Seismic isolation

Isolated building

Fixed-base building

Ball bearings

Isolated building

Fixed-base building

Ball bearings

Ground stationary

Ground stationary

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Ground moves to left

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Ground moves to right

▲ 14.1 Relative movement between the ground and a perfectly isolated building on ball bearings, and a fixed-base (conventional) building.

The concept of seismic isolation - often called base-isolation - is over one hundred years old. The first patent was filed in 1909 by an English physician who proposed talcum powder as the means of isolating load-bearing walls from their foundations. The application of seismic isolation to real projects did not begin until the late 1970s. Since then, approximately 2000 buildings world-wide have been isolated, mostly in Japan, US, Europe and New Zealand. Japan experienced a dramatic upsurge in seismic isolation following the 1995 Kobe earthquake. Now 1500 buildings are isolated. Of those, and in contrast to statistics from other countries, half are condominiums.1 With the trauma of Kobe still relatively fresh in their minds, some Japanese are prepared to pay a premium for the increased safety seismic isolation provides. Almost all recently constructed hospitals in Japan have been seismically isolated; as is increasingly the case in the US and New Zealand.

The beauty of seismic isolation is its conceptual simplicity. Imagine a building founded on ball-bearings. During an earthquake the building remains almost stationary while its foundations, subject to the energy of earthquake waves, move violently to-and-fro with the ground. Rolling of the ball-bearings accommodates the relative movement between superstructure and ground (Fig. 14.1). Vertical isolation is unnecessary given the generally excellent performance of buildings to vertical shaking.

Although the concept is simple, its implementation is more complex. If the interface between superstructure and foundations is almost friction-less, what happens during a wind storm? Although TV reporters would flock to a building being blown along a street, its occupants would be less impressed. Any seismic isolation system must be wind resistant. It should also possess a centring mechanism to continuously force the isolated superstructure to return to its original position during

Acceleration response of building

Normal damping

Increased damping from isolation shaking. Finally, damping is necessary. It reduces the seismic response of the superstructure and reduces the relative horizontal deflections that bearings and other details need be designed for.

Reduction in response from increased damping

Period shift

Natural period of vibration T (secs)

▲ 14.2 A response spectrum showing how the peak acceleration of a fixed-base building, A, is reduced to B by the 'period shift' and increased damping of a seismic isolation system.

Using terminology introduced in Chapter 3, seismic isolation reduces the seismic response of a superstructure by increasing its natural period of vibration and increasing its damping. These two interventions are shown in Fig. 14.2 which illustrates the effect of isolating a typically-damped conventional fixed-base building with a natural period of 0.5 seconds. During a design-level earthquake its peak acceleration is given by point A. The provision of a horizontally flexible isolation interface increases its natural period to, say, 2.5 seconds. At this far longer period of vibration the seismic response is considerably less, and reduced even further by the additional damping provided between superstructure and foundations. Point B denotes the peak acceleration of the isolated building, typically 20 per cent to 25 per cent of A.

The ability of seismic isolation to reduce seismic response or peak accelerations of buildings has been confirmed by observation and vibration measurements during several earthquakes. During the 1994 Northridge, Los Angeles, earthquake the seismically isolated University of Southern California Teaching Hospital came through unscathed while the nine nearby hospitals were so badly damaged all had to be evacuated. The following account summarizes acceleration measurements for two buildings in separate quakes; the university hospital above, and a computer centre in Japan during the 1995 Kobe earthquake:2

The measured free field peak ground acceleration ... was 0.49 g, while the peak acceleration throughout most of the structure was less than 0.13 g, and the peak acceleration at the roof was 0.21 g due to structural amplification in the upper two storeys. By analysing a model of the hospital without isolation, Asher et al. concluded that accelerations throughout the fixed base structure would have ranged between 0.37 g and 1.03 g, and that damage to building contents and disruption of service would have been almost certain. The West Japan Postal Savings Computer Center experienced ground motions with a peak site acceleration of 0.4 g in the January 17, 1995 Kobe earthquake (DISI996). The Computer Center exhibited no damage, and the maximum recorded acceleration in the building was 0.12 g. A nearby fixed base building of approximately

the same height was also instrumented, and the maximum recorded acceleration at the roof was 1.18 g.

