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Metropolis Mega-Development: A Case Study in Fast-Tracked Performance-Based Seismic Design of High-Rise Concrete
Towers in Los Angeles
Saiful Islam, Ph.D. S.E. Sampson Huang, Ph.D. S.E.
Shafiq Ibrahim, P.E. Fengshuang (Rex) Zhang
Saiful/Bouquet Structural Engineers Pasadena, CA
Abstract
The Metropolis mega-development is a five-parcel block
mixed-use development in downtown Los Angeles, California
containing 4.1 million square feet of luxury multi-family
residential, hotel and retail space, making it the largest
development currently in Southern California. Metropolis is
comprised of four high-rise concrete core shear wall buildings
including a 19-story 350-room hotel, a 39-story 308-unit
residential tower, a 42-story 525-unit residential tower and a
57-story 725-unit residential tower. The 57-story tower is
currently the tallest all concrete high rise tower located in the
western United States. Of the four towers, only the hotel
tower was less than the 240 ft height limit prescribed in the
code for pure concrete shear wall buildings. As such, while
the hotel tower was designed using the prescriptive code
approach, the other three towers were designed using a
Performance Based Design Approach. This allowed the
towers to rely only on the core walls for its lateral resisting
system as opposed to the dual system consisting of shear walls
and moment frames that would be required if a prescriptive
approach would have been followed. The result, a more
efficient structural design which provides significant
advantages to the project in the form of reduced construction
costs, improved architectural freedom and predictable seismic
performance in a major earthquake. The purpose of this paper
is to present the design of this extremely fast-tracked mega-
project and the challenges that came with the fast-track nature
of this project.
Introduction
The Metropolis mega-development is a five-parcel block on
6.3 acres of mixed-use development in downtown Los
Angeles, California. Just two blocks from Staples Center and
L.A. Live, the development spans two full city blocks and
connects the financial and entertainment districts, while
adding to the vibrant skyline of downtown Los Angeles. It
contains approximately 4.1 million square feet of gross
building area, making it the largest development currently in
Southern California. The project is comprised of 1,560 luxury
residential units in three towers, 350 hotel rooms, and
approximately 74,900 square feet of restaurant and retail space
built in two phases.
The project was designed and developed in two phases as
shown in Figure 1:
Phase 1 of the project included 1.1 million square
feet of gross building area built on a 2.3 acre lot.
This initial phase included a 350-room 18-story hotel
tower and a 310-unit 39-story residential tower, both
with two levels of basement. See Figure 2.
Phase 2 of the project included approximately 3.0
million square feet of gross building area built on a
4.0 acre lot. This phase is comprised of two
residential towers, a 525-unit 40-story 449 foot tall
residential tower (R2) and a 725-unit 57-story 656
foot tall residential tower (R3), both with two levels
of below grade parking and retail at the ground level.
In addition to the towers, Phase 2 also included an
approximately 1.5 million square feet nine-story
podium structure with an amenities deck on the roof
and approximately 1,900 parking stalls. See Figure 3.
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Figure 1 - Metropolis Two Phase Construction
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Figure 2 - Metropolis Phase 1 Overall Plan
Figure 3 - Metropolis Phase 2 Overall Plan
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In Phase 1, the hotel tower was kept below 240 feet
(measured from grade) so that a prescriptive code approach
could be used for its design. However, the 39-story 465 ft tall
residential tower was designed using the Performance Based
Approach which falls under the alternate design approach
allowed by the Code. Since the performance-based design is
outside of the prescriptive requirements of the building code,
the City of Los Angeles requires that the design is peer
reviewed by a panel selected by the City which includes an
academic researcher, a practicing structural engineer and a
geotechnical engineer. See Figure 4 for an architectural
rendering of the Phase 1 towers.
Figure 4 - Metropolis Phase 1 Towers
The performance based design and associated peer review and
approval process is very rigorous and time-consuming and
typically extends the design phase schedule by several months,
if not more. This is typically a concern on any fast track
project and, in the case of Metropolis which is considered to
be on a super fast-track, this concern was further amplified.
