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TR.28.2.2 Check Irregularities

STAAD.Pro can check irregularities per the IS 1893 2016 and ASCE 7-05/10/16 seismic codes.

For IS 1893 2016, the program can check horizontal irregularities (torsional and reentrant corners) per Table 5 and vertical irregularities (mass irregularities and irregular modes of oscillation) per Table 6.

For ASCE 7-05/10/16, the program can check horizontal irregularities (torsional and reentrant corners) and vertical irregularies (mass). Irregular modes of oscillation are not considered for this code.

General Format

Note: This command is inserted after any pre-analysis PRINT commands. 
CHECK IRREGULARITIES CODE [ ASCE7 | IS1893 2016 ]
Note: Soft story checks per IS 1893 2016, Table 6(i) can also be performed by the program. Refer to TR.28.2.1 Soft Story Checking. For ASCE 7, soft story checking is performed per figure C12-3, Type-1.
Note: Only one code may be checked per irregularities in a single model.

Torsion Check per ASCE 7

Torsion checks per Cl. 12.3.2.1 of ASCE 7 2016 ( ref. fig C12.3-1 Type 1) are preformed by applying unit loads in each orthogonal direction to the control node of the diaphragm.

Example floor diaphragm torsion evaluation

The unit load, Fx, is applied at each diaphragm in the X-direction. After analysis, the program locates the extreme nodes of the diaphragm and then dmin and dmax are calculated. The ratio of dmax/dmin is evaluated and compared to the code limit of 1.5. This process is then repeated in the Z direction for this diaphragm. Then the similar process is applied to the other diaphragms in the structure.

Torsion Check per IS 1893

Torsion checks per Table 5(i)-a IS 1893 2016 are performed by applying a unit force in each orthogonal direction at two different distances (the design eccentricity, edi) as per Cl. 7.8.2. This results in evaluating four different conditions for IS 1893 2016. The analysis is run and the displacements of the two extreme ends of the diaphragm are extracted from the analysis results. This ratio of these displacements is then reported with a status based on the following limits:

Ratio Status Description
Δmax/ Δavg < 1.2 OK Indicates that the floor is regular and passes the irregularity checks.
1.2 ≤Δmax/ Δavg ≤1.4 Warning Indicates that the floor is irregular and requires a full 3D dynamic analysis in order to justify the structural configuration or requires a change in structural configuration as per Cl. 7.1, Table 5, Sl No. (i) sec-i.a and sec-i.b.
Δmax/ Δavg > 1.4 Fails Indicates that the floor is irregular and fails the irregularity checks. The structural arrangement must be changed as per Cl. 7.1, Table 5, Sl No. (i) sec-ii.
e di = { 1.5 e si + 0.05 b i e si - 0.05 b i (Design eccentricity per IS 1893 2016 Cl. 7.8.2)
where
esi
=
the static eccentricity at floor i, which is equal to the distance between the center of mass and the center of rigidity.
bi
=
the floor plan dimension of floor i perpendicular to the direction of force

Example floor diaphragm torsion for IS 1893

The unit load, Fx is applied at each diaphragm in the X-direction in combination with two cases of My = Fx× (edi - esi) representing the to cases for edi. After analysis, the program locates the extreme nodes of the diaphragm and then Δmin, Δmax, and Δavg values are calculated. The ratio of Δmax/Δavg is evaluated per the table above. This process is then repeated in the Z direction for this diaphragm for a total of four cases at each diaphragm. Then the similar process is applied to the other diaphragms in the structure.

Note: Checks mandated by IS 1893 2016, Table 5(i)-b are not implemented as the prerequisite modelling information to determine the torsional mode is not present in STAAD.Pro.

Reentrant Corners

The program will check for reentrant corners per Table 5(ii) of IS 1893 2016 and Cl. 12.3.2.1 of ASCE 7 2016 ( ref. fig C12.3-1 Type 2). The program first automatically identifies the boundary topology of the diaphragm. That is, the order in which analytical members and nodes of the boundary of the diaphragm are joined together to for a closed polygon. The program then determines the reentrant, or concave, corners of the diaphragm. The two analytical members joining the reentrant node is determined, their lengths are calculated, and then the projections of those lengths are calculated. To determine the ratios, the following are calculated:

Li×cos(α)/Lx ≤ 0.15

Li×cos(α)/Lz ≤ 0.15

If both members from the reentrant node are orthogonal, then for each member, only 1 ratio is calculated as Li/Lx for members oriented along the x-direction and Li/Lz for members oriented along the z-direction.

