Part-I: Seismic Analysis/Design of Multi-storied RC Buildings using STAAD.Pro & ETABS according to IS:1893-2016 - Rahul Leslie 181118

1
Seismic Analysis/Design of
Multi-storied RC Buildings
using STAAD.Pro & ETABS
according to IS:1893-2016
Presented by .
Rahul Leslie
Deputy Director,
Buildings Design,
DRIQ, Kerala PWD
Trivandrum, India
Part - I

2
Topics Covered:
• Computer modelling and analysis using
STAAD.Pro & ETABS for
– Seismic Coefficient method as per IS:1893
(Part 1)-2016*
– Response Spectrum method as per
IS:1893(Part 1)-2016* (Covered in Part-II)
• Miscellaneous points
* With references to IS:13920-2015 & IS:16700-2017 where
relevant
Seismic Analysis of Multi-storied RC Building Presented by Rahul Leslie

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Aspects of Computer Model:
• Modelling is done using analysis packages like
STAAD.Pro, STRAP, NISA Des. Studio, ETABS,
GT STRUDL, RISA-3D, MIDAS-Gen, etc.

4
Aspects of Computer Model:
• Modelling is done using analysis packages like
STAAD.Pro, STRAP, NISA Des. Studio, ETABS,
GT STRUDL, RISA-3D, MIDAS-Gen, etc.
• Model contains
• Beams
• Columns
• Shear walls
But not usually
• Slabs, except
• Flat slabs / Flat plates
• Sloped RC Roofs (in ETABS)
• Masonry wall infills
• Stair slabs
Foundation is represented by support points only

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Model for ETABS: B+G+4=6 stories

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ETABS Model

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ETABS Model

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Model for STAAD: G+4 = 5 stories

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STAAD Model

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STAAD Model

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Aspects of Computer Model (Cont…)
• A model must ideally represent the complete three
dimensional (3D) characteristics of the building, including
– geometry
– stiffness of various members
– supports
– load distribution
– mass distribution
• For models in the purview of IS:16700-2017*, rigid offsets
at beam-column joint region should also be considered. (7.2
(a), IS:16700-2017)
* Criteria for Structural Safety of Tall Concrete Buildings, applicable to RC
buildings in the range of 50 to 250m height

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Beams and columns
• Beams and columns are modelled by frame elements
• Plinth beams should also be modelled as beams
• Slabs are not usually modelled

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Beams and columns
• Stiffness of Beams and columns to be reduced as:
• Beams: Ieff = 0.35 Igross
• Columns : Ieff = 0.7 Igross
(6.4.3.1, IS:1893(Part 1)-2016) – New

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Beams and columns
• Stiffness of Beams and columns to be reduced as:
• Beams: Ieff = 0.35 Igross
(6.4.3.1, IS:1893(Part 1)-2016) – New
• However, nothing has been mentioned on stiffness
reduced factors for
• Shear walls
• Flat slabs / Slabs

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Beams and columns
• Alternatively, in IS:16700-2017*, the reduction factors are
given as :
a) For factored load cases –
• Slabs: Ieff = 0.25 Igross
• Beams : Ieff = 0.35 Igross
• Walls : Ieff = 0.7 Igross
b) For un-factored load cases –
• Slabs: Ieff = 0.35 Igross
• Beams : Ieff = 0.7 Igross
• Walls : Ieff = 0.9 Igross
(7.2 & Table 6, IS:16700-2017)
*(for buildings coming under its purview)

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Beams and columns
• Using two sets of stiffness reduction factors (SRF),
however, has its own issues:
 It will require analysis using two copies of the same Finite
Element model files,
 One with SRF for factored loads, and
 The other with SRF for un-factored loads
...unless the Analysis software packages in future come
up with facilities for optioning different SRF for different
load combination cases – (load combinations to be
covered later)
 In Response Spectrum method (to be covered later), the
models for factored and un-factored loads end up having
different (but very close) sets of mode shapes and mode
frequencies, thus having each model analysed with
different sets of modal parameters, though for the same
building – I presume it is okay and generally accepted

