Transcript E-Course on Indian Seismic Code IS:1893

Lecture 4
January 30, 2006
In this lecture

Z, I, Sa/g and R values for tanks
 Sudhir K. Jain, IIT Kanpur
E-Course on Seismic Design of Tanks/ January 2006
Lecture 4 / Slide 2
Base shear coefficient

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Seismic force V = (Ah) x (W)
Ah is base shear coefficient
Ah 
Z  I 
   
 2  R
Sa
g
Design
philosophy
 Sudhir K. Jain, IIT Kanpur
Structural characteristics
Depends on time period
and damping
E-Course on Seismic Design of Tanks/ January 2006
Lecture 4 / Slide 3
Base shear coefficient
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Tanks have two modes
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Seismic force
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Impulsive
Convective
In impulsive mode, Vi = (Ah)i x impulsive weight
In convective mode, Vc = (Ah)c x convective weight
(Ah)i and (Ah)c are base shear coefficient in
impulsive and convective modes, respectively
 Sudhir K. Jain, IIT Kanpur
E-Course on Seismic Design of Tanks/ January 2006
Lecture 4 / Slide 4
Base shear coefficient
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Impulsive base shear coefficient
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Convective base shear coefficient
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(Ah)c = (Z/2) x (I/R) x (Sa/g)c
Note, R has been used in (Ah)i as well as (Ah)c
Zone factor, Z
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(Ah)i = (Z/2) x (I/R) x (Sa/g)i
As per Table 2 of IS 1893(Part1):2002
I, R, (Sa/g)i and (Sa/g)c will be discussed here

First, (Sa/g)i and (Sa/g)c
 Sudhir K. Jain, IIT Kanpur
E-Course on Seismic Design of Tanks/ January 2006
Lecture 4 / Slide 5
(Sa/g)i and (Sa/g)c
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(Sa/g)i is average response acceleration for
impulsive mode
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Depends on time period and damping of
impulsive mode
(Sa/g)c is average response acceleration for
convective mode

Depends on time period and damping of
convective mode
 Sudhir K. Jain, IIT Kanpur
E-Course on Seismic Design of Tanks/ January 2006
Lecture 4 / Slide 6
(Sa/g)i and (Sa/g)c
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Sa/g is obtained from design spectra
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Figure 2 of IS 1893(Part 1):2002
These spectra are slightly modified for tanks
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See next slide
 Sudhir K. Jain, IIT Kanpur
E-Course on Seismic Design of Tanks/ January 2006
Lecture 4 / Slide 7
(Sa/g)i and (Sa/g)c
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Modifications are:
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The rising portion in short period range from (0 to
0.1 sec) has been made constant
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Very stiff structures have time period less than 0.1 sec
There may be modeling errors; actual time period may be
slightly higher
As the structure gets slightly damaged, its natural period
elongates
Ductility does not help in reducing response of very stiff
structures
Hence, rising portion in the range 0 to 0.1 sec is usually
disallowed by the codes.
Spectra is extended beyond 4 sec
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Since convective time period may be greater than 4 sec.
