2016_City of Dreams Hotel, Macau

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56 TheStructuralEngineer

Project focus

City of DreamsMarch 2016

Synopsis

This article describes how cutting-

edge parametric-based engineering

techniques have been used to

achieve the detailed design of 2500

complex steelwork connections

for the exoskeleton of the new City

of Dreams hotel in Macau, China. It

discusses the tools, methodology and

strategy employed by the engineering

team to automate the diffi cult and

time-consuming process of creating,

verifying and documenting the

geometrically challenging, large-scale

steel connections using finite-

element methods within an ambitioustimescale of just 12 months.

City of Dreams, Macau –making the vision viable

Emidio Piermarini EI, BEng, MEng, Engineer, BuroHappold Hong KongHayden Nuttall MSc, DIC, BEng, CEng, FIStructE, MHKIE, Director, BuroHappold Hong KongRob May CEng, MIStructE, PE, MHKIE, MHKIBIM, Associate Director, BuroHappold BathVictoria M. Janssens PhD, PEng, Senior Structural Engineer, BuroHappold Hong Kong

IntroductionAn extraordinary building is taking shape

in the City of Dreams entertainment

resort in Macau (a Special Administrative

Region of the People’s Republic of China).

The 42-floor twin-tower construction

incorporates an irregular-form, aluminium-

clad structural exoskeleton with

connections of such scale and complexity

that they are possibly the most analytically

and geometrically challenging large-scale

steelwork connections ever to be built

(Figure 1).

The project for Melco Crown

Entertainment by Zaha Hadid Architects

and BuroHappold is under construction

(Figure 2). When it opens in 2017 it will

provide the City of Dreams development

with a dramatic landmark building tocomplement the existing complex of hotels

Figure 1

City of Dreamshotel – architect’srendering Z A

H A H A D I D A

R C H I T E C T S


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and entertainment facilities on the “Cotai

Strip”. Housed within its 150 000m2 of

floor space will be a seven-storey atrium,

780 hotel rooms, suites and villas, various

restaurants, luxury retail outlets, gaming

and event facilities, and a sky pool.

BuroHappold carried out the general

structural design of the building, together

with the detailed design and construction

documentation of all the steelwork

connections. The structural design work

faced engineering challenges arising from

the typhoon wind climate, seismic design

requirements, complex load paths and

highly irregular geometry of the building,

but it is the uniquely complex problem of

the detailed design and documentation of

the thousands of dissimilar and irregular

steelwork connections of the exoskeleton

– and the innovative methodology used

to solve it – that are the subject of this

article.

StructureThe design concept for the City of Dreams

hotel is of a striking exoskeleton which

wraps around the two concrete cores,

bringing them together with a flowing mid-

section featuring three irregular-shaped

curved openings. Inside the building, the

free-form steel framework continues,

curving high above a huge atrium space

that is echoed by that of the sky poolabove.

Figure 2City of Dreams hotel –current progress (January 2016)

Figure 3Structural system

b) Exoskeleton

c) Total system


a) Concrete cores

In structural terms, the steel exoskeleton

and the two internal concrete cores act

together to provide lateral load resistance,

sharing wind and seismic loads in

proportion to stiffness. The gravity system

comprises composite beams and slabs

that span between the exoskeleton and the

cores with minimal internal columns (Figure

3).

There are approximately 2500

connections in the exoskeleton. The

members and connections are fabricated

from steel plate up to 150mm thick

using grades up to S460. Many of the

connections incorporate “offshore-quality”

plate to BS EN 102251 in order to ensure

adequate ductility and strength in the

through-thickness direction. Members are

generally bolted together at connections

in the flat regions and site-welded in the

free-form central zone and the corner fillets

(Figure 4).

Methodology With such complex and irregular geometry

it was clear from the outset that traditional

code-based methods and standard

drawing software would not be suffi cient

to design and document the exoskeleton

connections. Instead, the BuroHappold

team decided that the complex stress

states that exist where members merge

into the connections meant that finite-element (FE) analysis was the only viable

option to verify their structural adequacy.

It was also clear that standard software

packages would not have the functionality

required to create the constructiondocumentation, especially for the free-

V A D I M I

S M A G I L O V


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form central region.

