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Continuing
Education
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Use
the following learning objectives to focus your study while reading
this month’s ARCHITECTURAL RECORD / AIA Continuing Education article.
Learning
Objective:
After
reading this article, you will be able to:
1.
Describe
different types of structural seismic systems.
2.
Explain how seismic systems-including
base isolators, dampers, shear walls, and braced framing-function
and what types of buildings each serves best.
3.
Explain
seismic terminology.
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In San Francisco,
architects, engineers, and builders don't say the word "earthquake."
They refer to those sudden and violent vibrations of the earth's
crust as "events." The one that's discussed most in the Bay Area
is the 1,000-year event-an earthquake the equivalent of the famous
1906 quake, with a magnitude of 7.8. According to the U.S. Geological
Survey (USGS), another 1,000-year quake, would kill "thousands
of people…and economic losses might be in the hundreds of billions
of dollars." The USGS also predicts there's a 67 percent chance
of a quake with a magnitude of 7 or larger before 2020.
No wonder
people there and in other seismic zones, such as Seattle, Mexico
City, and even St. Louis, are becoming increasingly savvy about
the seismic components of the structures in which they live and
work. "We're not designing to meet code anymore, we're paying
attention to what the tenants want based on their own risk assessments,"
says Mark Miller of Kaplan McLaughlin Diaz (KMD) in San Francisco.
"Office building owners want to limit the threat to their employees
and, just as important, to their businesses. Shutting down operations
for just one day costs thousands."
For example,
in Emeryville, just across the Bay Bridge from San Francisco,
Pixar is building an animation studio, designed by Bohlin Cywinski
Jackson. The expensive seismic system they're using is perhaps
a case of overkill; the base isolators, paired with a concentric-brace
frame, far exceed code. For a company that stands to lose money
if its sensitive computer equipment fails, however, the expenditure
is worth it.
The information
and case studies presented here focus on the Bay Area, America's
prime earthquake territory. There's a building boom underway there,
thanks in part to dot-com companies, but also as a reaction to
the lean years of the early 1990s when virtually nothing was built.
The boom is an opportunity to advance the seismic design process
and even experiment with American and international innovations.
Like many
aspects of building science, designing structures to withstand
seismic forces is trial and error. "We have a long way to go in
terms of understanding how materials and structures behave and
react," says David Friedman, president of Forell/Elssesser Structural
Engineers. "With every quake we learn a little more about what
stands and what falls."
Picking
a system
An earthquake
is a series of forces, or energy, and the goal of any seismic
system is to somehow dissipate or absorb that energy, decreasing
the demand on the building structure. There are several ways to
design a building to endure an earthquake with minimal structural
damage. The seismic system that works best depends on the particular
project-its location, the design parameters, the owner's requirements,
building codes, and costs.
In California,
seismic design is governed by the Uniform Building Code, which
sets standards based on the amount of load the building is likely
to take and its proximity to fault lines. "Code mandates that
the building be stable enough immediately after an event to allow
safe egress, but it says nothing about getting back into the building,
unless the building is classified as essential-hospitals, airports,
and so on," Friedman says. How a building actually fares is determined,
in part, by the level of structural engineering that the building
owners, working with the architect and engineers, decide to achieve.
Factors that determine this level include permissible outage time,
extent and cost of repairs, and how the electrical, plumbing,
and other systems must hold up. It's likely that the new International
Code will integrate some of these performance-based standards
when it is next updated in 2003, he adds.
The architecture
also determines what type of seismic system is employed. "Sometimes
the architect says, 'The building must look like this. Find a
way to make it stand.' But the best results come when we work
together, finding the right solution for the right problem," says
Eric Ko, a structural engineer with Ove Arup & Partners in San
Francisco.
Seismic
devices
The case
studies presented here rely on four types of systems: shear walls,
braced framing, shock-absorbing devices or dampers, and base isolation.
These may be used alone or, for tall buildings, in combination.
Each additional system increases construction costs, though the
amount of increase varies. Shear walls and braced framing are
least expensive, dampers are higher in cost, but base isolation
is most expensive, depending on the number of isolators required
and type. "They also perform best," Friedman says.
