Structural Flare

 

Reinforced concrete shear walls, varying in thickness from 28 inches at their base to 20 inches at higher floor levels, are located on 33-foot centers within each of the three towers. The shear walls serve as the primary vertical and transverse structural system of all three buildings. Additional concrete core walls located within both legs of each building restrain the hotel in the longitudinal direction and help prevent out-of-plane buckling of the relatively narrow shear walls within both legs of the towers. Indeed, the requirement for such restraint was established very early in the design following rigorous buckling analyses of both legs. Post-tensioned 8-inch flat slabs span directly between the shear walls. This arrangement maximizes structural efficiency and creates a simple floor slab solution that can be built rapidly. The flat slab scheme also provides flexibility in the hotel room layout, accommodates a relatively tight 9-foot 10-inch typical floor-to-floor height, and accommodates flexibility of building services distribution and coordination throughout each floor.

Story-height steel trusses located within the Level 23 mechanical floors connect the legs of the tower and resist the large shear forces that occur where these meet above the central atrium. At ground floor level, the base slab is post-tensioned to resist the horizontal thrusts generated by the inclined legs (Figure 3).

 

Designing for Construction

 

Because of the extreme flare of the legs of the towers, the opposing shear walls tend to flex and drift laterally during construction. It was therefore crucial that the design carefully consider the construction sequence; a simple in-place structural analysis would be inappropriate. To this end, Arup conducted a series of construction stage analyses of various erection scenarios early in the design process in order to quantify the amount of displacement and locked-in stress that would accumulate within each tower under each scenario. The study concluded that substantial temporary works would be necessary to properly limit both effects, but that some degree of locked-in stress could not be avoided. During the Tender Process, the engineering team worked closely with the hotel contractors to devise a scheme that utilized temporary shoring to brace the inclined shear walls against the vertical walls to good effect (Figure 4). Additional studies demonstrated that by introducing prestress within both the vertical and inclined shear walls, these movements and resulting stresses could be further reduced. A final construction stage analysis of the chosen method of erection was then conducted to serve as a benchmark for construction. As the towers were erected, a real-time monitoring system was implemented to compare the actual stress levels against this benchmark, enabling back-analysis and design modification of the constructed tower in the event that the actual measurements deviated from the targets.

 

Measurements of the walls included the:

• angular rotation at the top of the tower

• maximum displacement of elevation in all three axes

• differential movement between vertical and inclined walls

• differential movement between adjacent wall bays

• differential movement among all three towers (which affects the support provided to the SkyPark)

 

In addition, immediate displacements arising from self-weight effects were offset via pre-camber during construction. However, because of their asymmetry, further complicated by concrete creep and shrinkage effects, the towers are expected to continue to deform laterally until these movements converge after approximately 30 years. These predicted long-term displacements have been accounted for in the design and specification of the vertical transportation systems, core layouts, building services, and building finishes and façade details.

 

SkyPark

 

At 1,115 feet long, 131 feet wide, and cantilevering 213 feet at an elevation of over 650 feet above grade, the hotel rooftop SkyPark is the world’s longest habitable cantilevered observation deck and has become a symbolic icon for Singapore in the process (Figure 5). This feature component boasts 2.5 acres of landscaped gardens comprising 250 trees up to 26 feet in height and over 650 plants, necessitating in excess of 81,000 cubic feet of soil; signature restaurants; a 495-foot long infinity edge swimming pool containing over 375,000 gallons of water, and an observation platform offering unrivalled 360 degree views of the Bay and surrounding city (Figure 6). To be successful, the design had to reconcile several competing demands: the aesthetic goal of creating an elegant park high above the towers, the minimum weight and efficiency demands required to limit the adverse effects on the towers below, and the constructability requirements necessary to enable pre-assembly and rapid construction on site.

 

Structural Optimization Studies

 

A number of framing options were considered for the SkyPark early in the design process. These included various truss, beam, steel diagrid, and segmented hollow concrete bridge alternatives, among others. All such schemes were assessed for both their weight and ease of construction. The design team, in conjunction with the fabricators and erectors, converged upon a system of post-tensioned segmental steel box girders 33 feet deep by 12 feet wide with 13/8-inch sidewalls and 23/8-inch flanges as the primary structural system for the cantilevering segment, and a system of primary steel bridge trusses for those segments spanning between the hotel towers. The bridges directly atop each tower are supported on raking steel "V" struts extending from the hotel roof directly over the concrete shear walls. The segments between each tower span as simply-supported trusses onto the ends of the adjoining segments.

 

The relative displacements of all three towers vary dramatically, owing to their subtle geometric differences. Movement joints located between each tower accommodate such differential movements arising from thermal expansion, wind-induced movements, and other time-dependent lateral movements generated from creep and shrinkage arising from the asymmetry of the towers. Careful detailing through the infinity edge pool accommodates these joints and gives the impression of a clean monolithic roof deck running continuously across the towers.

 

Another important consideration in the design of the SkyPark cantilever was its response to wind-induced vibrations, and vibrations arising from rhythmic human activity such as dancing and running. The introduction of a 5-ton tuned mass damper within the design of the cantilever served to mitigate these effects. Subsequent dynamic testing of the completed SkyPark has verified the performance of this system.

 

Construction Sequence

 

The SkyPark was prefabricated off-site in 14 primary segments. The bridging sections extending between adjacent towers each consist of three 400-ton bridge trusses that were pre-assembled at grade. These were then strand jacked into place over a span of 16 hours. The final cantilevering portion was likewise prefabricated in segments at grade to ensure proper fit-up. The entire assembly was then disassembled and each segment was hoisted in turn via strand jacks cantilevering from a moveable gantry attached from secondary beams at roof level, themselves spanning between the main box girders of the previously installed segment.

 

In total, over 7,000 tons of SkyPark steelwork was erected in just 13 weeks.

 

Conclusion

 

The overall form of the Marina Bay Sands Hotel was driven by the architectural requirement for a continuous atrium running the length of all three towers. The resulting geometric flare was resolved structurally via prestressed concrete base slabs and multi-story shear trusses located between opposing building halves through the mid-level mechanical rooms. The hotel’s asymmetry demanded detailed construction sequence analyses to assess the towers’ continual deflection during erection. Moreover, the unique form of each tower resulted in differential movements under imposed load, which further complicated the engineering design of the rooftop SkyPark and its support. Each of these challenges was addressed and resolved through close collaboration among the design, construction, and client teams (Figure 7).