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Learning

Earthquake Design
29 and
Earthquake Tip
Construction

What are the Essential Features of Confined Masonry Houses?

Scope of Construction of Buildings

with Confined Masonry

Confined Masonry (CM) system is suitable for

construction of low- and medium-rise buildings, and can
be implemented with relatively lower engineering

inputs and technical supervision. Some countries
permit construction of CM houses of up to six storeys

tall, supported by design codes and engineering inputs.

CM system is not intended for use in the construction of
commercial buildings or large span buildings.
Low Moderate High
Architectural Guidelines
Seismic Zone Seismic Zone Seismic Zone
Architectural configuration critically determines
(a)
overall earthquake behaviour of CM houses.
(a) Overall Geometry

The overall shape of houses should be regular in B L < 4B
both plan and elevation. Best performance is achieved
in CM buildings, when, in plan, they (i) are L
rectangular, (ii) have no significant re-entrant corners
and no interior cut-outs, and (iii) have plumb vertical (b)
walls along the entire building perimeter (Figure 1a). Figure 2: Preferred geometry of CM houses –
Buildings with re-entrant corners, cut-outs, vertical (a) maximum height, and (b) plan aspect ratio
offsets and large cantilever overhangs (Figure 1b)
perform poorly during earthquakes.
W
Plan aspect ratio, i.e., ratio of longer and shorter
plan dimension (L/B), of a CM house should not
exceed 4 (Figure 2); seismic gap can be used in W
buildings with elongated plan shapes. Typical storey
height should not exceed about 3m. Even though
W
buildings up to 6 storeys high have been built in high D
seismic regions in Mexico, Chile and Peru, the
D
recommended maximum number of storeys is 4, 3 and
Figure 3: Plan view of CM house showing tie-
2 in seismic zones III, IV and V of India, respectively
(for now as design codes are unavailable) (Figure 2). columns at all wall junctions – no wall panel
is left without a tie-column at each end in plan

Openings in walls deteriorate earthquake
performance of CM buildings. Hence, openings should

ideally be kept to a minimum in number and size.
Larger openings must be confined by cast-in-situ RC

elements all around, otherwise a wall with large

openings is considered as unconfined. Openings in all

storeys must be positioned at the same location along
(a) (b)
the height of the building.
Figure 1: Overall geometry features – (a)
desirable regular building, and (b) undesirable Structural Guidelines
irregular building Wall thickness required in a building will depend
on loads and masonry strength. This should be
(b) Wall Configuration carefully checked. Masonry walls should be at least
Walls in a CM house should be aligned preferably 110 mm thick; their height should not exceed 25 times
on a rectangular grid that is symmetrical in plan thickness. Storey height should not exceed 3 m, even if
(Figure 3). Further, all walls must run through from thicker walls are used. Ideally, all masonry walls
the base of the building to the roof, without being should be fully-confined with RC tie-columns on both
terminated at any intermediate level. sides of openings and at wall intersections, and RC tie-