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For seismic design, on each level and for both directions, Shearwalls checks the maximum story drift computed as per ASCE 7 12.8.6 and Equation 12.8-15 against the limits in ASCE 7 12.12.1.
12.8.6 indicates that he design story drift is to be computed at the center of mass, except for structures in seismic design category C, D, E or F having torsional irregularity (Horizontal Irregularity 1 in Table 12.3-1), which must use the largest story drift at the edges of the structure.
ASCE 12.3.1.1 indicates that all shear lines must satisfy story drift limitations for the building to be modeled with rigid diaphragms, therefore both the deflection at the center of mass and the maximum deflection at any shear line are both shown in the Story Drift table.
Maximum Deflection
The maximum deflection dxe in is the largest deflection on any shear line, calculated as described above. If deflections on a line have been equalised , it is the common deflection of all walls on the line. If not, it is the largest deflection for any segment on the line.
If the design is rigid diaphragm, torsionally irregular and seismic zone C or worse, the highest deflection of all shear lines that have exterior walls is used for the story drift check.
Deflection at Center of Mass
The values of δx and δxe are determined at the center of mass, calculated by straight line interpolation of the deflections at the two shear lines on either side of the center of the mass.
Importance Factor I
The importance factor I calculated from the Risk category entered in the site dialog is used, and is shown below the table.
Deflection Amplification Factor Cd
The values of the deflection amplification factor Cd from the Site dialog for each direction are used. Unless they were over-ridden, they are the values from ASCE 7 Table 12.2-1. They are shown below the table.
Warning Messages
Failure messages appear beneath the table for each of the conditions – torsionally irregular rigid, other rigid, and flexible diaphragms. In addition, if centre of mass is used and the centre of mass is within limits, but another shear line is not, a warning appears