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CHAPTER 4 43

EARTHQUAKE-RESISTANT DESIGN CONCEPTS

Chapter 4

BUILDINGS, STRUCTURES, AND

NONSTRUCTURAL COMPONENTS

TheNEHRP Recommended Seismic Provisions includes seismic design and

construction requirements or a wide range o buildings and structures and their

nonstructural components. This chapter presents an overview o those dierent

types o buildings, structures, and nonstructural components.

4.1 Buildings

Generally, a building can be defned as an enclosed structure intended or human

occupancy. However, a building includes the structure itsel and nonstructural

components (e.g., cladding, roofng, interior walls and ceilings, HVAC systems,

electrical systems) permanently attached to and supported by the structure. The

scope o theProvisions provides recommended seismic design criteria or all

buildings except detached one- and two-amily dwellings located in zones o

relatively low seismic activity and agricultural structures (e.g., barns and storage

sheds) that are only intended to have incidental human occupancy. TheProvi-

sions also specifes seismic design criteria or nonstructural components in build-

ings that can be subjected to intense levels o ground shaking.

4.1.1StructuralSystems

Over many years, engineers have observed that some structural systems perorm

better in earthquakes than others. Based on these observations, theProvisions

design criteria or building structures are based on the structural system used.

Structural systems are categorized based on the material o construction (e.g.,

concrete, masonry, steel, or wood), by the way in which lateral orces induced by

earthquake shaking are resisted by the structure (e.g., by walls or rames), and by

the relative quality o seismic-resistant design and detailing provided.

TheProvisions recognizes six broad categories o structural system:

Bearing wall systems,

Building rame systems,

Moment-resisting rame systems,

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CHAPTER 444


Dual systems,

Cantilever column systems, and

Systems not specifcally designed or seismic resistance.

In bearing wall systems, structural walls located throughout the structure provide

the primary vertical support or the buildings weight and that o its contents as

well as the buildings lateral resistance. Bearing wall buildings are commonly

used or residential construction, warehouses, and low-rise commercial buildings

o concrete, masonry, and wood construction. Figures 21, 22, and 23 show typi-

cal bearing wall buildings.

Figure 21 Wood studs

and structural panel

sheathing of typical

wood frame bearing wall

construction.

Figure 22 Typical low-rise

concrete bearing wall

building.

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Building rames are a common structural system or buildings constructed o

structural steel and concrete. In building rame structures, the buildings weight

is typically carried by vertical elements called columns and horizontal elements

called beams. Lateral resistance is provided either by diagonal steel members

(termed braces) that extend between the beams and columns to provide hori-

zontal rigidity or by concrete, masonry, or timber shear walls that provide lateral

resistance but do not carry the structures weight. In some building rame

structures, the diagonal braces or walls orm an inherent and evident part o the

building design as is the case or the high-rise building in San Francisco shown in

Figure 24. In most buildings, the braces or walls may be hidden behind exterior

cladding or interior partitions.

Figure 23 A three-story

masonry bearing wall

building.

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Moment-resisting rame systems are commonly used or both structural steel and

reinorced concrete construction. In this orm o construction, the horizontal

beams and vertical columns provide both support or the structures weight and

the strength and stiness needed to resist lateral orces. Stiness and strength are

achieved through the use o rigid connections between the beams and columns

that prevent these elements rom rotating relative to one other. Although some-

what more expensive to construct than bearing wall and braced rame struc-

tural systems, moment-resisting rame systems are popular because they do not

require braced rames or structural walls, thereore permitting large open spaces

and acades with many unobstructed window openings. Figure 25 shows a steel

moment-resisting rame building under construction.

Dual systems, an economical alternative to moment-resisting rames, are com-

monly used or tall buildings. Dual system structures eature a combination o

moment-resisting rames and concrete, masonry, or steel walls or steel braced

Figure 24 A high-rise braced frame

building in San Francisco, California.

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CHAPTER 4 47


rames. The moment-resisting rames provide vertical support or the structures

weight and a portion o the structures lateral resistance while most o the lateral

resistance is provided either by concrete, masonry, or steel walls or by steel braced

rames. Some dual systems are also called rame-shear wall interactive systems.

