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TR.31.2.11 IS:1893 (Part 4) 2015 Codes - Lateral Seismic Load

This feature enables one to generate seismic loads per the IS:1893 specifications using a static equivalent approach per Part 4 (2015) for industrial structures.

All industrial (plant and auxiliary) structures like main plants, process plants, water systems, and maintenance workshops are recognized as industrial structures.

The seismic load generator can be used to generate lateral loads in the X, Y, and Z directions. 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).
Note: This implementation is only applicable to industrial structures. It is not valid for stack-like structures.

General Format

DEFINE IS1893 2015 ( ACCIDENTAL )  LOAD PART4
1893-2015-P4-spec
weight-data

Refer to Common Weight Data for information on how to specify structure weight for seismic loads.

where

1893-2015-P4-spec =    ZONE f1 RF f2 I f3SS f4SA f11 } (ST f5) ( { DM f6DF f12 } ) (PX f7) (PZ f8)
Note: The ACCIDENTAL option should only be used for ST 4 structures.
ParameterDescription
ZONE f1 Zone factor based on Table 14 (Clause 7.3.2) FOR 5% damping and Table 15 for Other damping Ratios.
RF f2 Response Reduction factor. Refer Table 4 (Clause 8.6).
I f3 Importance factor as per Table 3 (Clause 8.5).
SS f4 Soil Type as per Annex B. (Clause 16) (Use if the SA parameter is not specified).
  • 1 = Type I, rock or hard soil
  • 2 = Type II, stiff soil
  • 3 = Type III, soft soil
Note: Use either SS orSA to specify site conditions. If both parameters are specified, SS is ignored.
ST f5 Structure Type as per Clause 8.1..
  • 1 = Category 1, Failure can cause loss of life
  • 2 = Category 2, Failure can lead to serious fire hazard
  • 3 = Category 3, Failure will not be a serious hazard
  • 4 = Category 4, All other structures
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. Damping is considered to be 5% for RC structures and 2% for Steel structures (Refer Table 2). Refer Clause 9.4 of IS:1893(Part 5) -2015.
Note: Use either DM orDF 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.
SA f11 Average response spectral acceleration coefficient corresponding to site specific spectra. Refer to Clause 7.3. Use if SS is not specified.
DF f12 Multiplying factor for calculating Sa/g.
Note: For additional details on the application of a seismic load definition used to generate loads, refer to GUID-6D9B5C48-9FFF-4548-BFB4-ACAE1E973743.

Notes

  1. If the ACCIDENTAL option is specified for an ST 4 structure, 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 the SELFWEIGHT, JOINT WEIGHT, and MEMBER WEIGHT commands you have specified.  The ACCIDENTAL option is not applicable to other structure types.

    The ACCIDENTAL 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 GUID-82BB33A3-5810-4CC0-A6D6-5EF0440C202B#GUID-82BB33A3-5810-4CC0-A6D6-5EF0440C202B

  2. By default, STAAD.Pro calculates natural periods of the structure in both X and Z directions respectively which are used in calculation for seismic force at each node. If PX and PZ are included, the program will consider these values for calculation of average response acceleration coefficient. If ST is used instead of PX and PZ values, then the program will calculate natural period using the Rayleigh method.

Example

DEFINE IS1893 2015 LOAD PART4
ZONE 0.36 RF 5 I 1.25 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 seismic force at each node is computed by STAAD.Pro for building structures as per IS: 1893 (Part 4) 2015:

V = Ah.W

where
Ah
=
the design seismic coefficient based on either clause 7.3.1 or 7.3.2. This value is calculated for each node.
  • For site-specific spectra, A h = S a g × I R
  • For standard specific spectra, A h = Z 2 × I R × S a g
W
=
Seismic weight of at each node.
Note: All symbols and notations in the above equation are as per IS: 1893(Part 1) 2016.
STAAD.Pro utilizes the following procedure to generate the lateral seismic loads:
  1. You provide seismic zone coefficient and desired 1893 specs through the DEFINE 1893 2015 LOAD PART4 command.
  2. The program calculates the structure period, T using the Rayleigh method.
  3. The program calculates Sa/g utilizing T.
  4. Calculate Ah from section 7.3.
  5. The program calculates V from the above equation at each node corresponding to the product of the seismic coefficient and mass at each node. W is obtained from mass table data entered via SELFWEIGHT, JOINT WEIGHT(s), MEMBER WEIGHT(S), and/or REFERENCE LOAD you provide through the DEFINE 1893 2015 LOAD PART4 command.

See TR.32.12 Generation of Loads for additional information.