Vertical Irregularities

a. Stiffness Irregularity

The non-uniformity of the stiffness along the height of the building is referred to as stiffness irregularity. To facilitate parking of vehicles, infill walls are avoided in the ground storeys of residential buildings (Figure 6.3). Also, open shop front demands the absence of infill walls in the front side of the ground storey. This leads to a soft storey, resulting in a sway mechanism under lateral load. Inelastic deformations will concentrate in this storey, with the remainder of the structure staying in the elastic range of response. The transfer beam in the first floor is stronger than the columns beneath, thus creating a situation of strong-beam–weak- column joints. Even well detailed columns will lose strength, stiffness, and energy absorption capacity due to the concentrated inelastic demand placed on this single storey. Thus, collapse of the building is likely under moderate to severe earthquake. Although lack of infill walls at the ground storey is due to functional requirement, it needs special design of the columns.

b. Mass Irregularity

Mass irregularity may be caused by variation of mass between floors. c. Vertical Geometric Irregularity

To avoid the monotony of a box type of structure, setback towers are provided. But this may create a vertical geometric irregularity.

d. Weak Storey

The open ground storeys frequently observed are examples of weak storeys. e. In-Plane Discontinuity

If the in-plane offset of a lateral force resisting element is greater than the length of the element, an in-plane discontinuity exists. For a column set back in the ground storey, although the offset is less than the length of the column, it is a case of in-plane discontinuity when the direction of lateral load coincides with the direction of offset.

6.3 LOCAL DEFICIENCIES

Local deficiencies are element deficiencies that lead to the failure of individual elements of the building such as crushing of columns, flexural and shear failure of beams, columns and shear walls etc. Unaccounted loads, inadequate confinement, unauthorized alterations, poor quality of construction, poor detailing, lack of anchorage of reinforcement, inadequate shear reinforcement, insufficient cover, inadequate compaction and curing etc. and environmental deterioration are reasons for local deficiencies. The observed deficiencies of the elements are described next.

6.3.1 Columns

Columns are the primary gravity-load carrying members for most RC buildings. Therefore, column failures have led to catastrophic collapses during the past earthquakes. Buildings designed only for gravity loads may have several inadequacies for seismic loads. The common deficiencies are discussed below. a. Inadequate Shear Capacity

Typical gravity and wind load designs normally result in a design shear force significantly lower than the shear force that can develop in a column during seismic loading. Hence, columns in the buildings not designed for seismic forces have inadequate shear capacity. The cross-sectional dimension of a column is frequently limited to 230 mm to flush it with the wall. This may be inadequate for seismic loading. Another common problem is artificial “shortening” of columns by adding partial height partition walls that restrict the movement of the lower part of the columns. The resulting short columns are stiff and attract much higher shear forces than they were designed to carry.

b. Inadequate Confinement of Column Core

Although the frame structures are supposed to be designed using the strong- column–weak-beam concept, the use of deep spandrel beams in the first floor leads to stronger beams compared to the columns. The ground storey columns often form plastic hinges during strong seismic loading. The concrete core in a plastic hinging region must be adequately confined to prevent loss of the shear and flexural strength of the column. The confinement requirement in a column is more stringent because of the high axial load and shear that typically need to be carried through the plastic hinging region. Frequently, 6 mm diameter ties are placed at 200 to 225 mm spacing in the plastic hinging region. The ends of the ties have 90º hooks with inadequate hook length instead of 135º hooks. Although in the drawings the hook end is shown to be bent to about 135º, in practice 90º hooks are provided. These hooks open, leading to loss of confinement. There are

numerous examples of failure of poorly confined columns during the Bhuj earthquake.

c. Faulty splicing of rebar

It is a common practice in gravity load design to provide the column splice just above the floor and that too designed for compression only (Figure 6.4). The column may be subjected to large moments or subjected to tension under seismic loading (especially when infill walls are added and the column serves as a boundary element for the wall), resulting in pull-out of the rebar.

d. Inadequate Capacity under Biaxial Loading

The problems of shear strength and confinement are more severe in corner columns, especially if the building has significant eccentricity between the centre of mass and the centre of rigidity. Corner columns need to have a higher degree of confinement if they are to survive the biaxial loading demands that are likely to occur in them.

Figure 6.4: Examples of lack of seismic detailing