To complicate matters further, since the

exoskeleton would be clad in aluminium,

all connections and associated plates

and bolts would have to be located

within the cladding zone defined by the

architect. This would inevitably constrain

and limit options for the geometry of the

connections and necessitate non-planar

solutions.

Finally, the timescale for detailed

design and documentation of all 2500

exoskeleton connections was just 12

months. Put another way, the team would

need to complete an average of 50

connections per week.

In response to this seemingly impossible

task, BuroHappold drew on its expertise

in parametric engineering and structural

optimisation developed on previous

projects, including the Aviva Stadium in

Dublin, Ireland2, and the Louvre Museum

in Abu Dhabi, United Arab Emirates3.

Essentially, the team’s solution was to

create a unique, bespoke computational

approach using application programming

interface (API) techniques to allow

effi cient processing of the huge number

of FE models required and, critically,corresponding three-dimensional

Input Parameters

(variables)

Grasshopper

Definition/Script

Output Model

Viewed In Rhino 3D

Figure 5Parametric definitions using Grasshopper visual programming for Rhino 3D Figure 4Zones of exoskeleton Figure 6Design process

(3D) visualisation of every connection

throughout. The approach allowed the

engineering team to focus on the quality

of the engineering solution, rather than on

cumbersome data handling and repetitive

number-crunching tasks, resulting in

significantly faster and reliable output. As

a result, the entire detailed design and

documentation process was completed on

schedule, in a fraction of the time that the

team estimated would have been required

using a more conventional methodology.

Tools

Parametric design is a process in which

problem parameters are defined as

variables and a series of functions applied

in order to find the solution(s). By varying

these parameters, many variations of

the same problem can be solved. In this

case, the problem was FE analysis of the

many and various steel connections in the

exoskeleton.

The modelling software Rhinoceros

3D (Rhino 3D)4 and its plug-in module

Grasshopper5 were chosen as parametric

design tools for the speed and accuracy

they would bring to the task.

The combination allowed the team tocreate the geometry for a large number of

complex 3D forms quickly and accurately

using visual programming techniques and,

crucially, to make changes to the geometry

by changing the parameters (Figure 5).

They could literally “see what they were

doing” in each step of the programming

logic and in the corresponding geometry

as it was being created, making the code

debugging process much easier and

quicker than it would have been using

traditional practices.

Rhino 3D was also used to model

the outer surface geometry as a clash-

detection study to show that the

connections were within the cladding

zone. Autodesk’s Robot Structural Analysis

(RSA)6 software was used to create

the local FE models for each unique

connection type.

In this context, it is worth noting that

the size and complexity of the structure

meant that the global analysis model for

the building, which was created using

MIDAS structural analysis software7, took

over 12 hours to run. Hence, it was not

viable to create and insert FE models of

all the connections into the global model,

as this would have increased the analysis

time even further, possibly by three or fourtimes. Similarly, if the models were inserted


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one at a time, there would be at least a

12-hour wait each time the team wanted to

investigate alternative arrangements for a

connection. The only practical alternativewas to create separate “local” FE models

of the connections and transfer, or map,

onto them the corresponding moments and

forces from the global model results file,

for all 105 load combinations.

To maintain the t ight programme, almost

every aspect of the local model generation

and analysis was automated using bespoke

Visual Basic scripts that linked MIDAS,

RSA and Excel with Grasshopper via their

APIs.

In achieving the solution to this

ambitious project, the BuroHappold

engineering team found themselves at the

cutting edge, using the software in ways

that had not been done before, sometimes

working at the limits of the products’

capabilities. The team maintained frequent

dialogue with all the software companies’

technical support teams throughout, which

proved to be highly productive for both

parties.

Strategy The engineering team’s strategy was to break

this immense problem into five key steps

(Figure 6). The first four months of the project

were spent developing bespoke Grasshopper

scripts for every step. This significant time

investment was justified many times over by

the huge time saving made in the subsequent

analyses and generation of documentation.

