Stiff structures versus flexible

The terms 'stiff' and 'flexible' are relative ones, and must be interpreted with care. Some of their effects depend in part on the height of structure concerned. Table 8.2 summarizes some of the comparative merits of stiff and flexible construction, some of which have been discussed in the earlier parts of this section. A few further points of comparison are highlighted by discussing fully flexible structures.

Fully flexible structures may be exemplified by many modern beam and column buildings, where non-structure has been carefully separated from the frame. No significant shear elements exist, actual or potential: all partitioning and infill walls are isolated from frame movements, even the lift and stair shaft walls are completely separated. The cladding is mounted on rocker and roller brackets (of non-corrosive material). Apart from the points listed in Table 8.2 it has further disadvantages. Floor-to-floor lateral drift and permanent set may be excessive after a moderate earthquake. In reinforced concrete the joint detailing is very difficult. There is no hidden redundancy (extra safety margin) provided by non-structure as in traditional construction.

To overcome the difficulties imposed by the deformability of more flexible construction over the years, there has been a trend to avoid using traditional moment resisting frames by various means such as shear walls (various forms), bracing (various forms), base isolation, and energy absorbing devices. These will:

• reduce lateral drift;

• reduce reinforced concrete joint detailing problems;

• help to ensure that plasticity develops uniformly over the structure;

• prevent column failure in sway due to the P-delta effect (i.e. secondary bending resulting from the product of the vertical load and the lateral deflection).

In conclusion, it can be said that in many situations either a stiff or a flexible structure can be made to work, but the advantages of the two forms need careful consideration when choosing between them.

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