Montagu Pedestrian Bridge
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The Montague Pedestrian Bridge bridge is the only link between the upper and lower parts of town when the river is in flood. The previous suspended bridge was washed away some time ago, leaving behind only the old bases and columns. 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.
Regarding the aesthetics of the bridge, the column positions were fixed. 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 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.
The design of an an aesthetically pleasing steel bridge structure in a scenic area such as Montague was challenging. The designers believe that the catenary shape of the top chord resembles 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.
Alwyn LE ROUX Pr Eng
On the right are photographs of the previous cable stay bridge just before it was washed away - 2008 11 13.
Submitted by William MARTINSON
All truncated references not fully cited below are those of Joanna Walker's original text and cited in full in the 'Bibliography' entry of the Lexicon.