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Assign section modifiers …
ETABS: For Beams

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Assign section modifiers …
ETABS: For Columns

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Assign cracked properties…
STAAD: For Beams

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Assign cracked properties…
STAAD: For Columns

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Supports:
• The type of support to be provided is decided by
considering the degree of fixity provided by the
foundation.
• Fixed Supports:
– Raft foundation: Support to be provided at the column
ends (located at top of the raft)
– Pile cap for multiple piles: Support to be provided at the
column ends (located at top of the pile cap)
– Isolated footing: When it is founded on hard rock, the
column end may be modelled as fixed (located at the top of
the footing)
– Single pile: Fixed support of the column is recommended
at a depth of five to ten times the diameter of pile,
depending upon the type of soil, from the top of pile cap.

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Supports (cont…):
• Pinned supports:
– Isolated footing: Support to be provided at the column
ends, (located at the bottom of the foundation).
• Spring supports:
– Spring supports can be provided with spring constants ,
eg., as per ASCE/SEI 41 (2006)
• In General
– Engineering judgement must be exercised in modelling the
support

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Slabs and Masonry walls
• The weight of slabs are distributed, as 2-way load
distribution, on the supporting beams.
• The weight of masonry walls are applied as uniform
load on the supporting beam

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Diaphragms
• Since the slabs are not modelled by plate elements, the
structural effect due to their in-plane stiffness (7.6.3 (b),
IS:1893(Part 1)-2016) can be taken into account as Rigid
Diaphragms by
– using ‘Master/Slave’ option (STAAD.Pro)
– assigning ‘Diaphragm’ action (ETABS , STAAD.Pro V8i
SS4 onwards)

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Diaphragms
• This method is to be resorted to, only if the slab is stiff
enough to act as a rigid diaphragm
• This is to be ascertained as per criteria specified in
7.6.4, IS:1893(Part 1)-2016

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Diaphragms
• Diaphragms have to be checked whether they can be
considered rigid (7.6.4, IS:1893(Part 1)-2016) by
– considering a floor independently,
– modelling the floor with shell elements & meshing it,
– applying a lateral load at its to be ‘Diaphragm centre’
– checking it’s deformation

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Diaphragms
• A floor diaphragm is considered to be flexible, if it deforms
such that the maximum lateral displacement measured from the
chord of the deformed shape at any point of the diaphragm is
more than 1 .2 times the average displacement of the entire
diaphragm (7.6.4, IS:1893(Part 1)-2016).

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Diaphragms
• This method is to be resorted to, only if the slab is stiff
enough to act as a rigid diaphragm
• This is to be ascertained as per criteria specified in
7.6.4, IS:1893(Part 1)-2016
• If the criteria is not met:
• The storey loads are to be distributed at the column points
of the floor, proportionate to the floor mass distribution
7.6.4, IS:1893(Part 1)-2016
• In tall buildings, the in-plane stiffness of the floor slab is to
be modelled (using meshed shell elements) -- 7.3.3,
IS:16700-2017 #
# for buildings in its purview

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Assign diaphragms: select all slabs in a storey and …
ETABS: Floor Diaphragm

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STAAD: Floor Diaphragm

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Slabs and masonry walls (cont…)
• Masonry infill walls have in-plane stiffness that can
influence the behaviour of the building under lateral load.
- Effects of infill walls are to be modelled, and then stiffness
irregularity* to be examined for (7.9.1, IS:1893(Part 1)-2016)
- Effects of infill walls are to be modelled, for analysis, if they
contribute to lateral stiffness# (7.3.4, IS:16700-2017)
• Masonry infills are modelled by
equivalent diagonal struts with
pinned ends (7.9.2.2, IS:1893(Part 1)-
2016)
* as per Table 6, IS:1893(Part 1)-2016
# for buildings coming under it’s
purview.

