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Learning

17 Earthquake Design
Earthquake Tip and

Construction

How do Earthquakes affect Reinforced Concrete Buildings?

Reinforced Concrete Buildings In most buildings, the geometric distortion of the slab
In recent times, reinforced concrete buildings have is negligible in the horizontal plane; this behaviour is
become common in India, particularly in towns and known as the rigid diaphragm action (Figure 2b).
cities. Reinforced concrete (or simply RC) consists of Structural engineers must consider this during design.
two primary materials, namely concrete with reinforcing
steel bars. Concrete is made of sand, crushed stone (called
aggregates) and cement, all mixed with pre-determined
amount of water. Concrete can be molded into any
desired shape, and steel bars can be bent into many

shapes. Thus, structures of complex shapes are (a) Out-of-plane

possible with RC. Vertical Movement
A typical RC building is made of horizontal (b) In-plane Horizontal Movement

members (beams and slabs) and vertical members Figure 2: Floor bends with the beam but moves
(columns and walls), and supported by foundations that all columns at that level together.

rest on ground. The system comprising of RC columns After columns and floors in a RC building are cast
and connecting beams is called a RC Frame. The RC and the concrete hardens, vertical spaces between
frame participates in resisting the earthquake forces. columns and floors are usually filled-in with masonry
Earthquake shaking generates inertia forces in the walls to demarcate a floor area into functional spaces
building, which are proportional to the building mass. (rooms). Normally, these masonry walls, also called
Since most of the building mass is present at floor infill walls, are not connected to surrounding RC
levels, earthquake-induced inertia forces primarily columns and beams. When columns receive horizontal
develop at the floor levels. These forces travel forces at floor levels, they try to move in the horizontal
downwards - through slab and beams to columns and direction, but masonry walls tend to resist this
walls, and then to the foundations from where they are movement. Due to their heavy weight and thickness,
dispersed to the ground. As inertia forces accumulate these walls attract rather large horizontal forces
downwards from the top of the building, the columns (Figure 3). However, since masonry is a brittle
and walls at lower storeys experience higher material, these walls develop cracks once their ability
earthquake-induced forces (Figure 1) and are therefore to carry horizontal load is exceeded. Thus, infill walls
designed to be stronger than those in storeys above. act like sacrificial fuses in buildings; they develop
cracks under severe ground shaking but help share the
5 load of the beams and columns until cracking.
Earthquake performance of infill walls is enhanced by
4 mortars of good strength, making proper masonry
3 courses, and proper packing of gaps between RC
Floor Level frame and masonry infill walls. However, an infill wall
2 that is unduly tall or long in comparison to its
thickness can fall out-of-plane (i.e., along its thin
1 direction), which can be life threatening. Also, placing
infills irregularly in the building causes ill effects like
Total Force

Figure 1: Total horizontal earthquake force in a short-column effect and torsion (these will be discussed

building increases downwards along its height. in subsequent IITK-BMTPC Earthquake Tips).

Roles of Floor Slabs and Masonry Walls
Floor slabs are horizontal plate-like elements,
which facilitate functional use of buildings. Usually,
beams and slabs at one storey level are cast together.
In residential multi-storey buildings, thickness of slabs Compression Gap
is only about 110-150mm. When beams bend in the
vertical direction during earthquakes, these thin slabs
bend along with them (Figure 2a). And, when beams Cracks
move with columns in the horizontal direction, the Figure 3: Infill walls move together with the
slab usually forces the beams to move together with it. columns under earthquake shaking.
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