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IITK-BMTPC Earthquake Tip 12

How do Brick Masonry Houses behave during Earthquakes? page 2
How to Improve Behaviour of Masonry Walls Choice and Quality of Building Materials
Masonry walls are slender because of their small Earthquake performance of a masonry wall is very
thickness compared to their height and length. A sensitive to the properties of its constituents, namely
simple way of making these walls behave well during masonry units and mortar. The properties of these
earthquake shaking is by making them act together as materials vary across India due to variation in raw
a box along with the roof at the top and with the materials and construction methods. A variety of
foundation at the bottom. A number of construction masonry units are used in the country, e.g., clay bricks
aspects are required to ensure this box action. Firstly, (burnt and unburnt), concrete blocks (solid and
connections between the walls should be good. This hollow), stone blocks. Burnt clay bricks are most
can be achieved by (a) ensuring good interlocking of commonly used. These bricks are inherently porous,
the masonry courses at the junctions, and (b) and so they absorb water. Excessive porosity is
employing horizontal bands at various levels, detrimental to good masonry behaviour because the
particularly at the lintel level. Secondly, the sizes of bricks suck away water from the adjoining mortar,
door and window openings need to be kept small. The which results in poor bond between brick and mortar,
smaller the openings, the larger is the resistance and in difficulty in positioning masonry units. For this
offered by the wall. Thirdly, the tendency of a wall to reason, bricks with low porosity are to be used, and
topple when pushed in the weak direction can be they must be soaked in water before use to minimise
reduced by limiting its length-to-thickness and height- the amount of water drawn away from the mortar.
to-thickness ratios (Figure 3). Design codes specify Various mortars are used, e.g., mud, cement-sand,
limits for these ratios. A wall that is too tall or too long or cement-sand-lime. Of these, mud mortar is the
in comparison to its thickness, is particularly weakest; it crushes easily when dry, flows outward
vulnerable to shaking in its weak direction (Figure 3). and has very low earthquake resistance. Cement-sand
mortar with lime is the most suitable. This mortar mix
provides excellent workability for laying bricks,
stretches without crumbling at low earthquake
shaking, and bonds well with bricks. The earthquake
response of masonry walls depends on the relative
Overturning strengths of brick and mortar. Bricks must be stronger
than mortar. Excessive thickness of mortar is not
Overturning desirable. A 10mm thick mortar layer is generally
satisfactory from practical and aesthetic
considerations. Indian Standards prescribe the
preferred types and grades of bricks and mortars to be
used in buildings in each seismic zone.
Soil Soil
Thick Wall (1½ brick) Short Wall (1 brick) Related Earthquake Tip
versus versus Tip 5: What are the seismic effects on structures?
Thin Wall (1 brick) Tall Wall (1 brick) Reading Material

IS 1905, (1987), Indian Standard Code of Practice for Structural Use of
Unreinforced Masonry, Bureau of Indian Standards, New Delhi
Inertia force IS 4326, (1993), Indian Standard Code of Practice for Earthquake Resistant
Large portion of wall from roof

not supported by Design and Construction of Buildings, Bureau of Indian Standards,
cross walls New Delhi

IS 13828, (1993), Indian Standard Guidelines for Improving Earthquake
Resistance of Low-strength Masonry Buildings, Bureau of Indian
Cross Wall Standards, New Delhi
Paulay,T., and Priestley,M.J.N., (1992), Seismic Design of Reinforced
Cross Wall Long Wall
Concrete and Masonry Buildings, John Wiley & Sons, USA

Authored by:
C.V.R.Murty
Indian Institute of Technology Kanpur
Kanpur, India
Sponsored by:
Short Wall Building Materials and Technology Promotion
Good support offered Council, New Delhi, India
by cross walls

Figure 3: Slender walls are vulnerable – height This release is a property of IIT Kanpur and BMTPC New
and length to be kept within limits. Note: In this Delhi. It may be reproduced without changing its contents
figure, the effect of roof on walls is not shown. and with due acknowledgement. Suggestions/comments
may be sent to: nicee@iitk.ac.in. Visit www.nicee.org or
www.bmtpc.org, to see previous IITK-BMTPC Earthquake Tips.
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