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Posts Tagged ‘design analysis

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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Posted on: June 6, 2009

1        Design Analysis

1.1    Alternative Analysis

1.1.1   Pontoon Alternatives

1.1.1.1  Pontoon Structure

Because the MFB would be subject to both physical forces and corrosion from both the atmosphere and the solution the bridge will float on, selection of the correct pontoon design and material was important.  Furthermore, the pontoons would have to provide the buoyancy force and stability that a MFB requires.  The materials available for the pontoon in the market are both vast and differ predominantly depending on environmental and social environments (as discussed in 2).

1.1.1.1.1      Barrel Design


Figure
4.1: Poly Drum
(ADCO Services, 2003)

 

Sealed short cylindrical drums, like that in Figure 4.1 and Figure 4.2, also known as barrels are regularly used in the construction of small rafts and pontoons (SookeRotary.com, 2009).  These drums are readily available as many industries utilize these containers for the transportation and storage of liquids (Newcastle City Council, 2006).  Barrels are available in different sizes and but the more common sizes are the 20L, 120L, 205L and 220L volume drums (Schutz DSL, 2009).

The physical size of a barrels provide little buoyancy and stability but do provide a streamline shape if orientated end into wind and water currents.

Using the Barrel Design for the MFB would mean using a product that had gone through the “test of time”, previously used for small rafts and pontoons, and was a readily available resource in Cambodia.


Figure
4.2: Steel Drum
(Global Industrial, 2009)

1.1.1.1.2      Cigar Design

Cigar-shaped pontoons are physically similar to barrels but have rounded ends and an elongated shape as seen in Figure 3.3.  The elongated shape provides superior levels of stability if orientated perpendicular to a bridge span (Fidler, 2008).  The increased displacement of this design over barrel design offer larger buoyancy forces, which can be utilized to support heavier bridge designs with fewer pontoons.  The streamline shape of this design limits the effects of water and wind turbulence if orientated end onto the current flows and thus reduces the load on the anchoring system


Figure
4.3: Canary North Quay Bridge, West India (Darkwaters, 2009)

Although the superior levels of stability that the Cigar Design could provide, because the shape was unavailable in Cambodia, unless it was manufactured out of synthetic (and costly materials), this structure was left for reconsideration for second-generation MFB’s.

1.1.1.1.3      Box Design

Box design pontoons are commonly used in calm water environments (Alaska Department of Fish and Game, 2006) and are frequently installed in boat mariners.  The design consist of a square or rectangular box section which provides the vessel for displacement on top of which is attached a flat surface used as the deck.  The large area of displacement of this design provides large load bearing capacity with the trade off to large susceptibility to surface currents and wind loading.  The stability of this design is poor due to the narrow displacement surface and so is generally installed between steel piles, which are used to locate and brace the bridge section.

Unfortunately, both the poor stability, and uncommon shape meant it was incompatible with both the environmental climate, as well as the environmental resources of Cambodia.

1.1.1.1.4      Displacement Design

The displacement design bridge is not regularly used in practice but does overcome the need to suspend a carry way, as seen in Figure 3.4.


Figure
4.4: Vera Katz Eastbank Esplanade and Floating Bridge
(Bridge Stories, 2009)

Unfortunately, this design is unsuitable for Cambodia due to the large effects water and wind currents would have on this structure due to its large longitudinal area.

1.1.1.1.5      Selection

Although the Cigar Design does provide increased superiority in terms of both stability as well as load, because of the unusual shape, considerations for this design will only be made for the 2nd generation MFB.  Thus, because both the Box and Displacement Design are inappropriate for the Cambodian environment, the classic Barrel Design is selected by default.

1.1.1.2  Pontoon Material

The materials used in the construction of floats available on the market internationally are vast and vary greatly in their cost and availability.  Not all materials listed in the conceptual analysis are appropriate for the use in the Tonle Sap, and is eliminated as necessary.

1.1.1.2.1      Concrete

Concrete is used in the construction of pontoons and offers good resilience to the effects of ultraviolet radiation (Hota, 2006).  Concrete is a versatile material, which can be cast into complex shapes (Concrete.com, 2009) but is reliant on reinforcement steel to provide enough toughness in an environment such as the Tonle Sap.

Unfortunately, the density of concrete is approximately 2300 kg/m3 (Jackson, 1996) and thus limits both the size of a pontoon, and the load bearing capacity.

