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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
The program now implements ASCE 7 12.8.4.3, the amplification of accidental torsional moment, for Seismic Design Categories C-F.
As this is required only for buildings with torsional irregularity, the program now automatically detects torsional irregularity 1a from table 12.3-1.
On each level of the building and in each direction the program determines if the largest of the two end wall storey drifts δmax is greater than1.2 times the average of the end wall storey drifts δave. In doing so it takes into account both +ve and –ve accidental eccentricities, and uses the worst of these to determine the irregularity and the resulting amplification factor Ax.
If an irregularity is detected on a particular level and direction, the program calculates the amplification factor from Eqn 12.8-14.
Ax = ( δmax / 1.2 δave )2
The factor is limited to a maximum of 3.0, and is only applied if it is greater than 1.0.
In the equation for torsion becomes ,
T = ( CL-CR +/- ae * Ax ) * F,
where CR = Centre of rigidity, CL - Center of load or mass, F -concentrated force, ae - accidental eccentricity:
Note that multiplying both the positive and negative accidental eccentricity by Ax always results in an increase in shearline force. If the inherent eccentricity is larger than the accidental eccentricity, it is true that in one direction the overall torsion is diminished, but that is always the least critical of the torsions that is rejected by the program. If the accidental eccentricity is larger than inherent, then both amplified torsions are needed as they are critical for shearlines to the right and to the left of the centre of rigidity.
If an amplification factor is applied in a particular direction, then the factor is output in the torsional analysis section of the log file, and the equations for torsion T adjusted to show ae multiplied by Ax.
If an amplification factor has been applied anywhere in the structure, a note appears under the Seismic Information table indicating a torsional irregularity was detected, the Ax factor was applied, and referring you to the log file for more details.