Conference on Structural Marvels - Reflections at Keppel Bay 07Sept10

1 IStructE Conference on Structural Marvels – Reflections at Keppel Bay

REFLECTIONS AT KEPPEL BAY

Lim Kok Kim, Teh Hee Seang and Madan Bikram Khadka

T.Y. Lin International Pte Ltd, Singapore

ABSTRACT Located along the southern coastline of Singapore, The Reflections at Keppel Bay is a premier waterfront development overlooking the spectacular open blue sea at the front and the lush greenery of Mt Faber at the back. The site is formerly a shipyard surrounded by a deep natural water harbour dotted with jetties, boat ramps and large dry docks built along its shoreline. The jetties and boat ramps have since been demolished and a vertical seawall put in place to enable the land boundaries to be pushed out and optimized via land reclamation. The pre-existing dry docks have mostly been retained to create water channels to enable the sea to extend right into the heart of the development. The initial authority planning restrictions had limited the height of buildings in Reflections to 28 storeys apparently due to vista view controls from Mt Faber. It was subsequently relaxed following the developer’s and architect’s appeal. The architect had shown that the towers would be located mainly away from the central vista view and emphasized that the iconic composition and its curves would greatly enhance the aesthetics of the area where sea channels outside is also the southern gateway into Singapore. The eventual design yielded 6 curved sky towers and 11 villa style apartments housing 1,129 new homes. The sky towers are the main feature of the design. The taller three are 41 storeys high whilst the shorter three are 24 storeys. They are arranged in pairs but with each rotated strategically for architectural reasons and also to ensure that views from the apartment units are maximized. Conceptualized by world renowned architect Daniel Libeskind, the alternating towers with double curvature symbolizes an ascending symphony of chords. Each pair of the towers are connected by skybridges at 3 levels and capped off with a steel tower crown each. The main structure is conceptualized and designed using reinforced concrete. This paper shall present mainly the challenges in the design and construction of the curved sky towers in concrete. The skybridges and tower crowns are in steel. It shall also discuss how the towers are designed to incorporate the many variations in apartment unit layouts contained within the curved body form, each staggered differently at every floor on top of one another. Special considerations arising from the continuous change in centre of gravity of the structure during construction causing lateral movements are presented. Construction and instrumentation monitoring methods used are also discussed. The figures and charts presented shall mainly be those of Tower Type 1B/3B (see Figure 2b) for consistency and clarity.


KEY WORDS curved towers; vertical curvature; curved concrete frame; inclined columns; construction engineering; creep; skybridges INTRODUCTION In 1995, T.Y. Lin International Pte Ltd was appointed as the C&S Consultant to engineer the redevelopment of Keppel Shipyard where the Reflections is now sited. The proposal is to convert the then pre-existing shipyard into an integrated premium waterfront residential development with a world class marina with commercial and hotel amenities. The entire site is about 32 hectares and the proposed plan is to have 2500 to 3000 residential apartment units developed in phases. Phase 1 started with regularization and improvement of the land through land reclamation. This commenced in year 2000 with the enhancement of infrastructure by constructing seawalls to regularize the land and link bridges along the shoreline to create a continuous promenade around the bay. Phase 2 is the development of the 1st residential parcel around the pre-existing docks ie. the Caribbean. Phase 3 is the construction of a cable-stayed bridge and the marina on Keppel Island. The Reflections is Phase 4 of the project and occupies the largest land parcel in the whole development. The site area is 83,591m2 and the permissible GFA is 193,400m2. The construction contract was awarded in January 2008 to Woh Hup Pte Ltd under a lump sum, semi design and build arrangement. The contract is for the full scope of works including foundation piling and the contract period is 48 months. Figures 1a & 1b show images of the site before and after re-development (ie. status as is today).