Cantilever column systems are sometimes used or single-story structures or in

the top story o multistory structures. In these structures, the columns cantilever

upward rom their base where they are restrained rom rotation. The columns

provide both vertical support o the buildings weight and lateral resistance to

earthquake orces. Structures using this system have perormed poorly in past

earthquakes and severe restrictions are placed on its use in zones o high seismic

activity.

In regions o relatively low seismic risk, theNEHRP Recommended Seismic

Provisions permits the design and construction o structural steel buildings that

do not specifcally conorm to any o the above system types. These buildings are

reerred to as structures not specifcally detailed or seismic resistance.

Figure 25 A tall steel

moment-frame struc-

ture under construction.

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In addition to these basic structural systems and the primary materials o con-

struction, theProvisions also categorizes structural systems based on the quality

and extent o seismic-resistant detailing used in a structures design. Systems that

employ extensive measures to provide or superior seismic resistance are termedspecial systems while systems that do not have such extensive design eatures

are typically called ordinary systems. TheProvisions also includes design rules

or structural systems intended to provide seismic resistance that is superior to

that o ordinary systems but not as good as that o special systems; these sys-

tems are called intermediate systems.

4.1.2NonstructuralComponents

In addition to the structural raming and the oor and roo systems, buildingsinclude many components and systems that are not structural in nature but that

can be damaged by earthquake eects. The types o nonstructural components

covered by theNEHRP Recommended Seismic Provisions include:

Architectural eatures such as exterior cladding and glazing, ornamenta-

tion, ceilings, interior partitions, and stairs;

Mechanical components and systems including air conditioning equip-

ment, ducts, elevators, escalators, pumps, and emergency generators;

Electrical components including transormers, switchgear, motor control

centers, lighting, and raceways;

Fire protection systems including piping and tanks; and

Plumbing systems and components including piping, fxtures, and equip-

ment.

The design and construction requirements contained in theProvisions are intend-

ed to ensure that most o these components are adequately attached to the sup-

porting structure so that earthquake shaking does not cause them to topple or all,

injuring building occupants or obstructing exit paths. For those pieces o equip-

ment and components that must unction to provide or the saety o building

occupants (e.g., emergency lighting and fre suppression systems), theProvisions

provides design criteria intended to ensure that these systems and components

will unction ater an earthquake. TheProvisions also includes recommendations

intended to ensure that nonstructural components critical to the operability o es-

sential acilities such as hospitals can operate ollowing strong earthquake shaking.

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4.2 NonbuildingStructures

TheNEHRP Recommended Seismic Provisions also includes seismic design crite-

ria or many structures that are not considered to be buildings. These structures

are called nonbuilding structures and include:

Storage tanks, pressure vessels, and pipe supports such as those commonly

ound in petroleum refneries and chemical plants (Figure 26);

Water towers;

Chimneys and smokestacks;

Steel storage racks (Figure 27);

Piers and wharves;

Amusement structures including roller coasters; and

Electrical transmission towers.

Some nonbuilding structures, however, are not covered by the design recommen-

dations contained in theProvisions because they are o a highly specialized nature

and industry groups that ocus on the design and construction o these structures

have developed specifc criteria or their design. Some such structures are high-

way and railroad bridges, nuclear power plants, hydroelectric dams, and oshore

petroleum production platorms.

Figure 26 Structures

commonly found in

petroleum reneries

and chemical plants.

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Just as it does or buildings, theNEHRP Recommended Seismic Provisions clas-

sifes nonbuilding structures based on the structural system that provides earth-

quake resistance. Some nonbuilding structures use structural systems commonly

ound in buildings such as braced rames and moment rames. These structures

are identifed as nonbuilding structures with a structural system similar to build-

ings, and the design requirements or these structures are essentially identical to

those or building structures. Other nonbuilding structures are called nonbuild-

ing structures with structural systems not similar to buildings, and theProvisions

contains special design requirements that are unique to the particular characteris-tics o these structures.

Figure 27 Seismic design criteria for

steel storage racks of the type used in

large warehouses and big-box retail

stores are included in the Provisions.

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4.3 ProtectiveSystems

Most o the seismic-resistant structural systems used in both buildings and

nonbuilding structures are variations o systems that were traditionally used in

structures not designed or earthquake resistance. Over the years, engineers and

researchers improved the earthquake resistance o these traditional systems by

observing their behavior in laboratory tests and actual earthquakes and incre-

mentally refning the design criteria to achieve better perormance. Nevertheless,

these systems are still designed with the intent that they will sustain damage when

subjected to design-level or more severe earthquake eects.

