A Program Management Approach: Constructability Challenges (Part 3)
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3.???Formwork Systems
Developing a logical concrete formwork system in coordination with the superstructure design is critical to achieving the demanding cycle times associated with super-tall buildings. Most center core designs are reinforced concrete, and the center core is typically on the critical path since it is ahead of the overall process of the individual floors.??
Formwork System Selection. The selection of the formwork systems can vary. The system can either be a platform that allows the entire core at every level to be cast in one time or a more lightweight and mobile jump-form system to cast be cast in sections. Whatever the case, they should have the ability to lift to the next level without using a tower crane. This is a very important consideration as the vertical elements shall always go in an uninterrupted, predetermined cycle time independent of other trades. The formwork system shall also provide an adequate working area to allow reasonable material storage and manpower to work on multiple elevations.??
One formwork system that allows the entire core to be cast in one time has been successfully used in China. This system consists of a large platform and requires a hydraulic jack system to lift the entire platform (which could weigh more than 1,200 tons). The hydraulic jacks that lift the platform can individually lift 500 tons and are synchronized, controlled, and monitored at the center control room located within the platform. Some platforms are even designed to allow tower cranes to be fixed and lifted together with the platform. The platforms provide ample area for material storage and workers and are ideal for steel-reinforced concrete (SRC) core design that requires lifting and in-situ welding works.??
The design of this type of platform requires a detailed review of the structural capacity of the core wall to locally strengthen it as necessary to support the lifting loads. In addition, clash detection reviews are essential to ensure the structure of the platform will not clash with the permanent structure in the core wall. Other challenges with this type of platform are that it takes more than two months to assemble on-site and is difficult to modify to suit a reduced core as the building progresses higher.??
Conventional Systems. The most commonly used formwork system for vertical reinforced concrete elements for super-tall buildings is the jump form system. The system is lifted by a hydraulic system as the building progresses. This type of jump form system is modularized and can be re-configured and modified. This system will not create a large working and storage area and its loading capacity is limited. To best utilize this system and achieve the required cycle time, the reinforcement is preassembled at the ground level. Apart from the structure framework, the majority of the system is made of wood. Welding and other hot works shall proceed with extreme care at this type of jump form.?
Formwork at the Burj Khalifa.?Burj Khalifa is a reinforced concrete structure, and its buttress wall-designed center core allowed the jump forms to maximize their potential. Three (3) sets of jump forms were installed at the center core, and each progressed independently, with no more than one level difference between them at any time during the construction.?
?The system together had multiple working platforms, and these jump forms were connected and accessible to one another despite the difference in progress. The center core “jumped” every 3 to 4 days and was completed one day ahead of schedule. Each wing also has one set of jump forms. The nose columns at the wing tips were not included in the system; rather, they were constructed in a more conventional slab-column sequence. As the wings were set back in a spiral stepping pattern, the forms were modularized to cope with changes in shape and area. The wings lagged the center core at the lower zone, sometimes more than 10 floors, but eventually merged with the core after several setbacks as the building height increased. The cycle time of the wings averaged 5 to 6 days, at times less.
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4. Structural Steel Erection
Depending on the structural scheme, the steel used in super-tall buildings can be complex. Whether it is a steel frame structure like Taipei 101 and CCTV or a concrete core with steel outriggers and mega-columns like Merdeka PNB 118 and the Wilshire Grand, the steel shapes are custom-made and require thick built-up plates and the associated welds. Thick plates are readily available worldwide, but not necessarily at high strengths unless ordered well in advance. The main challenges may include a scarcity of qualified welders and, in developing countries where steel is seldom used in building construction, a scarcity of qualified steel fabricators and erectors. For complex steelwork, the solution is to prequalify and select a competent steel contractor to do the work. The price of bringing in a foreign contractor may increase but it is the only tried and true solution.
Case Study, Bitexco Financial Tower Helipad, Ho Chi Minh City, Vietnam.?
Preassembly and mockups can also be used effectively with structural steel. The Bitexco Financial Tower in Ho Chi Minh City, Vietnam is currently the tallest in Ho Chi Minh City. The tower comprises many innovative and challenging features, most notably the 50th-story helipad that extends six (6) meters over the street below.?
Hyundai Engineering and Construction was the main contractor and Turner was the Project Manager. Cantilevering 22 meters on the 50th Floor, the 40-meter diameter helipad is one of the defining features of the Bitexco Financial Tower. With its extreme cantilever, erection above the street for the 250-ton helipad was a challenging task that took months to preplan and ultimately execute. The helipad is constructed of two 2.1 meters deep, tapered built-up structural steel girders acting as the primary members with smaller tapered and rolled sections cantilevered from these. The built-up structural steel sections were fabricated in Korea and the final fabrication took place in Vietnam. To ensure proper fit-up before erecting in place, the entire helipad was first preassembled offsite. The heaviest built-up sections were 20 tons, which dictated the size of the tower cranes used for the project.
Case Study, CCTV Headquarters Construction Sequence, Beijing, China. Developing a construction sequence and verifying that design intent is achieved is a key consideration for complex structures. This is particularly true for the CCTV headquarters, which is among the most structurally complicated buildings ever built. Due to the inclination of the towers, the building was erected out of alignment by calculated “preset” amounts such that the final structure, after loading, deflected to the design positions. It is composed of four interconnected building elements consisting of two inclined towers, a base that joins the towers at the bottom, and a cantilevered Overhang that connects the towers at the top. Ove Arup was the structural engineer and East China Architectural Design & Research Institute (ECADI) was the engineer of record. China State Engineer and Construction (CSCEC) was the General Contractor and Turner was hired as a construction management consultant.
The design provided estimated deflections of the structure based on design loads, assumed material properties, and an assumed construction sequence. CSCEC was responsible for recalculating the deflections based on the actual material properties and construction sequence. CSCEC used finite element analysis to determine the structure deflections. Iterative models based on construction sequence countered these deflections until convergence to the final design position was achieved. A simplified methodology was as follows:??
Acutely, absolute deflections exceeded 240 millimeters laterally and 100 millimeters vertically. To counteract, members were fabricated and erected in accordance with the model, and two presets were implemented:
The structure was monitored using advanced survey techniques to ensure actual behavior agreed with the anticipated behavior. When complete, CCTV’s erection remains one of the most complexes ever undertaken.