• The properties of the diagonal struts are modelled according
to the following:
- Modulus of Elasticity as per 7.9.2.1, IS:1893(Part 1)-2016
- Where fm is the compressive strength of masonry (MPa), given by
- Where fmo is the strength of mortar in the masonry, as per IS:1905-1987
and fb is the strength of bricks in the masonry, as per IS:1077-1992

to the following:
- For the cross section of the strut, the terms used are: ‘thickness’ is the
horizontal dimension of the diagonal strut cross section; and ‘width’ is
the vertical dimension of cross section, measured perpendicular to the
inclination of the strut.
- For URM infill walls without any opening, width, Wds of equivalent
diagonal strut is:
- Where Lds is the (diagonal) length of the strut and
- Where.. (continued on next page)

to the following:
- where (continued from previous page)
- Em = modulus of elasticity of the materials of the infill
- Ef = modulus of elasticity of the materials of the RC frame,
- Ic = moment of inertia of the adjoining column,
- t = thickness of the infill wall
- θ = the angle of the diagonal strut with the horizontal;

to the following:
- In case of infill walls with openings, no reduction in strut width is
required (IS:1893-2016, Cl. 7.9.2.2(c))
- But it is known that openings reduce the stiffness of the diagonal strut,
and the reduction is to be incorporated by suitably reducing the width of
the strut.
- The procedure given for calculating Wds is found to be developed by
Mainstone & Weeks (1971) by utilising the formulae by Smith & Carter
(1969) .
- For reduction factors to account for openings in the masonry infills,
many formulae have been developed. .. (continued on next page)

to the following:
- For example, Al-Chaar (2002) has developed a formula for the
reduction factor due to openings. In his procedure, the reduction factor
ρw the thickness of the strut to account for the effect of openings is
- where Ao = area of the opening
Ap = area of the infill panel (= l.h)
- Subject to the condition that if Ao ≥ 60% of Ap, then ρw should be
taken as zero.
- ρw is the reduction factor for Wds, as
Wdo = ρw .Wds
- As of now (without any Amendment being published), the above
reduction factor is not supported by the IS:1893-2016 code.

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Masonry walls (Table 6 (i), IS:1893(Part 1)-2016) :
• The Structural Plan Density (SPD) should be
estimated when unreinforced masonry (URM)
infills are used.
• When SPD of masonry infills exceeds 20%, the
effect of URM infills shall be considered by
explicitly modeling the same in structural analysis
(by diagonal struts).

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Masonry walls (Table 6 (i), IS:1893(Part 1)-2016) :
• The design forces for RC members shall be larger
of that obtained from analysis of:
• a) Bare frame, and
• b) Frames with URM infills, using 3D modeling of the
structure.
• In buildings designed considering URM infills, the
inter-storey drift shall be limited to 0.2 percent in
the storey with stiffening and also in all storeys
below.

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Shear walls
• Structural shear walls and Shear core which are integrally
connected to the frame and floor slabs, can be modelled by
plate elements
– ‘Surface elements’ (STAAD.Pro V8i and earlier)
– Shell elements (STAAD.Pro Connect Edition and later)
– ‘Wall element’ (ETABS)

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Other considerations
• Staircase slabs built integrally with the frame should be
modelled (5.4, IS:13920-2015; 8.1.2 , IS:16700-2017)*
• Provide sliding joints at the interconnection of the stairs with
floors, so that they will not act as diagonal bracing (5.5,
IS:4326-1993). If it is not providable, either of the following
may be adopted instead:
• Separated Staircases — staircase carried by a structure separated from
the building (with a vertical separation joint between the two), in
which one end of the staircase rests on a wall and the other end is
carried by columns and beams.
• Built-in Staircase — When stairs are built monolithically with floors,
RC walls are provided at either side of the stairs, extending through
the entire height of the stairs and to the building foundations.
*(for buildings coming under its purview)

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Other considerations
• Staircase slabs built integrally with the frame should be
modelled (5.4, IS:13920-2015; 8.1.2 , IS:16700-2017)
• Buildings with any irregularities listed in IS:1893(Part 1),
buildings with floating columns and set-back columns, a
detailed Non-linear analysis is to be done (5.5, IS:13920-
2016).
• As of now, the most practical approach to doing a Non-linear analysis
is the Non-linear Static Procedure (NSP), better known as Pushover
analysis

