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Is 1893 (Part 4) :2005 | Structural Load | Concrete
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STACK-LIKE
(3 BIS 2005
Sectional Committee, CED 39
This Indian Standard (Part 4) was adopted by the
Hinlalayan-Naga Lushai region, Indo-Gangetic Plain, Western Indi% Kutch and Kathiawar regions are geologically unstable parts of the country where some devastating earthquakes of the world have occurred. A major part of the peninsular India has also been visited by strong earthquakes, but these were relatively few in number occurring at much larger time intervals at any site, and had considerably lesser intensity. The earthquake resistant-design of structures, taking into account seismic data from studies of these Indian earthquakes, has become very essential, particularly in view of heavy construction programme at present all over the country. It is to serve this purpose that IS 1893 : 1962 ‘Recommendations for earthquake resistant design of structures’ was published and subsequently revised in 1966, 1970, 1975 and 1984.
in view of the present state of knowledge and in order to update this standard, the committee has decided to
This standard has been split into five
cover the provisions for different types of structures in separate parts. parts. Other parts in this series are :
Bureau of Indian Standards, after the draft finalized by the
Sectional Committee had been approved by the Civil Engineering
Pafl.2 Liquid retaining tanks-elevated and ground supported
Part”3 Bridges and retaining walls
Part 5 Dams and embankments
Part I contains provisions that are general in nature and applicable to a[l types of structures. Also, it contains provisions that are specific to buildings only. Unless stated otherwise, the provisions in Part 2 to Part 5 shall be read necessarily in conjunction with Part 1.
This standard contains provisions on earthquake resistant design of industrial structures including stack-like structures. Industrial structures are covered in Section 1 and Stack-like structures are covered in Section 2.
All sub-clauses under the main clause 0.0 of 1S 1893 (Part 1) are also applicable to this part except the 0.4.1.
In the preparation of this standard considerable assistance has been provided by BHEL, IIT Roorkee, IIT Bombay, [IT Kanpur, NTPC, EIL, TCE, DCE, NPC and various other organizations.
For the purpose of deciding whether a particular requirement of this standard is complied with, the final value, observed or calculated, expressing the result of a test or analysis, shall be -rounded off in accordance with IS 2: 1960 ‘Rules for rounding off numerical values (revised)’. The number of significant places retained in the rounded off value should be the same as that of the specified value in this standard.
IS 1893 (Part
4):2005
1.1 The industrial structures shall be designed and constructed to resist the earthquake effects in accordance with the requirements and provisions of this standard. This standard describes the procedures for earthquake resistant design of industrial structures. It provides the estimates of earthquake loading for design of such structures.
1.2 All sub-clauses under 1 of IS 1893 (Part 1) are
also applicable to this part except 1.1.
1.3 This standard
deals with earthquake
design of the industrial structures (plant and auxiliary structures) including stack-like structures associated with the following industries:
Steel, copper, zinc and aluminum plants;
Cement industrie~
Sugar and alcohol industries;
Glass and ceramic industries;
*Foundries;
Electrical and electronic industries;
Consumer product industries;
Structures for sewage and water treatment plants and pump houses;
r) Leather industries;
Mill structures;
and marine/potiharbour
v) Water and waste water treatment facilities; and
w) Paper plants.
This standard shall also be considered applicable to the other industries not mentioned above.
In addition to the above, the following structures are classified .as stack-like structures and are covered by this standard:
a) Cooling towers and drilling towem;
b) Transmission and communication towers;
c) Chimneys and stack-like structures;
Silos (including
silos used for urea
e) Support structures for refinery columns, boilers, crushers, etc; and
f) Pressure vessels and chemical reactor columns.
