...Diminished Housed Tenon??? Please, call this something else ...
Sorry, as David also suggested, that is the descriptive term employed as I understand it, have heard it described and seen. I don't make the the rules or vernacular semantics, nor the syntax in English...LOL
...if anything it is the opposite, increased yes, tenon no (full section) ...
A "tenon" ("Hozo"...in Japanese as just one example) can and often is "full section" and again from a perspective of timber framing historically and accross its vast cultural implications "tenons" do not have to be just a proportional element of a timber...they can be just the end of it, a "free component" of another member or a number of modalities still falling into the understanding of there given role as they play within context to the cultural practice of a region timber framing methods.
...housed no (no level bearing) and a slip through notch (not a fixing joint)
...
Respectfully Roger, a "housed joint" does not nor is historically always a "level" bearing surface. Not in European or any other timber framing culture that I know of by definition.
It is...a trapping and containment of a the primary joint often employed to not only compensate for irregularity in the receiving timber, but also to resist further the tectonic loads a joint will sustain during its given service life within a frame...
Though this type of tenon is a drop in service member (in this case a joist) it can more than work if employed properly and bear loads effectively, but I would agree not as effectively as some of the other offered more advanced and less common methods as suggested. This is (as the OP described in his query) more often an in situ repair method and/or augmentation to work in concert with other tying members to be fully effective.
...In the normal sense, diminished haunch, tusk tenons and soffit tenons have first lower down a lengthened penetration to provide a strong level bearing, second the diminishing length provides re-enforcement of the bearing while decreasing the cutting away of the receiving timber. This is especially important in maintaining bearing capacity in the beam that is notched to carry the joists...
In general, I fully agree with portions of that description of these types of tenon and there application.
They are however, the exception to normal practice, practical and/or common employment for joist which typically are just a "dropped" in full dimension "tenon" on the end of a joist...or...with a reduction to the joist in depth to reduce further the size of the mortise in the receiving member. In lesser forms of historical examples (often with splits and failures) this is a simple notching, but often in the better forms a adzed or drawn reduction accross the joist depth. Summner (or Summer, etc) beams this can be often found also...again a tenon and again full dimension.
...One other thing, normally joists are tying members ...
Statistically...not true at all. Neither accross Europe, nor in Afrcia, the Middle East or Asia.
In actuality accross the breadth of the timber framing, joist seldom play much of a any significant role beyond bearing support within the floor diaphragm contextually and any moment resistance they offer or do play a part in is through frictional load forces alone...This, of course, is not always the case, and there are (as you mentioned) forms of joist that can and do play a role as tying members within the floor diaphragm of a frame.
If I had to guess and/or estimate, if an actual inventory of all members play the service role of joist within timber frames are accounted for...over 60% of them (conservatively that I have examined in North America or globally) most are like the "Neda" (or Nebuto ?) of Japan (et al) and simply rest atop the receiving member only...No joinery of any kind at all...just gravity...
