Conference on Structural Wonders

23 November 2012, Singapore

SOUTH BEACH DEVELOPMENT

Russell Neil Cole, Soh Seng Siong and Edwin Ong Beng Koon

Arup Singapore Pte Ltd, 10 Hoe Chiang Road

#26-01 Keppel Towers, Singapore 089315

ABSTRACT

South Beach development was conceived as a response to Singapore’s aspiration to be a ‘city

in a garden’. In keeping with this aspiration to be a garden city development, the project takes

its primary cue from nature and its ability to provide natural filters and regulators that act in

harmony with the local climate. As a tree provides shelter from the sun combined with

beautiful dappled light, shelter from rain and an open natural environment beneath its boughs

– the development is covered with a similar environmental filter inspired by the protection

offered by a tree canopy. The basement/podium forms a green spine with interlinked

courtyards. The sunken garden entry plaza at the entrance to South Beach with connectivity

to the Esplanade MRT Station is an example of open and public space within a private

development. The towers are shaped to allow enhanced views towards the Central Business

District. The Tower Bases gracefully cut back to allow the Green Spine to stretch through the

site. Sky-terraces are cut from two towers and climb vertically as a cascade of green spaces.

The South Beach development is intended as an iconic landmark and a showcase of

sustainability strategies. A key feature of the design is the “environmental filter” canopy that

covers the open space, linking the two towers, covering the podiums and heritage buildings.

The microclimate within this canopy is designed so that it caters to the specific activities that

are envisaged on the site. The development represents a considerable challenge to the

planning and design skills of engineers. Sophisticated software is mated with technical know-

how to solve complex engineering problems from concept to design to construction. This

paper summarises some of the engineering challenges faced during the design of the

development; from deep excavation in poor soil condition to two high-rise towers with

sloping faces to an undulating, unsymmetrical large-span canopy.

KEYWORDS

Circular Diaphragm Wall, strut-less excavation, top-down construction, shear walls, sloping

structure, high-strength concrete, prestressed structure, large-span canopy.

INTRODUCTION

The South Beach Development site is approximately 3.5 hectares on plan. The site is

bounded by Beach Road at the north, Nicoll Highway at the south, at the east of the site

Middle Road and at the west of the site Bras Basah Road. The development comprises two

mixed-use towers, podium retail and office structures up to 5 storeys above ground, 3 levels

of basement and interface with the MRT exit at the south end of the site.

In addition to the new development, there are four conservation buildings within the

development which will be refurbished to new design functions.

The development is sitting on reclaimed land, comprising deep soft clay marine deposits

above the underlying Old Alluvium (OA) layer. Proximity to the MRT station and the

existing conservation buildings pose challenge to the design and construction of the earth

retaining structural system as the deflection limit becomes critical.

Some of the innovations include using 85m and 89m diameter circular diaphragm walls and

T-shaped walls with combination of top down and bottom up construction sequence to

minimize the need for temporary support system and also to control deflections.

Both mixed-use towers feature a sloping faces as shown in Figure 1. The North Tower

sloping face is oriented towards the East and the South Tower sloping face is oriented

towards the West. The sloping faces induces lateral loads to the floor plates at some the floors

where the column inclination changes. The two towers make extensive use of Grade 80 High-

Strength Concrete in the cores and columns.

Figure 1: Artist Perspective of South Beach Development

SITE GEOLOGY

The geology map showing proposed site location is presented in Figure 2. The geological

map was produced prior to reclamation of the Singapore Marina area. The approximate South

Beach project area has been highlighted for clarity. From the geological information, the

project area is in an area of Kallang Formation overlying Old Alluvium.

Figure 2: Geology Map Showing South Beach Location

In general, the site is underlain by very dense silty sands, silts and hard clays of the Old

Alluvium. Above the Old Alluvium are recent deposits of the Kallang Formation. The

Kallang Formation comprises soft to firm clays of marine, fluvial, estuarine origin and fluvial

sands.

Where the Kallang materials extend to depths below about +80 mRL, two distinct layers of

Marine Clay are apparent, separated by a stiffer layer of clay (the desiccated crust of the

lower Marine Clay) and thin layers of peaty and sandy deposits. These estuarine and alluvial

deposits are also found at the base of the Kallang Formation, between the Marine Clay and

the Old Alluvium.

