|
|
|
|
|
The allowable drift Da is calculated according to NBC 4.1 8.13.(3) for seismic design and NBC 4.1.3.5.(3) for wind design as a proportion of the storey heignt hs. For seismic design, for Normal and Importance categories it is 2.5% of hs. ,for High importance it is 2% and for Post-disaster it is 1%. If gypsum wallboard is present and used to resist shear , the drift ratio is 1% as per O86 11.8.8. The percentage used is shown in the legend to the Storey Drift table. For wind design, it is 1/500 hn = 0.2%. The drift limit in feet or meters is shown in the Story Drift table for each level of the building.
Story height
The story height hsx for each level is the wall height plus the upper floor thickness. The wall height on the gable end of the structure extends to the eave only, so that the triangular portion of the gable end is neglected. (The deflection calculation used to determine story drift assumes rectangular walls, and it can be assumed that the roof as a unit deflects the same amount as its juncture with the supporting wall).
Importance Category
The Importance category is input in the Site Information dialog.