STAAD.Pro Help

TR.31.2.9 IS:1893 (Part 1) 2002 & Part 4 (2005) Codes - Lateral Seismic Load

This feature enables one to generate seismic loads per the IS:1893 specifications using a static equivalent approach. Both Part 1 (2002) for building structures and Part 4 (2005) for industrial and stack-like structures are available.

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 1893 ( ACCIDENTAL ) LOAD ( PART4 )
ZONE f1 { 1893-spec-part11893-spec-part4  }
weight-data

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

(CHECK SOFT STORY)

where

1893-spec-part1 =    RF f2 I f3SS f4SA f11 } (ST f5) ( { DM f6DF f12 } ) (PX f7) (PZ f8) ( { DT f9 | GL f10 } )
1893-spec-part4 =    RF f2 I f3SS f4SA f11 } ST f5 ( { DM f6DF f12 } ) (PX f7) (PZ f8) ( { DT f9 | GL f10 } ) ( CS f13 ) ( AX f14 ) ( ES f15 ) CV f16 DV f17

where

ParameterDescription
ZONE f1 Seismic zone coefficient. Refer to Table 2 of IS:1893 (Part 1)-2002.
RF f2 Response reduction factor. Refer Table 7 of IS: 1893 (Part 1) -2002 or Table 3 of IS: 1893 (Part 4) -2005.
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 6 of IS: 1893(Part 1)-2002 or Table 2 of IS: 1893 (Part 4)-2005.
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 Clause 6.4.5 of IS: 1893 (Part 1) -2002 or Clause 8.3.2 of IS: 1893 (Part 4) -2005.
  • 1 = hard soil
  • 2 = medium soil
  • 3 = soft soil
Note: Use either SS orSA to specify site conditions. If both parameters are specified, SS is ignored.
ST f5
For IS 1893 Part 1, the program will calculate natural period as per Clause 7.6 of IS:1893(Part 1)-2002.
  • 1 = RC frame building
  • 2 = Steel frame building
  • 3 = All other buildings
For IS1893 Part 4, the program will calculate period as per Clause 14.1 of IS:1893(Part 4)-2005 for stack-like structures. For Category 1 Industrial Structures base shear is calculated as twice the base shear of other structures as per Clause 8.3 of IS:1893(Part 4)-2005.
  • 1 = Category 1 industrial structure (Part 4)
  • 3 = All other industrial structures
  • 5 = stack-like structures
Note: This parameter is optional for Part 1, but is required for Part 4.
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 Table 3 of IS:1893(Part 1)-2002.
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.
Note: See Note 'b' below.
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.
Note: See Note 'b' below.
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.
Note: Use either DT or GL to specify foundation depth. If both parameters are specified, DT is ignored.
GL f10 Y coordinate of ground level. A reduced lateral force is applied to levels below this height, per Clause 6.4.4.

Used for Y if the SET Z UP command is used.

SA f11 Average response spectral acceleration coefficient corresponding to site specific spectra.
DF f12 Multiplying factor for calculating Sa/g.
CS f13 Coefficient as given in Table 6 of IS:1893(Part 4)-2005. Valid only for stack-like structures for calculating fundamental time period per Cl. 14.1.
AX f14 Area of cross-section at the base of stack-like structures for calculating fundamental time period per Cl. 14.1 of IS:1893(Part 4)-2005.
ES f15 Modulus of elasticity of material of stack-like structures for calculating fundamental time period per Cl. 14.1 of IS:1893(Part 4)-2005.
CV f16 Coefficient of shear force given in Table 6 of IS:1893(Part 4)-2005. Required for stack-like structures (ST 5).
DV f17 Distribution factor for shear force given in Table 11 of IS:1893(Part 4)-2005. Required for stack-like structures (ST 5).
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, 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 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 GUID-82BB33A3-5810-4CC0-A6D6-5EF0440C202B#GUID-82BB33A3-5810-4CC0-A6D6-5EF0440C202B

    To 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.