Example reentrant corner topology

Note: Cantilever members that do not form part a closed polygon are identified. These members are ignored by the program when evaluating reentrant corners.

All dependent nodes that form part of a diaphragm must lie within the same plane. Further, the boundary dependent nodes must form a closed polygon. Improper modeling of diaphragms may yield incorrect irregularity status results.

Note: Member offsets both in plan as well as in elevation may result in the reentrant corner check to be incorrect. You must exercise caution while using offsets and should check such joints manually.

Mass Irregularities

The program will check for mass irregularities per Table 6(ii) of IS 1893 2016 and Cl. 12.3.2.2 of ASCE 7 2016 (ref. fig C12.3-2 Type 2). The mass of each floor is calculated as the total of all floor diaphragm masses at the same level. The ratio of each floor mass to the floor above and below is calculated. These ratios are compared to the code stipulated values.

Irregular Modes of Oscillation

Note: The program will check for irregular modes of oscillation per the IS 1893 2016 code. This check is performed only if you have include the static seismic IS 1893 2016 definition using zone 4 or 5.

Per Table 6(vii)-a, the sum of the percentage of mass participation for the first 3 lateral translational modes should contribute at least 65% in each principle plan direction. This is checked in both orthogonal directions.

Per Table 6(vii)-b, the time period for the fundamental modes in one direction should differ by at least 10%.

Note: Irregular modes of oscillation are not considered the ASCE 7 codes.

IS 1893 2016 Example

An example using IS 1893 2016:

FLOOR DIAPHRAGM
DIA 1 TYPE RIG HEI 3
DIA 2 TYPE RIG HEI 6
DIA 3 TYPE RIG HEI 9
CHECK IRREGULARITIES CODE IS1893 2016

ASCE 7 Example

An example using ASCE 7:

FLOOR DIAPHRAGM
DIA 1 TYPE RIG HEI 0
DIA 2 TYPE RIG HEI 5
CHECK IRREGULARITIES CODE ASCE7

IS 1893 2016 Example Output

An example output section of an IS 1893 2016 seismic irregularities check:

-IRREGULARITY CHECKS

                    STAAD.PRO IRREGULARITIES CHECK - (  IS1893-2016  )   v1.2 
                    *********************************************************

                         Including Amendment no. 2 November 2020
                    *********************************************************

--TORSION IRREGULARITY CHECKS

 Torsion Irregularity Check
 Ref: Table 5 (i)    - Ratio Limit(s): Lower-1.20 Upper-1.40
 ---------------------------------------------------------------------
 edi : Design Eccentricity
 esi : Static Eccentricity
 bi  : Floor/Diaphragm plan dimension perpendicular to force direction
 For Details Refer Clause 7.8 IS1893:2016-Part-1
 ---------------------------------------------------------------------


 Using edi = 1.5esi + 0.05bi
 ---------------------------
 Displacement of extreme points of diaphragm(dia.) in X dir.
 ------------------------------------------------------------------------
  Dia.  Node Max. Disp.  Node Min. Disp. Avg. Disp. Max./Avg.  Status
                 (mm)             (mm)      (mm)      Disp.
 ------------------------------------------------------------------------
    1      7    0.4984      2    0.4662    0.4823    1.0333     PASS   
    2     12    2.4871      9    2.3990    2.4430    1.0180     PASS   
    3     16    6.1637     13    6.0157    6.0897    1.0122     PASS   

 Using edi = esi - 0.05bi
 ------------------------
 Displacement of extreme points of diaphragm(dia.) in X dir.
 ------------------------------------------------------------------------
  Dia.  Node Max. Disp.  Node Min. Disp. Avg. Disp. Max./Avg.  Status
                 (mm)             (mm)      (mm)      Disp.
 ------------------------------------------------------------------------
    1      2    0.4984      7    0.4662    0.4823    1.0333     PASS   
    2      9    2.4871     12    2.3990    2.4430    1.0180     PASS   
    3     13    6.1637     16    6.0157    6.0897    1.0122     PASS   