The following standards contain provisions which, through reference in this text, constitute provisions of this standard. At the time of publication the editions indicated were valid. All standards are subject to revision and parties to agreements based on this standard are encouraged to investigate the possibility of applying the most recent editions of the standards indicated ~elow:
456:.2000
Code of practice for plain and reinforced concrete ~ourth revision)
800:1984
Code of practics for design loads (other than earthquake) for building structures:
(Part 1): 1987
(Part 2): 1987
materials (second revision) Imposed loads (second revision)
(Part 3): 1987,
(Part 4): 1987
SrTow loads (second
(Part 5): 1987
Special loads and load combinations
1343:1980
for prestressed
1888:1982
Method of load test on soils (second
1893 (Part
4) :2005
Criteria for earthquake resistant design ofstructures: Part lGeneral provisions and buildings
Earthquake resistant design and construction of buildings — Code of practice (second revision)
for design of reinforced
Code of practice for determination of bearing capacity of shallow
(fh-st revision)
construction of steel chimney:
Part 2 Structural
13920:
Handbook for structural engrneers — Application of plastic theory in design of steel structures
All sub-clauses under 3 of IS 1893 (Part 1) are also applicable to this stan-dard.
Response quantity due loads in X-direction
Response quantity due loads in Y-direction
Response quantity due loads in Z-direction
e. s>
Static eccentricity at floor, i
Response quantity due to imposed loads
Mass matrix .of the primary system
Total mass of all the equipment that are flexible mounted at different locations in the structure
Modal mass of mode, k
Total mass of all the equipment that are rigidly mounted at different locations in the structure
Total mass of structural system, which supports secondary system
Number of modes being considered
.4.16 are
Spectral acceleration coefficient
Super imposed dead loads
reinforced/
Standard penetration test value (SPT
prestressed concrete and structural steel.
value) of the soil
5. I Symbols and notations applicable to Section 1 are given as under:
Undamped natural period of vibration of the structure
Seismic weight of floor, i
perpendicular to direction of force
jth normalized mode shape
Index for closely spaced modes
Influence vector-displacement vector of
Mode shape coefficient
at floor,
Response quantity due to dead load
mode, k
Design eccentricity at floor, i
Mode vector value from the primary
system’s modal displacement at the location where the secondary system is connected
Peak response quantity due to closely spaced modes
Cross-modal correlation co-efficient
Modal damping ratio @j
Frequency ratio = ~
Absolute value of qua~tity in mode k
Maximum value of deflection
in rad/see,
Maximum value of deflection in X, Y, Z direction respectively
5.2 Symbols and notations applicable to Section 2 are defined as under:
Area of cross-section at the base of the structural shell
Coefficient depending upon the slenderness ratio of the structure
Coefficient of shear force depending on slenderness ratio, k
Thickness of pile cap or rafl
Maximum lateral deflection
Distribution factors for shear and moment respectively at a distance X from the top
Modulus of elasticity of pile material
Modulus of elasticity of material of the structural shell
Shear modulus of soil = pV,2
Shear wave velocity of the medium
Height of structure above the base
Height of centre of gravity of structure above base
Moment of inertia of pile section
Dvj D,m
l— ,),
Number of locations of lumped weight
Radius of circular ratl foundation
Spectral acceleration coefficient for
rock and soil sites
Characteristic length of pile
Weight lumped at ith location with the weights applied simultaneously with the force applied horizontally
Total weight of the structure including weight .of lining and contents above the base
i5i —
Lateral static deflection under its own lumped weight at ith location (chimney weight lumped at 10 or more locations)
0’, —
Modulus of sub grade reaction of soil in horizontal direction
6.1 Ground Motion
6.1.1 The characteristics (intensity, duration, etc) of seismic ground vibrations expected at any location depends upon the magnitude of earthquake, its depth of focus, distance from the epicentre, characteristics of the path through which the seismic waves travel, and the soil strata on which the structure stands. The random earthquake ground motions, which cause the structures to vibrate, can be resolved in any three mutually perpendicular directions. The predominant direction of ground vibration is horizontal.
Earthquake generated vertical inertia forces are to be considered in design unless checked and proven to be not significant. Vertical acceleration should be considered in structures with large spans, those in which stability is a criterion for design, w- for overall stability analysis of structures. Reduction in gravity force due to vertical component of ground motions can be particularly detrimental in cases of prestressed horizontal members and of cantilevered members. Hence, special attention should be paid to the effect of vertical component of the ground motion on prestressed or cantilevered beams, girders and slabs.
6.1.2 The response of a structure to ground vibrations
is a function of the nature of foundations, soil, materials, form, size and mode of construction of structures; and the duration and characteristics of ground motion. This standard specifies design forces for structures standing on rocks or soi Is, which do not
settle, liquify or slide due to loss of strength during vibrations.
