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Civil Engineering Magazine THE MAGAZINE OF THE AMERICAN SOCIETY OF CIVIL ENGINEERS

Bangkok Hotel Designed to Welcome

By Jean Thilmany

Having to forgo the use of a central core in the design, the engineers of the Rosewood Bangkok had to find innovative ways to transfer loads, withstand earthquakes, and deal with soft soils.

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The Rosewood Bangkok will be located in the Thai capital’s busy and growing central business district. Its setbacks, which comply with local building regulations, suggest the traditional Thai gesture called wai. Kohn Pedersen Fox

August 23, 2016—The wai , a gesture deeply rooted in Thai culture, is characterized by placing flat hands together with fingertips pointed upward and holding them near the heart. The Thai people use the wai daily in greeting one another and saying good-bye, as well as in apologizing or expressing gratitude. In translating this gesture to the design of a structure—the Rosewood Bangkok, a 33-floor luxury hotel now under construction in the central business district of the Thai capital—the New York City-based architecture firm Kohn Pedersen Fox Associates PC has created a form that is both familiar and welcoming. But it posed a number of challenges for the structural engineering consultant, the Thai business unit of the international engineering firm Aurecon.

The hotel, a $100-million project set to open in 2019, alludes to the wai in that it takes the form of two adjacent high-rise buildings, connected by glass-enclosed platforms, that are angled to suggest hands. The buildings' floor plates decrease in length as the towers rise, and these setbacks not only fulfill the architect's vision but also satisfy the requirements for visual sight lines stipulated in local building codes, according to Assawin Wanitkorkul, Ph.D., a technical director and executive of Aurecon's Thailand business unit, who responded in writing to questions posed by Civil Engineering online.

The two hand-shaped buildings will house the 159 guest rooms. "Between the two connecting high-rise structures . . . there is a large central opening that connects the exterior pool deck, gym, and spa spaces with the restaurant space 11 levels above," said Rutger Huiberts, an architectural designer with Kohn Pedersen Fox who also responded in writing to questions posed by Civil Engineering online. "To the sides of this dramatic cavern there is a very tall water wall as well as a series of 'sky villas' that look onto this unique space. Once completed, we expect the view from the restaurant over the pool deck below to be quite spectacular."

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The building is actually two structures housing the hotel rooms joined by glass-fronted platforms that house the hotel’s amenities. Kohn Pedersen Fox

While the views may be magnificent, the engineering involved in providing them has been challenging. The sloped design meant that, in contrast to traditional practice, there could be no central structural core in the 155 m tall reinforced-concrete building. Instead, Wanitkorkul explained, elevators and stairs are located along the back spine of each of the hotel structures, the spines functioning as offset cores.

The offset cores, together with the floor setbacks, created a unique shape that had to be supported in nontraditional ways, Wanitkorkul explained. "Every floor plate at each level is different, and this required us to take a tailored structural engineering approach," he said. That approach involves the use of four diagonal columns that both eliminate the need for transfer beams and support the structures' graded facade, Wanitkorkul said.

"The inclusion of the inclined columns, combined with the sloping design, meant transfer beams were not needed," Wanitkorkul added. "In fact, most of the guest-room floors are not able to accommodate deep transfer structures due to their limited floor-to-floor heights and long cantilever spans."

The diagonal-column approach also minimizes the size of the columns "while maintaining efficiency and even adding a special quality to the hotel guest rooms," Huiberts said.

Reinforced-concrete fin walls are placed at the top of each sloped column, connecting the columns at two points to act as gusset plates and provide a robust load path. They also transfer axial forces from the column above to the column below, Wanitkorkul said.

The floors themselves comprise posttensioned-concrete sections to minimize their depth and the amount of steel rebar required for each floor, Wanitkorkul added.

Bangkok (Krung Thep) is located in a seismically active area, and the lack of a central core also figured in the structure's seismic design. "With offset cores, inertia forces tend to push floor structures away from the core during seismic excitation," Wanitkorkul explained. "So we had to ensure proper connection details were included between the floor structures and the core walls to withstand the relevant tension forces."

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The structures have no central core; instead, elevators and stairways are located at the back of the buildings, which posed a number of formidable structural challenges for the engineers. Kohn Pedersen Fox

A slender glass structure joins the two hand-shaped buildings at the apex, and here the engineers added extra strength to the columns to enhance stability. "The apex at each pair of inclined columns is basically just glass," Wanitkorkul said. "While striking in appearance, it also means that . . . there is no floor structure to support the columns."

The engineers also had to contend with Bangkok's famously soft soil, Wanitkorkul said. To accommodate parking, the project called for a 24 m deep excavation, which he describes as "fairly deep" for the conditions. And here again, the lack of a central core posed problems.

"Usually basement slabs within a core area will provide support to the perimeter diaphragm wall against the lateral pressure from the earth," Wanitkorkul explained. Without central core slabs to rely on, the engineers had to find another way not only to limit lateral movements so as to minimize effects on neighboring buildings but also to limit the ingress of water that might seep around or below the wall. The foundation had to be able to withstand the gravitational building loads as well as the lateral forces generated by wind or earthquakes.

The engineers met these challenges with a roughly 1 m thick diaphragm wall and a 1,700 mm deep reinforced-concrete raft foundation supported by piles bored nearly 60 m into the soil, Wanitkorkul said. The team used a soil boring log to ensure that, once the piles were in place, water would not seep from beneath the toe of the diaphragm wall. They also placed water stops at the interface between each panel and the wall.

Those measures have been successful, Wanitkorkul said, as construction of the basement is now complete and there has been no sign of water seepage.

Construction has now reached the second floor, and the engineers have been called on to control diaphragm wall movement as the crews continue with the construction of the columns. Aurecon is currently studying the sequence of construction and working with the contractor to ensure that any deflection is minimal, Wanitkorkul said.

As the building takes shape, residents and visitors alike will soon see the wai , in building form, begin to grace the Bangkok skyline. "Bangkok has been growing steadily as a hub for both business and tourism over the past decade, and this development taps into this trend," Huiberts explained. "The building carefully connects interior and exterior at numerous points, allowing for a tight relationship with the urban surroundings and providing open-air, panoramic experiences from a multitude of both public and private vantage points."


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