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Other considerations for tall buildings
• The model should incorporate rigid end offsets at the joints
(7.2 (a), IS:16700-2017)
• ETABS does this automatically
• With STAAD.Pro V8i, one has to do this manually

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(7.2 (a), IS:16700-2017)
• P- Δ analysis to be done (7.2(d) & 7.3.9, IS:16700-2017)

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(7.2 (a), IS:16700-2017)
• P- Δ (P-Delta) analysis to be done (7.2(d) & 7.3.9, IS:16700-2017)
• STAAD.Pro & ETABS has conceptually different approaches
to P- Δ, and correspondingly the settings/parameters to be
provided also are different:
• In STAAD.Pro, the gravity loads (DL & LL) and the seismic loads
are to be combined using the REPEAT LOAD option instead of the
LOAD COMBINATION option, and then PDELTA analysis is run.
• In STAAD.Pro, only Seismic Coefficient Method can be included
with P- Δ analysis, not the Response Spectrum Method

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(7.2 (a), IS:16700-2017)
• P- Δ (P-Delta) analysis to be done (7.2(d) & 7.3.9, IS:16700-2017)
• STAAD.Pro & ETABS has conceptually different approaches
to P- Δ, and correspondingly the settings/parameters to be
provided also are different:
• In ETABS, a load combination is to be specified, which is considered
for the P- Δ analysis
• This load combination is the most critical one from among the codal
seismic load combinations, but with the seismic part omitted. In
IS:1893-2016 (with the most critical one in pink) these are
1.2 DL + 1.2 LL + 1.2 EL  1.2 DL + 1.2 LL
1.5 DL + 1.5 EL  1.5 DL
0.9 DL + 1.5 EL  0.9 DL

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ETABS Version 9.7 and earlier

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ETABS Version 2013 and later

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(7.2 (a), IS:16700-2017)
• Construction Sequence analysis to be done for buildings
taller than 150 m (7.3.13, IS:16700-2017)

Construction Sequence analysis

Construction Sequence Analysis Single Step Analysis
Construction Sequence analysis

68
(7.2 (a), IS:16700-2017)
• Construction Sequence analysis to be done for buildings
taller than 150 m (7.3.13, IS:16700-2017)
• Note: a case with floating columns has been demonstrated only
because the effects of construction sequence are most prominent in
such cases.

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Analysis as per IS:1893-2016
Seismic Coefficient Method
(Static Analysis)

70
Static analysis:
• The Design horizontal seismic coefficient Ah is
calculated from (6.4.2, IS:1893 (Part 1)-2016)
– Zone factor Z (Table 3 & Fig. 1, IS:1893 (Part 1)-2016)
– Importance factor I (Table 8, IS:1893 (Part 1)-2016)
– Response reduction factor R (Table 9, IS:1893 (Part 1)-2016)
– Horizontal Acceleration coefficient Sa/g

71
Static analysis:
• Where the Horizontal acceleration Sa/g is determined
from the Response spectrum curve (Fig.2A, IS:1893(Part
1)-2016) -- Modified

72
Static analysis:
• Where the Horizontal acceleration Sa/g is determined from the
Response spectrum curve (Fig.2A, IS:1893(Part 1)-2016 –
Modified)
• Separate Response Spectrum curves are given for Seismic
Coefficient Method and Response Spectrum Methods (Fig.2A &
2B, IS:1893(Part 1)-2016) – New
• Also the both the Response Spectrum curves are horizontal
straight lines after T = 4s. – Modified (Earlier, the code was
silent on the portion of the curve for T > 4s)
• But the above is insignificant in case of Fig.2A, IS:1893(Part
1)-2016 since the Seismic Coefficient Method is only applicable
for regular structures having T < 0.4s (6.4.3, IS:1893(Part 1)-2016) -
- New
• Unless, of course, the method is to be used for Base Shear correction (to
be covered later)