1.1.1.2.2      Steel Drums

205L steel drums are generally the standard in the petroleum industry (Perth Petroleum, 2009).  It is appropriate for the buoyancy device used for the MFB, but is disadvantaged to the plastic barrel because plastic is generally less reactive to seawater than steel.  However, the advantage of steel does not have the disadvantage of degeneration if exposed to ultra-violet light for prolonged periods (Askeland, 2008).

Despite the Tonle Sap is a freshwater lake (World Wildlife Fund, 2009), because of the reactivity of steel with salt water (Hall, 1997); the metallurgic process of converting steel into inox would increase the price above that which is accessible to Cambodians.

1.1.1.2.3      Non-Recycled Plastic Drums

Like steel drums, polyethylene (plastic) drums are often used in the construction of floats.  Plastic barrels are generally used in the chemical industry, which require a corrosion resistant storage container (IndiaMart, 2009).

The selection of polyethylene drums, which have been treated with UV stabilizers, would extend the lifespan of the product (ASM, 1995) and provide a more sustainable design. Compared with other materials, plastic drums are one of the lightest (yet cheapest) materials (Hansma, 2007) and therefore can be transported easily, thus reducing cost of freight.

Since the price of treating polyethylene drums with UV stabilizers (ASM, 1995) would be far cheaper than that of treating steel in metallurgic process (Hall, 1997), plastic drums would be preferred over steel drums.

1.1.1.2.4      Recycled Plastic Drums

Among the more environmentally sustainable materials, which could be considered suitable for the construction of pontoons, are recycled plastics such as polyethylene terephthalate, high-density polyethylene and polyvinyl chloride (Lenau, 2003).  These plastics are currently recycled and used to produce wheelie bins, which are comely, used in Europe for rubbish collection (Wheelie Bin Skins, 2009).

The production of recycled plastics relies on the collection of plastics currently thrown in waste deposits such as landfills (Waste Online, 2006).  The plastic is then subjected to various chemical and mechanical processes in specialized facilities.  Although this process has high initiation costs and logistics (Hinkley Center, 2002), increasingly complex and interlocked environmental regulations (AllBusiness.com, 2005) will mean increased demand in the long-term, thus good Return-On-Investment.

Fortunately, recycled plastic drums are readily available in Cambodia (EWB Australia, 2009).

1.1.1.2.5      Fiberglass Drums

A common composite material used in the marine environment for the construction of buoyancy devices such as boat hull design is fiberglass (Wiley, 1988).  Fiberglass is a matrix of thermosetting resin in which a reinforcement mat of glass fibers is laid (Techstreet, 2004).  Composites such as this require a mould in which the composite is laid into which determines the dimensions of the final product, as shown in Figure 3.5.  Fiberglass has the advantage of being super-lightweight and can be filled with low-density foam, which strengthens the product as well as providing buoyancy.


Figure
4.5: Vera Katz Eastbank Esplanade and Floating Bridge
(Bridge Stories, 2009)

Unfortunately, the expense of fiberglass (Taunton Press, 1997) restricts access to such material.  Nevertheless, consideration was given for second-generation MFB to include such materials.

1.1.1.2.6      Selection

Concrete was inappropriate as a floating device because of its restrictive density.  Steel drums proved to be less potent (and readily available) than Plastic drums in the Cambodian economic and environmental climate.  Fiberglass was another effective option, but was expensive.

Thus, recycled plastic drums were chosen by default.

1.1.2   Bridge Alternatives

A number of constraints limit the design of the bridge structure, which is supported by the pontoons.  The most influential component on the bridge structure design is the pontoons as these provide both the buoyancy and stability to a non-pile tethered bridge designs.

As the supporting force of a MFB is solely supplied by the pontoons, the bridge structure and the load intended for the structure cannot exceed the buoyancy force (AllExperts.com, 2006).  Therefore, a practical bridge structure needs to be lightweight to ensure an efficient safe working load is achieved.  The bridge structure is used both as a load-carrying frame, but also to locate the pontoons.  The secure locating of the pontoons to the bridge structure is paramount to ensure a safe design.

In designing the structure of the MFB, the design had to incorporate a centre of gravity in proportion to the stability provided by the pontoons.  Without properly addressing this issue, a bridge when loaded can become unstable and unsafe due to an increase in the height of the center of gravity (Gaudron, 1999).  The designs of floating bridges are generally of low modular type construction but can be as extravagant as the rolling bridge designed by Heatherwick Studio, which utilizes hydraulic rams to curl the structure (Figure 3.6).


