Ratcheting, Tension-Only Fuse Element Developed for Damage-Resistant Structures.

 

Seismic damage-resistant structures, such as jointed precast connections and rocking wall structures, usually require supplementary energy dissipation devices to limit peak displacements. Yielding steel fuses or buckling restrained braces (BRBs) are a common method of providing energy dissipation but have a tendency to create residual compressive forces upon joint closing which can counteract post-tensioning forces and reduce the initial resistance to rocking on subsequent cycles.

 

A ratcheting, tension-only fuse device has been developed to offer resistance to loading only in tension, while offering negligible resistance to compressive motion. This absence of compressive forces allows ready re-seating of a rocking connection to minimise residual structural displacements. Upon re-loading, fuse engagement will be more rapid due to the ratcheting mechanism, as the absence of residual compressive loads reduces the amount of elastic take-up before subsequent yielding occurs.

Two prototype ratcheting fuse mechanisms have been designed, constructed and experimentally tested at the University of Canterbury in collaboration with the UC Quake Centre. The designs were refined to provide ease of manufacture, eliminating complex and expensive machining processes, to reduce the overall construction costs and increase the chance of uptake. The first generation prototype was a linear ratcheting mechanism using a pawl and rack system, essentially a linear version of the ratcheting mechanism found in a socket handle or bicycle free-hub. The second generation prototype utilises an axisymmetric design, which enables a reduced tooth pitch and more rapid engagement, with less take-up on initial loading or re-loading. Both prototypes utilise a self-stabilising engagement mechanism. A spring force is used to provide initial engagement before tension forces provide an increased clamping force, ensuring robust and reliable engagement in field structures.

 

Experimental proof-of-concept testing on fourteen fuse elements has demonstrated the function of the ratcheting mechanism and assessed the hysteretic behaviour of the fuse element and the overall device with four different tooth pitch sizes.

High speed camera footage of the ratcheting mechanism was recorded to assess engagement timing, showing that the total time envelope for the ratcheting motion was typically less than 40 ms, which is substantially faster than the typical period of most structures. The project has been undertaken in Mechanical Engineering by Master of Engineering student Jarrod Cook, with Dr Geoff Rodgers, Associate Professor Greg MacRae and Distinguished Professor Geoff Chase. Jarrod Cook said he has enjoyed working on the project, with the highly applied nature of the work, the direct relevance to industry and the potential for uptake within the Christchurch rebuild being substantial motivational factors throughout his studies.

 

 

 

 

 

 

 

 

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