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Important Note – These are descriptions to changes implemented in WoodWorks Shearwalls for version 10.0 and may not reflect current behavior
ASCE-7 10 has provided less restrictive conditions for which flexible diaphragm assumptions can be made for seismic design, such that any light frame construction without concrete topping on the diaphragm can be idealised as flexible, so long as each shearline complies with storey drift limitations in 12.2-1 (which are ordinarily required to be met only at the centre of mass of the structure). For this reason, and in order to speed up processing time for complex structures, Shearwalls now allows you to choose whether to design for rigid diaphragms, flexible diaphragms, or both.
A data group called Diaphragm flexibility has been added, with the options Rigid analysis and Flexible analysis. You can select one or both of these, but cannot leave both unchecked.
They are both checked by default, and are saved with the project file.
When only diaphragms are chosen, the program does flexible diaphragm force distribution and shearwall design using these forces. Neither rigid distribution or design is performed, saving considerable processing time.
When only rigid diaphragms are chosen, the program first does flexible distribution (but not shearwall design), then rigid distribution and design. Flexible distribution is needed to provide the upper level forces used for rigid diaphragm analysis of multi-storey structures. Flexible load distribution is much less costly in terms of processing time than shearwall design, especially when deflections are used to determine shearwall stiffness.
Previously, the program used the walls designed for flexible diaphragms to get the rigidities to distribute loads to the shearlines for the initial iteration of the rigid diaphragm procedure. This is no longer done, and the length of each shearline is used as the starting rigidity for the line. Although this was done to eliminate the need for flexible diaphragm design prior to rigid design, the change has been made even the flexible method is chosen along with the rigid method.
Through experimentation, we determined that the rigidities estimated at via shearwall length the provided a better starting point for the rigid diaphragm procedure than the walls designed for the flexible method, because they are not as influenced by the force distribution using the flexible procedure. The flexible procedure in general is quite different than the rigid procedure and a starting point that is often too far removed from the eventual rigid distribution.