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- Δ 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. 1
The Building Sank evenly about 1 m
due to soil liquefaction. The displaced
soil caused a bulge in the road.
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.
(vi)Chemical analysis of water to be used in
making the Concrete mixtures.
2.3 Soft-first Storey:
Open ground storey (stilt floor) used in most
severely damaged or, collapsed R.C. buildings, introduced ‘severe irregularity of sudden
change of stiffness’ between the ground storey and upper storeys since they had infilled brick walls
which increase the lateral stiffness of the frame by a facto of three to four times. Such a building is
called a building with ‘soft’ ground storey, in which the dynamic ductility demand during the probable
earthquake gets concentrated in the soft storey and the upper storeys tend to remain elastic. Hence whereas the ‘soft’ storey is severely strained causing its total collapse, much smaller damages occurs in the upper storeys, if at all.
Fig. 4
Sway mechanisms with soft
storey ground floors (Izmit,
Turkey 1999
storey ground floors (Izmit,
Turkey 1999
Fig. 5
Soft first storey collapsed, upper
part of the building fall onto the
ground, (kachchh, 2001)
part of the building fall onto the
ground, (kachchh, 2001)
Fig. 6
Soft Storey (Open Plinth), Vertical
Split between two blocks (Bhuj)
Split between two blocks (Bhuj)
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 storey shears and moments: OR the shear walls are introduced in the stilt storey in both directions of the building which should be designed for 1.5 times the calculated storey shear forces.
Some remedial measures to counter the bad
performance are
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.
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.
Fig.9:-Floating columns
Fc= Floating Columns
Cb= Cantilever Beams
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.
2.5 Heavy Water Tanks on
the Roof:
Heavy water tanks add large lateral inertia
forces on the building frames due to the so called
‘whipping’ effect under seismic vibrations, but remain unaccounted
for in the design.
See the fall of such water tank in Fig.10
Fig.10
5 storey R.C. Collapse of open
plinth, water tank at top dislocated (Bhuj)
Recommendation:-
All projected systems above the roof top
behave like secondary elements subjected to roof level horizontal
earthquake motions which act as base motions to such projecting
systems. To account for such heavy earthquake forces,
IS:1893-2002 (Part 1) provides in clause 7.12 that their support system should be designed for five times the design horizontal
seismic co-efficient Ah specified in clause 6.4.2. Similarly any horizontal
projections as the balconies or the cantilevers supporting floating columns, the cantilevers
need to be designed for five times the design vertical co-efficient as specified in clause 6.4.5 of IS: 1893-2002 (Part 1)
2.6 Lack of Earthquake
Resistant Design:
Many buildings in Gujarat were not designed
for the earthquake forces specified in IS:1893, which
was in existence from 1962, revised in 1970, 1976 & 1984.
The applicable seismic zoning in Gujarat had remained the same
as adopted in 1970 version. It is the same even in 2002
version of IS:1893
(Part I). Inspite of that, the structural designers ignored the seismic forces in design. It may also be stated that most buildings are designed against lateral load in the transverse direction. Hence they collapse in the longitudinal directions. Proper arrangement of columns is shown in Fig. 11 which would give adequate seismic resistance along both axes of the building.
(Part I). Inspite of that, the structural designers ignored the seismic forces in design. It may also be stated that most buildings are designed against lateral load in the transverse direction. Hence they collapse in the longitudinal directions. Proper arrangement of columns is shown in Fig. 11 which would give adequate seismic resistance along both axes of the building.
Recommendation:-
It does not need emphasizing that all buildings including the multistoried buildings should be designed in accordance with IS: 1893 (Part 1) and IS: 4326 – 1993. The salient features of the design will be presented in Para 3.0 in this guide.
2.7 Improper Dimensioning
of Beams & Columns:
The structural dimensioning of beams and columns was inadequate in terms of provisions in IS: 13920-1993 and also for proper installation of reinforcements in Beam-Column joints as per IS: 456 and IS: 13920.
Recommendation:
The relative dimensions of beams & columns become very important in tall buildings from the point of view of provision of longitudinal & transverse reinforcement in the members as well as the reinforcement passing through and anchored in the beam-column joints, permitting enough space for
proper concreting and without involving any local kinking of the reinforcing bars. The practice of using small dimension columns like 200 or 230 mm and beams of equal width is totally unacceptable
from the reinforcement detailing view point. Infact for permitting the beam bars
passing through the columns, without any local bending then straightening (introducing kinks), the
proper scheme would be to use wider columns than the beams. Minimum dimensions of beams and
columns, also limiting aspect ratios of the two members, are specified in IS: 13920 which need to be
adhered to.