EARTH RETENTION SYSTEM

The earth retaining system structures (ERSS) for the development requires a robust retaining

wall system during basement construction to restrain excessive movements to the adjacent

services, conservation buildings, and Esplanade MRT station.

In choosing the type of earth retaining system for the South Beach, the following factors have

to be considered:

The site constraint

There are four conservation buildings on site that will be restored and strengthened

for future use. The ERSS shape and construction sequence have to be adapted to the

layout of the conservation buildings.

The condition of the soil/ground

The site consists of deep Marine Clay layer with varying contours. The ERSS has to

be designed such that the imbalanced soil pressure will not cause instability to the

system.

The control of ground movement

The ERSS for the development requires a robust retaining wall system to restrain

excessive movements to the adjacent services, conservation buildings and Esplanade

MRT station.

Water tightness

The location of the ground water table is about 14m above the final excavation level.

The water tightness has to be controlled to prevent excessive ground movement to the

adjacent structures.

Time and speed of construction and construction access

There are two tower buildings as the part of the development, namely the South

Tower and the North Tower at Zone 2 and Zone 4, respectively. The ERSS

construction has to accommodate an earlier start of construction of these towers.

Ease of quality control during construction.

Circular diaphragm wall is preferred as it allows the zones with towers to be constructed first.

It is also chosen due to its self-retaining property. It minimizes the use of internal struts to

retain the earth as it transforms the lateral earth pressure into circumferential compressive

stress, or hoop stress, within the diaphragm wall panels. As the result, the wall thickness can

be minimized and also, because of the minimal internal struts, it creates unobstructed

working space to facilitate the excavation works. In terms of ease of quality control, the

verticality of the panel can be assessed by the Koden test.

Owing to the layout of the existing conservation buildings, fully circular diaphragm walls

cannot be utilized. As the result, a fully circular, a dual-circular diaphragm wall, along with

two stretches of T-shaped panels are used.

The excavation plan has been divided into four zones, namely Zone 1, Zone 2, Zone 3 and

Zone 4 as shown in Figure 3.

Figure 3: Conservation Buildings and Zoning Layout

Zone 1 and Zone 2 comprise dual 85m diameter circular diaphragm walls, referred to as

‘peanut’. Zone 1 consists of a non-perfect circular wall, while Zone 2 is constructed as a fully

circular diaphragm wall. Zone 4 consists of an 89m diameter single circular diaphragm wall.

Zones 1, 2 and 4 are stitched together by two rows of approximately 40m and 93m long T-

panel walls.

For connectivity of all zones in the basement for permanent use, the wall between the zones

has to be removed. The construction sequence and design of the diaphragm wall panels have

to consider the permanent case where the hoop stress from the non-circular wall has to be

transferred to the permanent slab and beams in the basement. Two levels of ring beam are

used as a contingency measure at the circular and the peanut shaped diaphragm wall.

Even though the use of circular diaphragm wall is deemed to be most economical and

efficient, the shape imposes different kind of challenge. Figure 4 shows the diaphragm wall at

Zone 2 and Zone 4 after excavation to the pile cut off level.

The contour of the Kallang Formation/Old Alluvium soils at the site ranges, and as the result,

there is an unbalanced net soil pressures acting around the diaphragm walls. An in-house

structural program GSA is used to model the effect of the net soil pressure acting onto the

circular diaphragm walls. The net soil pressures are obtained from the in-house geotechnical

software FREW. The GSA gives compression stresses and bending moment of the wall, and

these results are compared to Plaxis results in order to obtain the bending moment for design.

Figure 4: Completed Excavation of Zone 2 and Zone 4

DIAPHRAGM WALL DESIGN

The diaphragm wall thickness is 1m at Zone 2 and Zone 4. Zone 1 diaphragm wall is 1.2m

thick considering the short distance to the Esplanade MRT station. For Zone 3, the 1.5m thick

T-shaped panel is used due to a more stringent deflection requirement along the stretch of the

wall.

a. Zone 2 and Zone 4

Zones 2 and 4 mainly consist of 6m panels with two rebar cages to ensure the

circularity of the wall.

b. Most of Zone 1 and part of Zone 2

Shorter 3m length panels are used for most of Zone 1, which is in near vicinity to

the existing Esplanade MRT station. A shorter panel is utilized to prevent the soil

collapsing. 3m panels are also used at the eastern side of the Zone 2 due to the

existence of fluvial sand layer that is prone to collapsing.

c. Connection between Zones

The design for panels that connect different zones has to consider the ease of

hacking and maintained connectivity. L-shaped panels are designed as permanent

panels at the zones connection, ensuring that both zones are connected by a single

panel. The design of the L-shaped panel also has to take into account the stress

concentration at the corner location.

d. T-shaped Panels

T-shaped panels are used due to its large bending and shear capacity. This is

especially important for the long-spanned wall at Zone 3 where deflection limit is

more stringent due to the existing conservation building Block 9. The flange and

web of the panel is 1.5m thick.

