Jim,

You are correct, there are a total of 20 backhead braces. Typically the older codes required calculating for an average brace stress while the new code wants to know the load on each brace. We compromise and establish the load on the brace supporting the largest area. It is a bit subjective and depends a lot on how the backhead is constructed. In #20's case, she has a doubler and t-irons which tend to distribute the load more evenly than a single sheet with brace rods welded in place. The average load is less than 8ksi, with 9ksi being the maximum allowed.

As to other locomotives, occasionally you will find under braced backheads. EBT #s 14 & 15 have a curious arrangement that differs from #12 and the bigger engines in that the brace t-irons are needed to support the upper portion of the backhead. Basically Baldwin eliminated the upper braces on the long t-irons. The upper braces are in place on #12 and the larger engines.

#20 has relatively small braces which accounts for the number needed. While larger braces could be used, it would require larger shell feet, crows feet and pins. We did make one larger brace for each side, the second brace from the outside. The t-iron rivets on the backhead were sufficient but we needed to make bigger crowsfeet and new feet for the shell with three rivets instead of two.

The original braces were located differently, and I suspect it was a later modification because the alignment was poor and two braces interfered with one row of crown stays.

#318 is insufficiently braced on her backhead, whereas my understanding is that #315 had this situation corrected by the Rio Grande at some point. My experience has been that insufficient or poorly distributed bracing usually occurs in early boilers - prior to 1910 - and new boilers built in the past 30 years or so. There are always exceptions, but in my experience, most original boilers built in the 20th century have pretty well designed backhead bracing.

Linn