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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.
ASCE 7 12.3.1.1 for seismic design permits wood structures to be designed using flexible diaphragm analysis as long as concrete topping greater than 1.5" thick has not been applied to the diaphragm, and the story drift requirements have been met on all shear lines. Note that ASCE 12.8.6 defines the design story drift for flexible diaphragms as the deflection at the center of mass, so the drift requirements for determining diaphragm flexibility are somewhat more stringent than those that must be followed in all cases.
SDPWS 4.2.5.2 requires that open-front structures, i.e those that are have significantly cantilevered diaphragms, be either modeled as semi-rigid or idealized as rigid, so flexible diaphragm analysis alone should not be used for these structures.
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.
ASCE 7 Method |
Diaphragm |
Torsional Analysis |
% load |
Inherent Eccentricity |
% used for accidental eccentricity |
Direct Shear Line Force Based on |
Simultaneous E-W and N-S |
ASCE 7 Provision |
|
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Seismic |
Yes |
100% |
Yes |
0% or 5%1 |
Stiffness |
No |
12.8.4.2 |
|
Seismic |
No |
100% |
No |
0% |
Tributary |
No |
12.8.4.1 |
|
Wind - Envelope (low-rise) |
Yes |
100% |
Yes |
0% |
Stiffness |
Yes |
Fig 28.3-14 |
|
Wind - Envelope |
No |
100% |
No |
0% |
Tributary |
No |
Fig 28.3-1 |
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Wind - Directional (all-heights) Case 1 |
Yes |
100% |
Yes |
0% |
Stiffness |
No |
Fig 27.3-8, Case 1 |
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Wind - Directional Case 1 |
No |
100% |
No |
0% |
Tributary |
No |
Fig 27.3-8, Case 1 |
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Wind - Directional Case 2 |
Yes |
75%3 |
Yes |
15% 2,3 |
Stiffness |
No |
Fig 27.3-8, Case 2; Eqn. 27.3-4 . |
|
Wind - Directional Case 2 |
Yes |
75%3 |
No |
15% 2.3 |
Tributary |
No |
Fig 27.3-8, Case 2; Eqn. 27.3-4 . |
1Accidental eccentricity applied only if the structure is torsionally irregular, i.e. the amplification factor Ax from ASCE 7 12.4.3 is greater than 1.0 for Seismic Design Categories C-F and greater than 1.2 for SDC B.
2For structures requiring dynamic analysis, you must input the eccentricity after calculating it using ASCE 7 Eqn. 27.3-4 .
3You can modify this value in Shearwalls as per Commentary C27.4.6.
4 The principal load case shown in the main part of the Figure; Shearwalls does not implement the torsional low-rise load cases given in Note 5.
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.