 Using edi = 1.5esi + 0.05bi
 ---------------------------
 Displacement of extreme points of diaphragm(dia.) in Z dir.
 ------------------------------------------------------------------------
  Dia.  Node Max. Disp.  Node Min. Disp. Avg. Disp. Max./Avg.  Status
                 (mm)             (mm)      (mm)      Disp.
 ------------------------------------------------------------------------
    1      3    0.2699      2    0.2383    0.2541    1.0622     PASS   
    2     10    1.0087      9    0.9347    0.9717    1.0381     PASS   
    3     14    2.0463     13    1.9160    1.9812    1.0329     PASS   

 Using edi = esi - 0.05bi
 ------------------------
 Displacement of extreme points of diaphragm(dia.) in Z dir.
 ------------------------------------------------------------------------
  Dia.  Node Max. Disp.  Node Min. Disp. Avg. Disp. Max./Avg.  Status
                 (mm)             (mm)      (mm)      Disp.
 ------------------------------------------------------------------------
    1      3    0.3916      2    0.2319    0.3118    1.2562   WARNING* 
    2     10    1.3253      9    0.9131    1.1192    1.1841     PASS   
    3     14    2.5782     13    1.8732    2.2257    1.1584     PASS   

   *** WARNING: The floor is irregular.  Please ensure conformance  
    with Cl. 7.1, Table 5, Sl No. (i) sec-i.a or sec-i.b.

ASCE 7 Example Output

An example output section of an ASCE 7-2016 seismic irregularities check:

                    STAAD.PRO IRREGULARITIES CHECK - (  ASCE7-2016   )   v1.0 
                    *****************************************************
 
 --TORSION IRREGULARITY CHECKS 
 
 Torsion Irregularity Check
 Ref: Fig. C12.3-1 T1- Ratio Limit(s): 1.20, 1.40
 ------------------------------------------------
 
 Dia.   Extreme Points of Dia in X        Extreme Points of Dia in Z
       Node    Disp.    Node    Disp.    Node    Disp.     Node    Disp.
               (mm)             (mm)             (mm)              (mm)
 ------------------------------------------------------------------------
    1      3   0.09130      1   0.09993      4   0.09484      1   0.10559
    2     15   0.29636     13   0.30993     16   0.29819     13   0.34410

             Diaphragm ΔX-max/avg ΔZ-max/avg Status
             --------------------------------------
                   1     1.0451   1.0537        OK    
                   2     1.0224   1.0715        OK    
 --GEOMETRY IRREGULARITY CHECKS 
 
 Re-Entrant Corner Check.
 (Ref: Fig. C12.3-1 T2- Ratio Limit: 0.15 )
 ------------------------------------------
 
     Node      Re-Entrant X-Proj  X-Proj/Lx Z-Proj  Z-Proj/Lz  Status
  Connectivity     Node    ( m)              ( m)
 ----------------------------------------------------------------------
         6->         5      0.0000  0.0000    7.0000  0.7778 Re-Entrant
         4                  1.0000  0.2000    0.0000  0.0000
        18->        17      0.0000  0.0000    7.0000  0.7778 Re-Entrant
        16                  1.0000  0.2000    0.0000  0.0000

                   Diaphragm:    Lx:       Lz:
                                 ( m)     ( m)
                   ----------------------------
                        1      5.0000    9.0000
                        2      5.0000    9.0000
 --MASS IRREGULARITY CHECKS 
 
 Mass Irregularity Check
 Ref: Fig. C12.3-2 T2- Ratio Limit: 1.50
 ---------------------------------------
 
 Dia.   Level     Mass       Above      Below     Ratio  Ratio  Status
        ( m)      ( kN)      ( kN)      ( kN)     Above  Below
 ---------------------------------------------------------------------
    1     0.000    341.643    253.287    Base      1.349   N/A     OK    
    2     5.000    253.287    Top        341.643    N/A   0.741    OK
Parent topic: TR.28.2 Floor Diaphragm
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