2.8 Improper Detailing of
Reinforcement:
In detailing the stirrups in the columns, no
conformity appeared to satisfy lateral shear requirements in the concrete of the joint as required
under IS 4326- 1976 and IS: 13920-1993. The shape and spacing of stirrups seen in collapsed and
severely damaged columns with buckled reinforcement was indicative of non-conformity even with the
basic R.C. Code IS: 456-1978.
Fig.13
Plan of Reinforcement in Beam & Column
Recommendation:
In respect of proper detailing of
reinforcement in beams, columns, beam column joints as well as shear walls, all the provisions in IS:13920
have to be carefully understood and adopted in design. The philosophy of over-design of beams in shear
to force flexural hinge formation before shear failure, confining of
highly compressed concrete in columns and the use of properly shaped shear stirrups with 135 degree
hooks are
some low-cost but extremely important provisions. For overall safety of the frame, design based on the
concept of strong-column,
weak-beam system
should be adopted as far as practical. It may be
mentioned that the full ductility details as specified in IS:
13920 permit the use of the High Reduction
Factor R=5 which would make the design economical. But if such ductility details are not
adopted, the Reduction Factor is permitted as only 3.0, which means that the design force will become 1.67
times the case when full ductile detailing is adopted which may indeed turnout to be more expensive
and at the same time brittle and relatively unsafe
Fig.13:- Detailing of reinforcement (Overlapping, Hoops
& Crosstie)
2.9 Short Column Detailing
In some situations the column is surrounded
by walls on both sides such as upto the window sills and
then in the spandrel portion above the windows but it
remains exposed in the height of the windows. Such a column
behaves as a
Recommendation:
To safe guard against this brittle shear
failure in such columns the special confining stirrups should
be provided throughout the height of the column at short
spacing as required near the ends of the columns.
2.10 Torsional Failures
Torsional failures are seen to occur where
the symmetry is not planned in the location of the lateral
structural elements as for example providing the lift cores at
one end of the building or at one corner of the building or unsymmetrically planned buildings in L shape at the street corners. Large torsional shears
are caused in the building columns causing there torsional shear failures (See fig.15).
Fig.15:-
Very unsymmetrical building
Recommendation:
Where site requirements of from functional requirements control the building plan shape, either it should be split into two
symmetrical rectangular blocks by providing separation sections of appropriate with between the blocks or the structural elements should be
so adjusted that the centre of stiffness and the centre of mass should coincide along both
axis of the building needless to say that any
non-coincidence of the centre of mass and centre of stiffness should be taken into design calculations as
per IS:1893
2.11 Pounding Damage of
Adjacent Buildings
Severe damage even leading to collapse are seen due to severe impact between two adjacent buildings under earthquake shaking
if the adjacent blocks of a building or two adjacent buildings are of different heights
with floors at different levels and with
inadequate separation. Such buildings can vibrate out of phase with each other due to very different natural frequencies thus hitting each other quite severely (see fig.16).
Fig.16:-
Pounding damage of adjacent buildings
Recommendation:
To avoid such pounding damage the amount of
separation between them should be liberally provided so as to cater for the combined
maximum out of phase displacements. A simple recommendation is given in IS:4326 (Cl.5.1.2)
for flexible as well as stiff buildings which must be adopted as a minimum to avoid the possibility
of pounding between two unsimilar buildings/blocks
2.12 Lack of Stability of
Infill Walls
The infill walls were not properly attached
either to the column or the top beams for stability
against out-of-plane bending under horizontal
earthquake
Recommendation:
Stability of infill walls is important in two
ways: first, they introduce their brittle failure
due to the diagonal compression in the panel and or
diagonal tension cracking; secondly, and more
important is their lateral stability under out of plane
earthquake force acting on their own mass. While
conducting the retrofitting studies of three lifeline buildings in Delhi, the 114 mm thick brick infill
walls have
turned out to be one of the main issues to handle while retrofitting the building so as to save
the inmates and the property inside from damage due to the failure of the infill walls. It has been
found that such walls will have to be contained with in pairs of vertical angles spaced at 1.2 – 1.5 m
apart. Therefore, while designing a new multistoried building, the stabilisation of the infill
wall panels should be properly considered either by providing confining angles near the top or by providing
slits on the vertical sides and stabilising by the means of vertical angles or channels.
2.13 Poor Construction
Quality:
The construction quality of the damaged R.C.
buildings was found to be much below that desired, as seen by the cover to reinforcement in the
damaged members and the bad quality of concrete in the columns in 150 to 300 mm length just below
the floor beams and within the beam column joints.
