Page 19 - EQTips_Eng

IITK-BMTPC Earthquake Tip 9

How to make Buildings ductile for Good Seismic Performance? page 2
Now, let us make a chain with links made of brittle Weak Beam
and ductile materials (Figure 3). Each of these links will Weak Column
fail just like the bars shown in Figure 2. Now, hold the
last link at either end of the chain and apply a force F.
Since the same force F is being transferred through all
the links, the force in each link is the same, i.e., F. As Strong
more and more force is applied, eventually the chain Beam
will break when the weakest link in it breaks. If the Strong
Column
ductile link is the weak one (i.e., its capacity to take load
is less), then the chain will show large final elongation.
Instead, if the brittle link is the weak one, then the
chain will fail suddenly and show small final
elongation. Therefore, if we want to have such a ductile Strong-Column Weak-Column
chain, we have to make the ductile link to be the Weak-Beam Strong-Beam
Design
Design
weakest link.
Figure 4: Reinforced Concrete Building Design:

Original Chain the beams must be the weakest links and not
the columns – this can be achieved by
appropriately sizing the members and providing
correct amount of steel reinforcement in them.
Brittle Links
Ductile Link
Quality Control in Construction
Loaded Chain The capacity design concept in earthquake-
resistant design of buildings will fail if the strengths of
the brittle links fall below their minimum assured
values. The strength of brittle construction materials,
F F like masonry and concrete, is highly sensitive to the

Ductile Link Brittle Links quality of construction materials, workmanship,
do not yield
stretches by supervision, and construction methods. Similarly,
yielding before special care is needed in construction to ensure that
breaking
the elements meant to be ductile are indeed provided
with features that give adequate ductility. Thus, strict
Figure 3: Ductile chain design. adherence to prescribed standards of construction
materials and construction processes is essential in
Earthquake-Resistant Design of Buildings assuring an earthquake-resistant building. Regular
Buildings should be designed like the ductile testing of construction materials at qualified
chain. For example, consider the common urban laboratories (at site or away), periodic training of
residential apartment construction - the multi-storey workmen at professional training houses, and on-site
building made of reinforced concrete. It consists of evaluation of the technical work are elements of good
horizontal and vertical members, namely beams and quality control.
columns. The seismic inertia forces generated at its
floor levels are transferred through the various beams Reading Material
and columns to the ground. The correct building Paulay,T., and Priestley,M.J.N., (1992), Seismic Design of Reinforced
components need to be made ductile. The failure of a Concrete Buildings and Masonry, John Wiley, USA
column can affect the stability of the whole building, Mazzolani,F.M., and Piluso,V., (1996), Theory and Design of Seismic-
but the failure of a beam causes localized effect. Resistant Steel Frames, E&FN Spon, UK

Therefore, it is better to make beams to be the ductile
weak links than columns. This method of designing RC Authored by:
buildings is called the strong-column weak-beam design C.V.R.Murty
method (Figure 4). Indian Institute of Technology Kanpur
By using the routine design codes (meant for Kanpur, India
design against non-earthquake effects), designers may Sponsored by:
not be able to achieve a ductile structure. Special Building Materials and Technology Promotion
design provisions are required to help designers Council, New Delhi, India
improve the ductility of the structure. Such provisions
are usually put together in the form of a special seismic This release is a property of IIT Kanpur and BMTPC New
design code, e.g., IS:13920-1993 for RC structures. Delhi. It may be reproduced without changing its contents
These codes also ensure that adequate ductility is and with due acknowledgement. Suggestions/comments
may be sent to: nicee@iitk.ac.in. Visit www.nicee.org or
provided in the members where damage is expected. www.bmtpc.org, to see previous IITK-BMTPC Earthquake Tips.
18