6.1.3 Thedesign approach adopted in this standard is to ensure that structures possess minimum strength to withstand minor earthquakes (< DBE) which occur frequently, without damage; resist moderate earthquakes (DBE) without significant structural damage though some non-structural damage may occur; and withstand a major earthquake (MCE) without collapse. Actual forces that appear on structures during earthquakes are much greater than the design forces specified in this standard. However, clucti[ity, arising from inelastic material behaviour and detailing, and overstrength, arising from the additional reserve strength in structures over and above the design strength, are relied upon to account for this difference in actual and design lateral loads.
Reinforced and prestressed concrete members shall be suitably designed to ensure that premature failure due to shear or bond does not occur, subject to the provisions of IS 456 and IS 1343. Provisions for appropriate ductile detailing of reinforced concrete members are given in 1S 13920.
In steel structures, members and their connections should be so proportioned that high ductility is obtained, as specified in SP 6 (6), avoiding premature failure due to elastic or inelastic buckling of any type.
6.1.4 The design force specified in this standard shall
be considered in-each of the two principal horizontal directions of the structure and in vertical direction.
6.1.5 Equipment and other systems, which are
supported at various floor levels of the structure, shall
be subjected to motions corresponding to vibration at their support points. in important cases, it may be necessary to obtain floor response spectra for analysis and design of equipment.
a) Earthquake causes impulsive ground motions, which are complex and irregular in character, changing in period and amplitude each lasting for a small duration. Therefore, resonance of
sinusoidal excitations, will not occur, as ~ would need titne to build up such amplitudes. NOTE — ~xccptional,resonance-likeconditionshave been
seen to occur between long distance waves and tall structures tbunded on deep soft soils.
b) Earthquake is not likely to occur simultaneously with maximum wind or maximum -flood or maximum sea waves.
c) Tbe value of elastic modulus of materials, wherever required, may be taken as for static
analysis unless a more definite value is available for use in such condition (see IS 456, IS 800 and IS 1343).
7.1 Categorization of Structures
To perform well in an earthquake, the industrial structure should possess adequate strength, stiffness, and ductility. Generally structures have large
capacities of energy absorption in its inelastic region.
Structures which are detailed
SP 6 (6) and equipment which are made of ductile materials~art withstand earthquakes many fold higher than the design spectra without collapse; and damage in such cases is restricted to cracking only.
as per IS 13920 or
a) Structures whose failure can cause conditions that -can lead directly or indirectly to extensive 10ssof life/property to population at large in the areas adjacent to the plant complex.
b) Structures whose failure can cause conditions that can lead directly or indirectly to serious fire hazard/extensive damage within the plant complex. Structures, which are required to handle emergencies immediately after an earthquake, are also included.
c) Structures whose failure, although expensive, does not lead to serious hazard within the plant complex.
d) All other structures.
Typical categorization of industrial structures is given in Table 5 .
NOTE — The term failure used in the definition of categories implies loss of function and not complete collapse. Pressurized
equipment where cracking can lead to rupture may be categorized by the consequences of rupture.
7.2.1 Dead Load
These shall be taken as per IS 875 (Part 1).
Industrial structures contain several equipment and associated auxiliaries and accessories that are
(SIDL)
permanently mounted on the structures. These loads response due to earthquake force (EL) is the maximum shall be taken as per equipment specifications. of the following cases:
=t 0.3 ELZ
These shall be taken as per 1S 87’5(Part 2).
* EL,
0.3 ELZ
Loaak
~+ ELZ
0.3 EL,
The earthquake load on the different members of a where x and y are two orthogonal directions and z is
structure shall be determined by carrying out analysis following the procedure described in 10 using the
by EL, and
denoted by ELZ.
7.3 Load Combinations
When earthquake-forces are considered on a structure, the response quantities due to dead load (DL), imposed load (lL), super imposed dead loads (SIDL) and design earthquake load (EL) shall be combined as per 7.3.1 and 7.3.2. The factors defined in 7.3.1 and 7.3.2 are applicable for Category 1 to 4 structures only under DBE (see 7.5).