73
Static analysis (cont…):

74

75

76
Static analysis:
• The time period of the structure is determined using
(7.6.2 (a) & 7.6.2 (c), IS:1893(Part 1)-2016)
– RC frames without brick infills
h = height of building in m
– RC frames with brick infills
d = base dimension in m
(parallel to direction of earthquake)

77
Static analysis:
• The time period of the structure is determined using
(7.6.2 (a) & 7.6.2 (c), IS:1893(Part 1)-2016)
– Buildings with RC walls (7.6.2 (b), IS:1893(Part 1)-2016) --New
Aw = total effectie cross-sectional area of
RC wall in the first storey in m2
Awi = cross-sectional area of RC
wall i in the first storey in m2
Lwi = length of RC wall i in the first
storey in m
(both for walls parallel to direction of earthquake)

78
Static analysis:
• The time period for irregular configurations is calculated for
parameters determined as per the following figures (Fig.5 ,
IS:1893(Part 1)-2016) --New

79
• Where the type of soils are
– Type I (Rock or Hard soil): N > 30, among other descriptions
– Type II (Medium soils): 10 ≤ N ≤ 30 for all soils
N >15 for poorly graded, among
other descriptions
– Type III (Soft soils): N < 10
(Table 4, IS:1893(Part 1)-2016)
• Where the N values are taken as the weighted average
of N values of soil layers up to 30m below ground level
(6.4.2.1, IS:1893(Part 1)-2016) -- New

80
Presented by Rahul LeslieSeismic Analysis of Multi-storied RC Building
6.4.2, IS:1893(Part 1)-2016,
description of (Sa/g) - (a)

81
6.4.2, IS:1893(Part 1)-2016,
description of (Sa/g) - (a)

82
Static analysis:
• Zone factor (Table 3, IS:1893(Part 1)-2016)
– Z = 0.10 for Zone II
– Z = 0.16 for Zone I II
– Z = 0.24 for Zone IV
– Z = 0.36 for Zone V

83
Static analysis:
• Importance factor (Table 8, IS:1893(Part 1)-2016)
– I = 1.5 for special buildings (including community halls)
– I = 1.2 for residential and commercial buildings
with occupancy > 200 persons -- New
– I = 1.0 for other buildings
• Response reduction factor (Table 9, IS:1893(Part 1)-2016)
– R = 3 for ordinary detailing (with ordinary detailed shear
wall, if any)
– R = 5 for ductile detailing (with ductile detailed shear wall, if
any) ie., as per IS:13920-2016
– R = 4 for ductile detailing with ordinary detailed shear wall
– R = 4 for ordinary detailing with ductile detailed shear wall
– R = 3 for ordinary detailing with ordinary detailed shear wall
– R = 5 for ductile detailing with ductile detailed shear wall

84
• The base shear is determined by (7.2.1, IS:1893(Part 1)-
2016)...
... but subject to the condition that VB ≥ (VB)min (7.2.2,
IS:1893(Part 1)-2016) – New ...

85
• Where (VB)min is to be determined from ρ, given as the
percentage of weight of the building (Table 7,
IS:1893(Part 1)-2016)...
(VB)min = ρW
... which is, in effect, as good as saying, for eg., for a
structure in Zone III, Medium soil, I = 1.0 and R = 5,
that T should be taken as not more than Tmin = 1.98s
(by back calculating from ρ to Sa/g, and Sa/g to Tmin)
Zone ρ
II 0.7
III 1.1
IV 1.6
V 2.4

86
• For tall buildings, ρ is determined from (Table 5,
IS:16700-2018)...
(VB)min = ρW
... for buildings of intermediate heights (ie., in the
range of 120 to 200m), interpolation is to be used
Zone
ρ for
H≤120m
ρ for
H≥200m
II 0.7 0.5
III 1.1 0.75
IV 1.6 1.25
V 2.4 1.75

