because columns are % based, if the container changes, everything inside changes. taking 960.gs for example, it's a fixed grid layout of 960px and 940px "usable" content area (10px margin each side). the problem is that media queries will affect this behavior (on smaller viewports), so they must be 'disabled'  at the same time the container is at a fixed width. 

I tried a couple of combinations (leaving the media queries alone, or setting min-width instead of width), and everything broke (as expected because of the reasons above). it's up to you then :)