Seismic Performance 2.0

Performance-based design using dampers and base isolators offers a wide range of benefits, including risk reduction and enhanced resiliency. How do you determine what’s best for your project?

A popular building industry myth is that modern, code-compliant buildings are “earthquake proof.” This is simply not true. The building may still be standing post-tremor, but it could be permanently leaning, the floors tilted and the interiors destroyed. Major repairs or reconstruction may be needed before the building is usable again.

Any building owner analyzing the investment in seismic resiliency needs to consider the major short- and long-term implications, including the time for structural repairs and the consequences of a long-term business shutdown.

“The building code focuses on occupant safety,” says Bryan Seamer, LPA’s Director of Structural Engineering. “It is not concerned about preserving the building owner’s investment or a long-term interruption of their business after the shaking stops.”

New technologies offer building owners options to design seismic resiliency that goes beyond code to meet the needs of the specific project. Performance-based design using dampers and base isolators is a less prescriptive approach, allowing for customization and innovation around the larger project goals.

“It’s creating your own recipe as opposed to following a cookbook,” Seamer says.

On several recent projects, LPA has worked with clients and construction companies to develop seismic strategies that focus on these next-generation energy dissipation strategies.

LPA’s integrated design approach, including structural engineers early in the process, has helped develop seismic strategies that fit the client’s specific priorities. In several cases — including a life -science company and a data-storage firm — the clients wanted to do more to preserve the function of their buildings following a big earthquake, recognizing that they could not afford for their facilities to be unusable for an extended length of time.

Energy dissipation systems may not work for everyone, but they are valuable options to protect buildings and assets in case of a seismic disaster.

An Analytical Approach

On both new builds and renovations, the process starts with a detailed seismic analysis, including research of the historical record and the local geology, which can determine the degree of risk and characteristics of major earthquake faults that may affect the site and the appropriate response. New tools allow designers to simulate earthquakes and forecast the effect of damage on building operations after the shaking stops. The risk analysis includes a review of potential losses and insurance coverage, which may not cover many related expenses. “Seismic performance can be engineered, based on probabilities and impact of different outcomes,” says Damon Dusterhoft, LPA Associate and Managing Director.

The data and a holistic analysis of the overall goals will help determine the best seismic options for a specific project. One size doesn’t fit all. Engineers are continually incorporating lessons learned from academic research and the analysis of earthquakes around the world. Traditional structural strategies (see sidebar on left) offer proven techniques to keep buildings standing, protect occupants and meet regulatory standards.

However, in the deeper analysis, traditional strategies often don’t meet the resiliency standards needed by building owners. Many clients find they can’t afford to lose operations for an extended period or spend millions to repair a damaged building. The prescriptive approach of meeting the regulatory requirements and little more won’t work for their business. Energy dissipation systems with dampers and base isolators provide enhanced resilience and more long-term value in developing a structure that will survive and remain functional after a major event.

Next-Generation Seismic Strategies

Dampers and base isolators perform a function similar to shock absorbers in a vehicle. During an earthquake, seismic energy is absorbed by these devices, which convert kinetic energy from ground shaking into heat energy. Like a tree’s ability to sway in strong winds, the principle behind these devices is to allow the structure to move without cracking or failing.

Fluid viscous dampers can be used to retrofit older, brittle buildings, adding enough seismic resilience to meet current standards. Made of sleeved sections containing steel pistons within a carbon-based fluid, the dampers’ internal components slide back and forth during an earthquake, absorbing seismic energy that would otherwise potentially damage the primary building frame.

Seismic dampers are a high-tech retrofit option and are most effective in mid- to high-rise moment frame buildings at a reasonable cost. “Seismic dampers respond well when the building has a lot of displacement,” says LPA Director of Structural Engineering Daniel Wang. “You don’t need to use them at every floor, so they provide a lot of bang for the buck.”

For the retrofit of Richard and Dion Neutra’s classic Tower of Hope, a 13-story, 26,000-square-foot structure in Garden Grove, California, LPA used dampers installed on only four floors near the base of the tower but not on the ground floor itself. The design preserved the original historic architecture, eliminated construction on the ground floor and saved nearly $3 million in construction costs compared to a traditional shear wall retrofit.