Beginning in the 1970s, engineers and researchers began to develop systems

and technologies capable o responding to earthquake ground shaking without

sustaining damage and thereby protecting the building or structure. TheNEHRP

Recommended Seismic Provisions presently includes design criteria or two suchtechnologies seismic isolation and energy dissipation systems.

Seismic isolation systems consist o specially designed bearing elements that are

typically placed between a structure and its oundation (Figure 28). Two types o

bearing are commonly used one is composed o layers o natural or synthetic

rubber material bonded to thin steel plates in a multilevel sandwich orm and the

second consists o specially shaped steel elements coated with a low-riction mate-

rial. Both types o bearings are capable o accommodating large lateral displace-

ments while transmitting relatively small orces into the structure above. When

these isolation systems are placed in a structure, they eectively isolate the

building rom ground shaking so that, when an earthquake occurs, the buildingexperiences only a small raction o the orces that would aect it i it were rigidly

attached to its oundations.

Energy dissipation systems are composed o structural elements capable o dis-

sipating large amounts o earthquake energy without experiencing damage,

much like the shock absorbers placed in the suspensions o automobiles. Energy

dissipation systems usually are placed in a structure as part o a diagonal bracing

system. Several types o energy dissipation system are available today including

hydraulic dampers, riction dampers, wall dampers, tuned mass dampers, and

hysteretic dampers.

Hydraulic dampers are very similar to automotive shock absorbers. They consisto a double acting hydraulic cylinder that dissipates energy by moving a piston

device through a viscous uid that is contained within an enclosed cylinder.

Friction dampers are essentially structural braces that are spliced to the structure

using slotted holes and high-strength bolts with a tactile material on the mating

suraces o the connection. When the braces are subjected to tension or com-

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pression orces, they slip at the splice connection and dissipate energy through

riction. Wall dampers are a orm o viscous damper that consists o vertical

plates arranged in a sandwich confguration with a highly viscous material. One

set o plates is attached to one level o a structure and another set to the adjacentlevel. When the structure displaces laterally in response to earthquake shaking,

the plates shear the viscous material and dissipate energy. Hysteretic dampers

dissipate energy by yielding specially shaped structural elements that are placed

in series with conventional wall or brace elements. Tuned mass dampers consist

o a large mass on a spring-like device. When they are mounted on a structure,

the lateral displacement o the structure excites the mass, which then begins to

move and dissipate signifcant portions o the earthquakes energy, protecting the

structure in the process.

Although seismic isolation and energy dissipation systems have been available or

more than 20 years, their use in new buildings has been confned primarily tovery important structures that must remain unctional ater a strong earthquake

and to buildings housing valuable contents such as museums or data centers. This

is because their use adds to the construction cost or a structure and most own-

ers have not viewed the additional protection provided by these technologies as

worth the additional cost.

Figure 28 The San Bernardino County Justice Center

in California was one of the rst base-isolated

buildings in the United States.

4.4 ExistingBuildingsandStructures

TheNEHRP Recommended Seismic Provisions primarily addresses the design o

new buildings and structures. However, the most signifcant seismic risks in the

United States today are associated with existing buildings and structures designed

and constructed prior to the adoption and enorcement o current seismic design

requirements in building codes. It is possible to upgrade these existing hazard-

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ous structures so that they will perorm better in uture earthquakes and some

communities in the United States have adopted ordinances that require seismic

upgrades o the most hazardous types o existing building.

Chapter 34 o theInternational Building Code and Appendix 11B o the ASCE/

SEI 7 standard include requirements aimed at improving the seismic resistance o

existing structures, typically as part o a signifcant expansion, repair, or alteration

o the building. These requirements are intended to prevent existing buildings

rom being made more hazardous than they already are (by either reducing their

current strength or adding mass to them) and to trigger a seismic upgrade o

these buildings when their expected useul lie is extended by a major renovation

project.

When a structurally dependent addition to an existing building is proposed,

Appendix 11B o ASCE/SEI 7 requires that the entire structure, including the

original building and the addition, be brought into compliance with the seismic

requirements or new construction. The upgrade requirement is waived i it can

be demonstrated that the addition does not increase the seismic orces on any

existing element by more than 10 percent unless these elements have the capacity

to resist the additional orces and that the addition in no way reduces the seismic

resistance o the structure below that required or a new structure.