  2. By default STAAD calculates natural periods of the structure in both X and Z directions respectively which are used in calculation for base shear. 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 depending upon the empirical expression given in IS: 1893 (Part 1)-2002 or IS: 1893 (Part 4)-2005.

  3. In the case where no rigid floor diaphragm is present, STAAD 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:

    12EI / L3

    where
    E
    =
    Young’s modulus
    I
    =
    moment of inertia
    L
    =
    length of the column

    The lateral stiffness for a shear wall (without opening) is calculated as:

    1 P h 3 12 E I + 1.2 P h A G

    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.

    where
    h
    =
    height
    A
    =
    cross-sectional area
    G
    =
    shear modulus of the wall

    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

Example

DEFINE 1893 LOAD
ZONE 0.36 RF 5 I 1 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

Methodology

The design base shear is computed by STAAD.Pro for building structures as per IS: 1893 (Part 1) 2002 equation 7.5.3 or for industrial structures as per (Part 4) 2005:

V = Ah.W

Where:

A h = Z 2 I R S a g

When site specific spectra is used per IS 1893 (Part 4) 2005, then:

A h = I R S a g

For stack-like structures, the design base shear is computed as per IS 1893 (Part 4) 2005 as:

V = CvAh.W·Dv

Note: All symbols and notations in the above equation are as per IS: 1893(Part 1) 2002 and IS: 1893 (Part 4) 2005.
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 LOAD command. Use the PART 4 command option to specify using IS: 1893 (Part 4) 2005.
  2. The program calculates the structure period, T.
  3. The program calculates Sa/g utilizing T.
  4. 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/or REFERENCE LOAD you provide through the DEFINE 1893 LOAD command.
  5. 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.

Soft Story Checking

Tip: When a rigid floor diaphragm is used, a soft story check may be initiated within that command without some of the limitations imposed on soft story checking in a seismic load.

As per the IS1893-2002 code Clause 7.1, to perform well during an earthquake a building must have simple and regular configuration, adequate lateral strength, stiffness and ductility.  This is because a building with simple regular geometry and uniformly distributed mass and stiffness in plan as well as in elevation, will suffer much less damage than buildings with irregular configurations.

According to this standard, a building can be considered irregular, if at least one of the conditions given in Table 4 - Plan Irregularities and Table 5 - Vertical Irregularities, of IS1893-2002 is applicable.

For IS 1893 2002, STAAD.Pro has implemented the methodology to find vertical stiffness irregularities, as given in IS 1893-2002 Table 5 Sl No. (1) i) a) and Sl No. (1) i) b), in the form of soft story checking.

  • Stiffness Irregularities: Soft Story – As per this provision of the code, a soft story is one in which the lateral stiffness is less than 70 percent of that in the story above or less than 80 percent of the average lateral stiffness of the three story above.
  • Stiffness Irregularities: Extreme Soft Story – As per this provision of the code, a extreme soft story is one in which the lateral stiffness is less than 60 percent of that in the story above or less than 70 percent of the average lateral stiffness of the three story above.

    Thus, if any story of a building is found to be soft or extremely soft, the building is likely to suffer much damage in an earthquake than a similar type of building but has more regular vertical stiffness.

Note: STAAD.Pro identifies column and shear wall (without opening) as vertical component for the purpose of computing lateral stiffness of the story. The vertical stiffness of a column is calculated as 12EI / L3 where E is the Young’s modulus, I is the moment of inertia and L is the length of the column respectively and that for a shear wall (without opening) is calculated as Ph3/3EI + 1.2Ph/AG (i.e., summation of flexural stiffness and shear stiffness, obtained as deflection of a cantilever wall under a single lateral load P at its top) where h is the height, A is the cross-sectional area, and G is the shear modulus of the wall (E and I are Young's modulus of elasticity and moment of inertia, respectively). The summation of lateral stiffness of all columns and shear walls at a particular floor level constitute the total lateral stiffness of that particular story or floor level. The program checks 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.