Shear
walls: These may be added to an existing building or designed
into a new one to add strength and stiffness to the building.
In commercial buildings, they are most frequently made of reinforced
concrete. Their placement and composition requires careful thought
about how seismic activity will affect a particular building.
On multistory buildings, for example, shear walls are heaviest
at the bottom floors where there is the most shear.
Architecturally,
shear walls have a strong impact. They require careful placement
of windows, doors, and wall openings. Their thickness, often 12
to 18 inches or more of concrete, eats up square footage and adds
weight. Says Navin Amin, with Middlebrook + Louie Structural Engineers,
"It's not so easy to have a slender, elegant structure when shear
walls are used alone for seismic resistance."
Braced
framing: Sometimes referred to as trussed framing, bracing
stretches diagonally within a bay to create a triangulated vertical
frame. As in roof trusses, the triangles are able to handle stresses
better than a conventional rectangular frame. They also add stiffness.
Braces are usually welded or bolted in place for a stiff and sturdy
joint that will not give in an earthquake. Bracing may be configured
as diagonals, chevrons, knees, or in X, V, or K shapes. Concentric
braces connect at the intersection of beams and columns. Eccentric
braces connect to the beam some distance from the beam-column
intersection. This configuration provides stiffness but prevents
a buckled brace from destroying the beam-to-column or brace-to-beam
joints.
Bracing
limits building design by requiring consistent floor design so
that the braces don't interrupt the usage of space. "Positioning
the bracing is an architectural challenge if what happens on each
floor is different," says Craig Hartman, faia, of SOM's San Francisco
office.
Dampers:
Damping is ordinarily performed by the failure of various building
elements. Every shattered window, buckled framing member, and
cracked piece of masonry is evidence of dissipated energy. Dampers
"literally soak up the energy of earthquake-induced motion," in
the words of one manufacturer. Instead of swinging back and forth
repeatedly as earthquake vibrations are transmitted, the building
is stilled as the motion of the dampers absorbs the energy.
There are
four basic types of dampers: visco-elastic, friction, metallic,
and viscous. Each employs some type of pumping component or piston
that operates against a friction device-pads or fluid-filled chambers-to
release energy in the form of friction or heat. Metallic dampers
absorb energy via the inelastic deformation of metal. Dampers
are normally incorporated into the bracing or paired with base
isolators.
Base
isolators: First used in the U.S. in the mid 1980s, base isolators
come in three varieties: high-damping rubber, lead-core rubber
(both elastomeric), and friction pendulum. All three have the
same effect: inserted under the building, they allow the structure
to move independently of the shifting ground below. Each type
of isolator performs differently.
Elastomeric
isolators, made of natural and synthetic rubbers, stretch with
the building as it responds to seismic forces. The rubber, as
it seeks its natural form, pulls the building back into place.
The all-rubber version is softer, allowing greater movement, but
the lead core absorbs some of the seismic energy and forces the
isolator back into place quicker. Friction pendulums offer a lower
profile than rubber isolators (8 to 12 inches versus 24 to 36
inches in height) for corresponding load and displacement values-a
valuable asset when floor height or excavation depth is limited.
They function like a ball on a plate: a curved slider, attached
either to the footing or the building above, slips around on a
concave steel plate. The weight of the building recenters the
slider on the plate after an event, righting the structure.
Among the
problems with friction pendulums is what one engineer calls "sticktion,"
the propensity of the slider to stick at the edges of the plate.
Also, in quakes of sufficient magnitude building edges can lift,
pulling the slider off the edge of the plate. There are disadvantages
to elastomerics, as well. The rubber can harden or stretch, making
it unresponsive. For this reason, isolators must be inspected
regularly.
Base isolators
are not appropriate for all buildings. The structure must be made
rigid so that it operates as a unit, involving the addition of
bracing, ties, or shear walls. Even so, it is a seismic strategy
that offers architects greater design flexibility than brace framing
or shear walls. Enough of the seismic force is absorbed by the
base isolators that the framing can be lighter, and base isolators
are suitable for renovation since most of the work is done at
the foundation level.