Recommendation:
Needless to say that if the quality of
construction is not commensurate with the quality of design, even a well planned and a well designed
building can show extremely bad behavior under earthquake shaking. It should be remembered
that during earthquake shaking all bad quality constructions will be revealed and nothing
can be kept hidden. Good quality of construction will include: proper mixing and quantity of water, good
quality sand and aggregates, designed quantity of cement in the mix,
proper mixing of all the ingredients with control on water cement ratio, adequate compaction in the
placement of concrete preferably by using vibrators, proper placement of steel with control on
the cover to steel and adequate curing before striking of the form work. The engineer incharge of the construction should
personally be present at site to supervise all operations. He should have appropriate sampling and
testing of materials carried out in a recognized laboratory, the results of test being kept in well
maintained register for inspection by quality audit team. He should organize the taking of sample of steel
reinforcement & concrete cubes in adequate numbers which should be tested at the specified age
of testing.
3. Some Important Codal
Design Provisions:
In the last few years the author has had the
opportunity of reviewing many reinforced concrete building designs prepared by well-established
consulting companies as well as individual 9 consultants and felt the need of preparing
brief guidelines so that no important Codal provisions are missed out and the various design details for
achieving better construction in the field and better ductile performance in the event of a great
earthquake are ensured. Thus a safe and ductile building
could be achieved.
3.1 Building Configuration
For achieving basic structural safety of
buildings under postulated earthquake forces the first important requirement is that the building
should be designed with symmetrical configuration both horizontally and vertically. In any case the
seismic force resisting elements must be planned symmetrically about the centre of the mass of
the building. IS:1893 (Part 1-2002) presents in detail in cl.7.1 the various types of irregularities
which should be avoided as far as possible or corrected by planning the structural resisting elements.
The present day requirements of large column free spaces inside can be met by designing strong frames
on the periphery of the building so as to resist most of the horizontal design seismic forces and
relieving the internal columns relatively from the earthquake forces. For this purpose shear
walls may be provided in the building perimeter to increase the stiffness in both principal axes
of the building as compared with the internal columns which could be designed basically for
vertical loads.
3.2 Calculation of Loads
The loads will include the following:
(i) Dead Loads: These will include the weight of all
components at each level, viz., roof including water tanks, Barsatis, Parapets, roof finishes,
slabs, beams, elevator machine room etc. and including all plasters and surface
cladding etc., and each floor level including fixed masonry or other partitions, infill walls,
columns, slabs and beams, weight of stairs, cantilever balconies, parapets and plastering
or cladding wherever used. The unit weights may be taken from IS:875 (Part 1) or
ascertained from the manufacturer.
(ii) Imposed Floor Loads: IS 875 (Part 2) deals
with the imposed loads on roofs, floors, stairs, balconies, etc., for various occupancies.
There is a provision for reduction in the imposed loads for certain situations, e.g. for large
span beams and number of storeys above the columns of a storey. The earthquake code IS:
1893 (Part 1)-2002 permits general reduction in roof and floor imposed load when
considering the load combination with the earthquake loading. But the two types of reductions,
that is, in IS: 875 (Part 2) and IS: 1893 (Part 1) are not to be taken together.
3.3 The Earthquake Load:
For working out the earthquake loading on a
building frame, the dead load and imposed load and weights are to be lumped at each column top
on the basis of contributory areas. The imposed load is to be reduced as specified in IS: 1893
(Part1)-2002 for seismic load determination. Let us call them Wi at ith floor and Wn at the nth level at
the roof level for a n-storey building. Hence the total load at the base of the building just above the
foundation will be n
W = Σ i=1 W i + Wo
where Wo is the weight of elements in the
ground storey.
3.4 Earthquake Resistant
Design
Now the following steps may be taken:
(a) Estimate fundamental time period Ta using
empirical expressions given in the Code IS: 1893-2002.
Ta = 0.075 H0.75, IS: 1893 Cl.7.6.1 for bare
frame along each axis
Tax = 0.09h/√d along x-axis IS: 1893 Cl.7.6.2
for frame with substantial infills
Ta z = 0.09h/√b, along z-axis, IS: 1893
Cl.7.6.2 for frame with substantial infills
where h is the height of the building and d
and b are the base dimensions of the building along x and z axis respectively.
(b) Calculate the design horizontal Seismic
coefficient Ah
Now compute the fundamental time periods T/x and T/z for the bare frame along the two axes by dynamic analysis. These are generally found
to be higher than Tax and Taz respectively.
The design horizontal coefficient Ah is given
by
Ah = (Z/2). (I/R). (Sa/g)
Take Z for the applicable seismic zone (IS:
1893 Cl.6.4.2),
Take I for the use importance of the building
(IS: 1893 Table 2),
Take R for the lateral load resisting system
adopted (IS: 1893 Table 7),
and take Sa/g for the computed time period
values T/x, Tax, T/z and Taz with 5% damping coefficient using the response spectra curves
IS: 1893 Fig 2 for the soil type observed. Thus four values of Ah will be determined as follows:-
In x-direction A/hx for T/x & Ahax for Tax
In z-direction A/hz for T/z & Ahaz for Taz
(c) Calculate the total horizontal shear (the
base shear)
The design value of base shear VB
VB = Ah W
as per 1893 Cl.7.5.3.
