2012 frontage increases (what, when and ching errors?) [506.2 and 506.2.1]
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 52 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.
THANKS

basically same question. why are they using 40/30 and not 55/30. net result is the same but it is confusing. what is the 40 public way showing? guess i need to look it up in the definitions or does anyone know where it ends?
second example seems like a clear case of needing to use the weighted average. not using it to keep things "simple"?

this is a clear weighted average example since they are using the equation. it seems to me that they have a typo in leaving out L1 of 80' in the Building Perimeter (F). so the first zero in the denominator should be 80 which would mean it is 9350 sf / 410 sf = 22.8'. right now they are using 330 feet instead of 410.
sort of a procedural thing but if they used Wn x Ln instead of Ln x Wn it would be a whole lot easier to keep track of the "truncated" W's where they plugged in 30. so ignore the red dot at the 20'. it is the value of L2.
bona fide typo or am i missing something?
THANKS

hi china,
i’ll drop in here. 1. there are errors in most everything on this issue. possibly the right answers but incorrect steps imho. 2. IBC code commentary is very good for this. but i believe i found 1 error there as well that i may submit at some point. 3. i have /never/ heard of anyone excepting a couple early post asking about this topic for the exams. like in over a year and a half. i seriously doubt you need to know it. that said, perhaps someone can follow up if they think it is worth spending anytime on outside of actual professional practice.
in short, i’d feel pretty confident ignoring it for the exams...
if it is important i have a couple summaries on the topic but i’d have to dig them out. but again i’m not sure it is worth anyone’s time outside of actual professional practice. 
I've noticed the same frustrating W issues reading through the BCI and also various practice problems. While extremely frustrating, for exam purposes, I've noticed that the result is always the same, you end up with >30 which amounts to W=30/30. My plan for the exams if this type of question comes up is to use the weighted average equation as described here:
https://prezi.com/yl4yzmgjiryr/howtocalculatebuildingareamodifications/
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