award winners and outstanding projects Spotlight Structural Innovations of Lotte World Tower By SawTeen See, P.E., C.E., Dist. M.ASCE, M. Eng., Leslie Earl Robertson, P.E., C.E., S.E., D.Sc., D.Eng., NAE, Dist. M.ASCE, AIJ, JSCA, AGIR, and Edward J. Roberts, P.E.

Leslie E. Robertson Associates was an Award Winner for its Lotte World Tower project in the 2016 NCSEA Annual Excellence in Structural Engineering Awards Program in the Category – New Buildings over $100M.

t 1,820 feet (555 meters) tall, core were needed to control the tower’s drift the 123-story Lotte World Tower and lateral accelerations due to wind loads. The stands as the first supertall build- 10-foot 9-inch by 10-foot 9-inch (3.3-meter by ® ing in , more than 1.8 3.3-meter) mega-columns at the ground level Atimes taller than the previous tallest build- (unbraced for the first eight levels) are compara- ing (located in the nearby city of Incheon). tively small compared to other towers of similar The building topped out in December 2015 height and even qualify as slender members. and the tower’s mast, dubbed the “Lantern,” LERA worked closely with the architects to was complete in March 2016. Designed by strike a balanceCopyright between the structural efficiency architects Associates gained by adding columns and the need to and structural engineers Leslie E. Robertson preserve open floor plans. Several structural Associates (LERA) of New York, the $2.5 designs were studied: a system of concrete billion tower and adjacent development fea- mega-columns with relatively small intermedi- tures a variety of usages, including office, ate steel columns at the perimeter; a system of retail, hotel, officetel (a combination office long-span spandrels with clear spans between and apartment common in Korea), parking, concrete mega-columns; and a combination of museum, and observation space. the two. The owner, Lotte, selected a system of The tower’s tapered shape – with sloping long-span spandrels for the office and officetel settlements between the perimeter and the concrete core walls in the middle third of the floors, with spans of up to 80 feet (24.5 meters) services core. The potential imbalance of building and columns sloping in two directions betweenmagazine mega-columns. At the building corners, the floor was partly addressed by vertically – creates a unique environmentS on every T floor. Rthe long-span U spandrels C cantilever T 46 feet U(14 cambering R the Emega-columns during con- Occupying the majority of the tower (from the meters) beyond the mega-columns while bend- struction, allowing forces to be transferred ground level to the 71st floor), the office and ing to follow the building’s curved floors. These through the outrigger members as the mega- officetel floors are steel framed with a slab-on- corners posed significant challenges for meet- columns settle further than the concrete core. truss deck, whereas the hotel floors (from levels ing the stringent deflection and vibration floor LERA recommended delaying the final con- 87 to 101) are concrete flat slabs with drops. The criteria. In response, LERA designed a series of nections of the outriggers to mitigate the force diagrid structure at the top of the building con- 1-story-high deflection control posts at every transfer in the short term, while also designing tains premium office, museum, and observation other floor, aligned with the cladding mullions. for the potential force transfer due to long- floors, which are also steel framed with a slab-on- Since these small members are not required term creep and shrinkage. truss deck. The tower sits on top of a 21.3-foot for strength, they were not fireproofed. Higher The tower topped out in 2016 and stands as thick mat (6.5 meters), with piles reinforcing up the building, the hotel floors are supported the fifth tallest building in the world.▪ the ground. These piles are not connected to the by perimeter steel columns that are spaced at mat, to conform to Korean building regulations. the module of the hotel room partitions and SawTeen See is the Managing Partner at Though the tapered shape of the building led transferred through belt trusses. LERA. She led the structural design of the to challenging structural complexities, it was The design for gravity and lateral loads from Lotte World Tower, , South Korea. effective at minimizing wind loads. wind and earthquakes is only one element Dr. Leslie E. Robertson, an internationally The tower’s primary lateral load and gravity of a grander structural design. For the Lotte acclaimed structural engineer, has system consists of eight concrete mega-columns, World Tower, robustness and redundancy transformed urban and rural landscapes concrete core walls and a series of outriggers and were foremost considerations in the design. through pioneering and innovative designs. belt trusses located at the mechanical, refuge, At the outset of the project, LERA’s team Among a plethora of notable projects, Dr. sky lobby, and hotel amenity floors. The belt of engineers studied various disproportion- Robertson led the structural design of the trusses transfer the diagrid “lantern” structure ate collapse scenarios, including the loss of World Trade Center, New York, NY. to the column configuration of the hotel floors, members in either the belt trusses or the as well as the columns of the hotel floors to the steel perimeter columns. As a result, some Edward J. Roberts is the Project Manager mega-columns at the officetel and office floors. belt truss members were increased in size at LERA. His expertise in 3D modeling Level 5 to Level 7 are hung from the lowest where required. has advanced the development of LERA’s belt truss. Only two levels of outriggers tying The effects of creep and shrinkage also had analysis software. the perimeter mega-columns to the concrete to be taken into account, such as differential

STRUCTURE magazine43 May 2017