Figure 1a: Site ‘Before’ Development (Original Shipyard)


Figure 1b: Site ‘After’ Development (Current Status) Design conceptualization of the Reflections project started in late 2005 with the engagement of Concept Architect, Studio Daniel Libeskind from New York. Assisted by DCA Architects, the local project QP, extensive effort was spent in juggling and refining the building configurations around the irregular yet interesting site to optimize layout. In addition to the uniquely shaped towers, the design also proposed gravity-defying building forms for the club houses, asymmetrically shaped skybridges and slanting tower crowns. Technical studies and feasibility tests of various structural schemes were carried out. Several tweaks to some of the originally proposed forms were made. They were mostly simplified in consideration of construction and engineering efficiency before the concept was crystallized for planning approval submission to the authorities. The final approved design nevertheless posed some pioneering engineering challenges, especially in the design and construction of a vertically curved tower. The challenge was made even more difficult by the need to fit and stack different types of residential apartment units within the unconventional building form. Apart from these, there were also other engineering challenges. The project had a huge basement with approximately 2km long of perimeter wall aligned mostly against the sea shoreline. The site also straddled over a large and critical underground box culvert drain near the middle. The live culvert is a major stormwater runoff discharge outlet for the southern catchment of Singapore. Its presence complicated the basement design and necessitated its exit ramps to cross under. This paper however shall focus only on the curved sky towers as they are the most unique and prominent feature in the overall design. The towers house the majority of the development dwelling areas and are anticipated to present the most difficult challenge to both the designer and builder. Engineering wise, one of the critical decisions to make in the beginning stage of design was to determine whether the structure should be built in steel or concrete. The usual cost, time and risk considerations came into mind. Ultimately, concrete was selected mainly because of cost. Another factor was the limited floor to floor height due to overall building height constraint. Local construction expertise, labour and material availability had a large influence on the overall cost. Figures 2a & 2b show the architect’s perspective of the overall project and its various tower types.


Figure 2a: Architect’s Perspective

Figure 2b: Tower Types


DESIGN General Description The actual form of the Reflections sky tower consists of a double vertical curvature that is about 500m radius at the front & back, and a gently curved side face that is tapered about two degrees inwards from bottom to top. On the taller tower, the entire floor plate shifts out by 3.9m at Level 19 and then shifts back by 4.3m at roof level. See Figure 3.

Isometric Form Front Elevation Side Elevation

Figure 3: Form & Shape of Taller Curved Tower 1B

The height of the taller tower is 174m to the top of the crown and the shorter is 116m. The transverse aspect ratio of the tall tower is about 1:9 and the typical inter-storey floor to floor height is 3.45m. The towers are paired alternately at different angles and orientation for architectural reasons. Towers 1A&1B face each other back to front, 2A&2B back to back; 3A&3B front to front. Each pair is linked by 3 skybridges at Levels 8, 15 & 22. All towers are clad in glass curtain wall and each is capped with a steel tower crown that is up to 49m in height. The structural system conceptualized for the tower is a dual system consisting of an inclined perimeter curved concrete frame tied to a vertical core shear wall via a rigid floor diaphragm. The system is suitable for both vertical load transfer as well as lateral load resistance purposes. The geometry of the curved frame induces a sustained bending moment throughout the height of the building under its own weight. This creates kick-out forces which have to be resolved in the floor diaphragm at every level. The floor structural system used is essentially a flat slab system with thickenings in the form of band beams to serve as chord restraints to the inclined columns. The band beams are thus designed to double up as struts and ties to resist the varying compression or tension forces at different levels. Figure 4 shows the lateral kick-out forces that are automatically generated in the floor diaphragm where columns are inclined. These occur in various directions towards the vertical lift core which serves like the spine of the curved body. They are the forces that will pull at the spine and cause it to deform differently under its own standing self weight.


Figure 4: Inclined Column Load Transfer

All columns and shear walls in the building are inclined except for the lift core wall. Those internal and along the vertically curved faces are inclined in one direction whilst those at the sides are inclined in 2 directions. The range of inter-storey column incline is up to 9 degrees. To maximize their contribution to direct frame action, they are aligned as much as possible. See Figure 5.