The pros and cons of going with performance based design
with extended design schedule versus going with a
prescriptive design approach, which would have cut down the
design and approval time but would have required a dual
system consisting of shear walls and moment frames, were
discussed at length. The introduction of the moment frames
would not only have significantly increased the building cost
but it would have also added significant time to construction,
not to mention the architectural and space planning impact
(due to very large moment frame columns and beams). In the
final analysis, it was clear that it was far better to go with
performance based approach and rely only on core walls for
lateral resistance as it yielded the most cost-efficient structure
which could be built faster and easier and provided the
greatest architectural and planning flexibility.
In Phase 2, the two towers and the 9-story podium structure
are functionally attached. However, structurally they were
separated from each other via seismic joints. This allowed the
two towers to be designed using a performance based design
approach while the podium structure was designed using a
code prescriptive approach. This also precluded the podium
structure, which supported a very heavy and extensively
landscaped amenities deck, from penalizing the two towers.
Furthermore, the seismic joints also allowed a clear load path
without any heavy transfer diaphragms and reduced the risk
category classification. It also allowed the tower design, which
was on the critical path because of performance based design,
to proceed while the design of the amenities deck on the
podium/parking structure was being completed, thus saving
months in the design time. Figure 5 shows the Phase 2 R3
Tower structural elements and exterior design.
Figure 5 - Metropolis Phase 2 R3 Tower
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The performance based design of the residential towers in both
Phases 1 and 2 were done in accordance with the “An
Alternative Procedure for Seismic Analysis and Design of Tall
Buildings Located in the Los Angeles Region” document
developed by the Los Angeles Tall Buildings Structural
Design Council (LATBSDC).
Structural System Description
Phase 1
The Residential Tower and Hotel of Phase 1 are reinforced
concrete structures with shear walls providing seismic force
resistance. See Figure 6 for a three-dimensional view of the
Phase 1 structure.
Figure 6 - Phase 1 Structure 3D View
The gravity system of the Residential Tower consists of 8-inch
post-tensioned slabs for all levels with the exception of the
below grade levels and the floors supporting either the
amenity deck or heavy mechanical equipment where
conventionally reinforced concrete slabs are more suitable. As
a common practice for flat-plate slab construction, shear stud
rails were used to increase the punching shear resistance at the
column-slab joint. To increase usable space and to reduce
material cost, high-strength concrete up to 8,000 psi
compressive strength was used for the vertical concrete
elements including walls and columns. All concrete slab
utilizes 5,720 psi concrete mix so that puddling is not required
at the column-slab joint per ACI318.
As shown in Figure 7, the lateral system of the Residential
Tower included one full-height central core wall and four six-
story concrete shear walls up to the amenity deck level. Three
separate mat foundations were introduced under the concrete
shear walls and individual spread footings were used to
support gravity columns outside of mat foundation. With the
39-story above-grade structure, the Residential Tower also
includes a two-story subterranean basement that is
encompassed entirely by perimeter basement walls that serves
to retain soil and to provide lateral support.
Figure 7 - Phase 1 Residential Tower Shear Wall System
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As a result of three sets of shear wall systems employed with
staggered top of wall elevation along the height of the
building, two major transfer diaphragms were introduced: one
is at the amenity deck where seismic forces start to unload
from the central core wall into relatively-stiffer six-story shear
walls; the other transfer diaphragm is located at ground-level
where a similar mechanism occurs with the rigidity of the
basement shear walls being much higher than the other taller
shear walls and the core walls. The two transfer diaphragms
were delicately designed to remain essentially elastic under an
MCE level seismic event and in turn multiple drag beams
were introduced at those levels to create a clear load path for
load transfer.
Phase 2
For the two residential towers in Phase 2, the gravity systems
are similar to Phase 1 except that higher strength concrete was
used at the gravity columns and slabs, up to 10,000 psi
concrete mix for columns and 6,000 psi concrete mix for all
slabs. In the nine-story podium structure, a post-tensioned flat
slab with drop panels was used at parking garage levels where
headroom is not sensitive for the parking spaces, which also
helped control the slab deflection.
