STEPS FOR SAFE DESIGN AND CONSTRUCTION OF MULTISTOREY REINFORCED CONCRETE BUILDINGS - PART 2
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 - 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).
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Fig.11:- Lateral
Strength of Building Frame
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All the upper floors weak in long directions (Izmit,
Turkey 1999)
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
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.
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 (see fig.13).
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 short column under lateal earthquake loading where
the shear stresses become much higher than normal length columns and fail in
shear. (See fig. 14)
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.
Fig 1 4 - Damage to buildings due to short column effect
on columns
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 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
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.
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:-
The load combinations for
analysis and design will be taken as follows:
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:
(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.
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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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.
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