I'm pretty sure what you're describing is a lean-on bracing system (in the absence of deck). Galambos (Guide to stability design criteria for metal strucures) discusses these systems.
The answer depends on the number of W24 being braced agains each other. As the number gets large, your unbraced...
In ten years of working on Canadian projects (mostly in Ontario, but also all over the country), I've not heard of a site wanting to use the US sizes instead of the Canadian ones. The Canadian metric sizes are generally readily available, up to 35M. (You should check availability of 45M and 55M...
MacGregor & Bartlett (2000) "Reinforced Concrete: Mechanics and Design" for Canadian readers:
http://www.chapters.indigo.ca/books/Reinforced-Concrete-Mechanism-Design-canadian-James-G-MacGregor-F-Michael-Bartlett/9780131014039-item.html?ikwid=reinforced+concrete+bartlett+macgregor&ikwsec=Books...