As both residential towers have a significant low-rise wing
(19-story in R2 and 25-story in R3) attached to the main
tower, as shown in Figure 8 and 9, a very simple and practical
structural lateral system was developed for these towers with a
main core shear wall for the main tower stack and a smaller
core that extends only through the lower stack wing to balance
the twisting of the towers. Similar to Phase 1, both towers
included two-story subterranean levels, however, with the
basement walls only partially surrounding the tower foot print
since the two towers share architectural functions with the
podium structure. To avoid drastic torsion behavior below
grade, individual basement shear walls were added at the
perimeter of the two towers where the basement retaining wall
did not occur. Different from the foundation system of Phase
1, a continuous mat foundation was provided under each entire
building footprint for the two Phase 2 towers and each
foundation employed two different thicknesses under the low-
rise wing and the main tower with a transition in between. A
delay strip, similar to Phase 1, was employed between the side
wing and main tower for both the R2 and the R3 towers.
Figure 8 - Phase 2 Structure and Seismic Joint Seperation
Figure 9 - Phase 2 Tower Architectural Rendering
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Performance Objectives
The performance based design of all three residential towers
followed the procedure described in the 2014 Edition of “An
Alternative Procedure for Seismic Analysis and Design of Tall
Buildings Located in the Los Angeles Region” by the Los
Angles Tall Buildings Structural Design Council. Table 1
shows the specific performance objectives used for the design.
For the service level earthquake, during which the building is
required to remain essentially elastic, linear response spectra
analysis was performed using ETABS with torsion and P-
Delta effect taken into consideration.
For the MCE level eartquake, nonlinear three-dimensional
time-history analysis was performed to assess and also to
validate the performance of the residential towers. Table 2
below summarizes the elements that were considered as
inelastic in the Perform-3D model and those that were treated
as elastic elements. The time-history analysis involved
analyzing for 7 pairs of ground motion at the MCE level
rotated in two orthogonal directions (14 analyses total).
Inelastic elements Elastic elements
Shear walls in flexure Coupling beams Slab beams for outrigger effects (slab-wall, slab-column connections)
Shear walls in shear Columns Diaphragm slabs of podium Foundations Slab column punching shear
Earthquake Intensity Performance Objectives
Service Level Earthquake (SLE) :
50% probability of exceedance in 30 years (43 year return period); 2.5% damping
Serviceability:
Building is to remain essentially elastic with minor yielding of structural elements, minor cracking of concrete and minor damage to non-structural elements. Repairs, if necessary, are expected to be minor and could be performed without substantially affecting the normal use and functionality of the building.
Maximum Considered Earthquake (MCE) :
2% probability of exceedance in 50 years (2,475 year return period); 5% damping
Collapse Prevention:
Building is to have low probability of collapse. Claddings and their connections to the structure must accommodate MCE displacements without failure. Extensive structural damage may occur, repairs to structural and non-structural systems are required and may not be economically feasible.
Table 1 – Earthquake Performance Objective for Performance Based Design
Table 2 – Nonlinear Model Elements
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For each of the towers, the potential location of the “plastic
hinge” in the core wall was carefully analyzed and special
confinement reinforcement was detailed accordingly within
this plastic hinge zone to ensure ductile behavior during even
the most critical earthquake.
Analysis Performed
Phase 1 R1 Tower
The Service Level Earthquake (SLE) evaluation was
performed by linear response spectrum analysis that assessed
the building behavior subject to multiple criteria, among
which the drift limit and coupling beam shear capacity check
are the most essential.
Figure 10 shows the drift profile of the residential tower for
the service level earthquake with a maximum drift limit of
0.5% that ensures that the building behaves elastically,
however the requirement is usually met and does not govern
overall structural design.
Figure 10 - Phase 1 Residential Tower SLE Drift Plot
Figure 11 shows the demand-capacity ratios (DCR) for one
coupling beam along the height of the building for the SLE
analysis. Since the coupling beam is a deformation controlled
member and is expected to yield under strong earthquakes, the
DCR limit for coupling beams under the SLE analysis is set to
be 1.5 as per the design criteria approved by the peer review
panel. The “kick” of the curve right above the amenity deck
also indicated that there is a major force transfer in the
diaphragm where the stiffer shear walls starts to absorb
seismic forces.
