Irregularity Analysis

Shearwalls now implements a more rigorous treatment of seismic irregularities from ASCE 7 12.3.2 and Tables 12.3-2 and 12.3-3 than before, and applies the irregularity analysis to determining

• whether the Equivalent Lateral Force (ELF) procedure used by Shearwalls is allowable for each Seismic Design Category, as per 12.3.3.1
• the new regularity criterion for limiting the value of the design seismic response parameter S_DS (see Update to ASCE 7-16 – Seismic Load Generation and Design ).
• the conditions in 12.3.4.2 whereby the redundancy factor ρ is equal to 1.0
• the application of the 25% increase in drag strut forces as per 12.3.3.4.

1. Input

   In the Site Information dialog:

   1. Irregularity Groups

      Checkboxes have been added for both the N-S and E-W force directions for the following single irregularities or groups of irregularities, which are checked if any one of them is deemed to exist:

      1. Horizontal Irregularities

         1a Torsional

         1b Extreme Torsional

         2 Corner, 3 Diaphragm or 4 Offset

         These correspond to the ASCE irregularities as follows:

         • Corner = Reentrant Corner
         • Diaphragm = Diaphragm Discontinuity,
         • Offset = Out-of-Plane Offset.

         If 1a Torsional is checked, then 1b Extreme Torsional is unchecked and vice-versa.

      2. Vertical Irregularities

         1a Soft story, 2 Weight or 3 Geometric

         1b Extreme Soft or 5a Weak Story

         4 In-plane Discontinuity

         5b Extreme Weak Story

         These correspond to the ASCE irregularities as follows:

         • Soft story = Stiffness – Soft Story
         • Weight = Weight (mass)
         • Geometric = Vertical Geometric
         • Extreme Soft = Stiffness – Extreme Soft Story
         • Weak Story = Discontinuity in Lateral Strength – Weak Story
         • In-plane Discontinuity = In-plane Discontinuity in Vertical Lateral Force Resisting Element
         • Extreme Weak Story = Discontinuity in Lateral Strength – Extreme Weak Story
   2. Files from Older Versions

      Previously the choices were

      Horizontal irregularity or in-plane vertical irregularity

      Other vertical irregularity.

      When opening project files from previous versions, if the first of these was checked, then 2 Corner, 3 Diaphragm or 4 Offset is checked for both directions. If the second was checked then 1a Soft story, 2 Weight or 3 Geometric is checked for both directions.

   3. Output

      The irregularities and directions selected are listed in the Site Information section of the Seismic Load Generation Details

   4. Use Detected Torsional Irregularity for Design

      A checkbox has been added called Use detected torsional irregularity for design. This means that if the program detects a torsional irregularity during the analysis of the structure for design, but one was not selected in the Site Dialog, it applies that irregularity to the determination of whether ELF is allowable, redundancy factor ρ, and the 25% drag strut force increase. Conversely, if no torsional irregularity is detected but one was selected in the Site Dialog, the torsional irregularity is not applied.

2. Verification of the Equivalent Lateral Force Procedure

   For Seismic Design Categories D, E or F with vertical irregularity 5b, and SDC E or F with horizontal irregularity 1b or vertical irregularities 1b or 5a, the program warns you that the Equivalent Lateral Procedure (ELF) is not permitted by ASCE 7 12.3.3.1. If only the horizontal Torsional irregularity 1b exists, this applies only if rigid diaphragm design is selected.

   The warnings appear if vertical irregularities 1b, 5a, or 5b are selected in the Site Information dialog, or if the following conditions are satisfied

   • Rigid diaphragm design is selected in the Structure Input view, and
   • Use detected torsional irregularity is not selected and Torsional Irregularity 1b is selected, or
   • Use detected torsional irregularity is selected and Torsional Irregularity 1b is detected by the program.
   1. Site Information Dialog

      If a combination of inputs leading to Seismic Design Category, and Irregularity inputs, in the Site Information dialog correspond to prohibited ELF, a message appears saying seismic design is prohibited by 12.3.3.1.

   2. Screen Messages

      When Load generation is invoked, if ELF is prohibited, warning messages appear on the screen informing you of the reason. Note that at this point, the Torsional irregularity has not been checked by the design routine, so only the input irregularities in the Site Information dialog are used.