Figure
4.6: Rolling Bridge designed by Heatherwick Studio
Geekologie (2007)

Furthermore, handrails were carefully considered for the purpose of safety.  The handrails of the MFB had to supply adequate support for users in the event the structure is hit by waves or wind (Bennett, 1999).  Furthermore, the handrails needed to be designed in such a way that they do not inhibit the flexibility of the structure.

1.1.3   Component Fixing Alternatives

1.1.3.1  Component Fixing Structure

After establishing all the various componentry, the next important consideration was the structure of the adjoining module.  In particular, because of the cyclic and fluctuating stresses caused by the constantly changing water levels (Food and Agriculture Organization of the United Nations, 2001), special consideration had to be made for the flexibility factor to ensure brittle fracture does not occur.  This consideration of environmental conditions, as well as of the loading capacities, led to the “modular design” of the MFB.

The modular design commonly seen in boat marinas (MarineTek, 2009) ensures the bridge structure has a moderate level of flexibility both in its structure and application sense.  A modular design consists of a long bridge span being constructed from many smaller individual modules, which allow for flexibility at the point where they connect to one another.  The modular design also has the additional benefit of its USB-like “plug-and-play” nature, which allows individual short spanning modules to be used where a long span would be impractical.


Figure
4.7: “Hot-Swap” Super-Fast Connectivity
(Scarlet Skunk Marine Services, 2009)

Furthermore, the connection of a modular design can also utilize “hot-swap” super-fast connectivity as seen in Figure 3.7, appropriate for a bridge that may need to be moved frequently, disassembled/reassembled quickly as seen in Figure 3.8.


Figure
4.8: Lacy V Murrow Bridge
(Seattle Daily Journal of Commerce, 2009)

1.1.3.2  Component Fixing Material

Note that the material used in the bridge structure would generally dictate the method of fixing components together in the bridge. For example:

1)      In relation to concrete structures, methods include bracketing or bolting

2)      In relation to plastic structures, methods include clipping, bracketing, or bolting

3)      In relation to wooden structures, methods include dowelling, bolting, roping, or bracketing

4)      In relation to metal structures, methods include welding, riveting, or bolting

Since 3.1 established the use of plastic drums, and 3.2 established the use of a wooden bridge, both (2) and (3) would be relevant.  Furthermore, in order to adjoin the plastic drum and the wooden bridge, steel brackets were introduced, thus the relevance of (4) too.

1.1.4   Anchoring Alternatives

Because the bridge was likely to “float” in the middle of a vast flood of water, consideration for anchoring was required.

Traditionally, the methods used to hold bridges in place were to either:

1)      Fix the bridge to a solid structure at either end as seen in Canary North Quay Bridge, West India (Figure 3.3) or;

2)      To tether the bridge to pylons which are driven into the sea floor (Figure 3.4)

Unfortunately, these methods are only useful if, for (1) either there is a solid structure at the termination points of the bridge; or for (2) the bridge is not required to be removable in the case of pylon-tethered designs.  Otherwise, as with the MFB, an alternative anchoring system would have to be employed.

As a result, the MFB uses “heavy weights” (conventional marine anchors), which are attached to the bridge, locked on to an area of the seabed.  Furthermore, because the MFB needs to accommodate to changing water levels driven by wind and tides in large bodies of water, a special method is devised (see 5.6) to adjust for changing water levels accordingly.

1.2    Selection Analysis

1.2.1   Benefits

There are a number of benefits to the MFB bridge design, as finalized above, which makes it ideal for the communities living on and around the Tonle Sap.

1.2.1.1  Generic Parts

The componentry material is undemanding with generic parts for the construction of each section.  This allows for standardized construction techniques (Berhow, 2005) to be used, which increase the economical benefits of this design by shortening the construction time needed.

1.2.1.2  Local Materials

The bridge uses materials that are easily sourced in Cambodia, including:

v  Plastic drums (EWB Australia, 2009)

v  Wooden decking (American University Washington D.C., 1996)

v  Bamboo railings (Biodiversity International, 2009)

As discussed in 2.1.1, locally sourced materials aid in reducing the material costs due to eliminating freight and tariff charges while providing the benefits of supporting the Cambodian economy.