In the final stage of the basement construction, the diaphragm walls between the four zones

will be removed to form a long basement more than 270m long as shown in Figure 5.

Figure 5: Section Cut through the Basements

NORTH TOWER

North Tower is located at Zone 4 of the excavation zone and comprises 33 levels of office

floors, 3 levels of sky gardens dividing the floors into three zones. The total height of the

building is approximately 190m (to the top of the sky garden) and the overall height

including facade is 218m.

The typical floor layout comprises a central core with perimeter columns at various centres.

The columns at the east elevation are sloping over the height of the building as seen in Figure

6, whilst the columns on the other three elevations are vertical. Columns are spaced typically

at 12m c/c on the east and west elevations to accord with the architectural planning grid.

Figure 6: North Tower East Elevation

The floor structural system consists of post-tensioned (PT) flat slab of varied thicknesses

spanning between the core wall and the perimeter frame. Perimeter post-tensioned band

beams are used to enhance the stiffness of the perimeter frame, whilst maintaining a shallow

structural zone. This results in a thinner structural zone when compared to ordinary

reinforced concrete or composite floor alternatives, allowing higher headroom and deeper

MEP zone for better flexibility in planning of service routings. Figures 7 and Figure 8 shows

a typical floor framing plan. Two-way PT slab is adopted for column spanning from 12m to

18m with slab thickness varying from 265mm to 375mm.

Figure 7: North Tower Typical Floor Framing Plan

Figure 8: 3-D View of North Tower Floor Framing Plan

265 thk

265~375 thk

SOUTH TOWER

The South Tower superstructure comprises 18 levels for hotel and 22 for residential plus

three, 9 meters high, Sky Garden levels located respectively 66, 88 and 132 meters above

ground (see Figure 9). The height of the concrete structure is 190m; the overall height,

including a structural steel crown, is 218m.

There are three different floor typologies: hotel, residential lower stack and residential upper

stack (levels 3 to 21, 23 to 31 and 33 to 45 respectively). There is basically no typical floor as

the sloping façade and several architectural solutions produce different layouts at every level.

A centrally located core, 16 perimeter walls and inclined columns on the west elevation resist

gravity and lateral loads. The core can be perceived as 5 macro-beams; at level 23, where the

floor plate is subdivided into two separate areas, the central macro-beam is terminated and

two macro-beams per side continue to the top.

Floor Plan - Residential Upper Stack (Level 37)

Floor Plan - Residential Lower Stack (Level 27)

Floor Plan - Hotel (Level 16)

Figure 9: South Tower – Selected Floor Plans and South Elevation

LATERAL STABILITY AND STRUCTURAL ANALYSIS

The lateral load on the building is primarily resisted by the central core. Under normal

circumstances, the perimeter beam-column frame will provide some additional lateral

stiffness. The structure for the towers has been analysed using software (ETABS), in order to

assess the lateral stability system. 3-dimensional models with the proposed structural

elements including slabs, beams, columns and walls have been modelled including the

sloping faces (see Figure 10 for North Tower and Figure 11 for South Tower).

Notional loads, equal to 1.5% of the characteristic dead weight of the structure and applied

horizontally, are higher than wind loads and govern the strength design with gravity.

Expected wind induced accelerations at the tower’s upper levels where assessed through wind

tunnel testing to ensure occupants comfort criteria are satisfied.

Figure 10: North Tower – ETABS Model

The building’s centre of mass is at varying offsets from the buildings’ centre of rigidity. As a

result, there is significant lateral displacement of the building due to gravity loading alone.

Thus the inclusion of P-delta in the analysis is very important. For the North Tower, the built

in P-Delta analysis of ETABS has been used to cater for this effect.