For design of the building and portions
thereof, the base shear corresponding to higher of Ahax and A/hx, similarly between Ahaz and A/hz will be taken as minimum design lateral
force.
(d) Seismic Moments and Forces in Frame
Elements:
Calculate the seismic moments and axial
forces in the columns, shears and moments in the beams by using the seismic weights on the
floors/(column beam joints) through an appropriate computer software (having facility for using
floors as rigid diaphragm and torsional effects as per IS: 1893:2002).
It may be performed by Response Spectrum or
Time History analysis. The important point is that according to IS: 1893 Cl.7.8.2., the
base shear computed in either of the dynamic method, say V/B shall not be less than VB calculated under
Cl.7.5.3 using Ahax and Ahaz. If so, then all shears, moments, axial forces etc worked out
under dynamic analysis will be increased proportionately, that is, in the ratio of VB/V/B.
(e) Soft Ground Storey
It must be designed according to Cl.7.10 of
IS: 1893-2002.
4. Method of Design
Structural design of various members has to
be done by Limit State Method, as per IS 456-2000 for which the following load combinations should
be used to work out the maximum member forces:-
Using
DL for DEAD LOAD
LL for LIVE LOAD
EQX for SEISMIC LOAD (X) DIRECTION
EQZ for SEISMIC LOAD (Z) DIRECTION
The load combinations for analysis and design
will be taken as follows:
1. (DL+LL)*1.5
2. (DL+LL+EQX)*1.2
3. (DL+LL+EQZ)*1.2
4. (DL+LL-EQX)*1.2
5. (DL+LL-EQZ)*1.2
6. (DL+EQX)*1.5
7. (DL+EQZ)*1.5
8. (DL-EQX)*1.5
9. (DL-EQZ)*1.5
10. 0.9DL+EQX*1.5
11. 0.9DL+EQZ*1.5
12. 0.9DL-EQX *1.5
13. 0.9DL-EQZ*1.5
The members (beams, columns, shear walls
etc.) and their joints will be designed for the worst combination of loads, shears and moments.
MATERIALS:
a) Cement: Ordinary portland cement conforming to IS 269
- 1976 shall be used along with fly ash after carrying out the design mix from
approved consultant.
b) Reinforcement: Cold twisted high yield
strength deformed bars grade Fe 415 conforming to IS:1786-1985, or preferably TMT bars of standard
manufacturer e.g. TATA Steel, SAIL or equivalent shall be used.
The following grades of concrete mix may be
adopted or as required for safe design:
(a) For RCC columns in lowest few storeys :
M35
(b) For RCC columns in the middle few storeys
: M30
(c) For RCC columns in the top few storeys :
M25
(d) For beams, slabs, staircase etc. : M20
(e) For raft foundation : M 20 or 25
(f) Max. Water cement Ratio : 0.45
(g) Minimum cement content : 300 kg/m3 of
concrete.
(h) Admixtures of approved brand may be used
as per mix design
CLEAR COVER TO ALL
REINFORCEMENT:
For mild Exposure and fire rating of 1 hr.
following clear covers may be adopted
(a) For foundation R.C.C.:
i) Footings : 60 mm.
12
ii) Raft : 60 mm.
(b) For columns : 40 mm
(c) For Beams : 25 mm or main bar dia.
whichever is more.
(d) For Slab : 20 mm.
4.1 Ductile Detailing
After designing the frame column-beam, shear
walls and foundation by limit state theory as per IS: 456:2000, all details of longitudinal
steel, overlaps, shear capacities, confining reinforcement requirements, stirrups and ties etc. shall be
worked out using the provisions of IS: 13920-1993.
The drawings should clearly
show all the adopted details.
5. Concluding Remarks
In a nut-shell, the seismic safety of a
multi-storeyed reinforced concrete building will depend upon the initial architectural and structural
configuration of the total building, the quality of the Structural analysis, design and reinforcement detailing
of the building frame to achieve stability of elements and their ductile performance under severe
seismic lading. Proper quality of construction and stability of the infill walls and partitions are
additional safety requirements of the structure as a whole. Any weakness left in the structure, whether in
design or in construction will be fully revealed during the postulated maximum considered earthquake for
the seismic zone in the earthquake code IS: 1893.
Acknowledgement:
The figures have been taken from various
sources to suit the text message and are anonymously
acknowledged.
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