i've seen a couple of folks post questions on why building codes illustrated is using a particular value for "W' in 506.2 Frontage Increase and also ask why on irregular plans the "Weighted Average" for W was not being used per 506.2.1. and i must admit i am confused on this issue as well.
2015 IBC commentary has a definition for this but while this is usually helpful i am not finding it to clear up the issue (can send it to anyone that would like to review this page).
anyway, i am posting a couple of pages from the 2012 and 2010 versions of building construction illustrated (2015 building code illustrated will be of no use.
not sure what i am missing but the definitions here seem a tad sloppy even in the equations. so -
when using 506.2 equation 5-2 i have seen folks use "W":
If = W/P - 0.25 (W/30) [for "frontage increase"]
and when using 506.2.1 equation 5 - 3 it would be best to use "Wn" so it is clear this is an individual width on the site as it pertains to a specific building footprint length
W = W1 x L1 + W2 x L2 + W3 x L3 ... Wn x Ln [for a W as a "weighted average'"]
now comes the sloppy part.
in equation 5 - 3 one DOES NOT use any Wn x Ln combination for ANY W value that is LESS THAN 20'. so any building edge that is less than 20' from an "authorized" clear area is not computed in this equation.
OTOH, in equation 5 - 2 the value of W is always (apparently) BETWEEN 20' and 30'. it has to be greater than 20' because this is the minimum W that code allows and it must be LESS than 30' because they use this as a "restrictor" in the equation to keep the increase at a maximum of 0.75 x Tabular Area (basically they don't want W/30 to be larger than 1).
but it is /super/ confusing when you look at these examples. for instance
1. do you use the MAXIMUM VALUE of Wn that is available on the site to run your numbers (with the understanding that you will modify W/30 down to 1 if the value is too high? building construction illustrated seems all over the map with this (examples below). i mean, don't you use the maximum value of W and if so do you just use this on orthogonal building footprints or something? it's unclear what value is being used.
2. when do you use the averaging equation? when the Wn on the site is a different value but the plan is orthogonal? or any time the plan is NOT orthogonal? building construction illustrated seems to never use the weighted average and instead seems to plug in some arbitrary W (always over 30' usually so one can't say they are putting in the maximum because something is over).
3. IBC 2015 commentary shows an example using "weighted average" on an orthogonal plan with different LENGTHS to the lotlines so i am wondering if weighted average should almost always come into play.
4. anyone know how the public way is defined and where all the dimlines for these are supposed to end and why there is never a dimension on any of them?
i realize it may be unlikely (?!) that these equations will play a part in the exam but if they do it would be nice to be familiar with them.
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