Bank of China

Statistics

Article by Michael W. Su

Although some seventeen years have passed since its opening, the Bank of China tower in Hong Kong remains unique in both its form and structure. Planning for this building by architect I. M. Pei and structural engineer Leslie Robertson began at the end of 1982. Construction started in 1985, and the building was opened to the public in 1990. Remarkably, its ranking as the tallest building in Asia and fifth tallest in the world at the time of its completion was achieved through the realization of a still singularly-innovative, composite steel and concrete structure that also gave rise to its distinctive form. Robertson has characterized the building's principal structural system as a "bundled vertical space truss" – in contrast to the cantilevered beam system of most tall buildings. The conception of this unusual structural system was motivated by several factors.

            When the Hancock Tower in Chicago was completed in 1970, it was discovered that the tower's signature perimeter trusses, which had only been installed for resistance against lateral loads, actually carried a significant portion of the gravity loads. This finding suggested the feasibility of a structural system defined principally by trusses, and only secondarily by columns, rather than the other way around. In other words, while most structures marshal columns for gravity loads and trusses, or shear walls, for lateral loads, e.g. – the cantilevered beam system, the Hancock Tower strongly indicated the possibility of reversing this hierarchy. Specifically, gravity and lateral forces could be simultaneously supported by a primary three-dimensional space-frame consisting of planar trusses if gravity loads were transferred to these trusses by secondary columns and spandrel beams. In effect, not only would the truss webs brace against lateral forces, but these same webs would carry gravity loads to the truss chords. Then, the chords would transfer the combined gravity and lateral loads to the foundation.

            Although the principal advantage of utilizing such a space-frame is its exceptional economy resulting from savings in weight, reduction in number of columns to the foundation, and preclusion of space-consuming concrete structural cores, the space-frame also has a principal disadvantage: the complexity of its connections and, hence, assembly. Further, since planar trusses are much more readily fabricated from steel, fireproofing of the material is necessary. However, in the case of the Bank of China tower, the economy of the space-frame was explicitly required because the clients sought to achieve as much floor area as Norman Foster's nearby Hong Kong and Shanghai Bank building, but with both a smaller site and only one third the budget. In response, Pei and Robertson conceived their pioneering composite space-frame, which combined the respective strengths of steel and concrete to attain the requisite economy without incurring the associated penalties.

            Somewhat in the manner of a composite between the 1974 Sears Tower's bundled tubes of increasing heights and the 1970 Hancock Tower's clearly-expressed diagonal bracing – both located in Chicago and with structures designed by the famed Fazlur Khan, the Bank of China tower consists of four bundled, triangular-sectioned, space-frame tubes that successively reach floors 28, 38, 51, and 70 to attain a final building height of 315m at the roof and an additional 54m atop two masts. Seen in plan, these tubes have sides that correspond to the sides of the four isosceles triangles resultant from dividing the building's 54m square plan into quadrants along the diagonals. Each tube can even be considered as the result of vertically stacking discrete space-frame modules 54m in height, which are formed by joining three vertical planar trusses by their chords at the three corners of each of the plan's four isosceles triangles, i.e. ­– two interior triangular trusses and one exterior square truss with X-bracing. The tallest tube thus consists of five such modules, the next consists of three, then two, and then one. However, since these modules have common trusses in the building interior, the overall space-frame structure can also be described as the union of eight different vertical planar trusses of varying heights and configurations: four perimeter trusses with one, two, three, and five X-braced square bays, and four interior trusses.

            While these trusses are fabricated from unexpectedly thin plates and with simple connections, e.g. – none of the steel plates are thicker than 10cm and the steel contractor estimated the erection to have required only half the welding of other buildings of comparable volume, the final "bundled vertical space truss" is actually constructed by embedding all the truss chords within the concrete of four massive corner columns and one center column. For added stiffness and fireproofing, truss webs are also filled and covered with concrete. Finally, although the building's asymmetric form results in non-uniform gravity and lateral loading, the truss chords are configured within the concrete columns such that any load eccentricities are either canceled out or well within the safety factors – including the overall torsional lateral load on the asymmetric building! In fact, the resulting vertical joints actually comprise the four corner columns and one center column supporting the entire building. By combining steel and concrete this way, the costly complications of joining truss chords of varying geometries are entirely avoided, even as the advantages of both materials are well harnessed.

            Interestingly, while the resulting composite corner columns rise directly from the foundation, the inner chords of the four interior trusses are joined by the center concrete column in such a way that this column does not begin at the foundation. Rather, it bears upon the 25th floor apex of an angled triangular truss defined by the lowest webs of two interior trusses and corresponding to one quadrant of the square plan. (With rising height, the central column is supported by two additional triangular trusses, which correspond to the two adjacent quadrants. Loads on the central column are thus distributed to the corner columns by three similar trusses at three different heights.) Elimination of this central column at the building base results in a building lobby of uncommon spaciousness. Beneath this lobby, however, is a concrete caisson foundation densely packed with some 110 caissons within the building footprint and four large caissons at the building corners.

            In addition to bearing the gravity loads of its own weight and the lateral loads of the tower, the space-frame carries gravity loads from individual floor plates through secondary perimeter columns that are partly supported by the X-bracing of the perimeter planar trusses and partly by the horizontal – but externally unexpressed – Vierendeel trusses located between each of the 54m tall space-frame modules. Although these Vierendeel trusses account for the slight discontinuity between the X-bracings on the building façade, they serve the necessary function of transferring gravity loads from the floor slabs to the corner columns where the transfer to the X-bracing is insufficient or uneven. With respect to gravity loads, therefore, the four corner and one center primary columns are augmented by 20 perimeter columns from the fourth to the 25th floor, 17 perimeter columns to the 38th floor, 12 perimeter columns to the 51st floor, and finally, 9 perimeter columns to the 70th floor.

            The result of Pei and Robertson's pioneering explorations into structural systems, materials, and assembly is a still physically-unique building that attains the remarkable efficiency of 100kg of steel per square meter without ceding its claims to record-setting heights. Indeed, the uniquity of the Bank of China tower is affirmed by the absence of close contenders, thus far, either in form, height, or structural system.