Tenth U.S. National Conference on Earthquake Engineering Frontiers of Earthquake Engineering July 21-25, 2014 Anchorage, Alaska 10NCEE

SEISMIC ISOLATION OF SINGLE FAMILY

HOMES: CURRENT TECHNOLOGY AND

FUTURE APPLICATIONS

A.W. Taylor1

ABSTRACT

Seismic isolation, also known as base isolation, is a method of protecting structures from

earthquakes by providing a flexible, sliding, or rolling interface between the ground and the

structure, thereby essentially de-coupling the motion of the ground from the motion of the

structure. The basic principle of seismic isolation have been understood for hundreds of years,

but it was only the early 1980s that commercially viable seismic isolation systems were first

developed for buildings. Since that time thousands of buildings around the world have been

constructed with seismic isolation systems. In the United States almost all isolated buildings

have been commercial, institutional, or historic structures; there are fewer than five isolated

single family homes in the United States. In contrast, in Japan more than 3,800 single family

homes have been constructed with seismic isolation systems. This paper explores the design and

construction of single family homes with seismic isolation systems. Methods of seismic

isolation for light-weight structures, such as single family homes, are reviewed. Practical

considerations for design of isolation systems are discussed, and approaches to seismic isolation

are proposed which could facilitate future applications of seismic isolation to single family

homes.

1Associate, KPFF Consulting Engineers, Seattle, WA 98101

Taylor AW. Seismic isolation of single family homes: current technology and future applications. Proceedings of

the 10th

National Conference in Earthquake Engineering, Earthquake Engineering Research Institute, Anchorage,

AK, 2014.

DOI: 10.4231/D3FX73Z5B

Seismic Isolation of Single Family Homes:

Current Technology and Future Applications

A.W. Taylor1

ABSTRACT Seismic isolation, also known as base isolation, is a method of protecting structures from

earthquakes by providing a flexible, sliding, or rolling interface between the ground and the

structure, thereby essentially de-coupling the motion of the ground from the motion of the

structure. The basic principle of seismic isolation have been understood for hundreds of years, but

it was only the early 1980s that commercially viable seismic isolation systems were first

developed for buildings. Since that time thousands of buildings around the world have been

constructed with seismic isolation systems. In the United States almost all isolated buildings have

been commercial, institutional, or historic structures; there are fewer than five isolated single

family homes in the United States. In contrast, in Japan more than 3,800 single family homes

have been constructed with seismic isolation systems. This paper explores the design and

construction of single family homes with seismic isolation systems. Methods of seismic isolation

for light-weight structures, such as single family homes, are reviewed. Practical considerations for

design of isolation systems are discussed, and approaches to seismic isolation are proposed which

could facilitate future applications of seismic isolation to single family homes.

Introduction

Seismic isolation, also known as base isolation, is a method of protecting structures from

earthquakes by providing a flexible, sliding, or rolling interface between the ground and the

structure, thereby essentially de-coupling the motion of the ground from the motion of the

structure. The basic principle of seismic isolation have been understood for hundreds of years,

but it was only the early 1980s that commercially viable seismic isolation systems were first

developed for buildings. The first modern-era building constructed with seismic isolation was

the William Clayton Building in Wellington, New Zealand (1981), and the first seismically

isolated building in the United States was the Foothill Communities Law and Justice Center, in

Rancho Cucamonga, California (1985). Since that time thousands of buildings around the world

have been constructed with seismic isolation systems.

Seismic isolation has not been widely adopted in the United States. No comprehensive registry

of base isolated buildings in the United States exists, but by the author’s estimate the total is less

than 250 buildings. Almost all isolated buildings in the United States have been commercial,

institutional, or historic structures; to the author’s knowledge there are fewer than five isolated

1Associate, KPFF Consulting Engineers, Seattle, WA 98101

Taylor AW. Seismic isolation of single family homes: current technology and future applications. Proceedings of

the 10th

National Conference in Earthquake Engineering, Earthquake Engineering Research Institute, Anchorage,

AK, 2014.

single family homes. By contrast, according to data compiled by the Japan Society of Seismic

Isolation [1], in 2009 there were more than 6,500 seismically isolated buildings in Japan. This

number includes more than 3,800 single family homes. Why are there so few base isolated

homes in the United States? There are probably several barriers to implementation, some of

them economic, and others technical. It is the objective of this paper to discuss the practical

challenges associated with seismic isolation of single family homes and to facilitate wider

adoption by suggesting methods for overcoming these challenges.

