‘Seismic fuse’ enhances earthquake protection

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A licensing agreement between US-based StarSeismic LLC and D&H Steel Construction Ltd will enable the Henderson-based company to start manufacturing Buckling-Restrained Braces (BRBs) that improve earthquake protection for buildings.

While BRBs have been tried and thoroughly tested internationally, the technology is still relatively new to New Zealand. D&H Steel believes that BRBs could be rapidly deployed in new construction projects and in retrofitting existing buildings that are at seismic risk.

D&H engineer John Frederickson says the US began developing BRBs in the late 1990s.
“BRBs are specifically detailed to yield reliably in both tension and compression. Seismic energy is dissipated with nearly symmetrical behaviour that takes advantage of the ductility of steel to give the building resilience during an earthquake,” Mr Frederickson says.

“This is a significant improvement over a conventional braced frame system, in which the braces tend to buckle in compression. The consequent damage to the building caused by this buckling often leaves owners reluctant to embark on expensive repair.
“Our US technology partner, StarSeismic LLC, bases the development of its brand of BRBs on an intense, multi-million dollar programme of testing. Each type and grade of BRB is certified with a specific seismic rating based on an engineering design that is calibrated against testing.

“This means that its performance during a seismic event can be accurately and reliably predicted.
“A useful analogy is to compare the BRB to an electric fuse. It’s designed to absorb and dissipate energy up to a predetermined level, beyond which the seismic fuse yields while the main gravity components of the building — its columns, beams and floor slabs — remain isolated from damage.”

The fuse analogy holds true when it comes to selecting the rating of the BRB to be used. Just as an over-strength electric fuse can cause as much damage as an under-strength fuse, BRBs are designed to specified load capacity and stiffness performance levels.
StarSeismic has compiled comprehensive BRB ratings based on extensive testing over more than 10 years.

When designing a new building, an engineer can use this data to size its columns, beams and foundations so that they are only slightly stronger than the capacity of the BRB. In the event of an earthquake, the “seismic fuse” dissipates the seismic energy without damaging the adjacent structure of the building.

In other seismic-resisting systems, the behaviour and design of the structure is usually governed by less optimised seismic components, which invariably leads to an over-strengthening of the foundations, columns and beams. No improvement in seismic resilience is necessarily assured, while an increase in building costs is more than likely.
“BRBs are particularly useful in the seismic retrofitting of older buildings. The BRBs are ‘tuned’ to absorb the earthquake energy to a point just below the capacity of the existing structure,” Mr Frederickson says.

“In many instances, the BRBs are light enough to be manhandled inside the building, making installation a much simpler and faster process.
“One of the key lessons learnt from the Christchurch earthquakes is that in addition to protecting the lives of the occupants from collapse, buildings also need to be resilient.
“This means that after an earthquake, the BRBs can be quickly checked and, if necessary, easily replaced, allowing for prompt occupation and ongoing use of the building. BRBs offer a reliable, quick and cost-effective way of achieving both objectives.”

By entering into a licensing arrangement with StarSeismic, D&H Steel is able to manufacture BRBs calibrated by rigorous testing to ensure that they perform reliably during an earthquake.

Indeed, testing performed on BRBs to date has shown they are capable of withstanding multiple seismic events without failure or loss of strength.
“And, of course, now that we can manufacture BRBs locally, there is a considerable reduction in lead times as well as freight costs,” Mr Frederickson says.