This question bothers me for quite a while and is about bicycle frame construction, specifically a BMX frame and even more specifically it's front triangle. It is a heavy duty frame by definition, and most common point of frame failure in this area, is lowest point around area D - and may happen even if type1 gusset is present. It's usually a crack along weld (which, if unnoticed, may result in tube C snapping off upon next considerable impact and is highly unwanted). I am looking into building a frame with type2 gusset, which appears more reliable overall, but unfortunately lack knowledge to confirm this.
I tend to think additional heat stress from second weld during type1 gusset install does not do good and useful weld surface does not increase a lot.
With type2 gusset, however, useful weld surface increases considerably, also holding all three tubes together, which should relieve stress on area D as well.
However, few reputable people suggest type2 construction would still need type1 gusset between tubes B and C, but cannot back up their argument with real data.
To summarize - by what amount does type1 gusset does actually improve situation here, and would it be any use along with type2 gusset? I would rather increase tube B wall thickness around weld area rather than add type1 gusset, if this proves an issue.
Frame tube material is 4130 chromoly steel, TIG welded. Tubes A and B are usually around 35~38mm outer diameter and have 1.25mm thick wall. Tube C wall is usually 2-3mm thick.
Frame experiences repeated cycles of 'sharp' dynamic loads (i.e. handling rider mass upon landing from height).