Ironically, I'm working on the AT&SF FT-J as we speak too... but in N scale. 

You may find this helpful, PJ Student's article re the FT-J (and the Ft-O&P) in HO.  http://magazine.trainlife.com/wp-content/uploads/2017/08/ptm_197804.pdf starting on pg 16.
On pg 17, he states the side sill web plates (what Student call the fishbelly) are "inset 16" from the edge of the deck side sill."  No word on spacing of other bellies.  

Incidentally, would love to see other drawing or images of the FT-J.  (So far KSHS has not replied to my request to see their's listed on line here https://www.kshs.org/archives/67594.)

All I have to work with is the grainy Cyril Durrenberger image of the Wt-J in this Student article, but it seems to materially differ from the Student version beyond what he mentions, in that it seems to have 5 subfloor braces that pierce the side sill web plates and extend to flush with the deck side sills - perhaps 3 center as I beam-ish (center and other 2 at the point of the fish belly bend) and the two towards the ends being c or box channels? 

Also, interested in the width of the exposed side sill web plates, not given in the Student article, except as the width of the fishbelly, based on the dimensions of the Train-Miniature donor car, plus scabs he used to join the deck.  

Other that this drawing online of an Ft-Ia (apparently same as Ft-18?)  I can't help much here I've not researched it - but very interested as that is also on my to do list!  http://old.atsfrr.org/resources/zebrowski/folio/ft-pdf/ft-i.pdf

-Jake Schaible

 

Thanks Steve.  Not sure why the link didn't take, but you are correct... add back the F at the end. 

Also, please ignore my erroneous mention of Ft-Ia being also Ft-18. F-18 seems to have been the Folio 211 page number. Pardon my mistake.

This is likely no mystery to anyone but me, but for weeks I've been perplexed by comparing the aforementioned PJ Student's article re building an Ft-J (v similar to the Ft-I).  Again, Student failed to include the 5 side sill supports that can be seen peaking out from under his article's grainy Cyril Durrenberger image of the ATSF Wt-J 2027?7.  My question is were added on later with one of the several rebuilds?  Or were they original, and Student just ignored this feature in his models.  

Charlie Slater was kind enough to share images of these feature, which show they were not - as I initially suspected - contiguous parts.  Rather they are compound built up plates riveted to the floor support / cross bar features, which seemed for a bit to bolster my hypothesis (and hope) that they were added in one of the later rebuild phases.   Other late images of converted Ft-I cars show these better:   https://upload.wikimedia.org/wikipedia/commons/c/c3/Atchison%2C_Topeka%2C_and_Santa_Fe%2C_Flat_Car_97834_%2815894678128%29.jpg

Torn on how to advance, I broke down and finally purchased a book long on my wish list : Hendrickson's "Santa Fe Open-Top Cars" (on sale for just $20 now at https://sfrhms.org/product/vol-7-open-top-cars-flat-gon-hopper-cars-1902-1959/)  No excuse for waiting so long, but at ~1/3 the list price?  Sold!  Anyway, reviewing the Ft-I & Ft-J cars (pg 25 - 28) all show these 5 side sill support features.  Only exception is a converted Wt-J ATSF 191240, caught in 1954.  However several feature of this car (including 8 stake pockets instead of 10 in the as built Ft-I &Ft-J) have me doubting it was part of the Ft-I or -J classes. 

So .... shoot.... now I need to make and fit 10 side sill plates per car... in N scale!   ;-)