I interpret these provisions to simply mean that the "full design live load"
must be used on two adjacent spans or on alternate spans - this would
disallow using 50% of live load in lieu of fully loaded or fully unloaded
spans based on one's judgement that a full pattern loading is unlikely.
Basically the code is saying it is acceptable to consider these two pattern
loads rather than numerous live load combinations for a continuous beam.
I would think that a "reduced live load" based on code allowable reductions
could still be considered as "full factored live load" as required by the
code. I would also think that both noted pattern loads would apply to
lateral load analyses when combined with live loads.
On another note, I think that ACI 8.9.1 (or UBC 1908.9.1) are misleading and
are missing appropriate limitations on use. The ACI commentary says that
those provisions only apply to gravtiy loads and not to lateral loads - but
the actual code does not limit theses assumptions to gravity loads?