Montagu Pedestrian Suspension Bridge in Montagu, Western Cape
The Montagu Pedestrian Bridge in Graaf Street is the only link between the upper and lower parts of town when the Keisie River is in flood.
In the 2008 floods the old cable suspension bridge washed away leaving only the old bases and columns. This made it difficult for pedestrians to walk to Montagu town as the low water bridge is used predominately for vehicles. WorleyParsons was requested to design a new bridge over the river. The old foundations and columns were investigated and found to be fit to support the new bridge, which suited the limited budget.
From previous flood levels it was clear that the bridge soffit needed to be raised above the existing columns. Although the additional free board was sufficient for letting the flood water safely through, the danger to light structures such as pedestrian bridges is that floating debris such as tree branches get stuck and create a build-up of smaller debris and eventually cause the collapse of the bridge. The bridge structure therefore had to be as robust as possible to withstand side impact. For this reason the deck was constructed in reinforced concrete. The structural steel used for the bridge weighed about 13.5 tons.
The design of an aesthetically pleasing steel bridge structure in a scenic area such as Montagu was challenging. Six alternative layouts were presented to the client for selection. The selected design consisted of a 4-span bridge comprising two main spans of 19.5m each and two side spans of 10.0m. The walkway width of 2.5m allows pedestrians and cyclists to cross easily when going in opposite directions.
The designers believe that the catenary shape of the top chord resembled the mountains in the background.
The circular hollow sections as main elements were selected for the "softer" appearance than that created by open hot rolled sections. The circular hollow sections also make maintenance much easier.
The bottom chord of the walkway is in double curvature, raising the pedestrians and cyclists gradually from the ends to the high point in the middle of the bridge, 1.5m above the old columns. The top chord has the shape of four catenaries (inverted arches). Strong V-shaped elements, over the columns, hold the bridge above the supports.
Construction of the new pedestrian bridge in Montagu was completed at the end of April 2012. The R2.66 million steel bridge is 59.25 meters long and around 2.5 meters wide and boasts with a concrete walking surface. The bridge, which was initially built in two pieces, was later divided and eventually transported in five pieces because of its magnitude. After two days of transportation, the bridge was lifted with cranes and assembled on site.
Steel bridges have many advantages including a life span extension of over 100 years, and they are adaptable and can readily be repaired after accidental damage. The new design made provision for a large clearance area below the bridge, around 1.5 to 2 meters. This was done to enable debris like trees to pass through effortlessly during floods.
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