STEPS FOR SAFE DESIGN AND CONSTRUCTION OF MULTISTOREY REINFORCED CONCRETE BUILDINGS - PART 1
1.
Introduction:
A large number of reinforced concrete
multistoreyed frame buildings were heavily damaged and many of them collapsed
completely in Bhuj earthquake of 2001 in the towns of Kachchh District (viz.,
Bhuj, Bhachao, Anjar, Gandhidham and Rapar) and other district towns including
Surat and Ahmedabad. In Ahmedabad alone situated at more than 250 kilometers
away from the Epicentre of the earthquake, 69 buildings collapsed killing about
700 persons. Earlier, in the earthquake at Kobe (Japan 1995) large number of
multistoreyed RC frame buildings of pre 1981 code based design were severely
damaged due to various deficiencies. Such behaviour is normally unexpected of
RC frame buildings in MSK Intensity VIII and VII areas as happened in Kachchh
earthquake of January 26, 2001. The aim of this paper is to bring out the main
contributing factors which lead to poor performance during the earthquake and
to make recommendations which should be taken into account in designing the
multistoreyed reinforced concrete buildings so as to achieve their adequate
safe behaviour under future earthquakes. The Indian Standard Code IS:1893 was
suitably updated in 2002 so as to address the various design issues brought out
in the earthquake behaviour of the RC Buildings. The paper highlights the main
provisions of this code.
2.
Causes of the Collapse of RC Frame Buildings and
Recommendations
2.1
Ignorance of the Architects and
Structural Engineers about the Contents of the relevant earthquake resistant
Building Codes :
Recommendation:-
The following BIS Standards will be
mainly required for the design of RCC Buildings. Architect’s and Structural
engineer’s design office should have the current copies of these standards
available in their offices and all their staff should fully familiarize with
the contents of these codes:-
1.
IS:
456 -2000 “Code of Practice for Plain and Reinforced Concrete”
2.
IS:
875 Part 1 “Unit weights of materials”.
3.
IS:
875-1987Design loads ( other than earthquake ) for buildings and structures,
Part2 Imposed Loads
4.
IS:
875-1987Design loads ( other than earthquake ) for buildings and structures
,Part 3 Wind Loads
5.
IS:
1904-1987 “Code of Practice for Structural Safety of Buildings: Foundation”
6.
IS:
1498-1970 Classification and identification of soils for general engineering
purposes (First Revision)
7.
IS:
2131-1981 Method of Standard Penetration Test for soils (First Revision)
8.
IS:
1905-1987, Code of Practice for Structural Safety of Buildings: Masonry
9.
IS:1893(Part-I)-2002 "Criteria for
Earthquake Resistant Design of Structures (Fifth Revision)”.
10. IS:13920-1993,
"Ductile Detailing of Reinforced Concrete Structures subjected to Seismic
Forces - Code of Practice"
11. IS: 4326-1993,
"Earthquake Resistant Design and Construction of Buildings - Code of
Practice (Second Revision)"
12. IS-NBC-2005:
National Building Code of India.
Note:
The design offices should keep in touch with BIS-CE division to keep track of
any amendments issued or further revisions.
2.2 Softness of
Base Soil:
The soft soil on which most buildings in Ahmedabad
were founded would have affected the response of the buildings in three ways:
(i)
Amplification of the ground motion at the base of
the building;
(ii)
Absence of foundation raft or piles;
(iii)Relative
displacement between the individual column foundations vertically and
laterally, in the absence of either the foundation struts as per IS:
4326 or the plinth beams;
(iv) Resonance or,
semi-resonance of the whole building with the long period ground waves;
(v)
In the absence of the beam at plinth or, ground
level, the length of ground storey columns gets increased, which increases the
flexibility of the ground storey and if the columns become ‘long’ the buckling
moments due to P- effect will increase bonding to cause collapse of the
columns.
(vi)
If the soil is sandy and water
table is high, it may liquify. See IS:1893-2002 Cl 6.3.5.2 and Table 1 for
minimum N (corrected values) for safety and carryout soil liquefaction analysis
by standard procedures available in the literature. The adverse effects of
liquefaction may be seen in Figs. 1, 2 & 3.
Fig.2 - This inclined building sank unevenly and leans against a
neighbouring building
Fig.3 - The
solid building tilted as a rigid body and the raft foundation rises above the
ground
Recommendation:-
Soil exploration at the buildings site must be
carried out at sufficient points and to sufficient depth so as to give the
following data:
(i)
Soil classification in various layers
and the properties like grain size distribution, fields density, angle of
internal fritting and cohesion a plastic and liquid limits and coefficient of
consolidation of cohesive sites.
(ii)
Position
of water table just before and just after monsoon.
(iii)SPT values
and CPT values.
(iv)
The output results should include
liquefaction potential, safe bearing capacity and the type of foundation to be
adopted, viz. (i) individual column footing of given width (ii) combined row
footing or (iii) raft foundation or (iv) Pile foundations.
(v)
Chemical
analysis of soil to find if it has any harmful elements to the concrete, if so,
precautions to be taken in making the foundations.
Chemical
analysis of water to be used in making the Concrete mixtures
2.3 Soft-first
Storey:
Fig.4 Fig.5 Fig.6
Fig.4 - Sway mechanisms with soft storey ground floors
(Izmit, Turkey 1999
Fig.5 - Soft first storey collapsed, upper part of the building
fall onto the ground, (kachchh, 2001)
Fig.6 -Soft Storey (Open Plinth), Vertical Split between two
blocks (Bhuj)
Some
times a soft storey is created some where at mid-height of the multi-storey
building, for using the space as restaurant or gathering purposes, see fig.8.
Such soft storey in building also collapsed in Kutch and Kobe earthquakes. For
such a case also, the storey columns should be designed for the higher forces
OR a few shear walls introduced to make up for the reduced stiffness of the
storey
Recommendation:-
In view of the
functional requirements of parking space under the buildings, more and more
tall buildings are being constructed with stilts. To safeguard the soft first
storey from damage and collapse, clause 7.10 of IS: 1893-2002 (Part 1)
provides two alternative design approaches
(i)
The dynamic analysis of the building is to be
carried out which should include the strength and stiffness effects of infills
as well as the inelastic deformations under the design earthquake force
disregarding the Reduction Factor R.
(ii)
The building is analysed as a bare
frame neglecting the effect of infills and, the dynamic forces so determined in
columns and beams of the soft (stilt) storey are to be designed for 2.5 times
the
Some remedial measures to counter the
bad performance are shown in Fig. 7.
2.4 Bad
Structural System:
The structural system adopted using
floating columns, for reasons of higher FSI is very undesirable in earthquake
zones of moderate to high intensity as in Zone III, IV & V since it will
induce large vertical earthquake forces even under horizontal earthquake ground
motions due to overturning effects.
Recommendation:-
The structural
engineer should provide for the load path in the building from roof to
the foundation. For example, a building with floating columns requires transfer
of the floating column loads to horizontal cantilever beams through shear
forces. The load path, therefore, is not vertical but changes from
vertical to horizontal members before reaching the foundation. Sometimes
similar situations arise within the frames where, for any reason, either the
beam is missing or a column is missing. These are structural discontinuities
and should better be avoided as far as possible. Other irregularities such as
those defined in Table 4 & 5 of IS: 1893-2002 (Part 1) become the cause for
large torsional moments and stress concentration in the buildings
which should better be avoided by the architect and structural engineer in
the initial planning of the building configuration. Otherwise, they should be
carefully considered in structural analysis and properly detailed in the
structural design.
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