      When shear wall design is invoked, similar warning messages appear, this time informed by the possible detection of the extreme Torsional irregularity.

   3. Design Results Output

      If ELF is prohibited, notes appear below the Seismic Information table and the appropriate Shear Results tables informing you of the reason and saying the results are for information only and should not be used to design the structure. A message appears in the Design Summary referring you to the Seismic Information table. The notes under the Shear results tables are displayed in red.

3. Redundancy Factor, ρ
   1. ASCE 7 Requirements

      For Seismic Design Categories D, E and F, structures with an extreme torsional irregularity (horizontal 1b), must have a ρ = 1.3 (in the case of E and F, the program will also warn that the Equivalent Lateral Force procedure is not allowed, see below). Otherwise, for these categories, satisfying one of two conditions allow ρ to be 1.0 rather than 1.3:

      Condition a – Involves removing elements and checking for story strength and existence of extreme torsional irregularity 1b with the elements removed.

      Condition b – The structure is regular in plan (no horizontal irregularities), and a check is made for sufficient perimeter framing.

   2. Existing Implementation

      Version 11 of Shearwalls detected the extreme torsional irregularity 1b to automatically assign ρ = 1.3 to structures in SDC D, E, and F, and to perform the checks for condition a with the elements removed. For condition b, it incorrectly used the Other vertical irregularity input to check for regularity of the structure.

   3. New Implementation
      1. Extreme Torsional Irregularity

         For rigid diaphragm design, the program assigns ρ = 1.3 for SDC D-F if

         • Use detected torsional irregularity is not selected in the Site Information dialog and Torsional Irregularity 1b is selected, or
         • Use detected torsional irregularity is selected and Torsional Irregularity 1b is detected by the program.
      2. Condition a

         The program detects the possible extreme torsional irregularity when elements are removed; the input in the Site Information dialog is not used to determine this irregularity.

      3. Condition b

         The plan irregularity check is made by checking for torsional irregularities 1a or 1b in the same manner described in Extreme Torsional Irregularity (i above). The structure is also deemed irregular if any other horizontal irregularity is checked in the Site information dialog.

4. 25% Drag Strut Increase
   1. ASCE 7 Requirements

      According to 12.3.3.4, for Seismic Design Categories D-F, the collector (drag strut) forces based on the diaphragm design force F_px from 12.10.1.1, is increased by 25% if the structure has any horizontal irregularity relevant to Shearwalls, or if it has vertical regularity 4, In Plane Discontinuity.

   2. Existing Implementation

      In Version 11, the increase was applied for SDC D-F to all drag struts in the building if you selected Horizontal Irregularity or In-Plane vertical irregularity in the Site information dialog

   3. New Implementation

      For Seismic Design Categories D-F

      1. Torsional Irregularity

         For rigid diaphragm design, the 25% increase is applied if

         • To all drag struts in the building if Use detected torsional irregularity is not selected in the Site Information dialog and Torsional Irregularity 1a or 1b is selected, or
         • Use detected torsional irregularity is selected and Torsional Irregularity 1a or 1b is detected by the program on the level with the drag strut.
      2. Input Irregularities

         The 25% increase is applied to all drag struts in the building if any of the other horizontal irregularities (than torsional irregularities) are selected in the Site Information dialog, or if Vertical Irregularity 4 – In plane discontinuity is selected.

      3. Horizontal Irregularity 4 – Out-of-Plane Offset

         The 25% increase is applied if on the level above the drag strut, anywhere on the level above the shearline with the drag strut, there is a shearline in that direction with no line on the level below. This is regardless if this irregularity is checked in the Site Information dialog.

      4. Output

         In the Collector Forces table (formerly Drag strut forces), a line has been added to the legend to explain the conditions under which the 25% increase is applied.

5. Limit on Value of S_DS

   Refer to 7d)iii, under Maximum SDS Value in the Determination of Cs and Ev (12.8.1.3), in for the application of the new regularity criterion to this provision.

6. Perforated Wall Note (Change 105d)

   A note has been removed from the Hold-down Design table regarding Irregularities not being checked.