Limiting seismic damage in new builds

06 Oct, 2021
 
Limiting seismic damage in new builds

Creating technology to protect buildings from earthquake damage is an ever-present challenge for researchers, engineers and concerned building owners in seismically active regions such as New Zealand. A significant new advancement in resilient connection technology has been developed in a collaborative research project between Auckland University of Technology (AUT) and the University of Auckland (UoA).

Researchers at AUT's Built Environment Engineering and the UoA's Civil and Environmental Engineering Department have developed a highly cost-effective joint system for steel framed buildings to prevent structural damage occurring during a severe seismic event, reducing overall damage and enhancing the building's ability to operate post event. This system is rigid under normal operating conditions, but becomes flexible without damage during a severe earthquake, to limit the forces generated within the structure and becomes rigid again when the shaking stops.

Dr Shahab Ramhormozian, Senior Lecturer in Structural and Earthquake Engineering in AUT's School of Future Environments has led the new research, which builds on lifelong research conducted by Associate Professor Charles Clifton of UoA.

The new technology is based on a significant enhancement of the Sliding Hinge Joint (SHJ) originally created by Associate Professor Clifton in 2005. This traditional Sliding Hinge Joint (SHJ) offered a significant advance over traditional rigid moment frames but had a limitation of suffering post-earthquake loss of strength and stiffness. This response means that during subsequent events, a building's lateral bracing system can experience increased displacements and have increased potential for damage. Post-shaking this could result in an 'out of plumb' building, colloquially known by engineers as "post-event residual drift". The exciting new work addresses these weaknesses recognised during testing and research carried out over the last fifteen years.

The new joint is called an Optimised Sliding Hinge Joint (OSHJ) and presents a significant step forward in technology with the creation of a joint that now truly deserves the moniker of a "low damage" steel-framed joint. This new variant is game changing as it enhances protection provided to buildings from suffering structural damage during large earthquakes. In subsequent aftershocks the joint continues to respond without significant loss of stiffness and facilitates building recentring.

How does it work?

The joint is rigid before the earthquake which helps it meet in-service requirements but becomes flexible or semirigid during the earthquake to prevent structural damage. It becomes rigid again at the end of the earthquake, typically retaining better than 85% of its pre-earthquake strength and stiffness, thereby achieving gold standard level in structural building resilience under the USA's Redi rating.

The building's structure is substantially protected during a severe seismic event, and shouldn't require inspection, retightening, or replacing the building's components. Although more research is required, this improved level of performance will reduce the cost of post-event damage to the primary structure to as low of 1% of the original construction value.

The semi-rigid beam to column connection, as shown in the figure, is configured to respond to the fact that during large events the lower portion of the joint tries to open and close. The technology 'allows' this rotation and behaviour to occur in a "controlled" mechanism. Within the joint, the top flange level collar plates between beams and column are fixed, protecting the concrete floor system from movement generated damage. The seismic forces driving the column displacements, forcing rotations to occur at the bottom flange to column connection. Between the bottom beam flange and projecting column plate, two high hardness shim materials facilitate sliding and dissipation of heat during these large events. By developing a specific tightening procedure and deploying special curved washers, the researchers have created a highly resilient joint that resists bending/damage such that its unlikely to require post event repair.

Selection of the OSHJ enhances performance of buildings in seismic regions

The Optimised Sliding Hinge Joint (OSHJ) is a true low-damage, versatile and cost-effective seismic solution for multi-storey steel buildings. According to data collated by MBIE, steel framed buildings represent circa 60% market, which when applied to the construction of non-residential construction conservatively means this technology could be extremely beneficial if applied to help protect at least $3 billion of new buildings in New Zealand. For steel framed construction, the new OSHJ connection could be efficiently selected to enhance Steel Moment Frames (beam to column connections). They can also be deployed in Concentrically Braced Frames (CBFs) or Buckling Restrained Braced Frames (BRBFs) protecting ends of diagonal bracing elements or be located at the end of concrete shear walls to control uplift without damage.

The development of the new joint and related learnings has enabled the researchers to collaborate with engineers from Beca to seek practical reviews of the systems to enhance its design, specification and buildability and in the process create a pathway into industry. Leveraging learnings from this process has enabled Dr Ramhormozian to co-author a design guide for engineers in practice "Optimised Sliding Hinge Joint (OSHJ): Design and Installation Guide for a Low Damage Seismic Resisting System" with Dr. Clifton, with funding from the Heavy Engineering Research Association (HERA). The purpose of the guideline is to gather learnings to assist structural design engineers to understand the OSHJ system, targeting improved building performance. The guideline helps engineers conceive and calculate joint size and features to dissipate seismic energy, and post seismic event assist the building recenter.

What are some of the OSHJs advantages in construction?

The joint's construction advantage is that the joint is fast to fabricate, simple to erect and does not require design and purchase of expensive special proprietary components. The joint is similar to existing beam to column moment joints that steel fabricators and engineers are familiar with. The OSHJ system simply requires addition of some high hardness shim plates and some special concave washers, neither of which introduce significant additional capital costs. It is therefore a very simple system which delivers outstanding seismic resilience for both new buildings and as seismic retrofit systems into existing buildings.

Proven performance of full-scale seismic tests with severe earthquakes

In 2019 and 2020, the researchers placed the OSHJ technology through its paces. The OSHJ underwent extensive testing at Tongji University, China. The real scale test setup was equipped with the OSHJ and exhibited unparalleled performance in beam/column joints. Following the extensive testing in one of the world's most powerful and sophisticated facilities was used to simulate a range of severe earthquakes which found that the joints did not require post event repair.

Where is the OSHJ being deployed?

This new evolution of the joint is being implemented first time in the design of the new $50 million Office building precinct designed by Beca structural engineers for Waikato's Tainui Group Holdings in Hamilton. As inter-generational building owners and investors, proven design outcomes supported by research has helped inform the selection of the OSHJ to provide a cost effective yet resilient structure for Tainui Group Holdings. The new technology will not only help them protect the investment of the Iwi in the building for generations to come but will also reduce anchor tenant ACC's exposure to the disruptive effects and downtime caused by seismic damage.

Sean Gledhill, Technical Director at Beca says the collaboration between staff at AUT, the University of Auckland and Beca enabled us to explore the benefits of this innovative new connection technology. "The researchers responded to our concerns and provided fact-based evidence, assistance, guidance and fulfilled a learned advisory component to our project's design team. They were incredibly receptive to our ideas and participated in the review and refinement of the OSHJ system working closely with fabricators and installers so that all parties could share knowledge and refine the methodology of design and detailing, fabrication and installation of the joints.

"This sharing of ideas has helped us inform our client of the benefits and considerations around the system, giving them confidence in the solution and its performance. This in turn inspired the project team of Engineers, Architects, Project Managers, Quantity Surveyor's and Builders in our collective drive toward creation of one of the country's most cost-effective yet resilient buildings. We hope this project seeds many more collaborations between our organisations and industry, as we believe responsible "research lead innovation" can help power our country forward."

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