ELY and earthquake loads in vertical direction
design spectra specified in 8. Earthquake loads
and y (horizontal) directions are denoted
7.3.2.2 As an alternative to the procedure in 7.3.2.1, the response (EL) due to the combined effect of the three components can be obtained on the square root of the sum of the squares (SIMS’)basis, that is
(ELK)2 + (ELY)2 + (ELZ)2
NOTE — Thecombinationproceduresof 7.3.2.1and7.3.2.2
applytothesameresponse quantity (say, moment in a coI umn about its major axis, or storey shear in a frame) due to different
components of the ground motion. These combinations arc to
be made at the member forcektress Iev-ets.
7.3.3 For structures under Category 1, which are
designed under MCE (see 7.5.1) and checked under DBE, all load factors in combination with MCE shall be taken as unity.
7.4 Increase in Permissible Stresses
In the plastic design of steel structures, the following
load combinations shall be accounted for: 7.4.1 Increase in Permissible Stresses in Materials
a) 1.7 (DL
+ SIDL
+ IL),
b) 1.7 (DL
+ S/DL)
+ EL,
c) 1.3 (DL
+ EL).
NOTE — Imposed load (ff,) in load combination shall not
include erection loads and crane payload,
Rcirrfor-ced
In the limit state design of reinforced and prestressed concrete structures, the following load combinations shall be accounted -for:
When earthquake forces are considered along with other normal design forces, the permissible stresses in material, in the elastic method of design, may be increased by one-third. However, for steels having a definite yield stress, the stress be limited to the yield stress, for steels without a definite yield point, the stress will be limited to 80 percent of the ultimate strength or 0.2 percent proof stress, whichever is smaller; and that in pre-stressed concrete members, the tensile stress in the extreme fibers of the concrete may be permitted so as not to exceed two-thirds of the modulus of rupture of concrete.
a) 1.5 (DL
b) 1.2 (DL
+ EL),
c) 1.5 (DL
EL), and
d) 0.9 (DL
+ SIDL)
NOTE — Imposed load (/[.) in load combination shall not
include erection load and crane payload,
7.3.2.1 When responses from the three earthquake
components are to be considered, the response due to each component may be combined using the assumption that when the maximum response from one component occurs, the responses from the other two components are 30 percent of the corresponding maximum. All possible combinations of the three components (ELX, EL and EL,) including variations
\ shall be considered. Thus, the
in sign (plus or minus
When earthquake forces are included, the allowable bearing pressure in soils shall be increased as per Table 1, depending upon type of foundation of the structure and the type of soil.
In soil deposits consisting of submerged loose sands and soils falling under classification SP with standard penetration N values less than 15 in seism”iczones III, IV, V and less than 10 in seismic zone 11,the vibration -caused by earthquake may cause liquefaction or excessive total and differential settlements. Such sites should preferably be avoided while locating new settlements or important projects. Otherwise, this aspect of the problem needs to be investigated and
appropriate methods of compaction or stabilization
adopted to achieve suitable N values as indicated
under Table 1. Alternatively, deep pile
foundation may be provided and taken to depths well into the layer, which is not likely to Iiquify. Marine clays and other sensitive clays are also known to liquefy due to collapse of soil structure and will need special treatment according to site condition.
7.5 Design Basis Earthquake (DBE)
Design basis earthquake (DBE) for a specific site is to be determined based on either : (a) site specific studies, or (b) in accordance with provisions of IS 1893 (Part 1).
7.5.1 Structures in Category 1 shall be designed for
maximum considered earthquake (MCE) (which is twice of DBE).
designed for DBE for the project site.
8 DESIGN SPECTRUM
8.1 For all important
deal ing with highly hazardous chemicals, evaluation of site-specific spectra for earthquake with probability of exceedence of 2 percent in 50 years (MCE) and 10 percent in 50 years (DBE) is recommended. All Category 1 industrial structures shall be analyzed using site-specific spectra. However, if site-specific studies arc not carried out, the code specified spectra may be used with modifications as per 8.3.2. If time-history analysis is to be carried out, spectra-compliant time- history shall be determined based on the site-specific spectra.
8.2 For all other structures not covered in 8.1, the
and all industries
spectra and seismic zone as given in Annex A and Annex B is recommended [these are in accordance with IS 1893 (Part 1) ].
The horizontal seismic coefficient Ah, shall be obtained using the period T, described as under.
