Structural Analysis
Turner brings unparalleled experience to the process of assessing structural systems and methods for the Greyhound Development. The team we have proposed has delivered buildings using the methods described below around the globe.
Components of Our Response to Question One
Figure G1 Traditional Top-Down Construction Sequence (source: Guidance on embedded retaining wall design – CIRIA, C760, 2017)
Top-Down Overview
Top-Down Construction aims to improve the construction timeline by utilizing the permanent basement floor slabs as the tie-backs for the selected earth retention system. With this approach, the deep foundation system is constructed per traditional methods; however, after excavation of each subgrade level, the associated structural slab is constructed to brace the earth retention system. This bracing, along with a leave out in the basement slab, allows for continued excavation of the following subgrade levels. While excavating and constructing below grade, some superstructure work can begin simultaneously, reducing the construction timeline.
Up-Down Overview
Expanding further on this methodology, Up-Down Construction allows for the entire superstructure to be completed prior to beginning substructure construction using Top-Down Construction techniques. This methodology requires additional pre-planning and design considerations to ensure the full superstructure lateral loads can be resisted at ground level prior to excavation, during substructure construction, and at the foundation level after excavation. These construction techniques will heavily depend on contractor experience/expertise with the type of construction and conflicts with existing in-place foundation obstructions (i.e., existing drilled piers, walls, etc.).
Conventional Top-Down Construction
As background, in a traditional top-down sequence, the entire structure foundations and basement walls are constructed from the ground surface, then upward and downward construction occur simultaneously across the entire site, as shown in Figure G1. In this traditional top-down method, often slurry walls are used to construct the basement walls. Also, in the traditional top-down method, the slabs are constructed across the entire basement as excavation proceeds, in order to provide lateral restraint to the basement walls during excavation.
Hybrid Top-Down Methodology
The development includes three parking levels below grade (excavation extending about 32.5 ft below existing ground surface). In a top-down construction sequence, the tower foundations/subterranean columns and first level deck are constructed first, then the excavation and shoring Support of Excavation (SOE) is performed below grade. This would enable the main superstructure of the towers to be erected while the below grade excavation and concrete slabs are simultaneously constructed. By starting the tower construction as soon as the tower foundations/basement columns and first level is placed provides a schedule enhancement for the vertical construction including tower structure, curtainwall, interior fit out, MEP, and ultimately speeding up the sign-offs and turn over by approximately six months.
As opposed to the traditional top-down method, in the proposed hybrid top-down method,
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The portion of the basement under the towers is constructed using a top-down technique, and the remainder of the basement (the portion not under the two towers) is constructed using conventional techniques.
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Rather than beginning with slurry walls to create the permanent basement walls and support of excavation, instead traditional soldier beams and lagging, with tie-back anchors, are used as temporary support of excavation.
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Rather than constructing the floor slabs for each subterranean level as excavation proceeds, instead the subterranean columns are temporarily braced and excavation proceeds to the very bottom; the floor slabs are constructed when construction comes back up from the bottom. All of this work during excavation proceeds simultaneously with the towers being constructed above grade. This is illustrated in Figure G2. Further details are provided below.
Figure G2 Schematic representation of hybrid top-down approach.
Plunge Columns
The approach to this hybrid top-down method is to install drilled piers, which serve as both permanent and temporary support for the towers, within both tower’s footprints, as shown in the logistic layout, with two crews drilling through the overburden at grade level. The drilled piers are referred to as “plunge columns.” The columns are constructed with the permanent steel column embedded in the center of the piers. Below the pressure slab on grade, the columns contain both the steel column at the center, and a rebar cage, as shown in Figure G3. During excavation of the piers, the hole will be stabilized such as with casing or drilling mud. The steel beam will be placed in position with tolerance as required for later construction of the permanent below-grade beams and decks.
Figure G3 Photo of steel about to be lowered into pier excavation. Note use of steel beam at center, and reinforcement cage for portion of pier that will be below lowest floor slab elevation. Cage in photo also includes Osterberg Load Cell at base.
Temporary Bracing
The plunge columns may be temporarily stabilized with steel frame elements to allow excavation to proceed to the bottom, as shown in Figure G4.
Ground Level Deck
The plunge columns will support the deck platform for construction of the higher floors in order to continue ‘up’ with the tower construction. At the beginning of the above-grade construction, the first-floor deck would encompass only the footprints of the two towers; the remainder of the first-floor deck would be constructed after the completion of the below-grade structure across the entire site.
The sequence described above for the below-grade construction is illustrated in Figure G5.
Figure G4 Photograph of steel bracing used to temporarily stabilize plunge columns.
