Those who live in the Bay Area may be familiar with The Great California Shakeout drill, which trains people to “drop, cover, and hold on” in order to protect themselves during earthquakes.
That’s great advice for people, but mission-critical computer hardware can’t dive under sturdy tables. For data centers in Silicon Valley, a strong level of proactive protection that can withstand seismic events is essential.
For this reason, RagingWire Data Centers recently announced it will debut in the Silicon Valley data center market by building a facility with a sophisticated seismic protective system that reduces earthquake induced vibrations. RagingWire’s SV1 Data Center will be the first data center in Santa Clara to incorporate such a system.
Planned to be over 70 ft tall, RagingWire’s four-story SV1 Data Center will be engineered to ride out significant ground shaking with no interruptions to the 16 megawatts of scalable critical IT load contained within the building. This resilient building design was previously used by RagingWire’s parent company, NTT, to help multi-story data centers successfully withstand major earthquakes in Japan.
Built to Protect People and Computers
Typical building design philosophy for seismic events allows structural elements to yield and deform as a means of energy dissipation. The goal is that in a design level event, the building occupants are safe, even if the building is damaged. This is often a sufficient level of protection for an office structure, where business continuity can be maintained by relocating the occupants, and any damage to the building contents are of secondary importance to the core business it supports.
For mission-critical buildings like data centers, it is also important to consider the protection and functionality of critical equipment and servers, which are not easily replaced. In seismically prone regions, data centers are sometimes designed to reduce building damage during a design level earthquake or reduce time required to reoccupy the building. However, for data centers, protecting the contents of the building may be as critical as the structure itself.
In buildings dealing with important IT services, it is important to maintain the full functionality of IT equipment, mechanical equipment, power supply, etc. For this reason, NTT's data centers in Japan adopt a high-performance seismic base isolation system, which can be used to dramatically reduce earthquake-related accelerations, to protect both the building structure and critical equipment.
This design philosophy and technology met its intended performance objective during the Great East Japan Earthquake in 2011, which struck with a magnitude of 9.1. The Tokyo data centers of NTT Communications (the parent company of RagingWire) withstood this major seismic event with no damage.
Base Isolation: What Is It?
A base isolation system is a collection of structural elements that creates a flexible interface between an isolated “superstructure” and a lower structure that is “fixed” to the ground. These elements typically consist of special bearing devices that can allow for sliding, yielding, or damping behaviors. The combination of these systems can reduce the structural accelerations and forces applied to the superstructure and its contents during a seismic event.
During an earthquake, the substructure (or foundation), is rigidly attached to the ground. So as the ground shakes, so does the foundation. However, with the presence of the isolation system above the foundation and below the superstructure, the motion of the superstructure is significantly reduced and will essentially remain in its position, while the fixed lower structure and the earth move below it.
The reality is that the superstructure will still undergo its own dynamic movement; however, the accelerations associated with this shaking can conservatively be assumed to be on the order of 60% less than that of an equivalent non-isolated or “fixed base” building.
The tradeoff for this added flexibility is that the interface must withstand large displacements. Depending on the system design and expected seismicity, a base isolated structure can experience building displacements ranging from two feet to five feet of movement. Accommodating this movement is a challenge that must be solved by careful coordination amongst the design team, to allow the building and its infrastructure attached to the isolated structure above the fixed foundation to move without damage.
At RagingWire’s SV1 Data Center, the base isolation system will be comprised of Triple-Friction Pendulum (TFP) bearings and Fluid Viscous Dampers (FVD) to further assist in energy dissipation and reductions in overall building displacement.
The Base Isolation Device: Triple-Friction Pendulum Bearings
A pendulum bearing is a curved sliding type isolation device. The sliding behavior of the device is activated when the earthquake force overcomes the friction between the internal device components.
The curved nature of a pendulum bearing enables the building to have some ability to revert back to its original position after an earthquake. A triple-friction pendulum bearing is comprised of five sliding components that piece together to create three pendulum behaviors. An inner slider is sandwiched by two concave plates which are also sandwiched by two larger outer concave plates. The plate interfaces are assigned different levels of friction to promote staged movement.
The result is that during smaller earthquakes the device will exhibit high stiffness to prevent excessive movement of the building. This stiffness will gradually decrease and transition to a low stiffness region to allow maximum acceleration reductions during a design level seismic event.
In a large earthquake, the TFP will transition back to high stiffness behavior to limit excessive displacements that could pose major damage to the isolation system and superstructure.
Supplemental Energy Dissipation: Fluid Viscous Dampers
Fluid viscous dampers (FVD) are velocity-dependent devices that provide passive energy dissipation by passing a perforated piston head through chambers filled with viscous fluid. It is similar to a car shock absorber, but for structures.
These devices can be implemented as part of a non-isolated building to reduce floor to floor lateral building displacements and accelerations during either a seismic event or high wind event (actually, dampers are frequently used in high rise structures to reduce building vibration resulting from high winds). In an isolated building, the FVD device is connected on one end to the isolated superstructure and on the other end to the fixed lower structure.
The piston head is pulled through a viscous fluid as the superstructure moves during a seismic event. As this piston moves faster, the resistance of the device increases.
The FVDs at RagingWire’s SV1 Data Center will be used to reduce overall building movement to a practical level while maintaining the acceleration reduction of the isolation system.
Putting It All Together in RagingWire’s SV1 Data Center
For the SV1 project, PARADIGM Structural Engineers and NTT Facilities worked together with RagingWire to design a base isolation system capable of meeting the performance of a Japanese NTT data center in seismically vulnerable Silicon Valley.
The SV1 Data Center will be built to reduce superstructure accelerations and displacements to minimize both building and equipment damage, thus allowing the facility to quickly return to operation after a design level event, or even maintain operating without interruption.
Due to the high seismicity of the Silicon Valley region, a non-isolated structure at the SV1 site would experience large accelerations which increase throughout the height of the building. These accelerations mean the equipment in non-isolated buildings can be at a greater risk of damage than those in an isolated structure. Inside the building, racks of servers are subject to toppling, while the rooftop equipment is at risk of detaching from its moorings and rupturing the cooling loop. However, seismically isolated structures can significantly reduce the effects from ground motion to the building, its occupants and all of the critical infrastructure it contains.
This cutting-edge design will reduce the impact caused by nearby or distant earthquakes and substantially minimize the impact to the server racks and infrastructure, allowing the structure to maintain functionality after a design level event.