Improving
steel-moment frames
The 1994
Northridge Earthquake, which originated northwest of Los Angeles,
proved that steel-moment-resistive frames don't hold up. While
the flanges of the beams and columns are fully welded in this
framing system, allowing greater continuity and distribution of
moment forces, the 1994 event resulted in cracking along the webbing
of the columns and buckling throughout the assembly.
Since the
Northridge quake, these connections have been tested extensively
by private research firms and at the four principal federal seismic
research centers. On shake tables, which simulate seismic vibrations,
and in sophisticated computer-modeling programs, new twists on
the moment connections are tested. "There are so many ways of
putting a building together, seismically speaking, it seems ridiculous
that so much attention is given to steel-moment frames," Eric
Ko says. "But architects aren't willing to give up the flexibility
that this system allows."
Among the
variations are dogbones, in which the flanges of the beam adjacent
to the column are shaved, focusing the deformation onto that area
of the beam instead of the joint. Damage to joints and columns
is more serious and more difficult to fix than isolated beam damage.
Another method for improving the performance of steel-moment frames
is the slotted-web moment connection in which slots are cut at
the top and bottom of the beam webbing. Like the dogbone, this
localizes damage to the beams. A circular cut at the end of each
slot arrests cracking that might continue through the web.
Slotted-web
connections were used by Robert A. M. Stern Architects and Gensler
at the 18-story Gap Embarcadero in San Francisco. The specifications
for the slots, based on the size and position of the beam, are
given to the fabricator, who makes the necessary cuts. A building
as tall as this Gap requires a combination of seismic systems
to keep it standing after an event. The architects created a four-sided
concrete core which acts as a centrally located shear wall. It
resists overturning forces and provides stiffness for the rest
of the structure.
The central
spine doesn't interfere with openings-important when the view
is as good as it is from this building, located near the waterfront.
The core measures 157 by 30 feet at the base, but shrinks to 90
by 30 feet at the top. Its walls taper upward from a thickness
of 18 inches to 12 inches on top. The core holds the elevators,
restrooms, and storage areas. Steel wraps around the core and
is tied to it by collector beams that "gather" seismic forces
and direct them into the concrete. In case of a quake, 90 percent
of the force will go into the core.
A dance
An architect's
job in a quake zone is to make choices that will mitigate the
effects of earthquake energy on a building and its contents. "A
shear wall cracks, a dogbone buckles, base isolators sway, dampers
move in and out, and bracing bends and stretches," Friedman says.
"The key is something has got to give somewhere."
As seismic
studies progress, however, that something that has to "give" doesn't
have to result in a less functional, flexible building. Just as
important, it doesn't have to make a building that's clunky and
unattractive. "A building should be able to dance with the forces
that affect it," KMD's Miller says. "There's no reason why architecture
in the Bay Area can't be as graceful as in a place where there
is no seismic threat."
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Case
Study
New
International Terminal at San Francisco International Airport
Completion:
Spring 2000
Construction cost: $500 million
Architects: Joint Venture Architects (JVA), including
Skidmore, Owings & Merrill (SOM), DelCampo & Maru, Michael
Willis and Associates
Engineer: SOM
Seismic
concerns, while important, weren't top priority when design
work began on the New International Terminal in 1993. Then,
in 1994, the Northridge quake proved yet again that nature
wins out over architecture, despite engineering know-how.
"We suddenly realized we had to increase the building's
seismic criteria," says Craig W. Hartman, faia, an SOM partner
at the firm's San Francisco office.
In
California, building codes rate structures such as airports
and hospitals as essential: they may sustain minimal damage
but must continue to operate after a severe quake. In addition,
because the terminal spans the entrance road to the domestic
terminals, its ability to withstand seismic vibration was
even more important. "Nobody wanted anything to come down
and block that road," says Peter L. Lee, one of the project
engineers.
The
terminal's cantilevered roof rests on 2 sets of 10 columns.
The innovative, winglike trusses span 860 feet and encompass
1.2 million square feet. To keep the trusses stable and
allow the use of delicate steel framing and a window wall
at the front facade, SOM's seismic engineers selected friction
pendulum base isolators-267 of them tucked beneath the building's
columns.
