It is important that you review in your project specifications if there is any specific requirements on deflections, as that will affect the railing design.
Attached is a spec section of a recent project. Section 1.4.E. is titled Rigidity, limiting the deflection with concentrated or uniform top load to L/120. The other ‘title’ it could be under is ‘deflections’. Generally any deflection criteria is found under the Performance Requirements of the spec section.
Determining the ‘L’ (length) is a bit tricky, but on a simple span, post to post for a top rail, is the actual length between supports. For a cantilever use the cantilever length times two. For a 42” tall post the length ‘L’ is then 84” for purposes of determining deflection limits. The reason is the base of the post is similar to the center of a simple span, the free end is similar to the ends of a simple span, at least from a deflection stand point.
Going forward we will now be putting a note on our drawings similar to following: Per IBC 2015 no criteria established for railing deflections, VIVA Systems designed for deflections of 1.5" or less in either direction