TR.31.2.11 IS:1893 (Part 1) 2016 Codes - Lateral Seismic Load
This feature enables one to generate seismic loads per the IS:1893 specifications using a static equivalent approach per Part 1 (2016) for building structures.
The seismic load generator can be used to generate
lateral loads in the X and Z directions only. Y is the direction of gravity
loads. This facility has not been developed for cases where the Z axis is set
to be the vertical direction (See the
SET Z UP
command in
TR.5 Set Command Specification).
General Format
DEFINE IS1893 2016 ( ACCIDENTAL ) LOAD
1893-2016-spec
wall-definitions
Refer to Wall Area Definitions for information on defining walls.
weight-data
Refer to Common Weight Data for information on how to specify structure weight for seismic loads.
where
1893-2016-spec = ZONE f1 RF f2 I f3 { SS f4 | SA f11 } (ST f5) ( { DM f6 | DF f12 } ) (PX f7) (PZ f8) ( { DT f9 | GL f10 } ) (HT f13) (DX f14) (DZ f15)
Parameter | Definition |
---|---|
ZONE f1 | Seismic zone factor. Refer to Table 3 (Clause 6.4.2) of IS:1893 (Part 1)-2016. |
RF f2 | Response reduction factor. Refer Table 9 (Clause 7.2.6) of IS: 1893 (Part 1) -2016. |
I f3 | Importance factor depending upon the functional use of the structures, characterized by hazardous consequences of its failure, post-earthquake functional needs, historical value, or economic importance. Refer Table 8 (Clause 7.2.3) of IS:1893 (Part 1)-2016. |
SS f4 |
Rock or soil sites factor.
Depending on type of soil, average response acceleration coefficient
Sa/g is calculated corresponding to 5% damping. Refer to Table 4
(Clause 6.4.2.1) of IS:1893 (Part 1) -2016.
Note: Use either
SS or
SA to specify
site conditions. If both parameters are specified,
SS is ignored.
|
ST f5 |
Structure type of the seismic-resisting system.
The program will calculate natural period as per Clause 7.6.2 of
IS:1893(Part 1)-2016.
Note: *ST 4 requires that
wall data be entered in order to determine the natural
period.
|
DM f6 |
Damping ratio to obtain
multiplying factor for calculating Sa/g for different damping. If no
damping is specified 5% damping (default value 0.05) will be considered
corresponding to which multiplying factor is 1.0. Refer to Clause 7.2.4 of
IS:1893(Part 1)-2016.
Note: Use either
DM or
DF to specify
damping. If both parameters are specified,
DM is ignored.
This should be a value between 0 (zero) and 1.0, inclusive. For example, 7% damping should be specified as 0.07. |
PX f7 | Optional period of structure (in sec) in X direction. If this is defined this value will be used to calculate Sa/g for generation of seismic load along X direction. |
PZ f8 | Optional period of structure (in sec) in Z direction. If this is defined this value will be used to calculate Sa/g for generation of seismic load along Z direction. |
DT f9 |
Depth of foundation below ground
level. It should be defined in current units. If the depth of foundation is 30
m or more, the value of Ah is taken as half the value obtained. If
the foundation is placed between the ground level and 30 m depth, this value is
linearly interpolated between Ah and 0.5Ah. Refer to
Clause 6.4.5 of IS:1893(Part 1)-2016.
Note: Use either
DT or
GL to specify
foundation depth. If both parameters are specified,
DT is ignored. See Node d below.
|
GL f10 |
Y coordinate of ground level (or
global Z coordinate for
SET Z UP ). A reduced lateral force is applied to
levels below this height, per Clause 6.4.5.
Note: Use either
DT or
GL to specify
foundation depth. If both parameters are specified,
DT is ignored. See Node d below.
|
SA f11 | Average response spectral acceleration coefficient corresponding to site specific spectra. Refer to Clause 6.4.7 of IS:1893(Part 1)-2016. |
DF f12 | Multiplying factor for calculating Sa/g. |
HT f13 | Height of the building. Refer Clause 7.6.2 (a) and Fig. 5 of IS 1893 2016 |
DX f14 | Base dimension of the building in X direction at the plinth level. Refer Clause 7.6.2(b) or (c) |
DZ f15 | Base dimension of the building in Z direction at the plinth level. Refer Clause 7.6.2(b) or (c) |
Notes
-
If the
ACCIDENTAL
option is specified, the accidental torsion will be calculated per the IS 1893 specifications. The value of the accidental torsion is based on the center of mass for each level. The center of mass is calculated from theSELFWEIGHT
,JOINT WEIGHT
, andMEMBER WEIGHT
commands you have specified.The
ACC
option along with accidental eccentricity factor (generally 0.05 as per IS 1893 code) needs to be provided in the 1893 seismic primary load case (i.e.,1893 LOAD X / Z f1 ACC f3
). f2 can be negative. See TR.32.12.2 Generation of Seismic LoadsTo consider horizontal torsion in cases where a floor diaphragm is present in the model, the
ACCIDENTAL
option should not be specified. Instead, dynamic eccentricity along with accidental eccentricity should be provided in the 1893 seismic primary load case (i.e.,1893 LOAD X / Z f1 DEC f2 ACC f3
). For equivalent seismic analysis, f2 is 1.5 and f3 is 0.05 as per IS 1893 code. f1 is always positive or zero, however f2 can be negative. If f2 is 0.0, only accidental torsion will be considered for this particular load case. -
By default, STAAD.Pro calculates natural periods of the structure in both X and Z directions respectively which are used in calculation for base shear. If
PX
andPZ
are included, the program will consider these values for calculation of average response acceleration coefficient. IfST
is used instead ofPX
andPZ
values, then the program will calculate natural period depending upon the empirical expression given in IS: 1893 (Part 1)-2016. -
In the case where no rigid floor diaphragm is present, STAAD.Pro identifies columns and shear walls (without openings) as vertical components for the purpose of computing lateral stiffness of the story.
