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IITK-BMTPC Earthquake Tip 21

Why are Open-Ground Storey Buildings vulnerable in Earthquakes? page 2
structure. The Code suggests that the forces in the
columns, beams and shear walls (if any) under the
action of seismic loads specified in the code, may be
obtained by considering the bare frame building
(without any infills) (Figure 4b). However, beams and
columns in the open ground storey are required to be
designed for 2.5 times the forces obtained from this
bare frame analysis.
For all new RC frame buildings, the best option is to
avoid such sudden and large decrease in stiffness
and/or strength in any storey; it would be ideal to
build walls (either masonry or RC walls) in the ground
storey also (Figure 5). Designers can avoid dangerous
Photo Courtesy: The EERI Annotated Slide Set CD, Earthquake
Engineering Research Institute, Oakland (CA), USA, 1998. effects of flexible and weak ground storeys by
(a) 1971 San Fernando Earthquake ensuring that too many walls are not discontinued in
the ground storey, i.e., the drop in stiffness and
strength in the ground storey level is not abrupt due to
the absence of infill walls.
The existing open ground storey buildings need to be
strengthened suitably so as to prevent them from
collapsing during strong earthquake shaking. The
owners should seek the services of qualified structural
engineers who are able to suggest appropriate
solutions to increase seismic safety of these buildings.

(b) 2001 Bhuj Earthquake

Figure 3: Consequences of open ground storeys
in RC frame buildings – severe damage to
ground storey columns and building collapses.

Direct flow of forces
The Problem through walls
Open ground storey buildings are inherently poor
systems with sudden drop in stiffness and strength in
the ground storey. In the current practice, stiff masonry Figure 5: Avoiding open ground storey problem –

walls (Figure 4a) are neglected and only bare frames are continuity of walls in ground storey is preferred.
considered in design calculations (Figure 4b). Thus,
the inverted pendulum effect is not captured in design.
Related - Earthquake Tip
Infill walls not Tip 6: How Architectural Features Affect Buildings During
Earthquakes?
considered in Tip17: What are the Earthquake Effects on Reinforced Concrete
upper storeys
Buildings?

Reading Material
IS 1893(Part 1) (2002), “Indian Standard Code of Practice for Criteria for
Design of Earthquake Resistant Structures,” Bureau of Indian

(a) Actual building (b) Building being assumed Standards, New Delhi
in current design practice

Figure 4: Open ground storey building – Authored by:
assumptions made in current design practice C.V.R.Murty
are not consistent with the actual structure. Indian Institute of Technology Kanpur

Kanpur, India
Improved design strategies Sponsored by:
After the collapses of RC buildings in 2001 Bhuj Building Materials and Technology Promotion
earthquake, the Indian Seismic Code IS:1893 (Part 1) - Council, New Delhi, India
2002 has included special design provisions related to
soft storey buildings. Firstly, it specifies when a This release is a property of IIT Kanpur and BMTPC New
building should be considered as a soft and a weak Delhi. It may be reproduced without changing its contents
storey building. Secondly, it specifies higher design and with due acknowledgement. Suggestions/comments
may be sent to: nicee@iitk.ac.in. Visit www.nicee.org or
forces for the soft storey as compared to the rest of the www.bmtpc.org, to see previous IITK-BMTPC Earthquake Tips.

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