The ASCE/SEI 7 standard contains similar requirements or building alterations

such as cutting new door openings into walls, cutting new stairway openings in

oors, or relocating braces within a structure. Such alterations trigger a require-

ment to bring the entire structure into conormance with the seismic require-

ments or new buildings unless the alteration does not increase the seismic orce

on any element by more than 10 percent, the seismic resistance o the structure is

not reduced, the orces imposed on existing elements do not exceed their capacity,

new elements are detailed and connected to the structure in accordance with the

requirements or new structures, and a structural irregularity is not created or

made more severe.

Both ASCE/SEI 7 and theInternational Building Code require a seismic upgrade

o an existing structure when an occupancy change will result in a higher risk to

the public. An example o such an occupancy change would be the conversion o

a normally unoccupied warehouse building into condominiums or an emergency

shelter intended to provide living space or the public ater a disaster. Furtherdiscussion o occupancies and the design requirements associated with them is

contained in the next chapter.

Although both ASCE/SEI 7 and theInternational Building Code require that exist-

ing buildings and structures be upgraded to comply with the requirements or

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new structures under some circumstances, it oten is impractical and technically

impossible to do this or many structures because they are constructed o systems

and materials that are on longer permitted by the building codes and or which

suitable design criteria are no longer available. In order to obtain literal compli-ance with the requirements to upgrade such structures, it would be necessary to

demolish the nonconorming elements and replace them with new conorming

construction, which is seldom economically practical. Recognizing this, FEMA

has developed a series o publications specifcally intended to help engineers iden-

tiy the likely perormance o existing nonconorming buildings and design eec-

tive means o upgrading these structures. Several o these publications have since

evolved into national consensus standards issued by the American Society o Civil

Engineers and they are widely accepted by building ofcials as suitable alternatives

to the requirements o the building code or existing structures.

One such standard, ASCE/SEI 31-02, Seismic Evaluation of Existing Buildings, isbased on FEMA 310 and employs a tiered methodology that enables engineers to

determine whether buildings are capable o meeting either lie saety or immedi-

ate occupancy perormance objectives. The lowest tier o evaluation provides a

simple checklist to assist the engineer in identiying defciencies that are known to

have caused poor perormance in buildings in past earthquakes. Higher tier evalu-

ations utilize progressively more complex analytical procedures to quantitatively

evaluate an existing buildings probable perormance.

ASCE/SEI 41-06, Seismic Rehabilitation of Existing Buildings, is based on sev-

eral FEMA publications (notably, FEMA 273/274 and FEMA 356) and provides

design criteria or the seismic upgrading o existing buildings to meet alternative

perormance criteria ranging rom a reduction o collapse risk to the capability to

survive design-level earthquakes and remain unctional. FEMA 547, Techniques

for the Seismic Rehabilitation of Existing Buildings, is an important companion

document to ASCE/SEI 41-06; it provides engineers with alternative structural

techniques that can be used to eectively upgrade existing buildings.

Many jurisdictions have adopted ordinances that require owners o some types

o buildings known to be particularly hazardous to perorm seismic upgrades o

these structures. The targets o such ordinances include unreinorced masonry

buildings, older precast concrete tiltup buildings, and wood rame buildings with

weak frst stories or inadequately attached to their oundations. Some o these

ordinances adopt technical provisions contained in theInternational Existing

Buildings Code produced by the International Code Council as a companion pub-

lication to the IBC. Other ordinances permit the use o the ASCE 41 procedures or

speciy other acceptable procedures developed or that particular community.

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Owners oten elect to undertake upgrades o buildings independent o require-

ments contained in the building codes or locally adopted ordinances. These

upgrades may range rom incremental projects that address specifc building

defciencies to complete upgrades intended to provide perormance equivalentor superior to that anticipated by the building code or new construction. The

International Building Code includes permissive language that enables such

upgrades so long as the engineer designing the upgrade can demonstrate that the

proposed changes do not create new seismic defciencies or exacerbate existing

seismic defciencies. FEMA 390 through 400 suggest some ways to incrementally

improve a buildings seismic perormance and FEMA 420 is an engineering guide

or use with those publications.

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