Beyond 4 sec, 1/T variation is retained
 Sudhir K. Jain, IIT Kanpur
E-Course on Seismic Design of Tanks/ January 2006
Lecture 4 / Slide 8
Sa/g
Sa/g
Sa/g
(Sa/g)i and (Sa/g)c
Spectra of IS 1893 (Part 1):2002
Modified spectra
For 5% damping
 Sudhir K. Jain, IIT Kanpur
E-Course on Seismic Design of Tanks/ January 2006
Lecture 4 / Slide 9
(Sa/g)i and (Sa/g)c
Expressions for design spectra at 5% damping
Expressions for spectra of
IS 1893(Part 1):2003
Expressions for spectra for tanks
For hard soil sites
Sa/g = 1 + 15 T
0.00  T < 0.10
= 2.50
0.10  T < 0.40
= 1.00 / T
0.40  T  4.0
For hard soil sites
Sa/g = 2.50
T < 0.40
= 1.0 / T
T ≥ 0.40
For medium soil sites
Sa/g = 1 + 15 T
0.00  T < 0.10
= 2.50
0.10  T < 0.55
= 1.36 / T
0.55  T  4.0
For medium soil sites
Sa/g = 2.50
T < 0.55
= 1.36 / T
T ≥ 0.55
For soft soil sites
Sa/g = 1 + 15 T 0.00  T < 0.10
= 2.50
0.10  T < 0.67
= 1.67 / T
0.67  T  4.0
For soft soil sites
Sa/g = 2.5
T< 0.67
= 1.67 / T
T ≥ 0.67
 Sudhir K. Jain, IIT Kanpur
E-Course on Seismic Design of Tanks/ January 2006
Lecture 4 / Slide 10
(Sa/g)i and (Sa/g)c
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Sa/g values also depend on damping
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Multiplying factors for different damping are given in Table
3 of IS 1893(Part 1)
Recall from Lecture 2, higher damping reduces base shear
coefficient or design seismic forces
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Multiplying factor =1.4, for 2% damping
Multiplying factor = 1.0 for 5% damping
Multiplying factor = 0.8 for 10% damping
This multiplier is not used for PGA
 Sudhir K. Jain, IIT Kanpur
E-Course on Seismic Design of Tanks/ January 2006
Lecture 4 / Slide 11
Damping
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Damping for impulsive mode
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5% of critical for RC tanks
2% of critical for steel tanks
These are kept in line with IS 1893(Part 1)
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Clause 7.8.2.1 of IS 1893(Part 1) suggests 5% damping for
RC and 2% damping for steel buildings
However, IBC 2003 suggests 5% damping for all
tanks
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It suggests 5% damping for all types of buildings also
 Sudhir K. Jain, IIT Kanpur
E-Course on Seismic Design of Tanks/ January 2006
Lecture 4 / Slide 12
Damping
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Damping depends on material and level of
vibration
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Higher damping for stronger shaking
Means that during the same earthquake,
damping will increase as the level of shaking
increases
We are performing a simple linear analysis, while
the real behavior is non-linear
Hence, one fixed value of damping is used in our
analysis
 Sudhir K. Jain, IIT Kanpur
E-Course on Seismic Design of Tanks/ January 2006
Lecture 4 / Slide 13
Damping
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IS 1893(Part 1), needs to have a re-look at the
damping values
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Accordingly, damping values for tanks can also
be modified
 Sudhir K. Jain, IIT Kanpur
E-Course on Seismic Design of Tanks/ January 2006
Lecture 4 / Slide 14
Damping
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Damping for convective mode
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In Table 3 of IS 1893(Part 1):2002
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0.5% of critical for all types of tanks
Convective mode damping does not depend
on material of tank or type of liquid stored
Multiplying factor for 0.5% damping is not given
Values are given for 0% and 2% damping
Linear interpolation shall not be done
Multiplying factor = 1.75, for 0.5% damping
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In Eurocode 8 this multiplying factor is 1.673
In ACI 350.3, this factor is 1.5
 Sudhir K. Jain, IIT Kanpur
E-Course on Seismic Design of Tanks/ January 2006
Lecture 4 / Slide 15
Importance factor, I
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Importance factor, I for tanks is given in Table 1
of the Guideline
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This Table is reproduced here
Type of liquid storage tank
I
Tanks used for storing drinking water, non-volatile
material, low inflammable petrochemicals etc. and
intended for emergency services such as fire fighting
services. Tanks of post earthquake importance.
1.5
All other tanks with no risk to life and with negligible
consequences to environment, society and economy.
1.0
NOTE: Values of importance factor, I given in IS 1893 (Part 4)
may be used where appropriate
 Sudhir K. Jain, IIT Kanpur
E-Course on Seismic Design of Tanks/ January 2006
Lecture 4 / Slide 16
Importance factor, I
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I = 1.5, is consistent with IS 1893(Part 1)
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IS 1893(Part 1):2002 suggests, I = 1.5 for
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Hospital buildings
Schools
Fire station buildings, etc.
