Mixed systems

Frame members

Moment frame

Shear wall

Plan (Typical Floor)

▲ 5.49 An example of a mixed system comprising shear wall and moment frame.

Moment frame

Shear wall

Plan (Typical Floor)

▲ 5.49 An example of a mixed system comprising shear wall and moment frame.

Just as an artist mixing different coloured paints avoids a selection with a murky appearance, so an architect is careful about mixing structural systems to resist seismic forces. (The term .mixed systems . applies where two or more different structural systems act together in one direction.) An example of mixed systems is where shear walls and moment frames act in parallel in the x direction in Fig. 5.49.

The problem with mixed systems, like the murkiness of an artist's colour, is a lack of clarity. With more than one system designers find it difficult to comprehend the force paths. Only through sophisticated engineering analysis can the combined structural behaviour of mixed systems be understood. The reason is that different systems are inherently structurally incompatible. With reference to Fig. 5.49 . the wall is much stiffer and stronger than the moment frame. Even though a designer might intend the frame to contribute significantly to seismic resistance, because of its relative flexibility its effect is negligible. The wall effectively resists all the horizontal forces. Once the walls suffer damage the frames will pick up some force, but are they strong enough? A mixture of structural systems to suit architectural planning or some other imperative leads to a structural combination that is unintelligible to all except a structural engineer with a powerful computer - the antithesis of a structure with simple and clear force paths. Generally, architects should avoid mixed systems, except as outlined below.

Shear wall

A successful mix of structural systems occurs in high-rise buildings where walls and moment frames work together to resist horizontal forces. The inter-storey deflections or drifts of a moment frame are greater near its base than further up its height. The converse situation occurs with a shear wall (Fig. 5.50). Where acting in parallel with a wall, a frame resists most of the upper forces including inertia forces from the wall itself. As the forces travel towards the base of the combined structure they gradually move via floor diaphragms into the wall. Near the base of the building the wall resists most of the force. In the high-rise example of Fig. 5.51, each system compliments the other.

Another reason for mixing systems is to increase the r edun-dancy of seismic resisting structure. The concept of internal forces finding alternative paths to the foundations in the event of one or more primary structural members failing is attractive - especially in the light of the widespread failure of steel moment frame joints during the 1995 Northridge earthquake as mentioned earlier. Elsesser suggests:

'If carefully selected, multiple systems can each serve a purpose; one to add damping and to limit deflection or drift, the other to provide strength. Multiple systems also serve to protect the entire structure by allowing failure of some elements without endangering the total building.'8

Several of the mixed systems he recommends incorporate structural devices, like dampers which are discussed in Chapter 14.

Shear wall Moment frame

▲ 5.50 The different deflected shapes of a shear wall and a moment frame resisting seismic forces.

▲ 5.51 A mixed structural system. A central shear wall core with a perimeter moment frame under construction. Office building, Wellington.

Some codes reward redundancy by allowing lower design forces. However, while increasing redundancy can be as straight forward as increasing the number of shear walls acting in one direction it can also be achieved by mixing structural systems. But due to the potential incompatibility problems outlined above and the need for a decision on an acceptable degree of redundancy to be informed by sound engineering judgment, mixed systems should first be discussed between an architect and an experienced structural engineer before being adopted.

Another form of mixed systems can occur where two or even three structural systems are placed above each other in a multi-storey building. For example, shear walls in the bottom few storeys might support and be replaced by moment frames that rise to roof level. Such changes in structural systems up the height of a building can introduce interesting architectural opportunities but they require especially careful structural design. The potential danger is the creation of one or more 'soft storeys ' (refer to Chapter 9). The structural engineer must therefore rigorously apply the Capacity Design approach to achieve an overall ductile building. This is conceptually realized by adopting the principle of never allowing a weaker or more flexible structural system to be beneath one that is stronger or stiffer.

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