Seismic Isolation of Light-Weight Structures

A useful way to understand the behavior of a seismically isolated building is to idealize the

structure and isolation system as a single degree of freedom (SDOF) oscillator. Since the

superstructure is much stiffer than the isolation system, the superstructure is treated as a rigid

body with mass M, and the isolation system is treated as a linear spring with stiffness K. All

isolation system behave nonlinearly to some degree, but the behavior of the structure can be

approximated using the equivalent secant stiffness of the isolation system at the target

displacement for the design earthquake. The fundamental frequency, T, of a SDOF oscillator

with mass M and stiffness K is T = 2π√(M/K), as illustrated in Fig. 1 below.

Figure 1. Single degree of freedom (SDOF) oscillator, and calculation of fundamental period, T.

In general, the greater the fundamental period of a seismic isolation system, the greater the

effectiveness of the system at reducing earthquake forces in the superstructure. Thus, examining

Fig. 1, the most effective isolation system will be achieved when the mass is maximized and

stiffness is minimized. For a structure of given mass M, however, there can be practical

limitations on the lateral stiffness of of the seismic isolation system. First, very low stiffness

systems tend to exhibit large displacements during design-level earthquakes. Such large

displacements may not be acceptable because there may be other practical or economic

restrictions on the maximum range of displacement. These can include the available distance to

property lines or adjacent structures, or the cost of features that will accommodate large

displacements, such as isolation moats, building joints, gap covers, seals, and utility connections.

Second, the ability to achieve low stiffness may be limited by the cost of the seismic the isolation

bearings themselves. Invariably, seismic isolation bearings that accommodate large

displacements are substantially more expensive than those designed for smaller displacements.

For most commercial and institutional structures, the inherent mass of the superstructure is great

enough that a sufficiently large ratio M/K can be achieved to create an effective isolation system

Stiffness K

Mass M SDOF Oscillator Vibration Period

K

MT 2

without resorting to extraordinary measures to decrease K. In the case of single family homes,

however, it may be difficult to achieve a sufficiently high value of M/K without providing an

extremely low value of K. Another option, obviously, is to increase M rather than decrease K.

This option is also discussed below.

Seismic Isolation Devices With Low Stiffness

Suppose a building were supported on an isolation system consisting of completely frictionless

sliding or rolling bearings. Such a system would, theoretically, completely disassociate the

horizontal motion of the ground from the superstructure; perfect seismic isolation in the

horizontal plane would be achieved. The lateral displacement capacity of such a system, however

would need to be unlimited because there would be no restraint on lateral movement of the

structure. In practice, some form of lateral stiffness must be provided, not only to control lateral

displacement of the isolation system, but also to restore the structure to its original position after

an earthquake.

Seismic isolation systems have been developed use electromagnetic or pneumatic levitation to

create nearly frictionless support conditions. While such systems may be effective, this paper is

limited to passive isolation systems that do not require activation, control, or a source of power.

Rolling Systems

The Cross Linear Bearing (CLB) is actually a pair of roller bearings assembled into a single unit.

The lower roller bearing is guided along a lower track that is mounted on the foundation. The

upper roller bearing is guided along an upper track that is mounted on the superstructure, and

oriented perpendicular to the lower track. The lower and upper roller bearings are attached

together into a single unit, so that the entire cross linear bearing assembly provides lateral

freedom of movement of the superstructure with respect to the ground in any horizontal

direction. Fig. 2 is a photograph of a large-scale CLB for commercial construction. Smaller

versions of the CLB can be specified for residential construction.

Figure 2. Cross Linear Bearing (Photo: DIS, Inc.)

Another system, Seismic Ball Bearings, provides a true rolling interface between the ground and

an isolated structure. The ball bearings are made of steel with a very high hardness rating.