Tanks are kept at same importance level
 Sudhir K. Jain, IIT Kanpur
E-Course on Seismic Design of Tanks/ January 2006
Lecture 4 / Slide 17
Importance factor, I
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Footnote below this Table is given to avoid
conflict with I values of IS1893(Part 4)
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IS 1893(Part 4) will deal with industrial structures
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Some industries assign very high importance
factor to tanks storing hazardous materials
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Not yet published
Depending on their own requirements
For such tanks, Importance factor (I) will be as
per part 4
 Sudhir K. Jain, IIT Kanpur
E-Course on Seismic Design of Tanks/ January 2006
Lecture 4 / Slide 18
Response reduction factor, R
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R values for tanks are given in Table 2 of the
Guideline
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This is reproduced in next two slides
 Sudhir K. Jain, IIT Kanpur
E-Course on Seismic Design of Tanks/ January 2006
Lecture 4 / Slide 19
Response reduction factor, R
Elevated tank
R
Tank supported on masonry shafts
a) Masonry shaft reinforced with horizontal bands *
b) Masonry shaft reinforced with horizontal bands and vertical bars at corners
and jambs of openings
1.3
1.5
Tank supported on RC shaft
RC shaft with two curtains of reinforcement, each having horizontal and vertical
reinforcement
1.8
Tank supported on RC frame#
a) Frame not conforming to ductile detailing, i.e., ordinary moment resisting
frame (OMRF)
b) Frame conforming to ductile detailing, i.e., special moment resisting frame
(SMRF)
Tank supported on steel frame#
1.8
2.5
2.5
# These
R values are meant for liquid retaining tanks on frame type staging which are inverted pendulum type
structures. These R values shall not be misunderstood for those given in other parts of IS 1893 for building
and industrial frames.
* These tanks are not allowed in Zone IV and V
 Sudhir K. Jain, IIT Kanpur
E-Course on Seismic Design of Tanks/ January 2006
Lecture 4 / Slide 20
Response reduction factor, R
Ground supported tank
R
Masonry tank
a) Masonry wall reinforced with horizontal bands*
b) Masonry wall reinforced with horizontal bands and vertical bars at
corners and jambs of openings
1.3
1.5
RC / prestressed tank
a) Fixed or hinged/pinned base tank (Figures 6a, 6b, 6c)
b) Anchored flexible base tank (Figure 6d)
c) Unanchored contained or uncontained tank (Figures 6e, 6f)
2.0
2.5
1.5
Steel tank
a) Unanchored base
b) Anchored base
2.0
2.5
Underground RC and steel tank+
+
4.0
For partially buried tanks, values of R can be interpolated between ground supported and underground
tanks based on depth of embedment.
 Sudhir K. Jain, IIT Kanpur
E-Course on Seismic Design of Tanks/ January 2006
Lecture 4 / Slide 21
Response reduction factor, R
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R values for tanks are smaller than buildings
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As discussed earlier, R depends on
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This is in line with other international codes
Ductility
Redundancy
Overstrength
Tanks possess low ductility, redundancy and
overstrength
 Sudhir K. Jain, IIT Kanpur
E-Course on Seismic Design of Tanks/ January 2006
Lecture 4 / Slide 22
Response reduction factor, R
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First let us consider, elevated tanks on frame
type staging
Staging frames are different than building
frames
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Hence, following footnote to Table 2
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These R values are meant for liquid retaining tanks on frame
type staging which are inverted pendulum type structures.
These R values shall not be misunderstood for those given
in other parts of IS 1893 for building and industrial frames.