Figure 5: Typical Structural Floor Plan Special Design Considerations There are a multiple of apartment unit types contained in both the typical tall and short towers. Service installations that depended on gravity flow (like sanitary pipes) were confronted with difficult problems because they had limited space and could not run down vertically. Their relative position changes at every floor due to the shifts in the tower curvature. They had to be strategically arranged and cranked at suitable levels so as to avoid eating up too much space and cutting through critical structures. The shafts are eventually located to abut against shear walls (which have a common flat plane throughout its height) or away from beams in the critical transverse frame direction. See Figures 5 & 6.


Figure 6: Elevation of Sanitary Stack Layout The towers are connected with skybridges at different levels, orientated in different directions. They are not aligned to any common frame direction. As such, fixing and coupling them to the tower frame was not practical as it would cause undesirable torsion. To avoid any undesirable rotational restraint to one another, the bridges are fixed at the short tower end and released at the tall tower end with multi-directional sliding bearings. Lift shaft verticality and cladding stretch tolerances were major considerations due to the continuous lateral movements and deformations under self weight. This is a result of the gravity load eccentricity in the curved geometry and natural creep and shrinkage characteristics of concrete. This inherent eccentric load will always cause the tower to displace one side and the displacement is irreversible (See Figure 7). With concrete, the sustained stress further complicates matters as the structure creeps over time and causes the building to deflect continuously even after it has been completed. This will happen until the creep and shrinkage are fully expired which may take many years. For this reason, the design of the tower was very much focused on ensuring that its stiffness, hence lateral deformation was extensively evaluated and restricted well within typically acceptable serviceability limits. Construction engineering analysis was prescribed to determine the appropriate pre-cambering requirements to negate the parts of lateral deformation that can be reasonably predicted for purposes of achieving the best target ‘straightness’ or profile, especially for lift shaft verticality on the tall towers.


Figure 7: Irreversible Lift Shaft Lateral Displacement Figure 8 shows the difference in the deformed shape pattern of the tower when superimposed dead loads are applied on a fully assembled structure (normal analysis) compared to when they are imposed one floor at a time, like in actual construction. Note the shape of the deformation curve is different. The maximum eccentric load deformation is relatively greater around the ‘belly’ of the structure (at mid-height) because this is where the curved body is strained the most. In actual construction however, the bulk of self weight elastic and early creep deformation in a fully assembled model analysis is cancelled if every new level is re-aligned to its designed profile.

Figure 8: Deflection Patterns (Normal Analysis vs. Construction Sequence)


Figure 9 shows the natural deformation of the tower subjected to its centre of gravity shifts at every floor, without alignment correction. The total deformation is made up of 2 components, one from immediate elastic storey deformation and the other from creep of concrete which is time-dependent. The immediate elastic component (plus a fraction of ‘short term creep’) will automatically be cancelled out during construction as the vertical alignment is re-set at every floor to the ‘designed profile’. However, the ‘long term creep’ that happens after re-alignment is carried out will accumulate over time. This is the time-dependent component of deformation that needs to be ascertained by construction engineering and corrected by pre-camber.

Figure 9: Total Horizontal Displacement (Construction sequence with no initial displacement correction)

Design Criteria, Loads and Material Tower lateral stiffness was one of the most critical considerations in the conceptual design of the structure. This is in acknowledgement of and to cater for the sensitivities, tolerances and serviceability requirements of architectural and service installations. Human comfort due to building acceleration (especially in the soft transverse direction) was also a major concern considering that apartments at the uppermost storeys are mostly multi-million dollar penthouses. The tower lateral stiffness was designed to limit its lateral movement to a maximum drift ratio of H/500 for both Gravity P-delta (elastic+creep) plus transient peak wind motion. The peak acceleration response for all towers was checked by wind tunnel test and was verified to be well within internationally recommended limits. (Please refer to Design Performance below.) For strength, the design lateral loads considered are those due to wind and notional load as in accordance to the Singapore Building Regulations. There is no earthquake design requirement in Singapore although the island does experience occasional minor tremors from some strong distant earthquakes in the region. The magnitudes of such are typically relatively small and its impact falls generally within the notional load design requirement of the tall buildings of this order. The 50-year return period basic design wind speed used is 33m/s and the notional load requirement is 1.5% of