8.3.1 When using site specific spectra, the seismic
coefficient shall be calculated from the expression :
8.3.2 When using code specific spectra, the seismic
co-efficient shall be calculated from the expression:
to site specific spectra.
NOTE — Structures in Category 1 shall be designed for seismic force twice that found using the provisions of this clause.
z. zone factor, given in Annex
s~g =
accordance with Table 2 of IS 1893 (Part 1)].
spectral acceleration coefficient for rock and soil sites given in Annex B [This is in accordance with Fig. 1 of IS 1893 (Part l)]. importance factor given in TabIe 2 is relative importance assigned to the structure to take into account consequences of its damage. response reduction factor to take into account the’margins of safety, redundancy and ductility of the structure given in Table 3.
A [This
values are to be taken as
9.1 Modelling
The mathematical model of the physkcal structure shall include all elements of the lateral force-resisting system. The model shall also include the stiffness and strength of elements, which are significant to the distribution of forces. The model shall properly represent the spatial distribution of the mass and stiffness of the structures, as well as mass of equipment cable trays and piping system along with associated accessories, 25 percent of the live load shall also be included as suitably distributed mass on the structure.
9.1.1 Soil-Structure
The soil-structure interaction refers to the effects of the supporting foundation medium on the motion of structure. The soil-structure interaction may not be considered in the seismic analysis for structures supported on rock or rock-like material.
Interaction effects between structure and equipment shall be considered as under:
Bet-ween Structure
For Category 2, 3 and 4, simplified considera- tions as per 9.2.1 may be used.
For Category 1, detailed considerations as per 9.2.2 shall be adopted.
For the purpose of 9.2, the following notations
M,=
total mass of the structural system on which the
system is supported,
Piles passing through any soil but rcstiog on soil Type I
Piles not covered S1 No. (i)
iv) Combined / Isolated RC’C footings with tie beams
7.4.2 )
of Soil Mainly
I Rock or Hard Soils:
1[ Medium
Well graded gravel and sand
gravel mixtures with or
clnyey sands poorly graded
or sand clay mixtures (Cl),
CW, SB, SW and S(’) having N above 30, where
N is the standard penetration
soils with N between
30, and poorly
or gravelly sands with little or
no fines (SP) with N>
All soils other than SP with
N<1O
bearing pressure shall be determined
in accordance with IS 6403
or IS 1888,
2 If anv increase in bearing Dressure has alrcadv been Dermitted lor forces other than seismic forces. the total increase in allowable
bearing-pressure
when seis;;c
force is also inciuded shall not exceed the limits specified
soils of smaller
Nvahses are met, compaction
adopted Io.achieve these values or deep pile foundations going to strooger
strata should be used.
The piles should be designed for lateral loads neglecting
resistance ofsoi[
layers liable to Iiquify.
Indian Standards may also be referred:
a) IS 1498 Classification
of-soils for general
b) IS2131
of standard penetration
test for soils.
c) IS 6403
Code ~f practice for determination
of bearing capidy
SIMI1OWfouodatioos.
d) 1S 1888 Method
of load tests on soils.
Isolated RCC
footing without tie beams or unreinftmced strip
foundation shall not be permitted
in sotl soils with N <10,
for Various -Industrial
(see 7.1 )
factor may be assigned to different
structures at the discretion of the project authorities
total mass of all the equipment that are rigidly mounted at different locations in the structure, and
total mass of all the equipment that are flexible mounted at different locations in the structure.
9.2.1.1 Wherever equipment are rigidly fastened to
the floor, the equipment mass (MJ shall be taken as lumped mass at appropriate locations. No interaction
between the structures and equipment shall be considered.
Ms + M.
No interaction between the structures and equipment shall be considered. In such case MF should be considered as lumped mass at appropriate locations.
9.2.1.3 If A4~/(A4~+ MS)20.25, interaction between
the flexibly mounted equipment and the structure shall be considered by suitably modelling the flexible equipment support system while considering the equipment as lumped mass.
9.2.2 Decoupling criteria as given below shall be used
9.2.2.1 For the purpose of this clause, the following
notations shall be used.