Seismic isolation of the seven-storey university hospital reduced its peak ground acceleration to approximately half that of the ground. But as Bill Robinson points out, seismic isolation is more effective than that:

This 7-storey hospital underwent ground accelerations of 0.49g, while the rooftop acceleration was only 0.21 g - a reduction by a factor of 1.8. The Olive View Hospital, nearer to the epicentre of the earthquake, underwent a top floor acceleration of 2.31 g compared with its base acceleration of 0.82g, a magnification by a factor of 2.8. A comparison between the hospital seismically isolated with lead-rubber bearings, the University Teaching Hospital, and the unisolated building, the Olive View Hospital, shows an advantage by a factor of 1.8 X 2.8~5 in favour of the isolated hospital.3

The performance of the Japanese computer centre building shows an even larger reduction of peak acceleration. Not only does the superstructure of a seismically isolated building experience less peak acceleration than its foundations but, unlike a fixed-base building, it does not amplify accelerations significantly up its height.

Similarly impressive reductions in superstructure accelerations were measured in a building during two lesser recent Japanese earthquakes.1 Not only do the occupants and contents of a seismically isolated building experience far less severe accelerations, any motion is felt as gentle back-and-forth or side-to-side movements with a period of vibration of between 2.0 to 3.0 seconds. This is a very gentle ride compared to the intense flinging and whipping motions within conventional buildings. In isolated buildings almost all of the relative movement between foundations and roof occurs at the plane of isolation. The superstructure tends to move as a rigid body in stark contrast to a fixed-base building where significant inter-storey drifts occur. No wonder seismic-isolation is an attractive option for buildings required to be functional immediately after a damaging quake, or housing expensive or irreplaceable contents.

Although conceptually compelling and with an impressive, albeit limited track-record, seismic isolation is definitely not a panacea for all seismic ills. The following considerations limit its applicability:

• Flexible buildings, generally more than ten-storeys high with natural periods of vibration greater than 1.0 seconds may not benefit sufficiently from the 'period shift' to between 2.0 and 3.0 seconds.

• Sites underlain by deep soft soils have their own long natural periods of vibration with which an isolated system with a similar natural period could resonant.

Internal rubber layer

▲ 14.3 A cut-away view of a lead-rubber bearing.

Highly polished stainless steel surface

Highly polished stainless steel surface

Centre position

Centre position

Top plate remains horizontal while the slider under it rotates

Top plate remains horizontal while the slider under it rotates

Displaced position

Displaced position

▲ 14.4 A section through a Friction Pendulum™ bearing.

▲ 14.4 A section through a Friction Pendulum™ bearing.

▲ 14.5 A Friction Pendulum™ bearing at the base of a column. The temporary locking brackets are yet to be removed.

(Reproduced with permission. © Skidmore, Owings & Merrill, LLP).

▲ 14.5 A Friction Pendulum™ bearing at the base of a column. The temporary locking brackets are yet to be removed.

(Reproduced with permission. © Skidmore, Owings & Merrill, LLP).

• Relatively wide horizontal separation gaps to allow for relative movement between superstructure and foundations in the order of 400 mm may represent a serious loss of usable floor area for buildings on tight urban sites.

• The first cost of a seismically isolated building is generally a few percent more than that of a fixed-base building. But, as Ron Mayes points out, if a client decides against earthquake insurance, income from saved annual premiums that is invested can repay the cost of isolation within three to seven years. When a damaging quake occurs possibly uninsurable business disruption costs 'will overwhelm any first cost consideration especially if the building contents have any significant value'.4

• Some current codes still take a very conservative attitude to this system that has been rarely tested in real buildings. They limit its potential advantages and dampen designers' enthusiasm by what some consider overly onerous analytical and testing procedures. Fortunately this situation is improving.

A seismically isolated building still requires vertical structures such as shear walls or moment frames. Like any other building, wind forces need to be resisted and even though isolated, seismic forces require adequate force paths. But, whereas considerable damage is expected to the ductile structure of a fixed-base building in a design-level earthquake, the structure of an isolated building would normally remain undamaged. Seismic isolation also allows buildings with normally unacceptable configurations (Chapters 8 and 9) or with absolute minimum vertical structure to perform adequately.

The horizontal flexibility and centring capability of a seismic isolation system is usually achieved by a sliding or shearing type of mechanism. The controlled rocking capability of slender structures has also been utilized in several structures, including in the Hermès building mentioned in Chapter 3. A range of special structural components facilitate these mechanisms. Lead-rubber bearings, with internal steel plates to limit vertical deflections caused by forces from the columns or walls they support, are popular (Fig. 14.3). As the top of a bearing is displaced sideways the elasticity of the rubber provides an elastic restoring force. Damping is provided by an internal lead (Pb) plug, or by another damping device. Sometimes the rubber is formulated with inherently high damping characteristics. Friction pendulum dampers, like those used in the San Francisco International Airport, require less vertical space (Figs 14.4 and 14.5). The gentle

Superstructure

Pin joint Damper

Superstructure

Cantilevered pile within an oversized casing to allow movement

▲ 14.6 Schematic representation of an isolation system where cantilevered piles provide horizontal flexibility and centring forces.

