Assignment 2 Bridges: Virtual & Real
Constructing Environments Semester 2/12
Below: A location the map showing its surrounding context.
The South Melbourne Station Pedestrian Bridge is located near the South Melbourne station, a former railway station on the St Kilda line. The track is now used for route 96 on the light rail line (City of Melbourne, 2012). The station was opened in 1858 and line through was opened in 1857. So subsequently it can be assumed, although no data was found that the bridge must have been opened somewhere between 1858 and 1857 (Heritage Victoria, 2011).
In addition to the assumption of construction dates due to construction time of South Melbourne station. It can also be assumed that the bridge was designed in the 1850s as the arched bridge truss was a typical design used for bridges during this period. For example the Princess Bridge built in 1850 was very similar to the South Melbourne bridge. At the time the Princess Bridge was the longest single span bridge in the world . The bridge was later rebuilt in 1888 to what it is today (Heritage Victoria, 2011). The main construction material used on the Pedestrian Bridge is Blue Stone. Blue Stone was a favored building material during the 1850s Victorian Gold Rush. In Melbourne due to difficulties with carving it was predominantly used for foundations, cobblestone roads, walls, gutters and bridges. Such as in on the Princess Bridge as mentioned above and Hawthorn Bridge. Left: An image of the Blue Stone used on the bridge.
South Melbourne Station Pedestrain Bridge Cnr. Ferrars &Dorcas St.
Above: The Richmond Bridge in Tasmania is the oldest bridge in Australia built in 1825 by convicts and is Australiaâ€™s oldest functional bridge. As I am from Tasmania and leave near this particular bridge I have used this bridge as precedent. Although constructed in sandstone it is very similar to the South Melbourne bridge. Compression Lines
Pinned Support Rx
Above: Free-body Diagram
The Arched Bridge: Has been a prominent bridge design for more than 2000 years. The design works by itâ€™s semicircular structure sophisticatedly distributing compression through its entire form and diverts weight into its two abutments which are the components of the bridge that directly take pressure. Arch bridges provide high resistants to bending forces.
South Melbourne Station Pedestrain Bridge
Left: Sketch of the weight path of bridge. The arched bridge is always in compression and the weight path of the bridge is always pushed out along the curve of the bridge towards the abutments. The tension is the bridge depends on the curvature as there is not a lot of curvature in the South Melbourne Bridge it is not under a great deal on tension.
Cnr. Ferrars &Dorcas St.
The New market Racecourse bridge is located in the Melbourne suburb of Flemington. The Bridge which used for trains is located just bellow the City Link free way. Located north of the bridge is the Flemington Bridge Station which was opened in 1885 so it can be assumed the New market bridge would have be opened around the same time (Heritage Victoria, 2012).
Above & Left: Images taken underneath the bridge of the support system and the wood slating.
Left: A map of the surrounding context which identifies the bridges proximity to the City Link freeway.
The design of the bridge is a truss bridge system which is a common type of bridge design in the 1980s because of its economical construction and efficient use of materials. The bridge is made from wrought iron as appose to steel it has a very low carbon content and was particular popular in truss design and also the material is readily available in Melbourne.
Pinned Support Weight
Above: Free-body Diagram demonstrates the weight loads
The truss bridge, is a simple skeletal structure. I theory individual part of a simple truss are only subject to tension and compression forces but not bending forces. Because of the trusses small beams put together can support a large amount of weight but can also span over a great distance. The Racecourse Bridge had to consider large amounts of weight for trains coming in contact with the bridge. Below: A weight path diagram of the dispersal of weight of a truss bridge. The bridge has been designed with King Post trusses which is one of the most simplistic forms of truss. It involves two posts angled leaning together into a common vertical support. This type of design is effective as weight is dispersed evenly for maximum support.
Racecourse Bridge New Market
Above: Images taken a site visit of the trusses.
In undertaking the bridge design I first decided to redesign the Racecourse Bridge (figure 1). It has a Kings post truss system so I remodeled that looking at the pictures from my site visit. It was not long and the successful design was letting the trucks across. However I had difficulties keeping the price down. Figure 2 demonstrates how I tired a different design and used a variety of materials in the different parts of the bridge that were in compression and tension. Budget was still my greatest changeling. I soon realized that one of the reasons my prices were sitting in the $250,000-$300,000 price range was because the span of bridge was 20m appose to 16m. The second I made this alteration to my setting my budget automatically dropped bellow $210,000 and then it was just a matter of trial and error trying to drop the budget further which I was able to by another $22,000 please see next page for final design.