Staging frames are non-building frames and are
different than building frames
 Sudhir K. Jain, IIT Kanpur
E-Course on Seismic Design of Tanks/ January 2006
Lecture 4 / Slide 23
Response reduction factor, R
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There are critical differences between building
frames and non-building frames
International codes clearly differentiate between
these two types of frames
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Building frames have rigid diaphragms at floor
levels
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Frames of staging do not have rigid diaphragms
In buildings, seismic weight is distributed along
the height at each floor level
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In elevated tanks, almost entire seismic weight is
concentrated at the top
These are inverted pendulum type structures
 Sudhir K. Jain, IIT Kanpur
E-Course on Seismic Design of Tanks/ January 2006
Lecture 4 / Slide 24
Response reduction factor, R
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Moreover in buildings, non-structural elements,
such as infill walls, contribute significantly to
overstrength
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Staging are bare frames
In view of this, for staging with SMRF, R = 2.5
as against R = 5.0 for buildings with SMRF
With R = 2.5, base shear coefficient for elevated
tanks on frame staging matches well with other
international codes
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See next slide
 Sudhir K. Jain, IIT Kanpur
E-Course on Seismic Design of Tanks/ January 2006
Lecture 4 / Slide 25
Response reduction factor, R
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Comparison for frame staging
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Zone and soil parameters are same used in Lecture 2
IBC 2003;
Frame staging, R = 3.0
Base shear coefficient
0.5
0.4
Guideline;
Frame staging, R = 2.5
0.3
IS 1893:1984;
All types of staging, K = 1.0
0.2
0.1
0
0
0.5
1
1.5
2
2.5
3
Time period (sec)
 Sudhir K. Jain, IIT Kanpur
E-Course on Seismic Design of Tanks/ January 2006
Lecture 4 / Slide 26
Response reduction factor, R
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Let us now consider, elevated tanks on RC shaft
They possess less redundancy and have single
load path
RC shafts are usually thin shell and possess low
ductility
There are analytical and experimental studies on
ductility of hollow circular sections used in RC
shafts
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Some references are given on next slide
 Sudhir K. Jain, IIT Kanpur
E-Course on Seismic Design of Tanks/ January 2006
Lecture 4 / Slide 27
Response reduction factor, R
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Studies on ductility of shaft
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Zanh F A, Park R, and Priestley, M J N, 1990, “Flexural
strength and ductility of circular hollow reinforced concrete
columns without reinforcement on inside face”, ACI Journal
87 (2), 156-166.
Rai D C, 2002, “Retrofitting of shaft type staging for elevated
tanks”, Earthquake Spectra, EERI, Vol. 18 No. 4, 745760.
Rai D C and Yennamsetti S, 2002, “Inelastic seismic demand
on circular shaft type staging for elevated tanks”, 7th
National Conf. on Earthquake Engrg, Boston, USA,
Paper No. 91.
Rao M L N, 2000, “Effect of confinement on ductility of RC
hollow circular columns”, a Master’s thesis submitted to Dept.
of Earthquake Engineering, Univ. of Roorkee, India.
 Sudhir K. Jain, IIT Kanpur
E-Course on Seismic Design of Tanks/ January 2006
Lecture 4 / Slide 28
Response reduction factor, R
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These studies have revealed that ductility of
shaft depends on
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Thickness of wall (ratio of outer to inner diameter)
Axial force on shaft
Longitudinal and transverse reinforcement
Some results from these studies on ductility of
RC shafts are discussed in next few slides
 Sudhir K. Jain, IIT Kanpur
E-Course on Seismic Design of Tanks/ January 2006
Lecture 4 / Slide 29
Effect of Axial Load on Ductility
Figure from Rai (2002)
Ast/Ag = ratio longitudinal
reinforcement to concrete
area.
P = axial load on shaft
fc’ = characteristic strength of
concrete
Ag = gross area of concrete
Hollow circular section
 Sudhir K. Jain, IIT Kanpur
E-Course on Seismic Design of Tanks/ January 2006
Lecture 4 / Slide 30
Response reduction factor, R
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In this figure, curvature ductility is plotted as a
function of longitudinal reinforcement
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These results are for inner (Di) to outer (Do)
diameter ratio of 0.94.
