Major Design Considerations

1. Objective

Bureau of Indian Standards IS: 13920 : 1993 recommends for special design to ensure adequate toughness and ductility (with ability to undergo large inelastic reversible deformation) for individual members such as beams, columns and walls and their connections and to prevent other non-ductile types of failure.

As a general rule, to maintain overall ductile behaviour of structure with minimal damage, it is necessary to provide the following combinations (Pillai and Menon, 2012):

Strong foundations and weak superstructure.
Members stronger in shear than in flexure.
Strong columns, and beams with little over-strength.

2. Means of Providing Ductility

Some of the main design considerations in providing ductility include:

Using a low tensile steel ratio (with relatively low grade steel) and/or using compression steel.
Providing adequate stirrups to ensure that shear failure does not precede flexural failure.
Confining concrete and compressions steel by closely spaced hoops or spirals, and
Proper detailing with regard to connections, anchorage, splicing, minimum reinforcement, etc.

3. Requirements of Stability and Stiffness

Under a severe earthquake, large lateral deformation and oscillations are induced resulting in formation of reversible plastic hinges at various locations. Thus a structural system should be designed to ensure that the formation of plastic hinges at suitable locations may, at worst condition, result in the failure of the individual element rather than progressive collapse.

Apart from the stability, the structure should have sufficient stiffness to limit the lateral deflection or drift. As per code the inter-storey drift is to be limited to 0.004 times the storey height to account for stiffness.

4. Requirements of Materials

As mentioned earlier use of relatively low grade steel is recommended. Further, lower the grade of steel, higher is the ratio of the ultimate tensile strength ( f_u) to the yield strength ( f_y). A high ratio of f_u/f_y is desirable, as it results in an increased length of plastic hinge and thereby an increased plastic rotation capacity.

For all buildings, which are more than three storeys, in height, have to use M20 as a minimum grade of concrete. Low density concrete lead to poor performance under reversed cyclic loading, whereas very high strength concrete is associated with lower ultimate compressive strain which adversely affects ductility.

5. Foundation

It is most important in the design to ensure that the foundation of a structure does not fail before the possible failure of superstructure. The maximum seismic forces transmitted to the foundation shall be governed by the later loads at which actual yielding takes place in the structural elements transforming the later loads to the foundation. Thus to ensure a safe foundation, it has to be ensured the foundation is stronger than the superstructure. Such a design concept is necessary to provide for ductile behaviour of the superstructure without serious damage to the foundation.

6. Flexural Yielding in Frames and Walls

As reinforced concrete is less ductile in compression and shear, dissipation of seismic energy is best achieved by flexural yielding. Thus it is necessary to avoid weakness of structure in compression and shear in relation to flexure.

In a structure composed of ductile movement-resisting frames and/or shear (flexural) walls, the desired inelastic (ductile) response is developed by formation of plastic hinges (flexural yielding) in the members, as shown in Fig. 29.7.

Figure 29.7 Formation of plastic hinges in a ductile structure

In ductile frames, plastic hinges may form in the beams or in the columns (Fig. 29. 7a). It is always desirable to design the frame such that the plastic hinges form only in the beams rather in columns. The reasons for such a condition are as follows:

Plastic hinges in beams have larger rotation capacities than in columns.
Mechanisms involving beam hinges have larger capacity – absorptive capacity on account of the larger number of beam hinges (with large rotation capacities) possible.
Eventual collapse of a beam generally results in a localised failure, whereas collapse of a column may lead to a ‘global’ failure, and
Columns are more difficult to straighten and repair than beams, in the event of residual deformation and damage.

Ductility and strength assessment of an entire structure requires non-linear analysis, considering material and geometric non-linearities