The world’s longest tunnel slide will open in late June at the Orbit sculpture in the former Olympic park in London. The structural engineer behind the feat spoke to New Civil Engineer about designing the unusual structure.
Buro Happold Engineering associate structural engineer Tim Finlay said the key fun engineering part of the structure was controlling the sliding velocity. “It’s a combination of gradient, but also curvature to make sure that you’re getting a fun experience,” he said
Typical speeds of around 4m/s to 5m/s are possible for slides of this nature with the Orbit slide being on the upper end of the scale, explained Finlay.
He said that straight slides were less predictable when trying to control speed. On straight slides, moisture has a detrimental impact on the speed, making the ride less enjoyable. However, when conditions are drier, the speed can’t be controlled and riders could accelerate out of control.
Finlay said that by introducing curves into the design, the rider’s speed was controlled far more predictably. He explained that by balancing the curvature and gradient of the slide, on a ‘fast’ day a rider would be thrown up onto the side of the slide and their equavalent weight will push up the friction value controlling their speed.
“Your co-efficient of friction stays the same but because your equivalent weight is pushing you out to the side you add a sort of feedback loop from the curvature which then controls the speed much better,” he said. “Yes you might go a little faster, but you don’t lose control.
“On a slow day you might go a little bit slower, but you don’t drop into the boring category.”
To try to control moisture levels within the tube, fans have also been incorporated into the design to produce warm air to dry the inner surface.
The design of the slide was carried out in Grasshopper, a parametric design and analysis programme, which combined a physics model of friction with geometry of the slide.
“Using Grasshopper we were able to generate a route that continuously combined gradient and curvature to give an output of a correct sliding velocity,” he said.
“That route then needed to be intertwined around the existing structure of the sculpture to make sure it firstly, didn’t clash with it, and secondly, it didn’t go too far away from it.”
Initial analysis models were verified by physical tests carried out at Pyramidenkogel in Austria, a similar, but shorter slide.
The 180m long slide has been constructed from 30 separate, stainless steel sections each measuring between 5m and 9m long. These 800mm diameter, 3mm thick tubes were fabricated by Wiegand in Germany and subsequently shipped to the UK. The 12 curves have been formed from a series of 300mm long straight cylinders with facetted ends. A video showing its construction can be seen here.
The slide is supported by the existing structure and is connected with clamps to the existing circular hollow (CHS) sections to ensure as little impact as possible is made. To install the sections, the slide is being lifted into place by a team of specialist abseilers, using a complex procedure involving ropes and pulleys which has been specially designed for the project.
Team behind the slide
Artists: Anish Kapoor and Carsten Höller
Structural engineer: Buro Happold Engineering
Steelwork fabricator: Wiegand