Updated: Nov 21, 2020
MSIKABA BRIDGE MODEL
Of all built structures, I am completely in awe of cable-stayed bridges. The people with the imagination and engineering intellect, who design and construct these functional art forms, seem so overlooked when compared to top architects, interior and fashion designers, many of whom enjoy global celebrity status. The one exception I can think of is celebrated structural genius Santiago Calatrava whose work is probably known more by those, who are familiar with images of his structures than the man himself.
So when Modelart was awarded the contract to model the Msikaba Bridge located in the Eastern Cape of South Africa, it was as though the stars had aligned. Renowned international consulting engineering firm SMEC designed this bridge.
It is well acknowledged in the real world that long span bridges are precarious structures to build. The combination of unpredictable wind forces and gravity work against the project every step of the construction phase. Only when the two ends of the deck meet in the middle and are connected, can the crewmembers relax in the knowledge that the assembly is finally at maximum structural-integrity.
The Msikaba Bridge spans 580 meters between the two concrete pylons measuring 126.7 meters high and just 6.5 x 3.5m at the base of each splayed leg. The cables are arranged in four sets of seventeen with a diameter of 267mm nearest the pylon, becoming incrementally thicker towards the center of the deck at 419mm. The cable length is 95m nearest the pylon and 302m furthest from the pylon. The combined weight of the cables 2500 ton and the bridge mass is 2700 ton. The highest point from bridge deck to the valley floor is 192.3 meters.
The sheer planning and logistics of delivering these enormous mostly pre-fabricated structural components to site is complex in the extreme. And that is the easy job! One then has to then position and assemble oversized components dangling over a hollow gorge well in excess of one hundred meters below, in a process that becomes incrementally more difficult as the assembly evolves. The people who embrace and drive these projects need to be commended for their bravery and dedication in making these seemingly impossible projects possible.
The privilege of being awarded the contract to build a model the Msikaba Bridge evokes a sense that the result needs to be as close to a document of the real life structure as possible, for it to have integrity in the mind of those who will view it. To achieve this, we would need to scale the cable thicknesses correctly. The deck needed to have the correct arched upward curve of 1,18% or 1,8m to the center from each end. The cross section profile of the deck its self needed to be true to the design at just 2m thick. A 2,5m high transparent wire mesh barrier flanked the pedestrian way, it too needed to be represented at a scale of 1: 500.
The complexities of modeling this bridge shares some parallels with the real life structure and turned out not to be as simple as we had imagined. We chose our normally sufficiently rigid egg-crate substrate to build the two landscape components onto. This is our standard uncomplicated process of assembling purpose made CNC profiled components, designed on the same principal as steel castellated beam structures.
The bridge components were all made as separate sub assemblies. The Pylons were CNC profiled in engineering plastic with pre-machined holes correctly located and angled to receive each cable. We created tenons extending from the base of the pylon legs as part of the same process that slotted into mortise holes in the robust concrete/plastic base structure, to ensure a firm fix and no risk of movement.
The bridge deck; at just over 10% longer and with a similar aspect ratio as a one-meter steel ruler for comparison sake. A seemingly simple structure and the least of our concerns at the outset, turned out to be a brain tease. The 4mm thick deck needed to have a structural integrity of its own to accept the pre profiled balustrade and extruded side profiles along the length of the deck. It needed to hold the form of the arched camber, ramping up into the center of the bridge and down the far side. After four or more attempts we finally achieved a good result by infusing two fine skins of polycarbonate top and bottom, over a honeycomb core at a slightly exaggerated camber that would settle into the correct shape under its own weight.
At the time of submitting our cost proposal, we had no idea how we would represent the cables, it transpired! The challenge was that they needed to be perfectly straight as if to be in tension, mindful too that the longest cables are 604 mm long. The obvious choice was to use piano wire, but this failed dismally as the weight of just one strand of 0,8mm wire measuring 604mm in length, would both sag excessively under its own weight, and distort the deck profile. We learned too, that it is not possible to get a perfectly straight strand of Piano wire. Our next stroke of genius was to experiment with elastic strands; the type that is used in the weaving of stretch fabrics. In theory, this seemed to be the perfect solution and that we were out of the woods. Practically though, it was impractical. In tension between the bridge deck and pylon, it had the opposite effect of the piano wire, by lifting the deck structure out of shape. The sixty-eight cables holding the span of the bridge would equal a lifting force of in excess of ten kilograms. This too would not work! There was some relief in abandoning rubber, due to the niggling suspicion that it would perish over time, resulting in ongoing maintenance for Modelart’s account. There are some beautifully woven imitation rope strands used in the model ship building industry, which we bought. This too presented challenges with calibrating the tension of four sets of seventeen cables. Each time one was fractionally over tensioned the rest would go limp. After countless ideas and hours of experimentation, we did find a solution that worked.
In the process of construction we needed to shift the model when the unthinkable happened. The egg-crate substrate was not adequately rigid and bent very marginally inwards when lifted from the ends. The result was that the pinched bridge profile was squeezed upwards by just a few millimeters but sufficient for everything secured to it to distort or pop loose. This was an unmitigated disaster because this would mean that the model would not be able to be moved once permanently installed. The intention of the model as briefed by the client, was that it needed to be able to be displayed at multiple venues as required in future. We had come so close to believing that we had solved all of the issues, only to have to remedy yet another unanticipated setback. The enthusiasm that the Modelart team had at the outset devolved into feeling of being deflated. There was no backing out and we got back to work! We found a costly but reliable solution in having a 100mm thick board of GPET honeycomb skinned with high impact fiber reinforced plastic top and bottom. This is a material system and method, used in the shells of bullet trains and composite aircraft. It did the trick perfectly when secured beneath the original egg-crate.
It was a great relief to the Modelart team when we were advised that the model which was transported by road over a distance of one thousand kilometres to the client, SANRAL’s HQ in Nelson Mandela Bay with zero issues. Three years later, it remains on display in its original pristine state.