[Indie]

8-10 lbs would take out an 8x8 inch square steel beam...as in break it in half.

[rickylc]

I also found this in trying to educate myself (perhaps from the same page you found? howstuffworks?)

Unfortunately, C-4 will keep making headlines for years to come. Because of its stability and sheer destructive power, C-4 has attracted the attention of terrorists and guerilla fighters all over the world. A small amount of C-4 can do a lot of damage, and it's fairly easy to smuggle the explosive past light security forces. The U.S. military is the primary manufacturer of C-4, and it tightly guards its supply, but there are a number of other sources for similar explosive material (including Iran, which has a history of conflict with the United States). As long as it is readily accessible, C-4 will continue to be a primary weapon in the terrorist arsenal.