Mini Project: Circular Pedestrian Bridge
Of late we’ve been really busy this term. One of our classes, 511 Advanced Steel, we took as a 4th year elective but its been really tough and its taken up nearly half our time. We also have two other term projects instead of final exams which have made life very chaotic staying till midnight or so every night for the past 2 weeks or so. We’ve been so busy that I don’t think we’ve cooked since the beginning of the month 3 weeks ago. On the bright side, this means we only have two finals which should be quite easily managed.
A few weeks ago I thought up this idea for a pedestrian bridge which I’m sure has been thought up before by someone else but I wanted to see how it would visualize so I drew it up in SketchUp. This is the circular pedestrian bridge.
Circular Pedestrian Bridge
Pedestrian bridges are one of the most interesting bridges to design due to their differencing nature from highway or long span bridges. Pedestrian bridges are relatively short spanned structures and are significantly lighter such that the majority of the weight is the self weight of the structure. Highway bridges by contrast are large, heavy structures that are required to take the load of large vehicles. They may be short highway overpasses that span 30m or long cabled stay bridges like the Alex Fraser Bridge.
Pedestrian Bridges are very public structure and unlike highway overpasses which are functional bridges, pedestrian bridges must have some aesthetic impact. Since pedestrian bridges are seen up close they must compliment the landscape and appear as sculptures or art. Such distinctive structures make for interesting projects.
Most often, bridges are required to span a single distance, such as a river without support in the middle. A highway overpass for pedestrians is different such that the meridian in-between highway directions allows for a pier. This allows for the distance to be split into two spans.
The Circular Pedestrian Bridge is different from a typical bridge because of its single pier at midspan with a cable stayed deck. Cable stayed bridges are most economic for long spans which this is not but given the geometry of the bridge might be feasible. Mostly, it’s for the aesthetics.
The bridge consists of a single pier at midspan with a unique pre-cast concrete torus. A concrete deck is suspended through this 9m torus supported by 18 cables on either side. The total span of the bridge is 58.8m and the deck is 4.8m wide. Concrete works best in compression and is weakest in tension. A concern that the torus will collapse upon itself due to its own self weight is accounted for by the position of the deck and the cables that while supporting the deck, pull the torus together.
The deck is cast concrete with underlying longitudinal and transverse steel beams which is also where the cables connect into. The deck could potentially be engineered woods but I’m not very knowledgeable about that material right now. Framing the bridge are two typical concrete abutments. Since the bridge is quite light and flexible, there’s a concern that people walking across it will induce significant vertical and lateral vibrations. These are difficult to account for except by making the bridge stiffer. This term we’ve also learned a little on how to model in the structural analysis program SAP2000. It’s not the most intuitive program and while I’m told it’s an industry standard, there are significant features missing from it. I thought it would be nice to see what modal shapes the bridge would make and if the forces would work but SAP isn’t the right program to effectively model this bridge in.
Vehicle View
On the roadway the bridge appears as a single, short tower from a distance. As a vehicle approaches, the shape of the torus appears due to differences in sun shading. The overall shape of the bridge is not indifferent from a small cable stayed bridge.
Pedestrian View
The pedestrian view provides a very different aspect of the bridge with a circular gateway created by the torus. Although I neglected railings along the edges of the bridge, this view shows how the cables enhance the circular entrance aspect.
3/4 View
Perhaps the most interesting view of the bridge is from the 3/4 at the top of the highway embankment. This view, presents the torus shape with the floating pedestrian deck. Although the deck is pin-fixed at the pier, the general appearance is that it floats through the torus.
For a pedestrian highway overpass this is a significant structure. While it may not be cost effective, it displays an interesting exercise in creating a different type of bridge. Further investigation would be required to ensure that the bridge can withstand gravity and seismic forces, how fragile the torus is, and how effective the cables are in supporting the deck.