It is important to understand that bespoke

programming, however skilled it might be, does

not replace engineering expertise. Rather,

it augments it by handling large amounts of

data effi ciently and releasing engineers to

focus on optimising the design. Accordingly,

visualisation, manual verification and

Figure 8Visualisation of data for similar connections

Figure 9Developingconnectionarrangement


a) b) c)

R U P E R T

I N M A N

Figure 7Grasshopper script to identify similar connections


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b)

acceptance were considered essential and

built into the process throughout.

The five steps were:

1. Identify similar connections

2. Develop the connection arrangement3. Find and map forces from global analysis

model

4. Analyse the connection

5. Generate analysis reports and construction

documents

Step 1: Identify similar connections

The first task was to confirm the number

of unique connection types required by

identifying those that were similar, in order to

reduce fabrication and erection time. With up

to nine elements connecting at each node,

and each element potentially having a different

section shape, section size and/or curvature,

this was not an easy task.

To identify unique connection types, a

Grasshopper script was written to interrogate

the exoskeleton member geometry Rhino

3D file created previously to help build the

global structural analysis model. It contained

the member centreline geometry and the

associated section shapes and sizes.

The script was used to search this file for

all intersections of centrelines (to locate the

connections) and to collect and organise the

relevant geometric data, such as the number

of intersecting members, whether members

are straight or curved, the member shapes

and sizes and the angles between adjacent

members. Thus organised into a programming

library, the data could be easily and accurately

compared to determine similarity of the

intersections, allowing for cases where the

geometry is handed (Figure 7).

Once the unique connection types had

been identified, the script visualised the

geometric data from the Rhino 3D file,

allowing the team to verify the similar

connection information easily (Figure 8). From

a total number of over 2500 connections, this

reduced the number of unique types to about

400.

Step 2: Develop the connection arrangement

Next, the principles of the connection weredeveloped through engineering judgement

based on the load paths (Figure 9a) and a

Grasshopper script was created to allow the

designer to rapidly conceive a connection’s

geometry to meet architectural and fabrication

constraints before sending the connection to

be analysed.

Mindful of the fabrication and erection

challenges that such massive connection

nodes would present, 3D study models of

each connection were created using Rhino

3D to ensure that the connections could be

readily fabricated. The models show the “plate-

by-plate” fabrication sequence for appropriate

clearance at every stage, including edge

distance tolerances and room for site welding,

testing and bolt tightening (Fig. 9b).

The connection designs also had to

accommodate extraordinary architectural

constraints. Zaha Hadid Architects had

provided a Rhino 3D model of the inner

surface of the cladding zone that all the steel

elements and connections had to fit inside(Figure 10a). For simpler connections, with

little or no curvature, the connections and

associated plates and bolts were similar in size

to the steel elements; therefore, clashes were

relatively easy to manage. However, in the free-

form central zone, multiple members typically

meet with high curvature, leading to complex

intersections (Fig. 10b). For this reason, the

connections are necessarily significantly

larger than the individual members. Given the

architectural envelope was not only tight but

also varied in depth where it was in double

curvature or warped, clashes were a real

possibility (Fig. 10c).

Since the constraints of fabrication

would often oppose those presented by

the architecture, the team realised that

Figure 10Workingwith architecturalenvelope

a)

c)


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applying them to the local connection

models was significant. Once again,

Grasshopper’s capability as a tool

for creating and visualising geometry

offered a number of benefits in terms of

speed and reliability.

With a script similar to that used in

Step 1 to identify unique connections,

data including connection geometry,

bar/node numbering, section sizes and

member orientations were transferred

to Grasshopper from the global analysis

model and mapped for each connection

under consideration (Figure 13). The

corresponding forces/moments were

then also extracted. The volume of

data this created was so large that it

was split into 55 separate files, each

containing up to five million sets of bar

forces/moments.

The bespoke scripts allowed

designers to search for any set of

forces/moments from the entire data

set and instantly visualise them on

screen. In-built vector transformation

tools could then be used to map the

forces/moments onto the local model.

The task would have been much more

diffi cult and time-consuming without the

powerful visualisation functionality that

Grasshopper provides, allowing as it did

for “visual debugging” of the script.

