Aspect Ratios

Aspect Ratio-This is very technical

An aspect ration is the ratio between the height and the width.   For example, a shear wall that is 8 feet long and 4 feet wide has an aspect ration of 2h/1w (the height is twice as long as the width).

To use the values listed in the shear wall tables above, a shear wall must have a 2/1 aspect ratio or less.  If the aspect ratio is greater 2/1 but less than 3.5/1, the per linear foot of earthquake resistance of the shear wall must be reduced by what is called a reduction factor.

This maximum 3.5/1 aspect ratio translates into an 8 ft. shear wall 27.5″ wide.  Any narrower than this and you have a post.

The way you figure out the aspect ratio of a shear wall is to divide both the height and the width by the width.  For example: if a shear wall is 64″ high/18″ wide, we divide both the height and the width by 18 to get a ration of 3.5/1.  If the wall is 96″ tall then 96″/18″ = 5.3/1.  This is a post and not a shear wall.

Once we determine the aspect ratio, assuming it is less than 3.5/1, we use a reduction factor of twice the width 2w/h (height).  So if we have a shear wall that is 96″ tall and 32″ wide the reduction factor is twice the w/h = 2 x 32/96 = 0.666.  The allowable shear in the table is multiplied by this factor to get the reduced shear capacity of the narrow wall.

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