Displaced bearings

Plane of isolation Crawl space

▲ 14.8 Lead-rubber bearings on cantilever reinforced concrete columns. Bhuj Hospital, India (Reproduced with permission from Adam Thornton).

Plane of isolation

Cantilever column

▲ 14.7 Two possible locations for an isolation plane.

Cover rides up when gap is closed Cover plate Cover plate fixing

Exterior surface

Exterior surface

Seismic gap

Retaining wall

Seismic gap

Retaining wall

▲ 14.9 Possible section through the perimeter of a seismically isolated building.

curved dish, which causes the column above to slide back down towards the lowest point, provides the centring mechanism. Two buildings in New Zealand are founded on long piles separated from the surrounding ground to achieve the long natural period necessary for seismic isolation (Fig. 14.6). A structural engineer should discuss the advantages and disadvantages of each isolation system with the architect before a final choice is made.

When designing a seismically isolated building, an architect must collaborate with the structural engineer over a number of unique design decisions. The most fundamental is the location of the plane of isolation. Should the isolators be placed in a crawl-space under the lowest floor level or can they be located at the top of cantilever ground floor columns (Figs 14.7 and 14.8)? If the first configuration is chosen, a 'moat' to accommodate a wide separation gap is required together with a suitable cover. The moat cover is designed to offer insignificant restraint to relative movement across or along the gap (Fig. 14.9). For an elevated isolation plane the design challenge involves detailing some

Section

▲ 14.11 San Francisco City Hall.
▲ 14.12 Parliament Buildings, Wellington.

vertical architectural elements, like the cladding that crosses the plane of isolation, to allow horizontal movement without restraint or damage.

Services connections to an isolated building must also accommodate relative movement. Gas, water and sewer pipes, as well as electrical and communications wiring require the capacity to undergo design movements without rupturing (Fig. 14.10) . Even elevator shafts and their pits that project beneath ground floors need to be separated from the ground. Nothing must compromise unrestrained horizontal movement between the isolated superstructure and foundations.

Seismic isolation is a viable option for both new buildings and those requiring seismic upgrading or retrofitting. Notably in the US and New Zealand, a number of load-bearing masonry buildings, including some of historical and cultural importance, have been seismically isolated (Figs 14.11 and I4.I2)5,6 In many situations, including the previous two examples, seismic isolation alone is insufficient to guarantee adequate retrofitted seismic performance. New structural elements, like braced frames or shear walls, are also required. Seismic isolation of existing masonry buildings requires significant construction effort in the foundation area. Every wall and column of the existing building needs to be sliced through to form the plane of isolation. After upgrading existing foundations and bases of the walls, bearings are inserted. They support the entire weight of the building while simultaneously providing horizontal flexibility, and often damping as well.

Several modern buildings have also been retrofitted using seismic isolation. When the Victoria University of Wellington discovered that, even after a moderate to major quake, their library building would be severely damaged, books spoilt and the library inoperable for several years, a decision was made to improve the building's seismic resistance

▲ 14.13 An area of the podium is propped while excavation is completed prior to installation of lead-rubber bearings. Library, Wellington.
▲ 14.14 A lead-rubber bearing inserted at the base of a reinforced concrete column.

Moment frame

Damper

Diagonal brace

Section

▲ 14.15 Dampers at each level of a moment frame to reduce accelerations and drifts.

(Fig. 14.13). In what is best described as sophisticated seismic surgery, each of sixteen tower-block columns in turn had their gravity forces supported on temporary steel props and hydraulic jacks. After two cuts with an abrasive wire a concrete block at the base of each column was removed and a lead-rubber bearing inserted (Fig. 14.14). A moat constructed around three sides of the building basement allows for horizontal movement. It is covered by a sliding plate.7 The whole operation was completed without disrupting library operations. The only disappointing feature of the project is how the architectural detailing for seismic isolation denies both the radical surgery undertaken and how the library is now flexibly attached to its foundations.

In summary, seismic isolation is conceptually the best method for outwitting a quake. Although there are situations for which isolation is unsuitable, where it is applied most of the earthquake energy that would otherwise cause extensive damage to the building fabric and its contents is prevented from entering the building. This is the closest we come to 'earthquake-proof' construction. Isolated buildings ride through intense and violent shaking of major quakes with no more than sedate and gentle movement.

Moment frame

Damper

Diagonal brace

Section

▲ 14.15 Dampers at each level of a moment frame to reduce accelerations and drifts.

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