To make sure I had selected the most appropriate and economical truss system I surveyed a few other designs. Figure 3 is an example of one of the successful truss system I reviewed. Although with this particular design I altered the settings to see its fool potential.
The Virtual Bridge
Above: Is my completed design. This is the most effective economical design. The style of truss is called a Pegram truss and is a combination of the Warren truss and the Parker truss. What makes this truss different to others is the difference in upper and low chord length and the panels are not square. To the right is an example of a Pegram truss in the Gimlet Bridge in Ilhado.
Above: Although hard to read proof of the price which was $187,021.20 which is the budget of $210,000 by $22, 979.
A combination of Hollow carbon steel and Quenched and Tempered steel is used in compression around the edge of the design. In the trusses where there is high compression I had to use thicker materials that had a higher compressive strength. The bottom truss is a solid carbon steel as this material has the high tensile strength.
In order to decrease my price I used a system of trial and area testing the minimum amount of material that could be used I virtually tested every member of the truss for the most economical material. Finally I was satisfied that I had the most economical design.
The Virtual Bridge
Comparison Comparing the three bridges for part A. It is clear the similar qualities of both the design and the compression and tension properties are similar in the Racecourse Bridge and in my West Point bridge design. The members of both these trusses are under similar compression and tensile strength. The truss configuration for both designs have angled member pressing together creating vertical strength.
The Virtual Bridge
The Arch bridge is similar in its compressive qualities. The truss bridge allows for the bridge to span over a longer distance which is more appropriate for the design. For the arch bridge to span a further distance there would have to be many abutments adding cost to the design. Although for an aesthetic point of view the arch bridge does create a beautiful view especially when it is in water.
Bridge: Racecourse Bridge, New Market. Span of bridge: There are four car lanes at 3.657m wde, two trams lanes and two footpaths that run under the Bridge. Therefore it can consdiered that it would be 25.6m is the span of the bridge. 15kn
About B ∑M1=(Fvcxd)+(Fvd+d)+(Fvaxd) =(-15x14.25)+(-15x11.35)+(Evax25.6) Fva=15KN
Defelection shape of bridge.
Take moment about A ∑M1= (15x11.35)+(-15x14.25) + (Fvbx 25.6) ∑M1=(-170.25)+(-213.75)+(Fvbx25.6) ∑M1=-384+(Fvbx25.6) Fvb=15KN
x <11.35m at 11m 11.35 >x <14.25m at 13m x < 14.25m at 15m
Fav = 15kN
x<11.35m at 11m x
Fav = 15kN
shear force ∑Fy=0
Bending moment ∑ moments through axis x
-Vx+15 = 0 vx=15 = Fav = 15kN
+Mx - (15)x = 0 Mx= Fav x = 15x M(11)= Fav (11) + 155KN Fvb=15kn
11.35m > x< 14.25m at 13m A
shear force ∑Fy = o -Vx+15-15=0 Vx = 0 kn please over page for further workings
Fav = 15kN
Bending Moment ∑Mxz = 0 Mx + 15(13-11.35) - (15)(13) = 0 Mx= 195 - 24.75 = 170.2KNM x>A.25m at 15m
Fdv=15kN Fav = 15kN
Span 25.6m Sheat force ∑Fy= 0 -V + 15 - 15 -15 = 0 Vx = -15kn Bending Moment ∑Mx = 0
M+15(15-14.2) +(15)(11.35) - (15)(15) Mv = 225 - 11.25 - 170.2 = 43.5kNm
Websites: Melboune Bridges, Herritage Victoria, Viewed 4th of October 2012, http://www.dpcd.vic.gov.au/heritage Trains and Trams, City of Melbourne, Viewed 6th of October 2012, http://www.melbourne.vic.gov.au/Pages/default.aspx Bridge Design, Melbourne Government, viewed 4th of October 2012, http://www.environment.gov.au/heritage/places/national/richmond/index.html Melbourne historics, OnlyMelb, viewed 11th of October 2012, http://www.onlymelbourne.com.au/melbourne_details.php?id=10448 bridges melbiurne Truss systems, Truss design, viewed 11th of October 2012http://heritage.portphillip.vic.gov.au/Local_History_services vic herritage Images: Page 1: Google Maps, viewed 18th of September 2012, www.googlemaps.com Page 2: Richmond Bridge, Tasmainan Herriatge, viewed 18th of Semptember 2012, www.tasherrtage.com. au Page 3: Google Maps, viewed 18th of September 2012, www.googlemaps.com Page 6-8:Images from west point bridge design are screen shots taken through out the process. Site Images: Students own taken on the 13th of Semetmber 2012 . Diagrams: Students own