Jin Mao Tower

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In-Depth Analysis by Michael W. Su

Like the Petronas Towers of the same year, the Jin Mao Tower in Shanghai relies upon a principal structure of reinforced concrete to counter extreme seismic and wind loads. Although the height of the Jin Mao Tower is somewhat more modest, its program and material renders the design of the Jin Mao Tower no less ambitious than its Malaysian contemporary. Without being able to divide its core services or shift mechanical spaces off floor plates – as with the Petronas Towers, and yet required to scale comparable heights and accommodate mixed uses on multiple levels like hotel, offices, and shopping mall, architect Adrian Smith and engineer Mark Sarkisian of Skidmore, Owens & Merrill found it most structurally efficient to complement an already unusual octagonal core of high strength concrete with a novel combination of eight composite steel columns and eight strategically placed “super-columns” of steel sections filled with high strength concrete. Because both the bedrock beneath the site is, for all intents and purposes, out of reach, and the top soil cover is of soft sand, the tremendous weight of the resulting structure is borne by an uncommonly thick (4m) concrete mat foundation that is, in turn, supported by an astounding 1,062 tubular steel piles about 1m in diameter and driven to the record-setting depth of over 80m. This concrete mat is so unusually thick as to have called for its own cooling system in order to control the concrete temperature during the heat-intensive curing process. (Such thicknesses would not be repeated until, among other examples, Taipei 101’s 5m thick foundation was poured.) The result is that the Jin Mao Tower confidently scales heights of 366m to the top habitable floor and 421m to the top of its highly decorative, 23m tall stainless-steel assemblage of roof-covering “cap truss” and spire by resisting earthquakes of magnitude 6 and winds of 60m per second (200km per hour) with no more than 70cm of deflection. Since concrete cores with solid shear walls are more effective at countering lateral motion than cores of braced moment frames – as in the later Taipei 101 Tower, the deflection of the Jin Mao Tower is adequately controlled by a simple but innovative damping system of specially designed shear bolts that move within damping channels. Also, the mass of the 57^th floor swimming pool apparently contributes its own damping effect.

      In comparison with cores of the more common rectangular cross section, the shear walls of an octagonal core can be formed with less stringent corner bracing since the octagon has an additional axial direction for resisting lateral forces. As well, the octagonal cross section generally responds more effectively to eccentric lateral loads due to its axial symmetry. By placing the eight super-columns in closely-spaced pairs that are centered on each facade with separation distances similar to the sides of the core octagon, a novel cruciform structural configuration is formed which minimizes shear lag, yields tremendous lateral stiffness, and even affords large expanses of column-free prospects to boot. The remaining eight composite steel columns are distributed in pairs among the four corners of the building mostly to support the gravity loads from this portion of the floor plates. Finally, all perimeter columns are tied to the core by three sets of two story tall steel outrigger trusses located on floors 24, 51, and the highest floor, 87. If the Petronas Towers could be considered to have a structure of axisymmetric perimeter with an asymmetric core, therefore, the Jin Mao Tower could certainly be described as a structure of asymmetric perimeter with an axisymmetric core. Likewise, the cruciform configuration with added corner columns bears more than passing structural similarity to the nine squares of the Sears Tower. Most interestingly, this structural arrangement provided an opportunity for the designers to give the Jin Mao Tower its most distinctive feature: its inversion of solid and void.

      The lower 50 floors of the Jin Mao Tower are given over to offices, while the upper 32 floors are dedicated to the 555 hotel rooms of Hyatt International: the highest hotel in the world. Whereas the services for the offices, i.e. – elevators, mechanicals, electricals, and staircases, are fully enclosed within the octagonal service core from ground to floor 53, astoundingly, from floor 53 to the roof, the designers effectively turned the solidity of the core inside out by shifting all the services for the hotel to spaces between two opposite pairs of super-columns. Concentric hallways, spiraling balconies, and an elevator bank behind a dramatic, ascending monolith of curved glass are cantilevered off the core to form an atrium 27m wide and 110m high. The effect is a void that seems to soar into the sky – from the middle of the sky.