Likewise, the plates on which the balls roll are made of a hard, abrasion-resistant grade of steel,

and the surfaces of the plates are ground flat after the plates are heat treated. The ball bearings

are typically about 50 mm (2 in.) in diameter, and each ball can carry a rated load of 14.7 kN

(3,300 pounds). A Seismic Ball Bearing unit is shown in Fig. 3. In this unit 9 ball bearings are

contained within an assembly of two plates, called a bearing “race.” The race maintains the

position and spacing of the balls during assembly. The bearing is shown in position between two

hardened steel plates.

Figure 3. Seismic Ball Bearing unit with nine balls, used in shake table testing at the University

of Nevada, Reno, USA (Photos: KPFF Consulting Engineers.)

Rolling bearings have extremely low coefficients of rolling resistance. Rolling resistance values

on the order of 0.0025 of the supported load are common. Thus, rolling bearings come close to

providing “frictionless” support of structures. Lateral resistance is so low that it is usually

possible to neglect the lateral resistance and energy dissipation created by rolling bearings in

computer models.

Because rolling bearings have low rolling resistance and they do not provide any restoring force,

rolling bearings must always be used in conjunction with other devices that provide lateral

stiffness and energy dissipation. Without these other devices, displacements of a pure rolling

isolation system would be unpredictable and would likely exceed the displacement range of the

rollers. Rolling bearings have been used in combination with elastomeric bearings and various

types of damping devices, or with lead-core elastomeric bearings (LRB) that provide both lateral

stiffness and energy dissipation in a single device.

Sliding Systems

One of the simplest, and least expensive, types of seismic isolation bearings consists of a low-

friction material sliding on a smooth, flat, surface. Such bearings are often referred to simply as

“sliding bearings” or “flat sliders.” In almost all cases the low friction material of choice is

polytetrafluoroethylene, abbreviated PTFE (known by its commercial name as Teflon), and the

smooth surface is polished stainless steel, usually in sheet form, fastened to a steel backing plate.

Fig. 4 shows a small-scale flat sliding isolator for residential construction. The load rating of this

bearing is about 44.5 kN (10,000 pounds) The coefficient of friction for PTFE materials varies

over the range 0.03 to 0.15 The coefficient of friction varies as a function of contact pressure,

sliding velocity, grade, and brand of material. It is advisable to consult the bearing manufacturer

for data on friction properties.

Figure 4. Small-scale PTFE sliding bearing for residential construction. PTFE disc slides on

flat, polished stainless steel surface (Photo: KPFF Consulting Engineers).

Another type of sliding isolation system system incorporate a spherically-shaped polished

stainless steel surface, or “dish”, and a matching articulated slider coated with a low-friction

material. Such spherical sliding bearings are often referred to by the trade name Friction

Pendulum System, or simply FPS bearings. FPS bearings are available in a simple single-dish

system, a double dish system (spherical surfaces above and below the slider), and a Triple

Friction Pendulum isolator (two outer concave dishes, two inner concave sliders that bare against

the two outer dishes, and an inner slider that bears against the two inner concave sliders). Fig. 5

illustrates a a single friction pendulum isolator. Friction pendulum isolators differ from rolling

bearings and flat sliding bearings in that the bearing exhibits lateral stiffness, due to the

resistance of the slider(s) rising up the side of the spherical dish(es).

Figure 5. Single friction pendulum bearing (Figures: EPS, Inc.)

Lateral Stiffness and Energy Dissipation

Friction pendulum devices provide sliding capability, restoring force, and energy dissipation in a

single isolation unit. In the case of rolling or flat sliding isolation bearings, a restoring force

must be provided by separate devices. In addition, some capacity must be provided for energy

dissipation, to control the maximum response of the isolation system. For lightweight structures

such as single family homes, the restoring force can be provided by elastomeric springs or even

coil springs. Energy dissipation can be provided either through yielding metal dampers, fluid

viscous dampers (also known as hydraulic dampers, or oil dampers), or sliding friction.

Elastomeric springs with low stiffness and large displacement capacity can be provided in the

form of laminated steel/rubber bearings (RB). These bearings may also support vertical loads, or

they may support no vertical load and act only as lateral springs. A photograph of a cut away of

a steel/rubber laminated bearing is shown in Fig. 6. Elastomeric bearings are usually fabricated

with natural rubber compounds. In some cases the rubber compound contains an additive that

increases the hysteretic damping provided by the rubber when the bearing undergoes large shear

displacements. In other cases increased damping is provided by installing a closely-fit central

lead plug, as shown in Fig. 6. This lead core deforms plastically in shear as the bearing

undergoes large shear reversals.

