Seismic Damping

The more one gardens, the more one learns; And the more one learns, the more one realizes how little one knows.

Vita Sackville-West

This post is definitely different from my previous ones. It will deal with buildings and earthquakes. It has nothing to do with wood or woodworking tools.

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Toranomon Hills Business Tower (foreground)

In my day job I am an executive with a large international Real Estate and Construction Project Management company in Tokyo, Japan. Without going into details that might violate non-disclosure agreements, my job involves managing all aspects of real estate acquisition and leasing, as well as the design, procurement, and construction of commercial and industrial projects. Mostly for non-Japanese Clients.

The photo above is one project I am involved in on behalf of a Client.

Tokyo is an expensive place to set up operations, and the real estate and construction processes are especially confusing for foreign companies. Ergo, the need for me and my teams.

My educational background is structural engineering, focused on seismic-resistant design. All of my Clients are very concerned about the earthquakes Tokyo experiences almost daily. There will be several magnitude 4~5 quakes here each year. This tends to keep people focused.

My point is that earthquakes are a constant threat taken seriously and for good reason. Accordingly, to one degree or another as building codes and Owners require, all buildings incorporate aseismic design features.

I have worked on buildings with expensive full-blown base isolation using rubber bridge bearings and hydraulic dampers similar to giant automotive shock absorbers, and other systems designed to dissipate damaging earthquake forces, but what I would like to show you today is a “slip-joint brace damper” just installed at a building near my office located in Marunouchi near the Imperial Palace (not the building in the perspective rendering above).

Notice how deep the beams are, and how thick the steel is. Although they don’t show up well in the photo, the columns too are massive. Much heavier than is typical in Western structures. I love Japanese structural steel!

Slip-joint Brace Dampers (white), Tokyo

I am not involved in this high-rise building, but a contractor I have used in the past named Obayashi Corporation 大林組 is the General Contractor. I was able to snap this picture of Obayashi’s jobsite while walking to my office from the subway station last week during a rare moment when the front gate was open and nothing was in the way.

The white columns and diagonal braces are the key to this seismic damping system.

The white paint is a fire-resistant intumescent coating designed to protect the steel from heat during a fire. Structural steel is very weak when exposed to fire, much more so than wood or concrete, so this sort of protection, while expensive, is necessary. The rest of the structural steel will be sprayed with a thicker, less-expensive and less-durable fire-resistant coating of one variety or another.

The diagonal braces in the photo are basically two steel plates bolted together face to face. The bolt holes are slotted to allow the bolts and plates to slip past each other when subjected to a certain amount of force.

The plates and bolts are contained in a steel pipe filled with high-friction oil to prevent the brace from buckling, prevent corrosion, and ensure the coefficient of friction between the plates/bolts remains constant for many decades in the future.

As the ground moves during an earthquake, the building moves with it, and the structural steel sways. The rectangular opening framed by connected beams and columns changes shape, becoming longer or shorter in the diagonal directions. Braces resist this “racking” movement.

As the plates and bolts in these dampers slip past each other, a great deal of friction is created converting the earthquake’s energy to heat, slowing down the racking motion, and controlling the harmonic vibration of the entire building.

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Although fixed-length braces are common in lighter, shorter structural frames, they are not usually a good thing in large structural steel frames because they tend to behave erratically and fail suddenly. This can be inconvenient.

The steel frame must be made strong enough without fixed-length bracing to absorb these forces without failing anyway to make a reliable structure. But if the frequency of the building’s movement back and forth and side to side begins to match that of the ground, then something called “resonance” can occur potentially doubling the forces acting on the building, forces powerful enough to suddenly and violently bend, break and topple the building. This can be inconvenient.

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The center image employs the damping braces as in the 2nd photo above.

An alternative to this sort of damping brace system is the more expensive “Base Isolation System”

So why would anyone use an expensive system like base isolation?

Base isolation allows the entire building, from its base up, to move opposite the ground motion in the horizontal direction, reducing the induced sway, racking, and damage to its interior and systems and equipment stored inside. This level of protection is necessary if the building must continue to function uninterrupted immediately after a large earthquake. Hospitals, R&D centers, Data Centers and other sensitive buildings with lots of expensive equipment that must be kept running no matter what are often worth the cost of base isolation systems.

But in the case of an office building like the one in the photo, the owner decided some interior damage, and the business interruption repairs would entail after the earthquake, would be acceptable.

The photos below show two components of the typical base-isolation system.

Base isolation bearings being tested. Very stiff in the vertical direction, but flexible in the horizontal direction. Two bearings are being tested in this photo, but they are never stacked like this in actual installations
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Hydraulic damper combined with base isolation bearings in the background to form a base-isolation system. The building’s entire weight is supported on these bearings. The dampers keep the building’s movement from getting out-of-hand, like shock absorbers on an automobile.

I hope you found this post interesting. Let me know if you want to see more stuff like this.

YMHOS

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