“;]
A4= mass matrix of the structural system,
(3, = ,jth normalized mode shape, OjT MOj = 1, and
U,,= influence vector, displacement vector of the structural system when the base is displaced by unity in the direction of earthquake motion.
of the dominant secondary
system modes and the dominant primary modes must
9.2.2.2 All combinations
be considered and the most restrictive combination shall be used.
9.2.2.3 Coupled analysis of a primary structure and
secondary system shall be performed when the-effects
of interaction are significant based on 9.2.2.9 and
9.2.2.11.
Coupling is not required, if the total mass of
the equipment or secondary system is 1 percent or Iem of the mass of the supporting primary structure. !fa coupled analysis will not increase the response of the primary system over that of a decoupled analysis
by more than 10 percent, then a coupled analysis is not required. However, the requirements of section
9.2.2.11 regarding the multiple supports should be
9.2.2.5 In applying sections 9.2.2.9 and 9.2.2.11, one
sub-system at a time may be considered, unless the sub-systems are identical anti located together, in which case the sub-system masses shall be lumped
When coupling is required, a detailed model
of the equipment or secondary system is not required, provided that the simple model adequately represents the major effects of interaction between the two parts.
When a simple model is used, the secondary system shall be re-analyzed in appropriate detail using the output motions from the first -analysis as input at the points of connectivity.
9.2.2.7 For applying the criteria of this section to
have a modal mass greater than 20 percent of the total
system mass, the total system mass is defined by
(r,)’
per 9.3), equipment or secondary system shall be
considered as per 9.2.2.4, 9.2.2.5 and 9.2.2.6.
9.2.2.9 When detailed analysis is to be carried out for
9.2.2.8 When carrying
out simplified
structures with equipment attached at a single point,
1S 1893 (Part
Response Reduction
‘), R for Industrial
Load Resisting
RC Moment—Resisting
Frame (OMRF)’)
ii) Special RC Moment—Resisting
Frame (SMRF)’)
iii) Steel Frame with:
iv) Steel moment resisting frame designed as per SP 6(6)
Walls’~
v) Load bearing masonry wall
a) Unreinforced
b) Reinforced with horizontal RC bands
c) Reinforced with horizontal RC bands and vertical bars at corners of rooms and jambs of openings
concrete shear walls!)
shear walls7)
Bu//ding.r )vith Dual Sysrems’)
shear wall with OMRF
shear wall with SMRF
with SMRF
Va]ues
of response reduction
factors are to be used for buildings
with lateral load
resisting elements,
elements built in isolation.
‘) OMRF in IS 13920.
are those designed and detailed
shall not be used in situations explained
in 4.15.2
1893 (Part l).
with shear walls also include boildings
having walls and frames, but where:
frames are not designed to-carry lateral loads, or
frames are designed to carry lateral loads but do not fulfd the requirements
of dual systems.
should be as per 1S 4326,
in zones IV and V.
shear walls are those designed and detailed as per IS 13920,
with dual systems consist of shear walls (or braced frames) and moment resisting frames such that:
the two systems are designed to resist the total design force in proportion to their lateral stiffness considering the interaction of the dual system at all floor levels, and
the moment resisting frames ar-edesigned to independently
resist at least 25 percent of the design seismic base shear.
not covered in Table 3, value oIJ{ shall be 2,
the coupling criteria shown in Fig. 1 shall be used. The mass ratio in Fig. 1 is the modal mass ratio computed as per 9.2.2.10 and the frequency ratio is the ratio of uncoupled modal frequencies of the secondary and primary systems.
9.2.2.10 For a secondary system dominant mode and
the primary system mode i, the modal mass ratio can be estimated by:
M,, =
@c,=
(I /Dci)’;
the mode vector value from the primary
system’s modal displacement at the location
from the ith normalised modal vector, (aCi),
(3Tc,M,,@ci= 1;
mass matrix of the primary system; and
total mass of the secondary system.
Multisupport secondary system shall be
rev iewed for the possibi Iity of interac~ion of structure
and equipment stiffness between the support points, and for the effect of equipment mass distribution between support points. When these effects can significantly influence the structure response, reference
=1-~
3’~T
$/ MS
=J-’S
MP+M
MCXMA
Explanetion
= frequency01 Uncoufxqd mode a of
secmdary
Irequency 01 unccwpled mode I of
primary s@em
shall “be made to specialized literature.