characteristic dead load. The building models were sent for a wind tunnel test [2]. A summary of the derived range of design forces for the towers are as follows: (i) Base Overturning Moment:

Load Source Tall Tower (MNm) Short Tower (MNm) Wind (from Wind Tunnel Test) 561-585 195-197 Notional 381-403 133-134 Eccentric Gravity 641-966 361-523

(ii) Base Shear:

Load Source Tall Tower (kN) Short Tower (kN) Wind (from Wind Tunnel Test) 5500-5700 2900-3800 Notional 5800-5900 3500-3700

(iii) Material:

Concrete (28-day Cube Strength) a. General: Grade 35 (All horizontal elements) b. Vertical structure (lower levels): Grade 60 c. Vertical structure (higher levels): Grade 40

Structural Steel a. Skybridge: Grade S355 b. Tower Crown: Grade S275

Design Performance Based on the loads and criteria prescribed, the building is expected to perform as follows: (i) Lateral Drift

Lateral drift analyses based on ‘Normal Analysis’ and ‘Construction Sequence Analysis’ carried

out are compared in Table 1. a. ‘Normal Analysis’ is based on load applied on a fully assembled model b. ‘Construction Sequence Analysis’ is based load applied on a progressively assembled model

according to construction sequence

(ii) Peak acceleration response

The 10-year peak acceleration response derived from wind tunnel analysis, based on 1% critical damping is as follows: a. Transverse, x-direction peak = 6.6 to 10 milli-g b. Combined peak = 10 to 12 milli-g

The recommended 10-year peak acceleration response limit is 19 milli-g [1].


TABLE 1

LATERAL DRIFT ANALYSIS

Load Normal Analysis

Construction Sequence Analysis[3]

Gravity (eccentric) During construction 30 years after completion

Elastic, SW 105* 165* - SDL 40 20* 20

LL 15 - 15 Creep, ST - 10* 10

LT 120 - 90 Wind, max 125 - 125 Total 300 195* 260 Target Limit (H/500) 310

Note: 1. * denotes cancelled out by construction re-alignment or pre-camber 2. SW = Self weight; SDL assumes 50% imposed during construction 3. Creep, ST denotes Short Term (Early) Creep; LT denotes Long Term 4. Results are for Tower 1B, H = 155m ie. to top of roof slab

CONSTRUCTION Construction of the curved towers presented several challenges. Firstly, the geometry of the form creates a continuously changing floor plate and changing inclination of columns at every floor. The temporary stresses, stability of the inclined structure and continuous movements arising from the time dependent properties of concrete are complex. The centre of gravity at every floor shifts as the tower is constructed upwards. The load history is dynamic and is dictated by the progressive load build-up on every floor. The load timing is important too as the deformation of the structure is affected by concrete age. The temporary deformations will be cumulative. The boundary conditions of the temporary structure are often not so definite. It was anticipated during the design stage that the construction of the curved structure would be carried out as ‘stand-alone’ without the deployment of any lateral strutting or props. Because of this, any lateral movement in the structure and stresses developed during the temporary stages will be locked-in and irreversible. It will not be possible or practical to make any alignment adjustments or corrections after it is built. The biggest concern arising from the above knowledge is whether the builder is able to construct the lift shaft to satisfy the stringent verticality tolerances of present day high-rise elevator systems. Most elevator systems can tolerate transient deformations but not many of them are designed to cope with permanent deformations. In the case at hand, permanent lateral deformation is inevitable. As such, construction engineering was prescribed and specified under the scope of the builder’s work. The design brief in the specifications called for construction analyses to be carried out as soon as the construction sequence and schedule are known and approved. The chief purpose is to determine the required construction pre-camber profile and demonstrate/verify that the achieved target profiles will be okay for the lifts. The builder is required to coordinate this with his appropriate lift sub-contractor to establish the acceptable tolerances and work towards achieving the required verticality accordingly