Figure 11: South Tower – ETABS Model

IN-PLANE HORIZONTAL FORCE INDUCED BY COLUMN’S INCLINATION

The four inclined columns at the eastern side of the building will induce some additional in-

plane horizontal force which has to be included in the slab design. The horizontal force could

be tension or compression depending on the column’s slope. The column’s inclination angle

is illustrated in Figure 12. The floor slabs from level 13 and above are under in-plane

compression force while the columns below level 10 are under tension. There is a huge

horizontal force induced in the slab at level 2, level 11 and level 12 because of the sudden

change in column’s angle. The horizontal force from the columns will be resisted by the core

wall. Therefore the floor slab must be stiff enough to ensure the transfer mechanism is robust

and direct.

Horizontal load transfer mechanism in the slab to the core wall is illustrated through strut and

tie action. The compression zone in the slab has to be robust and strong enough to direct

transfer the compression force to the core. The slab thickness at this zone has to be thickened

to ensure concrete is not crushed. The slab in tension zone could be resisted by applying PT

tendon in the slab.

In order to make sure there is no area in the slab overstressed by the in-plane forces, the

horizontal force should be modelled in the slab analysis and it has to be taken consideration

together other design load combination. 3D analysis RAM concept model is used for North

tower’s slab design which can consider the effect of in-plane force in the analysis.

Figure 12: In-Plane Horizontal Force Induced by Column’s Inclination

TRANSFER STRUCTURES

The South Tower irregular design, in both plan and elevation, required the introduction of

several transfer structures of different nature.

There are sixteen long RC walls along the perimeter of the hotel floors. However the long

walls are not architecturally suitable for the lower levels (car park, hotel lobby) and sky

gardens. This required the loads from the long RC walls to be transferred to rectangular

columns at the 3rd

storey and back to long RC walls at the sky gardens. Such transfers occur 7

times throughout the full height of the tower. Coordination was needed within the design

team to achieve vertical alignment for the 16 main perimeter walls/columns from the

residential floors above to the carpark in the basement.

A transfer of different nature arises where the inclined columns connect to the vertical

perimeter walls; this happens at different levels as the inclined columns follow different

elevations. A study of the generated in-plane forces was required and detailing issues to be

resolved. Critical areas where studied in detail using 3D modelling to better understand local

stresses’ behaviour (see Figure 13).

Figure 13: – Localized Stresses at Transfer

Starting from level 23 the floor plate is divided into two structurally separate areas; to support

the edge of one of the areas, three new vertical members, and a thus a transfer, had to be

introduced (two walls and one inclined column).

At these levels (residential), a limited structural depth is allowed in order to achieve a clear

ceiling height of 3000mm with a typical floor to floor height of 3600mm. To avoid large

deflections at the 5m cantilevered floor plate’s corners, additional transfer columns had to be

introduced at critical locations. These columns are transferred at level 23 (for the residential

lower stack) and level 33 (upper stack); they are not continuous through the sky garden at

level 32 thus to reduce their size and stress at transfer.

CANOPY

The canopy will be a very prominent architectural feature. The architectural intent is for the

canopy structure to be of a single layered structural system (as opposed to a truss system) and

as lightweight as possible

The proposed canopy has a doubly curved profile as shown in Figure 14. The curvature is

more prominent in the east-west direction and produces a ribboned/waved effect. The curved

profile of the canopy has valleys, peaks and undulations catering to the buildings and spaces

the canopy extends over. As the intent for the canopy structure is to be a single layered

structure, the structural efficiency of this roof is greatly dependent on the profile.

Figure 14: Canopy View from Memorial Park

A number of structural form-finding studies were carried out to find an efficient structural

form. It can be seen from Figure 15 that arching of the profile in a vaulted manner between

the structural supports yields the most desired structure. Under such a roof form the structural

elements will be loaded predominantly in tension and compression only. The form minimizes

the amount shear and hence resultant bending moment in the members which would

otherwise demand larger member sections for stiffness and strength requirements.

Figure 15: Basic Canopy Structure

As it is influenced by the profile and member arrangements, the canopy structure is equally

affected by the support conditions. The long span nature of the canopy means that any

measure to reduce the span will benefit the structural elements. The shape of the support

column can assist in this aspect by, for instance, using a flared column (see Figure 16) as

opposed to be single straight column. The studies also indicate the flared column shapes are

more beneficial to the lateral deflection performance of the canopy.

