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Present : Bret Lizundia, Project Manager from Rutherford and Chekene, Structural Engineers for New de Young
Frances Yang, Arch 229b Graduate Student summer of 2003 Location:
Job Site General
Project Info: $135
million construc cost, $200 mill total, difference is soft cost Project
split up into 2 bldgs. Main
bldg: 3-stories Base
isolated¡ªdecided by COFAM from day one. 270,000SF Steel¡ªcheaper
than concrete system Ordinary
Braced Frames above base isolation (elastic design level) Tower: 9-stories Fixed
base, 6¡¯ thick concrete mat foundation 35,000SF Concrete Exception
to city code height limit Doesn¡¯t
hold any art, so not under as strict seismic perf criteria For
public and educational program use Phased
in 3 parts because of size of plan Had
to underpin Asian Art Museum b/c was not demoed in time. The
Tower: Bret
explained the change in the tower scheme from H&D¡¯s original design.
It used to be a rectangle that twisted by rotating 37 degrees about its
vertical axis. This was changed so that the axis does not rotate.
Instead, the ¡°twist¡± effect comes from the shifting of the north and
south walls, in plane, in opposite directions. Thus, the rectangle
becomes a parallelogram. He illustrated this with a shoebox model,
saying that if you imagine pinching the top diagonal corners of the
shoebox, it then looks like it is twisting, but is really only shifting.
H&D liked how this change created a more dramatic tilt. There
were actually 4 different tower versions carried through 50% CD, including
one that changed from a rectangle to a triangle. The
effect of the twist in an earthquake is accommodated for by PT cables,
positioned vertically, from one side of the base of the wall, to the
opposite, far side of the top of the wall. Post-tensioning these
cables pulls down the top of the wall opposite the way that it wants to
cantilever over. In this way, the leaning corner is held back by the
post-tensioning force and prevents ratcheting of the towers during an
earthquake. These tendons are grouped in bundles of 12 into
galvanized ducts, which are greased and coated with plastic, thereby
unbonding it (allowing slip) from the concrete. Five of these ducts
will run up diagonally across the walls. There
was a time of debate between F&C, R&C and H&D over what
material to use for the tower. Designs for both materials were
carried for a while in DD. Once Swinerton received CDs, decision was
concrete. Steel would have req¡¯d waterproofing, also not used as
much in Europe. Concrete naturally waterproofs. Leaning system
not addressed in code. Would have had to use elastic design for
gravity moment, could not use ductile design, thus would req much more
steel. H&D ultimately decided they wanted the look of concrete.
Decision to go with concrete separate from decision on configuration of
twist. 80¡¯
of glass runs along long side of tower. Copper shroud around
tower¡ªveil effect. Copper expected to turn brown in 6-8 months.
Many different configs for elevator core with one or two elevators,
very tight space. Tower has only 3000SF floor plates, inefficient
use of space, but tower design was understood as architectural statement.
Base
Isolation: The
seismic design of the new de Young incorporates the most high-performance,
expensive, technically advanced lateral force resisting system in current
practice. The main building is completely supported by base isolators
under every column, which can be seen in an approx 4¡¯ space between the
basement level and the ground (ie., earth, not ground level).
The objective was to bring design accels/forces in this bldg down to
levels where art can be braced by conventional means, eg. strapping,
nailing, etc. There are two types of devices used in the base
isolation scheme. One isolator is made up of alternating layers of
an elastomeric material, ie. rubber, and thin metal plates, about 4ft in
diameter. The other kind is a slider, which is gigantic metal half
sphere that sits on a metal plate coated with Teflon, approx 6¡¯ in
diameter. When the building wants to move in an earthquake, these
are designed to allow the building to slide and dissipate the energy of
the earthquake through friction between the material surfaces. The
higher the load, the more frictional resistance develops in the devices.
In this way, the building is in essence isolated from the ground shaking
when an earthquake occurs. The ¡°separation¡± changes the bldg
period from about 0.5 seconds to 3 seconds, which brings down the
accelerations, ie. forces the bldg will experience. During an
earthquake, the earth will move back and forth under it, the friction and
damping devices will take out the energy, and very little force will
travel up to the structural elements. The
building movement allowed by the isolators must be designed for, as it
affects everything along the perimeter of the building. First, since
these isolators allow the building to move up to 3¡¯ laterally in each
direction, there is a 3 ft. ¡°moat¡± (waterless) around the perimeter of
the main building. The isolation system has also been augmented by
dampers installed across the moat, which also dissipate energy as the
building wants to move. Landscaping was designed to allow movement
of the concrete slab that covers and prevents people from falling into the
moat. All electrical, plumbing, mechanical, and other utilities that
must cross the moat were also designed for the potential 3¡¯ movement.
The respective consultants did this by replacing standard stiff pipes or
sheaves that would break with conduits that are flexible, collapsible,
and/or stretchable. Devices:
76 rubber bearings from Japan, 76 sliders from Richmond, 24 viscous Taylor
dampers (4 in crawl space, rest across moat) from NY. No friction
pendulum b/c more expensive. Dampers, 15¡¯ long, underwent rigorous
testing. $1 million worth of mech caps to hide seismic joint at the
tower/main bldg interface. Reqs
hanging elevator shafts, bumpers under elevators and designing for
higher forces in case elevator ¡°falls¡± Other: Catwalks
b/c archs didn¡¯t want holes in ceiling. Serve as platform access
to mech shafts instead of through access doors. $1 million for
catwalks. $100,000 for access doors. Also
below are the minutes from last year, which provided a basis for many of
my questions: Meeting Minutes -- November 3rd, 2002 Attendants: Andrew
Sparks - Structural Engineer, Rutherford and Chekene Role
in De Young Design: Leading
to Concrete: Contact
with H&D: Contact
with F&C: R&Cs
team structure for the De Young Museum: Effect
of Phasing on R&C: Structural
Issues: Solutions: This scheme was rather ground breaking, and as part of their original fee they had to support that it would be effective.
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