|
|
|
|
|
The program does not create a separate load case to determine shear line forces for drift-limit deflection; instead, it uses the ASD shear line forces multiplied by a factor α representing the difference in wind pressures between those used for serviceability and for MWFRS design as well as the difference in load combination factors.
Since wind pressures are given in ASCE 7 27.3.2 and 28.3.2 by P = d * Kz * Kd * Kzt * V2 * G * Cp, in which all terms except for V are the same, this factor is
α = PSERV / PMWFRS / 0.6 = (VSERV / VMWFRS)2 / 0.6
Rigid Diaphragm Method
The assumption that shear line forces are proportional to wind pressures is not strictly true for the rigid diaphragm method due to nonlinearity in the torsional distribution to shear lines. However, for typical asymmetric buildings with a significant torsional component, the difference in shear line forces using this approximation compared with generating and distributing reduced wind loads Wa is less than 1%.
Distribution to Segments within Line
The process of distributing forces to segments in a line such that they equalize deflections is performed independently for serviceability wind forces, that is, it does not factor each segment deflection by α, it factors the total shear line force then distributes this force to the segments.