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Choosing Rigid and/or Flexible Diaphragms
In the Structure input form, the program allows you to select whether to model your floors and roof as rigid diaphragms and/or flexible diaphragms. If you believe you either flexible or rigid diaphragms, then select just one of these methods. However, you may wish to keep both of these options checked for a semi-rigid situation, and also make sure that worst-case rigid vs.flexible is set in the Design settings. You can also leave the worst-case setting unchecked and examine the results for rigid and for flexible and to choose a wall which covers both situations manually.
Neither the NBC nor the CSA O86 give guidance as to whether flexible or rigid analysis should be used.
Force Distribution Parameters for Rigid and Flexible Diaphragms, Wind and Seismic Design
The following procedures are followed based diaphragm flexibility, wind and seismic design provisions, and wind load generation methods. Refer to Rigid Diaphragm Distribution for the definitions of inherent and accidental eccentricity.
Method |
% load |
Diaphragm |
Inherent Eccentricity |
% used for ae |
NBC 2015 |
NBC 2010 |
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Seismic |
100% |
Rigid |
Yes |
10% |
4.1.8.11.(11), Commentary 175 |
4.1.8.11.(10) |
Seismic |
100% |
Flexible |
No |
5% |
Commentary J174 |
Commentary J 172 |
100% |
Rigid |
Yes |
0 |
4.1.7.6; Fig 4.1.7.6.-A |
Fig. I-7, Commentary 36 |
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100% |
Rigid |
Yes |
0 |
4.1.7.9.(1).(a), Commentary 30-32 |
4.1.7.3 .(1).(a); Commentary 36,37, Fig. I-16 Case A |
*Shearwalls does not implement NBC 4.1.7.9.(1)(b), which calls for 75% of the wind load plus 15% accidental eccentricity, or clauses (c) and (d) which call for simultaneous loading in both orthogonal directions.
Output
Even if only one set of walls is designed for the worst-case of rigid and flexible diaphragms, and although one set of walls is always designed for the worst case of wind and seismic loading, the program outputs 4 separate set of results for rigid wind, rigid flexible,
All the intermediate calculations for torsional analysis for each load case are shown in the Torsional Analysis Details output report. Separate results are shown for each force direction, and each wind load case.