Contemporary architecture in seismic regions

Without entering the territory of Earthquake Architecture (Chapter 17), where ideas and concepts expressive of seismic effects inspire architectural designs, a brief examination of contemporary architecture in seismically active regions is warranted. Is the architecture of seismic regions different from that of seismically quiescent areas? Do the rules and recommendations regarding regularity, symmetry and so forth necessitated by seismicity, exercise a stultifying influence upon architecture and cause it to become less interesting and exciting?

Regarding the first question the relevant literature remains silent. No studies have, for instance, compared the architecture of, say, two cities with very different levels of seismicity, like New York and Los Angeles, to find evidence of any seismic factor that might influence their architecture. Such a study would be complex. And how does one even begin to assess architectural interest and excitement? So many factors influence built form - the immediate and wider built environment, cultural, climatic, constructional and historic aspects, to name but a few.

One starting point to engage with these questions is to review publications featuring the contemporary architecture of cities or countries located in seismic zones, such as Los Angeles and Japan. A cursory scan through the glossy pages of typical journals suggests that contemporary practicing architects are by no means overly constrained by ' seismic rules ' .6'7 Compositional irregularity is common among architecturally significant low-rise Los Angeles buildings. Their architectural exuberance is partially explained by the fact that many are located on green-field rather than urban infill sites. Off-set and sloping walls and columns, unsymmetrical, splintered, fragmented and curved forms abound. One example is the work of architect Eric Owen Moss. Two of his buildings on adjacent sites exemplify irregularity and complexity of form (Fig. 6.19). In few projects one is aware of a dominant or ordering structural rationale, but it is noticeable that a predominance seismic design and architecture 105

▲ 6.19 The Stealth Building whose cross-sectional form morphs from triangular to square along its length. Los Angeles.

of light-weight construction reduces the potential architectural impact of the requirements of seismic structure. Irregularity also features in Japanese examples with their juxtaposition of geometric forms, long cantilevers and sculptural qualities. Generally, Japanese construction materials are heavier and, while there is greater visual evidence of seismic resisting structure, once again there is no sense of, nor mention of architectural creativity having been hindered by seismic design requirements.

Irrespective of these findings there is no avoiding the fact that buildings in moderate- to high-seismicity regions require increased horizontal strength and stiffness in two orthogonal plan directions, as well as torsional resistance. Additionally, the requirement for a hierarchy of member strengths, particularly in moment frames with their weak beams and strong columns, set seismic resistant structures apart from structures in non-seismic regions. Seismic design entails meeting certain minimum requirements specified by the structural codes of each country. So why are these requirements not having a visible impact upon the architecture?

The first reason is that often seismic resisting structure, just like gravity structure, is concealed. Consider the San Francisco Museum of Modern Art, designed by Mario Botta and completed in 1995 (Fig. 6.20). Internal steel braced frames ensure the structure complies with the requirements of Californian seismic codes. The frames are concealed between

▲ 6.20 San Francisco Museum of Modern Art.
▲ 6.21 Braced steel structure of the San Francisco Museum of Modern Art during construction.
▲ 6.22 Separation joint between panels of brick masonry filled with flexible sealant. San Francisco Museum of Modern Art.

wall linings within the interior of the building and placed behind exterior masonry walls (Fig. 6.21). Although Botta's characteristic deep slots and recesses might signify that the exterior masonry walls are not load-bearing it is still difficult to appreciate that the building is steel framed. The ' solid masonry' is no more than a skin of loosely attached masonry panels separated from each other and adjacent structural elements to avoid damage when the building sways in a quake (Fig. 6.22). Not only are the seismic resisting elements completely hidden from view but one presumes wrongly that the building structure, at least in part, consists of load-bearing masonry.

Even if primary structure is concealed, keen observers can still recognize subtle tell-tail signs of seismic design. But these are little more than clues and so are difficult to discern during a building visit let alone from photographic images. For example, we have just noted the separation details between cladding panels at the San Francisco Museum of Modern Art. Window mullions in another building might be wider than normal to accommodate seismic movement. A newer building is set back from its boundaries to allow for seismic drift, the gap screened by flexible flashings. A large complex might be separated into structurally independent blocks. These finer details of seismic design certainly exist but are not particularly evident.

Light-weight construction is another reason explaining the lack of visible application of seismic requirements. Light-weight walls, perhaps

▲ 6.23 Comparison of the load path down a regular wall with that of an irregular wall at the Seattle Public Library. The additional structure required where forces change direction is not shown.

▲ 6.24 Seattle Public Library.

(Reproduced with permission from Maibritt Pedersen).

▲ 6.25 CCTV Headquarters, Beijing.

▲ 6.25 CCTV Headquarters, Beijing.

framed by wood or cold-formed steel studs, and wooden floors and roof attract little inertia force. Even if wind forces do not dominate horizontal loading, smaller forces mean less structure and make it easier and less expensive to design the complex force paths generally associated with exciting architecture.

When an architect breaks away from regularity and symmetry, force paths become more complex. More demands are made upon the elements of seismic systems. Expect increased numbers of transfer diaphragms, collectors and ties, and mixed systems, like moment frames working together with shear walls in the same orthogonal direction. Compare the force path through a geometrically irregular wall of the Seattle Public Library (opened in 2004), and designed by OMA, with that of a regular and continuous wall (Figs. 6.23 and 6.24). Additional structures to stabilize those areas where forces change direction along their path can be extensive and expensive. Complex force paths require sophisticated engineering design.

▲ 6.24 Seattle Public Library.

(Reproduced with permission from Maibritt Pedersen).

The new CCTV headquarters in Beijing provides one of the most extreme examples of structural rigour necessitated by complex force paths (Fig. 6.25). As one might expect given its scale, geometric complexity and its inherent instability, structural engineers undertook a huge number of exceedingly complex analyses to develop and optimize the structural system. Then they had to demonstrate through computer modelling satisfactory seismic performance for each of three increasingly severe earthquake scenarios.8

Undeterred by the intrinsic difficulty of answering the question posed at the beginning of this section regarding possible architectural limitations posed by seismic design requirements, a group of sixty fourth-year architectural students have undertaken what might be considered a pilot study. Half the students chose cities in the most seismically active areas of the world, while the other half selected cities with no significant seismicity. Every student then chose on the basis of class-decided criteria the best example of contemporary built architecture in that city and analysed any seismic configuration problems (these topics are covered in Chapters 8 and 9). When the students assessed the degree of architectural interest of each others' buildings without knowing the locations of the buildings, there was no discernable difference in the degree of architectural interest for both groups of buildings. Nor were there any significant differences in configuration irregularities between them.9

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