The time period of different industrial structures would vary considerably depending on the type of soil, span and height of the structure, distribution of load in the structure and the type of structure (concrete, steel and aluminum). It would be difllcult to give one or two
Accordingly, no simple guidelines can “be given for estimation of time periods of industrial structures.
9.3.1 The time period -shall be estimated
Eigen value analysis of the structural
model developed in accordance with 9.1 and 9.2.
all such structures.
9.3.2 For preliminary design, the time period can be
established based on its static deflection under mass
proportional loading in each of the three principal directions. This load is applied by applying a force equal to the weight of the structure or equipment at each mode in X, Y or Z direction. Where the founding soil is soft soil, the effect of the same shall be considered in the estimates for static deflection.
The time period T, would then be :
T = 21T ~ sec
FIG.1 DECOUPLING CRITERIAFOR EQUIPMENT OR SECONDARY SYSTEM ATTACf{MENTTO Tf[EpRfMARY SYSTEM
WITH SINGLEPOINT
1 .Om
Where 6 is the maximum value of deflection at any mode out of 6X6Y6= and ‘g’ is acceleration due to
gravity in the corresponding
10.2.2.1 The design eccentricity, ed to be used at floor i shall be taken as:
9.4 Damping
1.5 e,i + 0.05 bi
eti – 0.05 bi
The damping factor to be used in determining spectral acceleration coefficient (S,/g) depends upon the material and type of construction of the structure and the strain level. The recommended damping factors are given in Table 4.
whichever of these gives more severe effect.
e,i = static
distance between centre of mass and centre of
of Analysis Techniques
structures of Category
10.1.2 Detailed analysis shall be carried out for all
structures of Category 2 and 3 in seismic zones III, IV and”V.
10.1.3 Simplified analysis may be used for structures
of Category 2 and 3 in seismic zone 11.
10.1.4 Simplified analysis may be used for structures
of Category 4 in all seismic zones. However, those structures of Category 4, which could be identified as buildings, may be analysed as per provisions of IS 1893 (Part 1),
1, in aI1 seismic zones.
b, = floor plan dimension to direction of force.
of floor i, perpendicular
The factor 1.5 represents dynamic amplification factor, while the factor 0.05 represents the extent of accidental eccentricity.
NOTE — For the purposes of this clause, all steel or aluminium flooring system may be considered as flexible unless properly designed floor bracings have been provided. Reinforced concrete flooring systemat a level shall be considered rigid only if the total area of all the cut-outs at that level is less than 25 percent of its plan floor area.
three orthogonal (two horizontal and one vertical) components of earthquake motion. The earthquake motion in each direction shall be combined as specified in 7.3.
Seismic analysis shall be performed
10.2 Detailed
10.2.1 Seconda~
The analysis shall also include the influence P – A effect.
The effect of accidental eccentricity shall be considered for rigid floors/diaphragms. This shall be applied as an additional torsion force equal to product of the mass at floor level and 5 percent of the structure dimension perpendicular to the earthquake direction at the centre of mass of the floor.
10.2.4 Time-History
Time-history analysis of structures subjected to seismic loads shall be performed using linear analysis technique. The analysis shall be based on well-established procedures. Both direct solution of the equations of motion or model superposition method can be used for this purpose.
10.2.4.1 In model superposition method, sufficiently
large number 01 modes shall be used for analysis to include the influence of at least 90 percent of the total
seismic mass.
for DBE and NICE Conditions
NOTE — For combined structures, damping ratio coefficient
damping ratio coefficient is not evaluated, it shall be taken as that corresponding to material having lower damping.
shall be determined based on well
established procedures, if a composite
10.2.4.2 Modal
mass (MJ
of mode k is given by :
5 w, @,k
~ , 5 , w, (O,k)’
mode shape coefficient and
i, in mode
seismic weight of floor i.
I ().2.5
large number of modes shall be
used for analysis to include the influence of at least 90 percent of the total seismic mass. The model seismic mass shall be calculated as per the provisions of 10.2.4.1.