Figure G5 Sequence of hybrid top-down construction (from "Deep Foundations for Top-Down Construction of High-Rise Project" by Hudson, K.S., Hudson, M.B., and Arreguin, G., 2019 Deep Foundation Institute Annual Conference)
Excavation and Tie-Backs
The ‘down’ work includes the excavation below the first floor deck of the towers, and conventional excavation throughout the balance of the podium areas. As this operation continues downward to final grade, tie-back anchors are installed to retain the shoring, and plunge column bracing are installed. Dewatering is started when the excavation has proceeded close to the groundwater level. Excavated materials can be removed with temporary ramps, long boom backhoes or conveyors.
Pressure Slab-on-Grade, Foundation Walls, Subterranean Decks
Once excavated to bottom elevation the foundations for the non-tower portion of the development are constructed, the bottom pressure slab-on-grade is constructed across the entire development footprint, then workflow continues with basement walls and completion of supported concrete slabs for the B2 and B1 levels.
Summary of Techniques Used
The Hybrid approach consists almost entirely of conventional techniques, employed in a creative manner to allow speeding-up of above-grade construction. Table G1 provides a summary of the techniques used for the elements.
Table G1 Summary of techniques used for the hybrid top-down approach.
Renderings of our hybrid top-down approach to the Greyhound site. Click an image to expand.
Top-Down Construction References
Turner is well versed in top-down construction in Boston and Southeast Asia.
Bitexco Financial Tower, Vietnam
Turner delivered the 68-story, 262-meter (860-ft.), Bitexco Financial Tower (BFT), which was the tallest in the country when completed. In addition to the unique shape, project is also defined by the helicopter pad on the 50th floor. BFT utilized semi-top-down construction for its three basement levels, wherein the slurry wall was braced by a ring of slabs as excavation proceeded allowing an early start to the tower foundation construction (while podium excavation continued for several months). The first ring of the bracing slabs were cast ground floor proceeding downwards.
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The BFT Top-Down Foundation (Semi-Top Down) Sequence was as follows:
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Diaphragm wall panels and bored piles simultaneous constructed
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Steel “plunge” columns inserted into bored piles during pile construction
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Ground Floor slab “ring” cast inside of D-wall (supported by D-wall and first bay of columns)
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First basement level excavated across site
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First basement level slab ring cast
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Repeated slab ring casting and subsequent excavation below until excavation completed
The BFT Semi-Top-Down construction sequencing is shown in the following figures and photos.
Figure 1 Bitexco Financial Tower Foundation
Figure 2 Bitexco Financial Tower, Semi-Top Down Sequence
One Central HCM (Spirit of Saigon), Vietnam
One Central HCM (formerly Spirit of Saigon) is a two-tower residential development in Ho Chi Minh City, Vietnam. The two towers comprise of 46- and 55-story buildings and retail, all connected by a podium facing the future pedestrian area of the Ben Thanh Central metro station.
The two towers are distinguished by their cantilevered overhangs resembling dragon heads at the top of each structure. A cantilevered glass cube represents the pearls in the dragons' mouths under each "head." The two towers are 235 meters (770-ft.) and 225 meters (738-ft.) tall, respectively.
One Central HCM utilized semi-top-down construction for its six basements with supplemental cross bracing, wherein the slurry wall was braced by a ring of slabs as excavation proceeded allowing an earlier start to both tower foundations (while the podium excavation continued for several months).
The top-down sequence was started with the B1 slab, not ground floor, to allow for faster initial excavation (before having to excavate with overhead elements). The slurry wall can generally cantilever from the B1 level retaining the outside soil without bracing.
The basement excavation and concrete casting sequence for B2 to B6 levels is in the following tables.
Table 2 Extract of One Central HCM Semi-Top Down Sequencing
Figure 3 One Central HCM Top-Down Excavation and Sequence
Four Seasons Kuala Lumpur, Malaysia
Turner delivered the Four Seasons hotel in Kuala Lumpur, Malaysia. The 65-story hotel and residential tower is accompanied by a five-story luxury retail podium that opened in June 2018. Upon completion, Four Seasons Place became the 50th tallest building in the world, at 342 meters (1122-ft.) tall.
Figure 4 Four Seasons KL, Aerial View, View of basement soffit/beam finishing works, Basement Raft Rebar, and Tower raft views respectively
Taipei 101, Taiwan
Turner delivered the Taipei 101 development, including fast-track design and construction of this 101-story, composite steel frame commercial tower and retail podium.
The total built area is 370,000 square meters, with 214,000 square meters of office space, 83,000 square meters of retail space, and 73,000 square meters of parking space. Turner was involved in the project from the outset, managing the design, procurement and construction processes and coordinating the activities of multiple works packages and domestic and international contractors.
The top-down process was unique for the project as the larger retail podium proceeded as “up-down” while the tower works were conventional with strutting. Key features to this technique included:
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Required Podium Occupancy Prior to Tower Completion
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Different Approaches to Tower and Podium Basements
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Separated Slurry Wall Around the Tower Footprint
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Podium Construction Approach Top-Down
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Tower Construction Approach: Excavation/Strutting: Bottom-Up
Figure 5 Taipei 101, Combined Conventional Top-Down and Partial Up-Down.