Gravity resisting structure

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As explained above, the architectural integration of seismic and gravity resisting structure and architect-structural engineer collaboration is best begun early in the design process. In the early days of seismic design when suspended floors were cast-in-place rather than utilizing

(a) Two-way moment frames

(a) Two-way moment frames

Seismic and gravity frame

Direction of flooring span

Perimeter frame

(b) One-way moment frames

Seismic and gravity frame

Direction of flooring span

Perimeter frame

(b) One-way moment frames

Gravity only column

Area of floor supported by the perimeter frame

(c) Separate seismic and gravity resisting structure

▲ 6.7 Floor plans showing different degrees of separation between seismic and gravity resisting structure.

Gravity only column

Area of floor supported by the perimeter frame

(c) Separate seismic and gravity resisting structure precast concrete, seismic resistance of framed buildings was provided by two-way moment frames (Fig. 6.7(a)). Every column was a member of two orthogonal frames designed for gravity as well as seismic forces. Once designers appreciated the detailing and construction complexities and costs associated with ductile moment frames in the 1970s fewer structural elements were dedicated to resisting seismic forces. With reference to Fig. 6.7(b), if the floor structure spanning between the x direction beams is one-way there is no need for y direction beams to resist gravity forces. Since almost no floor loads are transferred to the two y direction perimeter frames they effectively can be designed to resist seismic forces only. Gravity and seismic resisting structure are therefore separated. In some buildings (Fig. 6.7(c)) the degree of separation between the two types of load-bearing structure is almost complete. Apart from a narrow perimeter ring of flooring carried by the perimeter frames interior columns support gravity forces while the perimeter frames resist all seismic forces. Not only do perimeter frames provide the best torsion resisting layout due to maximizing the lever-arm between them (see Fig. 5.5), but they possess two other advantages. Firstly, since seismic moment frame beams are relatively deep their perimeter location enables interstorey heights to be kept to a minimum. A potential clash between service ducts and the highly reinforced members is avoided. Secondly, by confining the large seismic resisting columns to the perimeter they are less disruptive to interior planning and can function as cladding elements, reducing façade costs.

The move away from two-way frames reduces structural redundancy. Now fewer elements provide seismic resistance. The search for economy of structure leads to the concentration of seismic structure rather than its more even distribution. Seismic and gravity structures are separated. The limit of this rationalization is reached with a seismic resistant structure consisting of two one-bay moment frames or two shear walls in each orthogonal direction (Fig. 6.8). Given the lack of redundancy, structural engineers need to be especially careful in the design, detailing and construction of the few critical structural elements and to design the slender gravity-only columns to accommodate horizontal seismic deflections without damage. Although these columns are not designed to resist seismic forces they do experience the same horizontal movements as the primary force resisting system.

Once the concept of separating seismic from gravity structure is accepted architects have more configuration options. No longer does a regular grid need to be superimposed upon a whole floor plan. Structural regularity in-plan, an indication of the degree of torsion and its resistance is assessed by moment frame, braced frame or shear y x

Seismic Forces Building
▲ 6.8 Seismic forces are resisted by two one-bay moment frames in each orthogonal direction. Shallow beams and slender columns support gravity forces. Office building, Wellington.
▲ 6.9 Slender gravity-only columns in the foreground with larger moment frame columns behind. Office building, Wellington.

wall location and not by its geometric symmetry as illustrated in Fig. 6.5. Gravity-only structural configuration can be treated more freely. It need not be 'gridlocked', and now that some columns do not resist seismic forces because of their slenderness and relative flexibility they are released from the strong column - weak beam rule. Columns can be slender and other materials explored (Fig. 6.9). Alternatively, columns can be enlarged but detailed as props - pin jointed top and bottom in each storey so as to not attract seismic forces to themselves and away from those moment frame columns designed to resist them.

As well as creating the opportunity to contrast the regularity and the large dimensions of seismic structure with more flexibly planned and slender gravity structure, separation of the two force resisting systems offers other design possibilities. For example, returning to Fig. 6.5 we see that the layout of structural walls is symmetrical about one diagonal. Moving along that imaginary axis away from the bottom left-hand corner one moves from opaqueness into openness or from possible intimate or private areas into a public or a more transparent realm. Peter Cook questions how structure is 'to be staccato, busy, cosy or symbolic of technicality? 3 The structural diversity or hierarchy resulting from the separation of these structural systems invites architectural exploitation in search of answers.

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