10.2.5.2 Modal
Tile peak response quantities (for example, member forces, displacements, storey forces, and shears and base reactions) should be combined as per complete quadratic combination (CQC) method as follows :
peak response quantity;
response quantity, in mode i (including sign);
response quantity, in modej (including sign);
cross-modal correlation co-efficient;
8(2(l+fl).fl’5
O.)j=
OJi=
(1-p’)’+
4(’p
(I+p’)
number of modes being considered;
modal damping ratio as specified in 9.4;
--# t
circular frequency, in ith
Alternatively, the ~ak response quantities may be combined as follows:
a) If the structure
does not have closely-spaced
modes, then the peak response quantity ( ~ ) due to all modes considered shall be obtained as:
absolute value of quantity, in mode k; and
number of modes being considered.
b) If the structure has a few closely-spaced modes [see 3.2 of IS 1893 (Part l)], then the peak response quantity 2“ due to these modes shall be obtained as :
where the summation is for the closely spaced modes only. This peak response quantity due to the closely spaced modes (2*) is then combined with those of the remaining well-separated modes by the method described in 10.2.5.2(a).
10.3 Simplified Analysis
Structures of category 2, 3 and 4 located in seismic zones II and 111 may be analyzed using the provisions of this clause, For all other industrial structures, the analysis procedure specified in 10.1 shall be used.
10.3.1 Simplified analysis shall be carried out by applying equivalent static lateral loads along each of the three principal directions. The equivalent static lateral loads shall be determined from design acceleration spectrum value (AJ calculated from 8.3.2 and 9.3.2. The static load at each node shall equal the product of its mass and the design spectral acceleration value.
11 DEFORMATIONS
11.-1 Drift Limitations
The drift limitations of horizontal and vertical members shall be taken as those specified in 1S 1893 (Part 1).
11.2 Separation Between Adjacent Units
Two adjacent buildings, or adjacent units of the same structure with separation joint in between shall be separated by a distance equal to the amount R times the sum of the calculated storey displacements as per 1“1.1of each of them, to avoid damaging contact when the two units deflect towards each other. When floor levels of two adjacent units or structures are at the same elevation levels, factor R in this requirement may be replaced by R12+25 mm.
12.I Foundations
The use of foundations vulnerable to significant differential settlement due to ground shaking shall be avoided for structures in seismic zones 111,IV and V. In seismic zones IV and V, individual spread footings or pile caps shall be interconnected with ties (see 5.3.4.1 of 1S 4326) except when individual spread footings are directly supported on rock. All -ties shall be capable of carrying, in tension and in compression, an axial force equal to AJ4 times the larger of the column or pile cap load, m addition to the otherwise computed forces. Here, Ah is as per 8.3.1 or 8.3.2.
12.2 Cantilever Projections
Towers, tanks, parapets, smoke stakes (chimneys) and other vertical cantilever projections attached to structures and projecting above the roof, shall be designed for five times the design horizontal acceleration spectrum value specified in 8.3.1
12.2.2 H.orizonta[
Al I horizontal projections like cornices and balconies
acceleration spectrum value specified in 8.4.
12.2.3 The increased design forces specified in 12.2.1 and 12.2.2 are only for designing the projecting parts and their connections with the main structures. For the design of the main structure, such increase need not be considered.
for five times the design vertical
SECTION 2 ‘STACK-LIKE STRUCTURES
Stack-1ike structures are those in which the mass and stiffness is more or less uniformly distributed along the height. Cantilever structures .Iike reinforced or prestressed cement concrete electric poles; reinforced concrete brick and steel chimneys (including multiflue chimneys), ventilation stacks and refinery vessels are examples of such structures. The guyed structures are not covered here.
Time period of vibration, T of such structures when fixed at base, shall be calculated using either of the following two formulae given (see 14.1 and 14.2). The formulae given at. 14.1, is more accurate. Only onc of these two formulae should be used for design.
Time period of structure,
measurement on similar structure and foundation soil
condition can also be adopted.
[S 1893 (Part
Table 5 Categorization
Air washer pump house
Ash water pump house
Ash water re-c.irculation
Ash/slurry
Ball mill and silos
and boiler house
storage handling/
dozirig buildings
Coal slurry settling pond
Condenser polishing
Control building (blast resistant)
towers (wet and dry) and control room
Corex gas station (tbr co-generation
storage tank (double walled)
Iiquetied
storage tanks with refrigerated