The lateral stiffness of a column is calculated as:
The lateral stiffness for a shear wall (without opening) is calculated as:
Which is the summation of inverse of flexural stiffness and inverse of shear stiffness, obtained as deflection of a cantilever wall under a single lateral load, P, at its top.
The summation of lateral stiffnesses of all columns and shear walls at a particular floor level constitutes the total lateral stiffness of that particular story or floor level. The program checks for a soft story of a building along both global X and Z directions respectively. This computation is valid only for those structures whose floors are treated as rigid diaphragm
-
Clause 6.4.5 of IS:1893 part-I -2016 stipulates that for underground structures and foundation at a depth 30m or below, the design horizontal spectrum (Ah or Ak ) value should be taken as half of the actual one for structures placed between ground level and 30 m depth the design horizontal acceleration spectrum must be interpolated between Ah and (0.5 Ah ). The reduction of Ah should be done on the potion of the structure (mass situated below
GL
) located below ground.You can provide
DT
orGL
parameter to tell the program what your actual depth of foundation below the ground level.Note: The parameterDT
should not be used to reduce Ah . OnlyGL
should be used.The program will then evaluate the multiplication factor on Ah and calculate the base shear. This reduces the actual base shear for underground portion and the base shear, VB is distributed into story shear of that portion (for static analysis).
- For the portion of the structure above the ground, the design lateral force at the ith floor, Qi :
- For the portion of the structure below the ground, the design lateral force at the jth floor, Qj :
Example
DEFINE IS1893 2016 LOAD
ZONE 0.36 RF 5 I 1.2 SS 1 ST 1 DM 0.05
JOINT WEIGHT
39 60 80 WEIGHT 100
LOAD 1 LOADTYPE Seismic TITLE SS_(+X)
1893 LOAD X 1
LOAD 2 LOADTYPE Seismic TITLE SS_(+Z)
1893 LOAD Z 1
LOAD 3 LOADTYPE Seismic TITLE SS_(+Y)
1893 LOAD Y 1
Methodology
The design base shear is computed by STAAD.Pro for building structures as per IS: 1893 (Part 1) 2016:
- You provide seismic zone
coefficient and desired 1893 specs through the
DEFINE 1893 LOAD
command. - The program calculates the structure period, T.
- The program calculates Sa/g utilizing T. For the Y direction, Sa/g = 2.5 per clause 6.4.6.
- The program calculates V
from the above equation. W is obtained from mass table data entered via
SELFWEIGHT
,JOINT WEIGHT
(s),MEMBER WEIGHT
(S), and/orREFERENCE LOAD
you provide through theDEFINE 1893 LOAD
command. - The total lateral seismic load (base shear) is then distributed by the program among different levels of the structure per the IS: 1893 procedures.
See TR.32.12 Generation of Loads for additional information.
IS 1893 2016 Implementation
MEMBER CRACKED CODE IS1893 2016
command. Refer to TR.20.10 Member Property Reduction Factors for details.- 7.2.2 - A minimum of lateral base shear which needs to be distributed to each floor (or each node of each floor) in a building is calculated per Table 7 and Clause 7.2.2. This is used to determine a minimum base shear value.
- 7.2.3 - An additional importance factor has been added.
- 7.2.4 - 5% damping should
be used for all structures, regardless of the material. The program will accept
other values in the
DM
parameter, but a warning will be issued in the output. - 7.2.6 - The user interface includes a list of response reduction factors taken from Table 10 of IS 1893 2016.
- 7.6.2 - Calculation of
the approximate time-period based on height of the building
- For point a, the time period is calculated as follows for reinforced-concrete and steel composite MRF builds: Ta = 0.080.75 .
- For point b and point c, the time period is a function of the wall area.
Therefore, the wall-data-pairs must
provided to correctly calculate the time period for ST 4 or 5
(reinfroced concrete structural walls or all
other
buildings).