Even with Grasshopper’s power

vector tools, mapping and translation of

the forces/moments from multiple files

was susceptible to error, so the team

used a two-step verification process

comprising visual and numerical checks

to ensure the extracted data were

correct (Figure 14).

For the visual check, the connectionwas displayed in 3D together with

multiple connections. These allowed parts

of the parametric scripts for one unique

connection to be copied or developed for

application to others.

For example, as a general design

principle, a 25mm edge distance tolerance

was allowed for members being site-

welded to the connections, to account for

erection tolerances. However, increasing

the thickness of a connection node in

order to maximise edge distance for

site welds would make it more likely that

the connection would clash with the

architectural envelope. In order to

explore this, the thickness was

defined as a parameter within the

Grasshopper script. The value

could then be adjusted until the

edge distance tolerance of 25mm

was achieved.

Thanks to Grasshopper’s

powerful visualisation, all these

changes occurred graphically and

in real time as the designer moved

the slider value up and down

(Figure 12). If the 25mm tolerance

could not be achieved because

of a clash with the architectural

envelope (as in the example

shown), the designer could rapidly

determine what value would

optimise the edge distance while

remaining within the architectural

envelope.

Step 3. Find and map forces from

global analysis model

With 105 load combinations and

up to nine members in a single

connection, the process of finding

the correct forces/moments inthe global model and correctly

finding an optimal solution meant being

able to explore the design space for

each connection rapidly. To address this,

BuroHappold engineers programmed the

geometry of each connection using a

parametric script with variables defined

for all dimensions that were likely to

need further study to meet architectural,

fabrication and construction constraints

(Figure 11). The more complex the

geometry of the connection, the more

complex the parametric script became, but

some guiding principles were common to

Figure 11Parametric connection definition and fabrication connection

Figure 13Global analysis model


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vectors showing the magnitude and

direction of the applied forces/moments.

This quickly displayed any missing data

and verified that the forces were acting in

the correct direction. Additional analytical

information from the global model, such

as bar/node numbers, section properties,

gamma angles and local axes, could

be displayed as well to ensure proper

mapping of bar information.

For numerical verification, an

equilibrium check was performed for all

load combinations to ensure no “out-of-

balance” forces/moments existed. Any

questionable load combinations or nodes

were then displayed graphically and further

interrogated.

Step 4: Analyse the connection

The accurate prediction of the resultant

stresses where multiple members intersect

was a major concern. Consideration

of even a simple cruciform example

illustrates the importance of accurately

predicting stresses where members merge

(Figure 15). At the start of the project, the

BuroHappold team had determined that

neither established code-based methods

nor bespoke first-principles methods

would readily capture the complex stress

states that exist in the many and varied

connections of the exoskeleton where

individual plates intersect and overlap,

especially in locations where multiple

plates up to 750mm wide merged into

a single plate. Given the geometric

complexity and sheer size of the

connection nodes, an FE approach was

the only viable method for verifying the

adequacy of the connections.

Almost every step of the connectionanalysis process was semi-automated

Figure 12Using parametric definition to meet multiple constraints

Figure 14Visualisation and numerical check of mapped forces


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to reduce set-up and processing time,

using bespoke scripts to link the various

software programs to Grasshopper though

their APIs. The scripts were used to

• generate the local FE model

• add extension bars and apply the forces

• apply analytical links and boundary

conditions

• run the analysis and extract results

At every stage, the engineer could

employ visual checks to ensure the correct

data were being used. Once the scripts

had been created, these local analysis

models took just a few minutes to run

(compared to 12 hours for the global

analysis using MIDAS), allowing the team

to run them as many times as they needed

to, in order to match plates’ thicknesses to

stress levels and optimise the connections.

The FE models were based on 2D shell

elements that incorporated all plates in the

connection together with an appropriate

portion of the incoming members. Beyond

this, bar elements were added to match

those in the global model and the mapped

forces/moments from the global model

were applied to these. Since the geometry

and the forces/moments in the local and

global models should match, it was easy

to check these visually and numerically

against one another.