The
nonstick, concave surface of the plate and the steel "slider"
that moves over it lets the building swing around during
seismic vibrations, while the weight of the structure evens
the terminal once the earth's crust calms down. The isolators
allow 20 inches of movement around the perimenter. "It's
a system that works best for a low building, one that lacks
strong overturn forces [a building's propensity to flip
over when it is rocked]," Lee says. "It's essentially a
squat, stiff building, despite the trusses."
Eccentric
and concentric braces reinforce the building's frame so
that it moves as a unit. The structure is also tightly knit
at roof and floor junctions.
While
the base-isolation system is unique, it is only part of
the story. The real engineering came in creating a seismic
moat-the 20-inch-plus seam around the building that allows
it to sway without crashing into the adjacent structures
or pulling the surrounding roadways along with it. "All
kinds of systems come through that seam," Hartman says.
"It required connections that could flex and stretch-not
exactly off-the-shelf items."
The
buildings that lead from the terminal to the boarding gates
and connect to incoming mass transit are fixed and will
not move in a quake. To keep these wings connected, the
floor plates, walls, and ceilings telescope. Metal plates
that slide over each other prevent the road in front of
the terminal, where passengers are loaded and unloaded,
from tearing away when the building moves.
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Case
Study
Hearst
Memorial Mining Center
Completion:
Fall 2001
Construction cost: $49 million
Architect: NBBJ San Francisco
Engineer: Rutherford & Chekene
Designed
by architect John Galen Howard for Phoebe Apperson Hearst
in memorial to her husband, George Hearst, one of the great
49ers (and father of William Randolph Hearst), some aspects
of this 1907 Beaux Arts building's interior are intended
to recall a mine shaft. But pace 800 feet to the east, and
you're standing on the Hayward Fault, determined by the
USGS to be the likely source of a major quake in the next
30 years.
"This
is the most historically significant building in a near-fault
environment," says Brendan Kelly of NBBJ. "It's worth preserving."
The
four-story laboratory and classroom building houses the
University of California at Berkeley's department of materials
science and mineral engineering. Due for renovation 20 years
ago, the building had become a warren of offices and labs.
"You could barely see the original features, like the two
light courts and central atrium," Kelly says.
While
sections of the building were closed after a structural
analysis in the 1970s, it wasn't until after the Loma Prieta
quake in 1989 that school officials took action. "The building
survived, but everyone realized its precariousness," he
adds.
The
university decided not only to preserve the structure, but
to update the labs and return the building to its original
state. They're also adding two 4,800-square-foot towers.
Construction began last summer.
The
new seismic system combines 24 fluid dampers and 134 high-damping
rubber base isolators, which allow 28 inches of displacement
around the building. New concrete beams and existing 2 1/2-foot-thick
masonry walls add rigidity and tie the structure together.
The
dampers and isolators work in concert; in a large event,
the isolators let the building sway, but the dampers prevent
it from swaying too actively. The 16-foot-long, 10-inch-diameter
dampers connect to the footings and the first-floor beams.
To
maintain the first-floor height, the contractors must remove
the existing footings and install new ones 11 to 27 feet
below, depending on the location. "This added a tremendous
amount of labor," says Douglas Robertson, a structural engineer
with Rutherford & Chekene. The isolators are placed atop
the new footings, while a grid of concrete beams rests on
the isolators. The building columns bear on these beams.
The tower additions are tied to the existing building at
the first and second floors with concrete beams.
Other
seismic precautions include strengthening the 7-foot-tall
parapets that ring the building by inserting rebar into
3-inch-diameter vertical cores drilled into the parapet
walls (called center-core reinforcing). The 20 chimneys
towering above the roofline, will be secured by prestressing
high-tension steel strands dropped from the concrete chimney
caps to anchor plates at the bottom. The historic Guastavino
tiles in the great hall will be painstakingly secured by
attaching them to fiberglass-composite stiffening ribs added
above the vaulted ceiling. Between the ribs, steel mesh
embedded in urethane foam will "grab" the tile.
It
seems the expense of the restoration is worthwhile. "I understand
the university president is getting phone calls from the
other campus departments," Kelly says. "Everyone wants to
move into Hearst when the building is done."