If ratio of axial load (P) to ultimate load (fck.Ag) is
0.1 then, curvature ductility is about 9 for Ast/Ag =
0.02
This value reduces to 3 for P/ (f’c.Ag) of 0.25
Now, let us see some results on effect of shaft
thickness
 Sudhir K. Jain, IIT Kanpur
E-Course on Seismic Design of Tanks/ January 2006
Lecture 4 / Slide 31
Effect of Shell Thickness on Ductility
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Effect of ratio of inner to outer diameter (Di/Do) is shown
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This result corresponds to P/(f’c.Ag) = 0.05
Very low axial force ratio
 Sudhir K. Jain, IIT Kanpur
E-Course on Seismic Design of Tanks/ January 2006
Lecture 4 / Slide 32
Response reduction factor, R
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For thin shaft with Di/Do = 0.95, curvature
ductility is 12
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For longitudinal steel ratio Ast/Ag = 0.02
This value increases to about 25 for thick shaft
with Di/Do = 0.8
Thus, thickness has significant effect on ductility
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A thick shaft has reasonably good ductility
 Sudhir K. Jain, IIT Kanpur
E-Course on Seismic Design of Tanks/ January 2006
Lecture 4 / Slide 33
Response reduction factor, R
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These analytical studies clearly indicate that
thin RC hollow sections possess very low
ductility
Issues connected with poor ductility of shaft,
inadequate provisions of IS 1893:1984, and
their correlation to behavior during recent
earthquakes is discussed in following paper:
Rai D C, 2002, “Review of code design forces for shaft
supported elevated water tanks”, Proc.of 13th Symposium
on Earthquake Engineering , Roorkee, Ed. D K Paul et al.,
pp 1407 -1418.
(http://www.nicee.org/ecourse/12_symp_tanks.pdf)
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 Sudhir K. Jain, IIT Kanpur
E-Course on Seismic Design of Tanks/ January 2006
Lecture 4 / Slide 34
Response reduction factor, R
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Based on all these considerations, R = 1.8 for
shaft supported tanks
With this value of R, base shear coefficient for
shaft supported tanks matches well with
international codes
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Comparison with IBC 2003 on next slide
 Sudhir K. Jain, IIT Kanpur
E-Course on Seismic Design of Tanks/ January 2006
Lecture 4 / Slide 35
Response reduction factor, R
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Comparison for shaft staging
Zone and soil parameters are same as used in Lecture 2
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IBC 2003;
Shaft staging, R = 2.0
Base shear coefficient
0.5
0.4
Guideline;
Shaft staging, R = 1.8
0.3
IS 1893:1984;
All types of staging, K = 1.0
0.2
0.1
0
0
0.5
1
1.5
2
2.5
3
Time period (Sec)
 Sudhir K. Jain, IIT Kanpur
E-Course on Seismic Design of Tanks/ January 2006
Lecture 4 / Slide 36
Response reduction factor, R
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Some useful information on RC shaft is given in
ACI 371-98
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ACI 371-98 , 1998, “ Guide for the analysis, design , and
construction of concrete-pedestal water Towers”, American
Concrete Institute, Farmington Hill, MI, USA.
It exclusively deals with tanks on RC shaft
It suggests same design forces as IBC 2003
It gives information on:
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minimum steel
construction tolerances
safety against buckling
shear design etc.
 Sudhir K. Jain, IIT Kanpur
E-Course on Seismic Design of Tanks/ January 2006
Lecture 4 / Slide 37
Response reduction factor, R
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We have seen comparison with IBC 2003
Comparison with other international codes is
available in following documents:
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Jaiswal, O. R. Rai, D. C. and Jain, S.K., 2004a, “Codal
provisions on design seismic forces for liquid storage tanks:
a review”, Report No. IITK-GSDMA-EQ-01-V1.0, Indian
Institute of Technology Kanpur, Kanpur.