The first step was to generate the local

analytical model in RSA (Figure 16). Thescript first created a Rhino 3D model of 2D

surfaces at the centre of the plates, which

could be planar or curved, and converted

these surfaces into RSA objects. It then

asked RSA to create the FE mesh of 2D

shell elements from these objects. Since

the FE mesh would be generated inside

RSA, the geometry of the surfaces created

in Rhino 3D needed to be of suffi cient

accuracy to avoid meshing problems,

which can occur when the meshing

algorithms cannot determine the intended

common boundary between adjacent

surfaces. Since the Rhino 3D geometry

was defined parametrically, the overall

geometry could be altered as necessary

until the connection was optimised and

the various fabrication/architectural

constraints had been met.

Once the 2D shell elements were

generated, the script automatically added

the bar elements to the model. The bar

geometry was extracted directly from the

global analysis model and placed in the

same virtual position in the local model.

As the bar forces had been mapped

inside Grasshopper, and the bar numbers

generated in the local model matched

the global model, the load combinations

and bar forces/moments could beautomatically applied using Grasshopper.

This again mitigated errors associated

with manual processes such as copy and

pasting tabulated data.

Under a conventional approach, the

definition of the analytical links between

the bars and the shell elements in RSA,

and the definition of boundary conditions

(analytical supports) would both have

been time-consuming manual operations.

Here, they were both scripted to happen

automatically, saving considerable time

for the project. The nodes of the FE mesh

were imported into Grasshopper, which

applied a script that used geometric

search algorithms to find the appropriate

nodes to which the bar elements should

be connected. This information was then

sent back to RSA and used to create

the analytical connections. The script

also automatically applied the required

boundary conditions to the local RSA

model in predetermined locations.

After the forces/moments for all load

cases had been applied, the models were

batch-processed. Finally, the sum of each

reaction was checked to ensure they all

equalled zero before the results were

prepared for extraction.

To avoid unnecessary handling of largeand cumbersome data files, and to speed

Figure 15Interaction of in-plane principle stresses and theoreticalvon Mises envelope for simple cruciform connection

Figure 16Connectionmodel

a) In Rhino 3D

b) Connected live to RSA


NB In both cases, the stress levels σ1 and σ2 for the incoming members of the cruciform are set at the yield stress of the material(p

y). When σ

1 and σ

2 are both positive or negative (right-hand case), the maximum stress in the overlapping region does not

significantly increase. However, when σ1 and σ2 have opposite signs (left-hand case), the maximum stress in the overlappingregion reaches √3 × p

y. This phenomenon is predicted by inspection of the well-known “von Mises failure envelope”.


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up the process, a script was developed to

extract stresses in batches to determine

the governing load cases. Stress maps

of these connections were interrogated

using a scale based on the maximum plate

thickness for a given selection of plates

(Figure 17). The stress maps were then

visually inspected to establish whether the

stresses in any areas were unacceptable.

If necessary, the plates’ thicknesses,

arrangements or grades were changed andthe whole process re-run until satisfactory

results were achieved.

Finally, the results were all individually

reviewed by BuroHappold engineers as

part of the verification and acceptance

process.

Step 5: Generate analysis reports and

construction documents

It was recognised early in the project

that, given the large number of unique

nodes, the generation of engineeringdocumentation for each connection

could be a laborious task. Since all the

visual data available to the designers

during the design process were created

in Grasshopper, the logical solution was

to transfer this to an Excel template after

the analysis was complete. By creating a

tool to automate this task, the team made

considerable time savings and provided a

comprehensive visual record of all steps

of the design process, ensuring that any

independent party could easily follow the

assumptions made and data used for the

design of each connection (Figure 18).

While documentation was not a primary

objective of the process, the Grasshopper

scripts generated rich and coordinated

data that could be easily extracted to

provide accurate and relevant information

for the fabricator.

After careful consideration of the

options, it was agreed with the contractor

that the construction information would

be issued in the form of 2D drawings for

the connections in the flat-sided areas

and curved corners, where the geometry

could be readily defined using conventional

drawing software, and as 3D digital models

for the free-form areas to assist the

fabricator in understanding the connection

geometry (Figures 19 and 20).