by adjusting his pre-camber. The analysis is also necessary to check whether there will be any temporary overstress situations in the partially and progressively constructed structure up from the foundation. For example, it is envisaged that the most critical condition for the concrete shear walls is when structure is in its temporary intermediate stage. This may happen when the structure reaches the apex of its curved profile somewhere near mid height. The walls or foundation piles may be in tension at that point when the vertical compression load component is at its lowest. Pre-cambering of the tower structure was implemented by the builder after the construction engineering analyses. The pre-camber profile was derived based on the amount of calculated creep deformation to be cancelled out at the appropriate target time. The ideal time should not be too distant in the future yet long enough to allow the bulk of the creep to be dissipated. As part of specified requirement, the builder had implemented an appropriate laser-guided instrumentation system to monitor the actual lateral movements of the structure. The monitoring prisms were placed mainly on the external wall of the lift shaft core as this was the most critical element. Prisms were also placed at each corner of alternate tower floors to monitor possible distortion. The instrumentation plan and system used by the builder is shown in Figure 10.

Figure 10: Instrumentation Plan & System Construction of the tower structure was mostly completed at the time of writing this paper. Eight out of nine skybridges were fully erected and the only outstanding structural works remaining are the erection of the steel roof crowns. The last progress report (August 2010) from the builder showed that they are 2.5 months ahead of schedule on the structure. The average tower floor cycle time achieved is 10 days. The instrumentation monitoring result of the actual movements showed that it is performing close to prediction. Method, assumptions and results from the builder’s construction engineering analyses and actual instrumentation survey are summarized as follows:


Method

(i) Builder carried out the construction engineering analyses using SAP 2000 non-linear version.

(ii) Initially, the natural total horizontal displacement of the structure was determined to assess its relative magnitudes at ‘Completion of Construction’ and ‘5 years’, ‘30 years’ after. See Figure 9.

(iii) The immediate elastic component of displacement plus a fraction of short term creep (automatically corrected during construction by repeated realignment) is then removed to determine the component of time dependent displacement (‘long term’ creep) that is locked-in and irreversible. This is the deformation that has to be corrected by pre-camber. See Figure 11.

(iv) The amount of pre-camber to set depends on the builder’s desired final target profile. Figure 12 shows the projected displacement curves at various intermediate and key stages of the structure. To attain the appropriate end ‘target profile’, the construction pre-camber can be set to reverse any of the projected displacement profiles eg. to achieve an end vertical profile at ‘Completion of Structure’, the pre-camber curve is simply the reverse of its projected displacement curve.

(v) In consideration of the lifts and expected creep expiry, the final ‘Pre-camber’ profile adopted is that of ‘5 years after completion of structure’. See Figure 13.

Figure 11: Components of Construction Sequence Total Displacement


Figure 12: Irreversible Short Term Creep Displacement (Tower 1B)

Figure 13: Pre-Camber and Target Profiles


Assumptions

(i) Boundary conditions: a. Base of tower is Fixed b. Floor diaphragm restraints at Basement Level 1 and 1st storey released. Pour strip

provided.

