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Learning

22 Earthquake Design
Earthquake Tip and

Construction

Why are Short Columns more damaged during Earthquakes?

Which Columns are short? The Short Column Behaviour
During past earthquakes, reinforced concrete (RC) Many situations with short column effect arise in
frame buildings that have columns of different heights buildings. When a building is rested on sloped ground
within one storey, suffered more damage in the (Figure 1a), during earthquake shaking all columns
shorter columns as compared to taller columns in the move horizontally by the same amount along with the
same storey. Two examples of buildings with short floor slab at a particular level (this is called rigid floor
columns are shown in Figure 1 – buildings on a diaphragm action; see IITK-BMTPC Earthquake Tip 17). If
sloping ground and buildings with a mezzanine floor. short and tall columns exist within the same storey
level, then the short columns attract several times
(b) larger earthquake force and suffer more damage as
Mezzanine compared to taller ones.
Floor The short column effect also occurs in columns
that support mezzanine floors or loft slabs that are

(a) added in between two regular floors (Figures 1b).
There is another special situation in buildings
when short-column effect occurs. Consider a wall
(masonry or RC) of partial height built to fit a window
Short Regular over the remaining height. The adjacent columns
Column Column behave as short columns due to presence of these
Tall
Colu mn walls. In many cases, other columns in the same storey
Sloped Ground are of regular height, as there are no walls adjoining
them. When the floor slab moves horizontally during
Figure 1: Buildings with short columns – two an earthquake, the upper ends of these columns
explicit examples of common occurrences.
undergo the same displacement (Figure 3). However,
the stiff walls restrict horizontal movement of the
Poor behaviour of short columns is due to the fact lower portion of a short column, and it deforms by the
that in an earthquake, a tall column and a short full amount over the short height adjacent to the
column of same cross-section move horizontally by window opening. On the other hand, regular columns
same amount Δ (Figure 2). However, the short column deform over the full height. Since the effective height
is stiffer as compared to the tall column, and it attracts over which a short column can freely bend is small, it
larger earthquake force. Stiffness of a column means offers more resistance to horizontal motion and
resistance to deformation – the larger is the stiffness, thereby attracts a larger force as compared to the
larger is the force required to deform it. If a short regular column. As a result, short column sustains
column is not adequately designed for such a large more damage. Figure 4 shows X-cracking in a column
force, it can suffer significant damage during an adjacent to the walls of partial height.
earthquake. This behaviour is called Short Column
Effect. The damage in these short columns is often in Partial
the form of X-shaped cracking – this type of damage of Short Height Regular
column
columns is due to shear failure (see IITK-BMTPC Opening Wall Column

Earthquake Tip 19).

Δ Δ

Short Portion of

column

restrained
Long from
Tall Column:

Attracts smaller Short Column: moving

horizontal force Attracts larger

horizontal force
Figure 3: Short columns effect in RC buildings

Figure 2: Short columns are stiffer and attract when partial height walls adjoin columns –

larger forces during earthquakes – this must the effect is implicit here because infill walls
be accounted for in design. are often treated as non-structural elements.
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