This was because the design intent for

the connections in the free-form area was

more diffi cult to communicate using 2D

drawings. Since the 3D information was

readily available, it seemed illogical to

convert this to conventional 2D drawings

that would have required multiple views,

sections and coordinates to define the

shape, position and orientation relative to

the finished structure. Rather, using the

Rhino 3D surface models that had already

been created for the clash-detectionstudies, 3D models that were geometrically

Figure 193D documentation for largestfree-form connection

Figure 17RSA von Mises stress plot and fabricated connection

Figure 18Example of calculation output


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Figure 20Example of complex 3D documentation

accurate in every sense (plate thickness,

plate geometry, plate hierarchy at plate

intersections, actual position/orientation

in the building) were provided for the

fabricator, who simply transferred them into

their own 3D construction model.

The approach was mutually beneficial

as it saved time for all parties in what was

already an aggressive schedule and helped

to minimise fabrication errors (Figure 21).

Conclusion

To meet the aggressive construction

programme for the City of Dreams

hotel project, BuroHappold needed to

develop a state-of-the-art approach to

the complex design and documentation

of the exoskeleton connections. Thisinvolved full FE analysis of more than 2500

connections and 105 load cases. The

whole process was run using bespoke

parametric Grasshopper scripts, which

successfully integrated MIDAS, RSA,

Rhino 3D and Excel. Due to the number of

unique arrangements, their highly irregular

shapes and the complex stress states

that exist where the members merge, the

exoskeleton connections are possibly

the most analytically and geometrically

challenging large-scale connections of

any building constructed to date (Figure

22).

The Grasshopper scripts not only

allowed the engineering team to process

vast amounts of data quickly; importantly,

they also incorporated “on-screen” visual

checks at all stages of the process tohelp eliminate errors. The scripts were

carefully designed to avoid being a so-

called “black box” set of tools, but rather

an extension of the engineer’s hand; cutting

out mundane tasks and allowing more time

to focus on problem-solving.

The initial decision to spend the first

four months of the 12-month programme

developing the process and writing/testing

the parametric scripts was a bold one, but

one which paid off later when some of the

connections were being created, analysed

and documented in less than one hour.

There was inevitably periodic updating

of the scripts throughout the project,

but the majority of the development was

completed in this early stage. Once set up,

this innovative design approach achieved

huge savings in man-hours and allowedBuroHappold to consistently deliver ahead

Figure 213D documentation via digital model and construction


a) Assembly details b) 3D setting-out


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1 British Standards Institution (2009) BS EN

10225:2009 Weldable structural steel for fixed

offshore structures. Technical delivery conditions,

London, UK: BSI

2 Shepherd P. (2011) ‘Aviva Stadium – the use ofparametric modelling in structural design’, The

Structural Engineer, 89 (3), pp. 28–34

3 Shrubshall C. and Fisher A. (2011) ‘The practical

application of structural optimisation in the design

of the Louvre Abu Dhabi’, Taller, Longer, Lighter:

Proc. IABSE–IASS Symposium, London, UK, 20–23

September

4 Robert McNeel & Associates (2016) Rhinoceros

References

3D [Online] Available at: www.rhino3d.com

(Accessed: January 2016)

5 Robert McNeel & Associates (2016) Grasshopper

[Online] Available at: www.grasshopper3d.com


6 Autodesk (2016) Robot Structural Analysis

Professional [Online] Available at: www.autodesk.

co.uk/products/robot-structural-analysisoverview


7 MIDAS Engineering Software (2016) midas Gen

[Online] Available at: http://en.midasuser.com/

product/gen_overview.asp (Accessed: January

2016)

of schedule.

Structural engineering in themodern era is challenged by projects

of increasing complexity, falling fees

and faster construction programmes.

The profession will not meet these

competing challenges successfully

without harnessing the best available

technology. The construction industry

is now largely a “digital” industry, with

the leading design teams, contractors

and manufacturers increasingly

creating and sharing digital information.

For structural engineers, parametric

and computational design are the tools

that will enable them to embrace this

complexity, avoid getting bogged down

in ever-increasing amounts of data and

devote more valuable time to what they

do best – engineering.

Figure 22Node fabrication in Guangzhou, China

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Documents

2016_City of Dreams Hotel, Macau