(www.iitk.ac.in/nicee/IITK-GSDMA/EQ01.pdf )
Jaiswal, O. R., Rai, D. C. and Jain, S.K., 2004b, “Codal
provisions on seismic analysis of liquid storage tanks: a
review” Report No. IITK-GSDMA-EQ-04-V1.0, Indian
Institute of Technology Kanpur, Kanpur.
(www.iitk.ac.in/nicee/IITK-GSDMA/EQ04.pdf )
 Sudhir K. Jain, IIT Kanpur
E-Course on Seismic Design of Tanks/ January 2006
Lecture 4 / Slide 38
Response reduction factor, R
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In the above two documents, following
international codes are reviewed and
compared:
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IBC 2000 (now, IBC 2003)
ACI 350.3
ACI 371
AWWA D-110 and AWWA D-115
AWWA D-100 and AWWA D-103
API 650 and API 620
Eurocode 8
NZSEE recommendations (From New Zealand)

Priestley et al. (1986)
 Sudhir K. Jain, IIT Kanpur
E-Course on Seismic Design of Tanks/ January 2006
Lecture 4 / Slide 39
Response reduction factor, R


Now we know Z, I, R and Sa/g for tanks
One can now obtain base shear coefficient for
impulsive and convective modes

An example follows.
 Sudhir K. Jain, IIT Kanpur
E-Course on Seismic Design of Tanks/ January 2006
Lecture 4 / Slide 40
Example

Example: An elevated water tank has RC frame
staging detailed for ductility as per IS: 13920 and is
located in seismic zone IV. Site of the tank has soft
soil. Impulsive and convective time periods are 1.2 sec
and 4.0 sec, respectively. Obtain base shear
coefficient for impulsive and convective mode.
Solution:
Zone: IV
 Z = 0.24 From Table 2 of IS 1893 (PART I):2002,
I = 1.5 From Table 1 of the Guideline
R = 2.5 for RC frame with good ductility (SMRF)
From Table 2 of the Guideline
 Sudhir K. Jain, IIT Kanpur
E-Course on Seismic Design of Tanks/ January 2006
Lecture 4 / Slide 41
Example on (Ah)i and (Ah)c
Impulsive time period, Ti = 1.2 sec, and soil is soft,
Damping = 5% (RC Frame)
 (Sa/g)i = 1.67/Ti = 1.67/1.2 = 1.392
(Clause 4.5.3 of the Guideline)
Convective mode time period, Tc = 4.0 sec and soil is soft
Damping = 0.5% (Clause 4.4 of the Guideline)
Factor 1.75 is to be used for scaling up (Sa/g) for 0.5%
damping (Clause 4.5.4 of the Guideline)
 (Sa/g)c = (1.67/Tc) x 1.75 = 1.67/4.0 x 1.75 = 0.731
 Sudhir K. Jain, IIT Kanpur
E-Course on Seismic Design of Tanks/ January 2006
Lecture 4 / Slide 42
Example on (Ah)i and (Ah)c
Base shear coefficient for impulsive mode
(Ah)i= (Z/2) x (I/R) x (Sa/g)i
= 0.24/2 x 1.5/2.5 x 1.392
= 0.10
Base shear coefficient for convective mode
(Ah)c = (Z/2) x (I/R) x (Sa/g)c
= 0.24/2 x 1.5/2.5 x 0.731
= 0.053
 Sudhir K. Jain, IIT Kanpur
E-Course on Seismic Design of Tanks/ January 2006
Lecture 4 / Slide 43
At the end of Lecture 4
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R values for tanks are less than those for
buildings.The basis for this is
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
Analytical studies
Provisions of international codes, and
Observed behavior of tanks
For tanks, slight modifications are recommended
for design spectrum of IS 1893(Part1)
Damping for convective mode may be taken as
0.5% for all types of tanks
 Sudhir K. Jain, IIT Kanpur
E-Course on Seismic Design of Tanks/ January 2006
Lecture 4 / Slide 44