Figure 16: Y-Shape Column Head

The canopy structure extends over the majority of the site. It is quite high above the ground

(average >30m) and with its undulating profile has a large tributary exposure to lateral wind

loading in the form of drag and friction forces. The canopy form does not lend to the

introduction of movement joints, nor is this desirable. So it also has to be designed and

detailed to thermal expansion and contraction.

Rectangular box sections are used for the canopy to enhance stiffness and buckling capacity.

The structural louvers in conjunction with the box ribbon sections form a virtual Vierendeel

truss system and together with the flared columns contribute towards lateral stability (see

Figure 17).

Figure 17: Canopy Vierendeel Truss System

WIND LOAD TEST

Wind tunnel study was conducted to assess the wind effect relevant to the structural and

serviceability design. A 1: 300 scale model of the proposed development was constructed

(see Figure 18) for the purpose of conducting wind tunnel studies to determine overall wind

load acting on the structure. The wind-induced peak accelerations throughout the towers

satisfy frequency dependent occupant comfort criteria.

Figure 18: Wind Tunnel Test Result and Wind Load comparison with CP3

ACKNOWLEDGEMENT

The authors are grateful for the contributions of Ms Zhou Zhi Qin, Mr Mauro Pellegrini, Ms

Le Hang, Ms Berlina Winata and Ms Fiona Yuen in the preparation of the paper.

ABOUT THE AUTHORS

Er. Russell Neil Cole Principal, Arup Singapore Pte Ltd

M Eng (Merit) PE Singapore

MIStructE Chartered Engineer

Email: russell.cole@arup.com

Russell has a background in façade and structural engineering and specialises in the design,

assessment and construction of all types of façade systems for a broad range of buildings. He

also has a strong interest in Building Physics and its application in hot climates. The projects

that he has undertaken cover a wide range and scope, from conceptual design and building

physics studies, to providing sub-contractor's details. The areas of investigations and

resolution of failing facades also form a large part of his work.

Russell founded the facade team in Singapore and ran it for many years before moving into

the leadership of the Buildings group. His projects are located in Singapore, Southeast Asia,

India and the Middle East, and past assignments in other markets where he had previously

worked, have given him the breadth and depth of experience in building engineering

throughout various markets.Russell has also contributed to technical papers on facades in the

UK, Hong Kong and Singapore.

Er. Soh Seng Siong Senior Associate, Arup Singapore Pte Ltd

BSc. (Hons) Civil & Structural Engineering

MSc. Civil & Structural Engineering

PE, PE (Geo) Singapore

Accredited Checker, Specialist Accredited Checker (Geo)

IES GeoSS TUCSS IStructE

Email: soh.sengsiong@arup.com

Soh Seng Siong has relevant abilities in the areas of design and supervision in civil, structural and geotechnical engineering projects. He has more than 30 years of experience in these disciplines and is a registered Professional Engineer (Civil), Professional Engineer (Geotechnical) as well as an Accredited Checker and Specialist Accredited Checker (Geotechnical).

Some of the major innovative designs are the triple donut configuration (circular diaphragm wall) for construction of deep basements, reinforced concrete ring beam strutting system for basement excavation adjacent to existing MRT station, 6 levels of top down construction, load bearing precast wall design and pile-raft foundation. Currently he is involved with the design of foundation and substructure of Capitol Project, Tanjong Pagar Mixed Development, Downtown Line 3 and Thomson Line MRT projects.

In the specialist consultancy service to BCA, he was involved in the underpinning of 30-storey building providing geotechnical and structural assessments.

Er. Edwin Ong Beng Koon Associate, Arup Singapore Pte Ltd

B Eng (Hons) P Eng Singapore MBA

IStructE

Email: edwin.ong@arup.com

An engineer since 1989, Edwin has been leading teams to plan and design the structural framework for buildings, preparation of tender and construction drawings and supervision of works carried out on the construction site. His experiences include coordinate design information between owners, other consultants and contractors for Design and Build Development, preparing tender document and contract administration for repair/retrofitting works and Civil Engineering projects for the Public Utilities Board and development of educational institute projects. He has also served as Resident Engineer at a major project at Changi Airport. Recent projects include the Sportshub, Marina Bay Sands Integrated Resort and City Square Mall.