(ii) Load History: a. Floor cycle = 10 days b. Superimposed dead load = 50% average upon structure completion

(iii) Concrete properties: a. Concrete time-dependent properties modeled according to CEB-FIP 1990

recommendations in SAP2000, with the following parameters: i. Cement Type Coefficient, s = 0.25 (normal cement) ii. Relative Humidity, RH = 80% iii. Notional size, h = 0.2 (200mm) iv. Shrinkage coefficient, βsc = 5 (normal cement) v. Shrinkage start age = 0

b. Elastic Modulus and Strength of Concrete

Concrete grade C35 C40 C50 C60 Cube strength (N/mm2)

35 40 50 60

Modulus of Elasticity (103 N/mm2)

33 34 36 38.5

Results Realistically, some of the above assumptions may not be completely representative or realized precisely at site. For example, the pour strips specified at Basement B1 and 1st storey which had to be closed off early. It was initially specified to eliminate uncertainties in the degree of restraint that can be achieved. There was also concern that the high restraint force may harm the floor structures. As such, initial analyses model excluded these restraints. Subsequently, the pour strips were closed off due to construction access and practicality issues. The age of concrete and timing of load also cannot be precisely captured. Such deviations are to be expected and can be resolved by calibration against actual survey readings. The actual survey results were conscientiously checked against the analyses model predictions throughout the construction. No calibration was necessary. The comparison can be said to be rather good overall. See Figure 14.


Figure 14: Actual (Surveyed Profile) vs. Predicted (Target Profile) curves


CONCLUSION The selection of reinforced concrete as the preferred scheme for the curved tower structure of Reflections has proven to be an appropriate and a good choice. Earlier reservations in its selection with regards to its variability, performance and constructability are solvable. Shortcomings anticipated can be resolved through proper attention during design, simple detailing and stringent quality control. Early anticipation of potential problems and rigorous, accurate engineering analyses are crucial in minimizing problems in the construction of complex structures. In concrete structures especially, the loading history, time-dependent properties and construction quality control of concrete are critical factors influencing the accuracy of prediction models. Established parameters and research models are available in technical literature and certain engineering software to simulate the behaviour of concrete structures at different ages. Construction engineering analysis is a must for building projects where the stress/strain build up is highly dependent on the sequence of construction and its temporary condition at intermediate stages. It is shown on this project that the stress and strain patterns along the structure can be significantly different. It was found using sensitivity studies that a shorter floor cycle time would have increased the short term lateral creep deformation of the structure by as much as 25%!, and the permanent stresses in the vertical structure are greater at lower levels when construction sequence is taken into account. Results of the actual survey readings of the tower lateral movements monitored throughout construction compared well with that of the predicted movements by construction engineering theoretical analyses. Allowance should be considered for deviations due to construction variations and tolerance. Monitoring should check on accuracy, rate of deformation and direction of movement to detect undesirable signs. Accuracy of assumptions and parameters that are difficult to ascertain in theoretical analyses can be adjusted/corrected by intuitive calibration during the process of construction by comparing with actual readings. The speed of construction using cast-in-situ concrete on this project does not appear to be a hindrance or disadvantage to the project construction schedule. The tower structures are all completed ahead of scheduled time. As a matter of fact, the flexibility and adaptability of cast in situ concrete has its advantages in the construction of buildings with extensive floor profile variations and little repetition. In the case of this project, none of the tower floors are similar. Awareness of potential problems, sufficient rigorous engineering studies, simplicity and clarity of details are probably the more critical factors influencing the constructability of structures. REFERENCES [1] Council on Tall Buildings and Urban Habitat (1995), Structural Systems for Tall Buildings,

McGraw-Hill, Inc. [2] Vipac Engineering and Scientists Ltd, Australia (2007), Wind Tunnel Test report on “Structural

Wind Load Study of Keppel Bay Plot 1 Towers Singapore, Revision 1”. [3] Woh Hup Pte Ltd / Meinhardt (Singapore) Pte Ltd (June 2008), Construction sequence analysis

report on Tower blocks, Reflections @ Keppel Bay.

Date post:	22-Dec-2015
Category:	Documents
Upload:	tan-yi-liang
View:	14 times
Download:	1 times

Conference on Structural Marvels - Reflections at Keppel Bay 07Sept10

Documents