers are first used to hold the two drums at a set distance of 1480mm apart.  Next, they are slid into the rectangular boxes on the brackets where they can then be fixed into place.  This creates the end drum piece.


Figure
5.5: Artist’s Visualization of how
a Timber fits into a Bracketed Drum

The second circumstance where the timbers are used in the design are to set the distance in between the two end drum pieces, which provides a platform for the deck to sit on.  To construct this, a water treated 4”x6” wooden timber of standard length (6000mm) has two 800mm sections cut from it.  The cut offs are then notched and become the connecting component between the cross sections and bridge span.  The notches measure 4” x 1” that allow for the location of these components securely to the cross timbers.  The notched pieces are then reattached to the underside of the remaining 4400mm timber at each end.  Several large cylindrical dowels are used in the connection of these two pieces, which create notched timber sections.  There are four notched timber sections per bridge module.  To ensure a high level of safety the notched timbers will be secured onto the cross timbers by a small recess and dowel between these two components.


Figure
5.6: Artist’s Visualization of a
notched timber section

The combination of a completed Figure 5.5 and Figure 5.6 is the underlying framework for the MFB, shown in Figure 5.7.


Figure
5.7: Artist’s Visualization of an
assembly bridge module

1.4    Wooden Decking

The decking for the bridge is constructed of a resource readily available in Cambodia: it is made of wooden tree trunks that have been treated to ensure suitable protection from water exposure.  The supplied tree trunks are 150mm in diameter and 5000mm in length.  The trunks are then cut through their length and sanded smooth to give two halves that are 5000mm in length.  The semi-circular tree trunks are then cut into pieces of 1100mm in length to be used as the decking.  The resulting width of the walkway is 1100mm and there are 29 pieces of deck spanning between the two end drums.  The wooden decking (unaccompanied) can be seen in Figure 5.8.


Figure
5.8: Artist’s Visualization of a
wooden decking

These individual pieces of deck are then nailed to the notched timber sections, convex down which creates a secure and sustainable deck surface, as shown in Figure 5.9.


Figure
5.9: Artist’s Visualization of an
assembly decking onto bridge module

1.4.1   Extension Pieces

A key feature of this design is the use of extension pieces as every second segment of bridge.  These extension pieces have a similar design as the segments described above; however, they have no drums attached to them.  The notched timber sections are spaced further apart, resulting in a wider deck; 1300mm compared to 1100mm.  The extension piece fits on the outside of the standard decking and is located onto the cross timbers in a similar fashion.

These extension pieces, shown in Figure 5.10, are constructed with 6 pieces of decking missing from each end to ensure the there is no overlapping of the decking materials.  The generic design of the notched timber sections allows the use of this component in the construction of both the standard and extension pieces.


Figure
5.10: Artist’s Visualization of an extension
piece (note wider decking is the only modification)

The use of these extension pieces reduces cost as it allows the use of one cross section with barrels for every two “bridge sections”, as shown in Figure 5.11.


Figure
5.11: Artist’s Visualization of an extension
piece and a standard piece interoperating

1.4.2   “Branching” Pieces

The construction of shortened end drum pieces which only have one drum attached in the middle allow for the addition of branched bridge sections perpendicular to the main span.  The branches are attached by several short lengths of rope secured between the branch section and the main bridge span, as shown in Figure 5.12.


Figure
5.12: Artist’s Visualization of a bridge branch piece

1.5    Handrails

Handrails are also to be included on the bridge to improve safety.  The handrails are made of bamboo and are 1300mm in height.  Eight bamboo poles are attached to the bridge segments (four to each side) and rope is strung from one to the other.  A singular rope section can be removed if a bridge section is to be joined perpendicular to another.

1.6    Anchoring

To locate the bridge in position a weighted pulley system is used.  This anchor system provides the lateral stability needed in an environment with changing water depths.  The system comprises of a two concrete weights, which are located onto the lakebed, either side of the bridge and are referred to as the dead weights.  Secondly a rope is attached to each dead weight and run through a common steel eye, located on the middle of the cross sections of the bridge structure, and then to a counter weight which is suspended under the bridge.

As shown in Figure 5.13, as the water level changes, the counter weight changes height as the bridge floats up and down which changes the length of the anchoring rope appropriately.


Figure
5.13: Artist’s Visualization of how the
Bridge reacts to changing water levels

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