I'll support Chandr advice,
XSteel 6.0 is wonderfull for steel structures detailing, fabrication and erection planning, it's also very friendly to use once you get his way of working (I kind tricky). But it's not a design software, as long as I know, and I'm I user, XSteel has modules only for connections check and based in ASD only.
The only restriction I could see concerning XSteel is that you should use his own generated drawings, post editing them under Autocad is painfull, and mustr say useless because any changes made to the model won't be reflected at your Autocad drawings.
One can't develop a complete design under XSteel you need a side program like Staad (grrrr), SpaceGas (XSteel can import the input files).
Another option for steel structures DETAILING is HyperSteel and this one have a major advantage it's an Autocad "ad on" object ARX based. I'm starting trying, soon I'll more info.
that's my 2 cents
cheers
Fred