Figure 11 - Phase 1 Residential Tower SLE Coupling
Beam Capacity Plots
In a parallel process with the SEL analysis, the building
behavior under the MCE level earthquake was studied using
the three-dimensional nonlinear time-history analysis using
the Perform-3D software that involves nonlinearity in several
types of structural elements as mentioned in Table 2. For the
central core wall, energy is dissipated by two critical “fuses”:
primarily via inelastic rotation of the coupling beams and
secondarily via flexural yielding of the shear wall vertical
reinforcement, thus the two critical inelastic behaviors have
been carefully designed and tuned such that design efficiency
and compliance to design criteria can be achieved. Figure 12
illustrates the structural fuse that was considered in the design
process and Figure 13 shows the typical rebar configuration
for the coupling beams.
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Figure 12 - Structural Fuse in Metropolis Phase 1 Residential Tower
Figure 13 - Coupling Beam Rebar Configuration
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Figure 14 shows the drift profile of the Phase 1 residential
tower for the MCE level earthquake analyses. As the
nonlinear computer model is analyzed with a total of 14
ground motion record, the drift limit set for MCE is 3% for the
average drift profile of all ground motions and 4.5% for any
one individual ground motion.
Figure 14 - Phase 1 Residential Tower MCE Drift
Coupling beam deformation is the most critical criteria that
determines the behavior and how efficient the energy
dissipation of the building and the maximum rotation occurred
near the amenity deck. As indicated in Figure 15, the
maximum rotation limit for average coupling beam rotation
from the 14 ground motions is 6%.
Tensile yielding of wall vertical reinforcement is the second
fuse that dissipates energy during a seismic event. Where the
tensile strain exceeds two times the yielding strain, special
confinement would be required to ensure ductility. With the
amenity deck as the major transfer diaphragm, Figure 16
illustrates the drastic increase of tensile strain near that level
illustrated the backstay effect that led to tremendous force
transfer between lateral resisting systems.
Wall shear stress check under MCE, in Figure 17, is another
critical behavior that needs to be fine-tuned. Shear failure is
often considered brittle and may cause catastrophic results.
Thus in the design criteria, wall shear was deemed to be force
critical and all wall shear demands were amplified by a 1.5
factor to ensure that shear failure does not occur. Figure 17
shows the average wall shear stress for all piers and for the 14
ground motions.
Figure 15 - Phase 1 Residential Tower MCE Coupling Beam Rotation
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Figure 16 - Phase 1 Residential Tower MCE Wall Tensile Strain Profile
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Figure 17 - Phase 1 Residential Tower MCE Wall Shear Stress Profile
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Phase 2 Towers
SLE and MCE level earthquake analysis for the two Phase 2
towers were performed in a similar manner to the Phase 1
Residential Tower. However, with the additional low-rise
wing attached to the main tower, the Phase 2 towers show
different behavior as it relates to the deformation and stress
distribution in the lateral system that in turn resulted in a
different design.
As shown in Figure 18, the two curves represent the drift
profile at opposite corners of the entire building and due to the
difference of stiffness in the major and minor shear cores, the
building underwent slight torsion behavior, but was still within
the acceptable limits.
In Figure 19, the major tower drift profile showed a set back at
the lower stack roof which possessed the similar trait to the
Phase 1 Residential Tower. However, to avoid the stress
concentration issue from Phase 1, the major and minor cores
in Phase 2 had been fine-tuned so that the backstay effect at
the roof of the lower stack is minimzed. Refer to Figure 20
and Figure 21 for the MCE level coupling beam rotation and
shear stress profile, respectively.
Figure 18 - Phase 2 R3 Tower SLE Drift Profile
Figure 19 - Phase 2 R3 Tower MCE Drift Profile
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Figure 20 - Phase 2 R3 Tower MCE Major Core Coupling Beam Rotation Profile
Figure 21 - Phase 2 R3 Tower MCE Major Core Wall Shear Profile