Figure 6. Cut-away view of small-scale steel/rubber laminated bearing with lead core, and a

similar bearing during testing (Photo: left KPFF Consulting Engineers; right DIS, Inc.)

Another way to provide damping of seismic isolation systems for single family homes is fluid

viscous (hydraulic) dampers, such as the one shown in Fig. 7. One end of the damper is

connected to the superstructure, and the other to the ground, so that the damper is activated by

lateral movement of the superstructure during an earthquake.

Figure 7: Small-scale fluid viscous (hydraulic) damper suitable for seismic isolation of single

family homes (Photo: KPFF Consulting Engineers)

Finally, damping may be provided through energy dissipation by friction between the sliding

surfaces of flat sliding bearings (Fig. 4), or the spherical sliding surface(s) of friction pendulum

bearings (Fig. 5).

Configuration of the Structure and Isolation System

At the risk of over-simplification, it can be said that the three primary factors governing the

behavior of a seismic isolation system are the lateral stiffness of the system, the damping

supplied by the system, and the mass of the superstructure. The designer of a seismic isolation

system can usual specify stiffness and damping independently through the choice of isolation

system components. Table 1 shows potential configurations of seismic isolation systems for

single family homes. Each of these systems provides the following characteristics which, in

most cases, can be adjusted independently: low lateral stiffness, a re-centering force, and system

damping.

Table 1. Potential configurations of components for seismic isolation of single family homes Elements Providing

Vertical Support

Elements Providing

Lateral Stiffness

Elements Providing

System Damping

A Flat sliding bearings Natural rubber bearings Flat sliding bearings

B Flat sliding bearings High-damping rubber bearings Flat sliding bearings,

high-damping rubber bearings

C Flat sliding bearings Natural rubber bearings Fluid viscous dampers

D Rolling bearings Natural rubber bearings Fluid viscous dampers

E Rolling bearings High-damping rubber bearings High-damping rubber bearings

F Rolling bearings Lead rubber bearings Lead rubber bearings

G Friction pendulum bearings Friction pendulum bearings Friction pendulum bearings

The structural engineer for the seismic isolation system of a commercial or institutional building

usually has little control over the mass of the superstructure. In the case of a single family home,

however, the engineer may specify that the first floor be constructed of a concrete slab, rather

than conventional framing with wood beams, joists, and plywood decking. A concrete slab at the

first floor will significantly increase the mass of most single family homes, and provide a rigid

diaphragm above the isolation system, two characteristics that improve the effectiveness of

isolation systems. Single family homes are often constructed on top of concrete slabs-on-grade,

so specification of a concrete slab at the first floor is in keeping with conventional construction

methods. Another important advantage of a concrete diaphragm is that is provides dead weight

to resist uplift of shear wall anchors caused by rocking of shear walls. If traditional wood

framing is used at the first floor, it is more difficult to engage sufficient dead load to resist wall

anchor uplift. Also, if the first floor diaphragm is wood, the in-plane stiffness of the diaphragm

should be explicitly considered in analysis. To achieve sufficient stiffness for efficient operation

of the isolation system, plywood sheathing that is 28 mm (1-1/8 inch) thick may be required,

rather than conventional 19 mm (3/4 inch) thick sheathing.

Fig. 8 illustrates a common configuration for a seismic isolation system, in which the isolation

bearings are located in a level just below the first floor diaphragm. For a single family home this

level may be either an occupied basement, or an unoccupied crawl space. Experience has shown

that many challenges arise from creating an occupied level below the plane of isolation in a

single family home: details of insulation, fire separations, utilities, stairs, ceilings, doors, and

other non-structural elements are all greatly complicated by the presence of the seismic isolation

system at the top of the basement level. It is strongly recommended, therefore, that the area

below the isolation level be limited to an un-occupied crawl space. This crawl space provides

space for movement of the isolation system, an area for flexible utility connections between the

ground and the superstructure, and, as is the case with conventional crawl space construction,

space for routing of ducts and other utilities.