1.2.1.3  Entrepreneurial Design

A key feature that differentiates the MFB from its competitors is its modular sections, which are joined together to form a long structure.  This allows for the easy extension or reduction in length and shape as the needs of the user change during the year.  As the engineers described it, the MFB allows for “plug-and-play” and “hot-swapping” (3.3.1).

1.2.1.4  Increased Safety

The use of short bridge sections in this modular design provides additional levels of safety and flexibility when compared to a longer more rigid structure.  The many joints between short sections allow for the flexibility of the structure when adverse weather conditions are experienced.  In the event of a wave affecting the bridge the multiple joints allow the bridge to flex and rotate in a variety of directions increases the stability of the structure when compared to a more rigid structure.

The placement of the drums on the ends of the cross section timbers allow for an increase in stability due to the wide footprint of the bridge structure.  The location of the drums helps to lower the center of gravity of the bridge, which is vital to ensuring a safe and sustainable design.

The location of the anchoring system onto the midpoint between the drums on the cross section timbers was a safety conscious decision.  The location of this ensures that the structure will not heel over when subject to a disturbed water state and decreases the swaying action of the bridge much like a keel on a boat.

1.2.2   Detriments

The MFB was designed in such a way as to be superior in every manner; nevertheless, because of the restrictive financial constraints on the project, several materials were chosen for the “first-generation MFB” that could be upgraded in the likelihood of a “second-generation MFB”.

This includes:

Cigar Design, rather than Barrel Design, which allows for increased stability and load; and

Fiberglass Drums; rather than Recycled Plastic Drums, which allows for increased strength and buoyancy (thus load)

Nevertheless, the engineering team intends that the second-generation MFB will have full “back-compatibility”, meaning it will integrated perfectly with first-generation MFB’s, thus reducing operational redundancy.

1.3    Budget Analysis

For a project of the immense size that the MFB aimed to be, and the suitability of the project on a far smaller scale, meant that cost estimations were made based on two criteria (Taylor, 1999):

1)      Precision: High level of repeatability, because if the MFB cost estimations were not precise, despite the project may be financially sustainable in the production of mass quantities, it would lack the consistency required for the production of smaller quantities of MFB’s; and

2)      Accuracy: Degree of closeness to the true quantitative value, because if the MFB cost estimates were inaccurate, the project may not have the funding to properly expand, if not implemented at all

In coming up with a budget proposal, we kept in mind the following qualitative characteristics in drawing up one:

Reliability: Synonymous with “Accuracy”, as described above; thus has representational faithfulness, and not biased;

Consistency: Synonymous with “Precision”, as described above;

Relevance:

  • To provide a forecast of revenue and expenses, as well as assets and liabilities
  • To provide a forecast of how the business may respond from certain strategies, and events (expected and unexpected)
  • Have both predicative and feedback value

Comparability: To enable actual financial operations of the business to be measured against the forecast;

Understandability: Stakeholders, without prior accounting and engineering knowledge, can understand and apply the budget in a systematic manner.  Obviously, this criterion assumes its users have a reasonable knowledge of business and economic activities and are willing to study the information with due diligence; and

Decision Usefulness: The budget must be provided in a timely manner as so not to inhibit decision usefulness.  This characteristic must be weighed up against the reliability criterion; since increased time would mean increased reliability, but may mean reduced decision usefulness. 

For each independent (that is, separately funded) implementation of the MFB, there are three stages of product development:

1)      Business start-up: In this period, working capital and labor service must be established.  Since the bridge is Do-It-Yourself, assuming no labor costs (other than those of training staff at EWB), working capital is the predominant levy.

2)      Business establishment: Once a community has established various MFB, it becomes established, and has a cash flow.  Because of the franchise-nature of the MFB system, the franchisor would receive a fee, which would go towards a marketing budget.  Note that economies of scale (with the exception of the steel bracket) is unlikely to be achieved because of the independent and autonomous nature of the MFB production system.

In order to increase perceived (as well as actual) professionalism, annual reports should be drawn up yearly, outlining the financial position, performance, and cash flow of each autonomous MFB system, and a consolidated report for the entire network of MFB’s (including the corporate head).

Because of the varying prices of componentry in Cambodia, the use of its own natural resources, and volatile economy, cost estimations could not be accurately provided in the design brief.  Nevertheless, price minimization was the objective at every stage of analyses.