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Case
Study
Chong-Moon
Lee Center for Art and Culture
Completion: Fall 2002
Cost: $125 million
Architects: Hellmuth, Obata + Kassabaum Inc. (HOK),
LDA Architects Inc., Robert B. Wong Architects and Planners
Design Architect: Gae Aulenti
Engineer: Forell/Elsesser
Built
in 1917, this four-story, 163,000-square-foot Beaux Arts
building, part of the city's Civic Center complex, was formerly
the public library. Now known as the Asian Art Museum, the
building will hold the city's collection of antique Asian
pieces, now housed at the de Young Museum.
"We're
told it is the city's most valuable asset, aside from its
real estate," says Mark Piaia, aia, of HOK. The city mandated
that in a 1,000-year quake, only 2 percent of the total
collection may be damaged. "The seismic criteria exceeds
design for hospitals and other 'essential' structures,"
he adds
The
seismic structure will consist of 120 lead-core rubber base
isolators, working in tandem with shear walls, added strategically
throughout the structure. The latter, 36-inches-thick at
their base, will be cast in place. "We had to work around
the existing openings to find locations for these," Piaia
says. "They eat up a lot of square footage." Metal struts
that stretch across the wings tie these to the main structure
The
20-inch-high, 40-inch-diameter isolators allow movement
of 30 inches in all directions. "You can specify or 'tune'
the isolators to give you whatever displacement factor you
want," says Paul Rodler, a structural engineer with with
Forell/Elsesser. "Softer isolators are used here for a greater
range of motion."
The
isolators are now being installed beneath the building one
column at a time. To do this, each column is first shored
up, the connection between the footing and the column base
is cut out, and the isolator is slipped in place. Then the
shoring is removed and the column is resettled. "The trick
in all this is that the building itself can't move more
than 1/10 inch during the process. Greater movement will
result in structural damage," Piaia says.
Also,
installing the base isolators this way meant sacrificing
some of the first-floor height. But digging down deeper
to repour the footings (as is being done in the Hearst Memorial
Mining Center) would have been prohibitively expensive.
The
seismic moat is straightforward-an air space capped by a
5 1/2-foot cover that wraps around the base of the granite.
There are no structures immediately adjacent to the center
and nothing for it to bump into, in case of a quake. The
seam is complex only in that it must jog around perimeter
details, including stairs, the dining terrace, and other
exterior features. It also contains flexible connections
for utilities and systems entering the building.
The
historical value of the building, combined with the priceless
collection it will hold, makes the expense of the seismic
system worthwhile. Says Piaia: "Investing in good seismic
design is like buying insurance."
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GLOSSARY
Base isolators:
These decouple the building from the ground, allowing each to
act independently in case of a seismic event. There are three
types: high-damping rubber, lead-core rubber, and friction pendulum.
Base shear:
The lateral force that is designed for at the building base. Braced
frame: A frame that uses triangulated elements (trussed) to brace
it against lateral forces.
Concentric
brace: A brace that stretches vertically from corner to corner
within a framing bay.
Dampers:
Like automotive shock absorbers, dampers consist of metal tubes
filled with silicon fluid and a piston rod that releases energy
by forcing the fluid into a series of chambers.
Dogbone:
Used on steel-moment frames, dogbones are the flanges of the beam
adjacent to the column joint that are weakened by shaving off
some of the steel on either side, creating a slender section about
two feet long (depending on the depth of the beam). If seismic
forces are strong, the beam deforms in this spot, thereby localizing
the damage to the frame.
Ductility:
Ability of a material to carry load after it is bent.
Eccentric
brace: A brace that stretches from some point along the beam
or the column, reducing stress on the corner joints.
Moat:
The seismic seam or buffer between a base-isolated building and
the surrounding structures or land. The moat must be large enough
to accommodate building movement in case of seismic activity.
Without a moat, the building is anchored to the surrounding structures,
limiting its movement and defeating the purpose of the isolators.
Shear
wall: A vertical wall that adds stiffness and rigidity to
a structure, usually made of concrete in commercial structures.
Steel
moment-resistive frame: A frame with joints between beams
and columns that are created with welds or, occasionally, high-strength
bolts to transfer the load among the members.
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Continuing Education