Metropolis Mega-Development: A Case Study in Fast-Tracked Performance-Based Seismic...

2017 SEAOC CONVENTION PROCEEDINGS

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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

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