Page 35 - EQTips_Eng

IITK-BMTPC Earthquake Tip 17

How do Earthquakes affect Reinforced Concrete Buildings? page 2
Horizontal Earthquake Effects are Different (which receive forces from columns) should be
Gravity loading (due to self weight and contents) on stronger than columns. Further, connections between
buildings causes RC frames to bend resulting in beams & columns and columns & foundations should
stretching and shortening at various locations. Tension not fail so that beams can safely transfer forces to
is generated at surfaces that stretch and compression columns and columns to foundations.
at those that shorten (Figure 4b). Under gravity loads, When this strategy is adopted in design, damage is
tension in the beams is at the bottom surface of the likely to occur first in beams (Figure 5a). When beams
beam in the central location and is at the top surface at are detailed properly to have large ductility, the
the ends. On the other hand, earthquake loading causes building as a whole can deform by large amounts
tension on beam and column faces at locations despite progressive damage caused due to consequent
different from those under gravity loading (Figure 4c); yielding of beams. In contrast, if columns are made
the relative levels of this tension (in technical terms, weaker, they suffer severe local damage, at the top and
bending moment) generated in members are shown in bottom of a particular storey (Figure 5b). This localized
Figure 4d. The level of bending moment due to damage can lead to collapse of a building, although
earthquake loading depends on severity of shaking columns at storeys above remain almost undamaged.
and can exceed that due to gravity loading. Thus, Damage
under strong earthquake shaking, the beam ends can Large Small
develop tension on either of the top and bottom faces. displacement displacement
at collapse
at collapse
Since concrete cannot carry this tension, steel bars are
required on both faces of beams to resist reversals of Damage
bending moment. Similarly, steel bars are required on distributed All damage
in one
in all
all faces of columns too. storeys storey
Strength Hierarchy
For a building to remain safe during earthquake
shaking, columns (which receive forces from beams) (a) Strong Columns, (b) Weak Columns,
should be stronger than beams, and foundations Weak Beams Strong Beams
Figure 5: Two distinct designs of buildings that
result in different earthquake performances –
Gravity Earthquake columns should be stronger than beams.
Load Load
Relevant Indian Standards
The Bureau of Indian Standards, New Delhi,
(a) published the following Indian standards pertaining to
design of RC frame buildings: (a) Indian Seismic Code
(IS 1893 (Part 1), 2002) – for calculating earthquake forces,
Stretching of member (b) Indian Concrete Code (IS 456, 2000) – for design of
and locations of tension Tension RC members, and (c) Ductile Detailing Code for RC

Structures (IS 13920, 1993) – for detailing requirements in
seismic regions.

Related - Earthquake Tip
Tip 5: What are the seismic effects on structures?
Tension
Reading Material
(b) (c) Englekirk,R.E.,(2003), “Seismic Design of Reinforced and Precast
Concrete Buildings”, John Wiley & Sons, Inc., USA
Penelis,G.G., and Kappos,A.J., (1997), “Earthquake Resistant Concrete
Amount of Structures,” E&FN SPON, UK
tension
Authored by:
C.V.R.Murty
Indian Institute of Technology Kanpur
Kanpur, India
Sponsored by:
Building Materials and Technology Promotion
Council, New Delhi, India

(d) This release is a property of IIT Kanpur and BMTPC New
Figure 4: Earthquake shaking reverses tension Delhi. It may be reproduced without changing its contents

and compression in members – reinforcement is
and with due acknowledgement. Suggestions/comments
required on both faces of members. 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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