Figure 8. Common seismic isolation configuration (Figure: KPFF Consulting Engineers)

For any seismically isolated structure, it is important to maintain the simplest possible joint

between the superstructure and the ground. Complicated covers, mechanisms, pop-up elements,

and “break away” or “sacrificial” jointing systems not only drive up the cost of the isolation

system, but also threaten to compromise the effectiveness of the isolation system. The

configuration shown in Fig. 8 above is highly recommended: a horizontal joint between the top

of a stem wall or other foundation enclosure. The gap in this joint can be filled with

compressible foam and the exterior can be weather sealed with caulk if necessary. It is important

this joint remain completely level around the entire perimeter of the house; steps in the joint and

sloped sections joint introduce significant complications in design and construction. Covering

the joint with “breakaway” elements can adversely affect operation of the isolation system,

especially for a light-weight structure such as a home. If a “breakaway” joint cover offers just

five pounds of resistance per lineal foot, then for a typical house with a perimeter of 200 feet,

constraint on movement of the isolation system totals 1000 pounds. Joint hiding systems that

depend on pushing soil or gravel away from the face of the building are not advisable for the

same reason.

With seismic isolation of commercial buildings, construction of flexible utility connections can

be challenging because hardware for large-scale utility pipes and conduits that are rated for large

ranges of movement are either not available, or very costly. With residential construction

flexible connections of utilities to the superstructure are much simpler, and can usually be

accomplished with “off the shelf” utility components. For example, electrical service, telephone,

and data lines can be connected with flexible conduit or wires in the crawl space, or by

conventional overhead service lines with sufficient slack to accommodate building movement;

water and gas connections can be made with flexible conduits in the crawl space; waste lines can

be connected with a section of flexible pipe, or with rigid pipe and a series of articulated joints.

Fig. 9 shows an example of a base isolated single family home that follows the principles

described above. The isolation system consists of seismic ball bearing units at most support

locations, with lead-core rubber bearings at select perimeter locations to provide lateral stiffness

and damping (configuration type F in Table 1). The mass of the structure is great enough that a

concrete diaphragm at the first floor was not required to achieve efficient operation of the

isolation system. Wood framing of the first floor was carefully planned so that dead weight of

the superstructure would be sufficient to control bearing uplift at most locations. At a few

locations supplemental dead weight was added to prevent uplift.

Figure 9. Seismically isolated single family home. Upper left: enclosed seismic ball bearing

unit supports timber girder; Upper right: lead-rubber bearing at a perimeter support

location provides lateral stiffness and damping, Bottom: seismic isolation gap at top of

foundation wall is level around entire building perimeter. (Photos: KPFF Consulting

Engineers)

Fig. 10 shows an example of a seismically isolated concrete slab for sensitive communications

equipment. While not a single family home, this same method of construction could be applied

to the foundation of a home. The slab is supported by twelve seismic ball bearing units, and at

each corner there is a lead-rubber bearing that supports no vertical load, but provides lateral

stiffness and damping to the system.

Figure10. Seismically isolated concrete slab for sensitive equipment under construction. The

steel housing for one of four lead-rubber corner bearings is in the foreground. Slab

weight is carried by Seismic Ball Bearing units distributed throughout the field of the

slab (Photo: KPFF Consulting Engineers).

A third and fourth example of isolated single family homes are not illustrated here because of

confidentiality agreements. Two different two-story homes were designed with seismic isolation

systems for two different clients. Neither home was constructed due to of loss of funding. The

isolation systems consisted of flat sliding bearings at most support locations, combined with non-

load-bearing elastomeric spring units at a few perimeter locations that provided a re-centering

force. System damping was provided only by friction of the flat sliding bearings. The isolation

level was a crawl space beneath the first floor. The seismic isolation gap at the perimeter was

level around the perimeter of both buildings.

Conclusions

The purpose of this paper has been to describe possible approaches to seismic isolation of single

family homes. Guidelines for selection of isolation systems have been provided and examples of

light-weight structures with seismic isolation systems have been summarized. It is hoped that

the discussions presented here will facilitate future application of seismic isolation to single

family homes in the United States.

Reference 1. JSSI (2012). Japan Society of Seismic Isolation, summary of response-controlled buildings in Japan, accessed

November 16, 2013, http://www.jssi.or.jp/english/rc_buildings.pdf