Vertical Distribution of SFRS Strength

1. Weak Story Restriction

   SDPWS Section 4.1.8 Vertical Distribution of Seismic Force Resisting System (SFRS) Strength has been added, which does not permit structures in Seismic Design Categories D, E and F to have a story with less total shear resisting strength than the story above unless the ratio of the story shear to the story strength on the lower level is less than the ratio of the strength of the story below to the one above. In mathematical terms, a weak story exists unless.

   V_r(i) / V_r(i+1) >= 1

   or

   V_r(i) / V_r(i+1) > V_(i) / V_r(i)

   1. Calculation
      1. Shear Resistance V_r

         The shear resistance V_r is the sum of the ASD-factored shear resistances of all shear walls in a force direction, also applying any applicable design factors as listed in Design Factors .

      2. Design Shear Force V_(i)

         The shear force V_(i) is the ASD-factored total shear force on the level; i.e. the sum of the design shear force in all shear lines in a direction, which includes any applicable factors such as the redundancy factor r.

         It is the vertically accumulated shear force that arises from building masses on the level under evaluation and all levels above.

   2. Output
      1. Seismic Information Table

         A section Weak Story (SDPWS 4.1.8) has been added below the Seismic Information table. For seismic categories , E and F, it shows:

         • a list of stories which have a lower resistance than the story above.
         • for any of those stories, the ratios V_r(i) / V_r(i+1 and V_(i) / V_r(i) .
         • a red failure message if any story fails the check.

         For seismic design categories A-C, a note indicates that the check is not required.

      2. Design Summary

         If this check fails for any level, a red failure message appears in the Design Summary referring you to the Seismic Information table for more details.

   3. ASCE 7 Weak Story Irregularity

      This is a check for a weak story that is in addition to the Weak Story irregularity restrictions from ASCE 7 Table 12.3-2 irregularities 5a and 5b and ASCE 7 12.3.3.1 and .2.

      The ASCE 7 checks are less stringent in terms of strength ratio, being applied when the weak story strength is 80% or 65% of the strong story, but they do not offer the proviso regarding the ratio of strength to resistance on the lower level.

      Please note that Shearwalls does not perform ASCE 7 irregularity analysis and does not check for irregularities 5a and 5b, so the addition of the SDPWS weak-story check does not fully cover all weak-story obligations. You must check for ASCE weak story irregularities even if SDWPS 4.1.8 passes.