COVER STORY
Lifting the main bridge
Synchronised hydraulic strand jacks were used to lift the skybridges which were assembled on the ground.
SKY BRIDGE CONSTRUCTION Various schemes were studied for the construction of the sky bridges. Eventually, the consultant’s design team decided to assemble the steel structure on the ground, using BONDEK structural steel decking together with a safety barrier, and lift it up by using synchronised hydraulic strand jacks. These jacks were installed on I-beams that were temporarily fixed on the roofs. As the external profile of the unit protrudes unevenly, the sky bridges were split into three portions - two cantilevered portions and the main bridge portion. The two cantilevered portions are located at the left and right recessed parts of the building surface and therefore could be easily installed from the edges of the respective floors. These two portions were installed using a tower crane and secured by temporary bracing. The temporary hoisting beam was installed on the roof to lift the main portion into position, using the hydraulic strand jacks, and it was then bolted to the two cantilever portions. Information was obtained from the Meteorological Service to ensure that the weather on the day of the lifting was good and there would be no rain
or strong winds that could cause the steel structure to sway, during the lifting operations.
QUALITY IN DESIGN, DETAILS AND SPECIFICATIONS The lateral stability of the building was checked, based on the static wind load, notional load, dynamic wind load and ground acceleration (0.03 g). The maximum deflection at the tips of the building was 72 mm (dynamic wind load) which is less than the allowable value of H/500 = 240 mm. In terms of the comfort level, the peak acceleration at the tip of building was 5.7 milligrams which is less than the allowable value of 20 milligrams. Therefore, the building is able to resist the lateral loads, in terms of deflection and comfort level. The lateral movements at the level of the sky bridges were monitored every month and the results were checked against calculated values. STRUCTURAL SYSTEM Superstructure A semi-precast structural system was chosen, comprising the following: • Cast-in situ concrete columns and beams mainly, precast concrete columns and beams where exposed • All load bearing walls - precast concrete lift walls / staircase walls / household shelter walls • Precast concrete volumetric house-
hold shelter with hollow-core wall system with infill concrete and reinforcement • All structural elements made of reinforced concrete for the 4th storey and below • Slab using precast prestressed concrete plank with cast in-situ topping supported by precast concrete / reinforced concrete beams • Sky bridge (structural steel truss) - 2.2 m deep main truss supported by pot bearings. - Main sky bridge has a weight of approximately 25 t. - Cantilever truss connected to wall and column. - 130 mm thick reinforced concrete slab with BONDEK on steel truss, on the top level. - Maintenance catwalk on the bottom level. - Planter boxes on the top chord of the truss. Design loading • Dead load - the unit weight of materials used for calculation of dead loads, as given in BS6399. • Superimposed dead load - finishes, planters, partition walls, M&E installations etc. • Live load - uniformly distributed loads used in the design are generally based on BS6399 and HDB requirements. • Wind load - wind loads acting on the structures are calculated using coeffi-
December 2016 THE SINGAPORE ENGINEER
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