IS 1893:2002 CRITERIA FOR EARTHQUAKE RESISTANT DESIGN OF
STRUCTURES PART1 GENERAL PROVISIONS AND BUILDINGS The Code is now
split into five parts Part Part Part Part Part 1 2 3 4 5 General
provisions and buildings Liquid retaining tanks - Elevated and
ground supported Bridges and retaining walls Industrial structures
including stack like structures Dams and embankments
Part 1 contains provisions that are general in nature and
applicable to all structures. Also, it contains provisions that are
specific to buildings only. The important changes as compared to
IS:1893-1984 are as follows: 1. Seismic zone map is revised with
only four seismic zones. Zone I is upgraded to Zone II. Killari
area is enhanced to Zone III. Bellary isolated zone is removed.
East coast is enhanced to Zone III and connected with Zone III of
Godavari Graben area.
2. Seismic zone factor is changed reflecting more realistic
value of peak ground acceleration.
3. Response acceleration spectra are now specified for three
types of founding strata viz. Hard, Medium and Soft. 4. The
empirical formula for calculating fundamental natural period T=0.1n
for moment resisting frames without bracing or shear walls is
replaced with Ta=0.075h0.075 for RC framed buildings. This formula
applies to bare frames e.g. in industrial plant buildings. The
formula for framed buildings with in-filled masonry walls is Ta =
0.09h/d0.5 where h and d are the height and base dimension of the
building along the considered direction of earthquake. 5. Revised
procedure first calculates the actual force that may be experienced
by the structure during the probable maximum earthquake, if it were
to remain elastic. Then response reduction due to ductile
deformation or frictional energy dissipation in the cracks is
applied via `response reduction factor' R in place of the earlier
performance factor K. The list of building systems and the
corresponding values of R is more exhaustive. The code procedures
for calculating base shear VB are summarized below: IS:1893-1984
-----------VB = K.C. h.W where K = Performance factor 1.0 for SMRF
(IS:4326 detail) and 1.6 for OMRF (IS:456 detail) C = Fundamental
time period dependant coefficient = 1.0 for T 0.35 sec h = .I.
0
= Soil-foundation system dependant coefficient = 1.2 for
isolated footings without tie beams in medium soils, piles in soft
soils, combined or isolated footings with tie beams in soft soils =
1.5 isolated footings without tie beams in soft soils I =
Importance factor = 1.5 for hospitals, schools, cinema halls,
monumental structures, telephone exchanges, radio, fire, railway
power stations and 1.0 for others 0 = Zone dependant design seismic
coefficient Zone 0 II 0.02 III 0.04 IV 0.05 V 0.08
W = Seismic weight of building = Dead load + appropriate amount
of live load = Dead load + 25% for LL up to 3 kN/sq.m 50% for LL
> 3 kN/sq.m
0% for LL on roof
IS:1893-2002 -----------VB = Ah.W [Z/2].[Sa/g] =
-----------[R/I] = Zero period acceleration value for the Maximum
Considered Earthquake II 0.10 III 0.16 IV 0.24 V 0.36 coefficient
for Hard, Medium or Soft soil, 5% 1.00/T for T > 0.40 (Hard:
GP,GW,SP,SW,SC with 1.36/T for T > 0.55 (Medium: All with 10
1.2*average storey drift ii) Re-entrant corners : if projection
beyond re-entrant corner is > 15% of plan dimension in that
direction iii) Diaphragm discontinuity: if open areas > 50% of
gross enclosed area or change in effective diaphragm stiffness from
one storey to next > 50% iv) Out-of-plane offsets:
discontinuities in lateral load resisting paths v) Non-parallel
systems In Elevation -----------i) Soft-storey: Lateral stiffness
< 70% of that in in the storey above or < 80% of the average
lateral stiffness of three storeys above ii) Mass: seismic weight
of any storey except roof < 200% of adjacent storeys iii)
Geometic: horizontal dimension of a lateral force resisting element
> 150% of that in adjacent storey iv) In plane discontinuity: In
plane offset of a lateral force resisting element > length of
that element v) Weak-storey having lateral strength < 80% of
that in the storey above Additional requirements for some of the
irregularities are specified:
Soft Storey: The columns and beams of the soft storey are to be
designed for 2.5 times the storey shears and moments calculated
under seismic loads besides the columns designed and detailed for
the calculated storey shears and moments, shear walls placed
symmetrical in both directions of the building as far away from the
center of the building as feasbible to be designed exclusively for
1.5 times the lateral storey shear force calculated as before.
Non-Parallel Systems: Earthquake effects about the two orthogonal
axes must be combined: a. Ex 0.3Ey b. Ey 0.3Ex 8. More load
combinations are required: Basic Combination 1) 1.5(D+L) 2)
1.2(D+LE) 1.2(D+L+EXP), 1.2(D+L+EXN), 1.2(D+L-EXP), 1.2(D+L-EXN),
1.2(D+L+EYP), 1.2(D+L+EYN), 1.2(D+L-EYP), 1.2(D+L-EYN) 3) 1.5(DE)
1.5(D+EXP), 1.5(D+EYP), 4) 0.9D1.5E 0.9D+1.5EXP, 0.9D+1.5EYP, where
D L E X Y P N : : : : : : : Dead Live Earthquake along X axis along
Y axis Positive eccentricity Negative eccentricity 0.9D+1.5EXN,
0.9D+1.5EYN, 0.9D-1.5EXP, 0.9D-1.5EYP, 0.9D-1.5EXN, 0.9D-1.5EYN
1.5(D+EXN), 1.5(D+EYN), 1.5(D-EXP), 1.5(D-EYP), 1.5(D-EXN),
1.5(D-EYN) Expanded Combinations
This results in 25 actual combinations but can be reduced
rationally. Limits of static analysis ------------------------Zone
II Regular buildings