Kurilpa Bridge Analyzed

Phil "Floating Bones" Earnhardt sent the following great link, I forward and expand on it a bit:

Oasys Software markets a series of products used for the design of buildings, bridges, retaining
walls, and other structures. They publish a variety of case studies on their website at http://www.oasys-software.com/solutions/case-studies.html . The studies cover noteworthy structures from around the world including The Beijing National Aquatic Centre (the "Water Cube") and The Singapore Marina Bay Sands Resort. It's a wonderful website with beautiful images and some cool math.

Of interest to us is their report on The Brisbane Kurilpa Bridge, winner of the World’s Best Transport Building award at the World Architecture Festival 2011 Awards. detailing the structural role of its tensegrity-like network of struts and cables. They confirm that the bridge is not a tensegrity, but assert that its network of prestressed struts and cables contribute to the structural integrity of the footbridge.

They define the tensegrity elements as follows:

•Masts: fabricated tubular steel sections up to 30m long with section sizes 610-905mm diameter
•Major mast cables: high-strength spiral wound galvanised wire ropes 30-80mm diameter
•Spars: circular hollow sections up to 23m long with section sizes 457-508mm diameter
•Spar cables: high-strength stainless steel spiral wound cables 19-32mm diameter

They then elaborate:

Pairs of major raking tubular steel masts spring from the main support piers on either side of the main span, setting the locations of an approximately coplanar array of minor secondary masts. The major and minor masts are offset from the perpendicular both longitudinally and transversely, thus both preventing cable/mast or cable/cable clashes and providing the signature “randomness” without significantly reducing structural efficiency. Secondary cables connected to flying struts, themselves pure tensegrity elements (supported only by cables), provide lateral restraint to the masts.

"Pure tensegrity elements?" If your criteria is "supported by cables." But that is not our definition of tensegrity--that is the criteria for a tension structure. But set that aside to read the true function of this cable/strut array:

The tensegrity array of flying struts and cables that hovers above and beside the deck fulfils three critical functions:
•It suspends the canopy, allowing it to float above the deck with no apparent means of support.
•It laterally restrains the tops of all the masts, preventing them from buckling sideways under the loads arising from the suspension of the deck plus lateral and seismic forces.
•It works in unison with all the masts and cables to resist twisting and lateral forces arising from patch loads on the deck (i.e. crowds), wind and earthquakes

The report also discloses additions to the struts to dampen oscillation.

External ring impact dampers were used. TMDs were considered, but the difficulty of tuning to each mast frequency made them impractical. The ring dampers were designed in close consultation with the architect for minimal impact on aesthetics and lighting, and ability to be maintained or replaced at a future date. Each has a steel annulus weighing between 100kg and 250kg (depending upon mast size and frequency), supported by three steel cables just over half-way up the mast, and able to move relative to it. Impacts between the mast and the ring dissipate the energy that any vortex shedding excitation puts into the masts, and are softened by visco-elastic pads that absorb energy and reduce sound.


Thanks, Oasys, for the details, a new application of tensegrity-like networks: to "resist twisting and lateral forces" of an otherwise straightforward, gravity-dependent structure.


Links:

Oasys Software Case Studies: http://www.oasys-software.com/solutions/case-studies.html
Discussion on the wiki: http://tensegrity.wikispaces.com/Kurilpa+Bridgehttp://tensegrity.wikispaces.com/Kurilpa+Bridge

Thanks to Katrina for letting me know about the award.