87
• The base shear is determined by (7.2.1, IS:1893(Part 1)-
2016)...
... but subject to the condition that VB ≥ (VB)min (7.2.2,
IS:1893(Part 1)-2016) – New ...
• Design lateral force for each level is determined by (7.6.3,
IS:1893(Part 1)-2016)...
... Where Wi is the seismic weight (to be covered) of the i’th storey at
height hi

88
A Simple Example
A six storied structure

89

90
Lumped mass model

91
height 18 m
Period 0.075x(18)0.75 = 0.6554 sec
(Assumed to be open structure)

92
Levels W (kN) h (m) Wh2
SW*(Wh2/
SWh2) Qi (kN)
1 0 0 0 0 0
2 23.57 3 212.13 1.5541 0.0632
3 23.57 6 848.52 6.2163 0.2529
4 23.57 9 1909.17 13.9866 0.5691
5 23.57 12 3394.08 24.8651 1.0117
6 23.57 15 5303.25 38.8516 1.5807
7 23.57 18 7636.68 55.9464 2.2763
SW 141.42 (kN) 19303.83 141.42 kN 5.7539 kN
Vb 5.7539 (kN)
height 18 m
Period 0.6554 sec
Sa/g 1.5258
Ah 0.0407
= (ZI)/(2R) x sa/g
= (0.16*1)/(2*3) * 1.5258

93
The forces are applied …and analysed

94
The forces are applied

95
Seismic Coeff. method
ETABS:
 Define & Apply Seismic
parameters:
• Direction
• T
• Z, I, R
• Soil Type
STAAD:
 Define Seismic parameters:
• Z, I, R
• Structure Type or
Tx & Tz
• Soil Type
• Damping ratio ξ
 Apply
• Direction (X, Y,Z), factor

96
ETABS: Seismic coeff. method

97

98

99

100
STAAD: Seismic coeff. method

101

102

103
Masses to be included:
• For seismic analysis, the effective masses to be
included for analysis are (7.4.1, IS:1893(Part 1)-
2016) :
– Full dead load
– 0.25 times Imposed Loads having intensity ≤ 3 kN/m2
– 0.5 times Imposed Loads having intensity > 3 kN/m2
– 0.2 times Snow Loads exceeding 1.5 kN/m2 -- New
– Imposed Load on roof need not be considered
(7.3.1, 7.3.2 , 7.3.5 & Table 10, IS:1893(Part 1)-2016)
• The earlier edition clause that Live load reduction for upper
floors (as per 3.2, IS:875(Part 2) - 1987) shall not be applied
further for mass calculation in now missing – it was
mentioned in 7.3.3, IS:1893 (Part 1) – 2002

104
Add Seismic Masses
EATBS
• Select loads to
combine
STAAD
• Add self wt., Joint
loads, Member loads,
Floor loads

105
Define Mass Source:-
ETABS: Seismic masses

106
STAAD: Seismic masses

107

108

109

110

111

112

113

114

115
STAAD.Pro V8i SELECT 4

116

117

118

119

120

121

122
Results of Seismic Analysis –
Bending Moment & Shear Force
• Gravity Loads – Bending Moment
• Gravity Loads – Shear Force
• Seismic Loads – Bending Moment
• Seismic Loads – Shear Force

123
Load combinations: will be covered later

124
ETABS: Run Analysis

125
ETABS: Gravity Loads  Bending Moment

126
ETABS: Gravity Loads  Shear Force

127
ETABS: Seismic Force in Z direction  Bending Moment

128
ETABS: Seismic Force in Z direction  Shear Force

129STAAD: Run Analysis

130
STAAD: Gravity Loads  Bending Moment

131
STAAD: Gravity Loads  Shear Force

132
STAAD: Seismic force in Z direction  Bending Moment

133
STAAD: Seismic force in Z direction  Shear Force

134
Analysis as per IS:1893-2016
Response Spectrum Method
(Dynamic Analysis)
Continued in Part-II with …

135
Continue with Part-II
rahul.leslie@gmail.com

Part-I: Seismic Analysis/Design of Multi-storied RC Buildings using STAAD.Pro & ETABS according to IS:1893-2016 - Rahul Leslie 181118

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