Page 61 - EQTips_Eng

IITK-BMTPC Earthquake Tip 30

What is Important in Foundations of Earthquake-Resistant Buildings? page 2
Seismic Design of Foundations
No structure can perform well, if it does not have a Collumn Footings
good foundation supported on strata that is stable Pile Cap
during earthquakes. All principles applicable in
foundation design of structures subjected to gravity
loads, are applicable in foundation design of Liqu efiable
soil layer
earthquake-resistant structures also. Concepts of
foundation engineering, like Bearing Capacity and
Settlement Criteria, are relevant to earthquake-resistant
buildings also. Thorough geotechnical investigations
at the site are a must for most design projects. In (a)
addition to traditional Standard and Cone Penetration

Tests, other in-situ tests (e.g., Shear Wave Velocity Test Drag due to
and Pressure-meter Test) may be performed. soil layer
above

Depending on geotechnical conditions, structural liquefied layer
configuration and loads, a suitable type of foundation
must be chosen. If soil type is hard, isolated footings
may suffice under individual columns. But, these Liquefied
foundations must be tied to each other with beams at soil layer
top of footings or within the footing depth to resist
relative movement between column bases (Figure 3).
On the other hand, if soil underneath is soft, other (b)
foundation types may become necessary, e.g., raft or
pile foundations. Figure 4: Pile foundation design for liquefiable
If the site is susceptible to liquefaction, either soil – (a) before liquefaction, and (b) after
ground improvement must be undertaken or the liquefaction
foundation must be carefully designed, such that it can
carry the load even after the vulnerable soil layers Capacity Design of Foundations
have liquefied (Figure 4). For instance, in case of pile It is difficult to inspect and repair foundations
and well foundations, layers susceptible to liquefaction after a severe earthquake. Further, damage to
should be neglected in estimating stiffness and foundation can be detrimental to the stability of the
strength of the soil system. structure. Hence, in seismic design, column damage in
In case of lateral spreading, investigations beyond columns is preferred over foundation damage during
the property boundaries (lines) of the building under strong shaking. This is achieved by adopting Capacity
consideration may become necessary, especially in Design Concept (See IITK-BMTPC Earthquake Tip 9); the
when plots are small. Also, lateral thrust offered by foundation system needs to be designed for loads
liquefied soil layers must be included in estimating higher than the ultimate flexural capacity of columns
force demands on foundations. (Figure 4) or of structural walls.

Isolated Footing
Tie beam Related - Earthquake Tip
Tip 9: How to make buildings ductile for good seismic performance?

Tip 31: Why do buildings sink into the ground during earthquakes?

Resource Material
Kramer,S.L., (1996), Geotechnical Earthquake Engineering, Prentice

Hall, Inc., New Jersey, USA
Towhata,I., (2008), Geotechnical Earthquake Engineering, Springer-
Verlag, Berlin

Authored by:

C.V.R.Murty

Indian Institute of Technology Jodhpur, India
Sponsored by:

Building Materials and Technology Promotion
Council, New Delhi, India

Figure 3: Good foundation design practice in This release is a property of IIT Kanpur and BMTPC. It may

non-liquefiable soil conditions – RC tie- be reproduced without changing its contents with due

beams between columns at the top of the acknowledgement. Suggestions or comments may be sent
to: nicee@iitk.ac.in. To see all IITK-BMTPC Earthquake Tips, visit
isolated footings
www.nicee.org or www.bmtpc.org.