Base isolators, which are usually installed between the building and its footings, also absorb side-to-side motion in the ground while the building structure above grade experiences only a small fraction of that movement. They reduce the movement of the building as the earth shakes. (Base isolator technology is not suitable for taller buildings; isolators have a limited ability to tolerate the rocking motion tall buildings experience during large earthquakes.)

Base isolators can add initial expense compared to other systems, but this added cost is partially offset through a reduction in the amount of structural steel needed for the frame and offer significant savings on materials when higher levels of seismic performance and resilience are needed. Base isolators can also reduce damage in even the most extreme earthquakes better than other systems.

“Base isolators are most cost-effective for buildings that can’t afford any downtime,” Dusterhoft says. “They give your building a good chance to remain operational after a sizable earthquake.”

For a client that has significant critical data storage needs, it was imperative that the building remain operational after a major seismic event. A base isolator system was ideal, giving the client a high level of confidence in continuous operation and functional resiliency even in a high seismic zone.

One of our clients is leveraging base isolation to manage the risk of losing critical data from research efforts. The building is designed to anticipate the earth moving and remain unaffected structurally while maintaining all utility connections. “The company determined that the work going on inside was too valuable to risk any chance of a disruption,” Dusterhoft says. “Investing in the infrastructure to maintain the data resources made economic sense.”

Dampers and base isolator systems may not be ideal for every project, but when the potential to reduce or eliminate long-term damage to the building is factored into the analysis, along with the ability to remain operational in the wake of a disaster, they are often the best option to provide value and security.

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SEISMIC PERFORMANCE GOALS

A building designed or retrofitted for seismic resiliency balances these attributes:

• Strength – Strong walls, columns and joints so that buildings don’t break.
• Ductility – Structural elements that can bend without breaking. This is the antonym of “brittleness.”
• Flexibility – The building can safely sway without damage to the structure or its contents. This is the antonym of “stiffness” and “rigidity.”
  
• Economy – Right-sized solutions that meet the owner’s performance and budgetary goals.
  

Structural retrofit designs typically focus on adding or strengthening key components such as shear walls, frames and diaphragms (the floors and roof) while adding ductility and ensuring that non-seismic building elements have the flexibility to sustain earth tremors without suffering significant damage.

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TRADITIONAL SEISMIC DESIGN

Traditional seismic resistance strategies typically fall into one of three categories, which help owners meet the public safety requirements in building codes.

Shear walls: Shear walls are designed to resist lateral loads caused by earthquakes. Composed of plywood, reinforced concrete, reinforced masonry walls or even steel plates, shear walls resist the forces caused when building floors and roofs move horizontally relative to one another.

Shear walls are common in homes and large multi-story buildings. Thicker and/or longer walls provide greater strength and stiffness but can also lead to higher floor accelerations. A common retrofit strategy for concrete shear wall buildings is to apply shotcrete, notes Daniel Wang, a Director of Structural Engineering with LPA. The drawback is that additional concrete adds weight and, as a result, horizontal seismic shear force to the structure.

Moment frames: A moment frame is an arrangement of vertical columns and horizontal beams — either concrete or steel — in a grid with specially designed rigid connections or joints that resist rotation during the horizontal movement of floors during an earthquake. Properly constructed, a moment frame structure absorbs the energy of a major earthquake at its joints by bending back and forth over many cycles of shaking. A well-designed moment frame will remain standing after a significant seismic event but will suffer permanent damage where the structural frame is deformed. It is similar in concept to a fuse in an electrical system that prevents overload of the system. After the 1994 Northridge, California, earthquake, many steel moment frame buildings suffered serious damage in the form of steel or weld cracking at beam-to-column joints. This led to major changes in the design of these critical connections.

Brace frames: A braced frame structure is designed to withstand earthquake forces with diagonal steel sections that connect building floors to one another. The diagonal braces add stiffness to the building and resist seismic forces in axial tension and compression. Unlike moment frames, braced frame members experience little to no bending during an earthquake.

Modern braces are engineered to provide ductility through deformation of the brace or the large gusset plates that connect the brace to the building’s primary frame. By sequestering the damage to the brace members, the building’s main beams and columns are protected, preventing collapse. In large earthquakes, the braces or their connection plates are designed to be damaged beyond repair and would need to be replaced before the building could be reoccupied —a time-consuming and expensive task.