Tacoma Narrows Bridge

Ok.. I'm in the beginning phases of what I hope will be a very fruitful endeavor of recreating the Tacoma Narrows Bridge in GMAX to get rid of the existing default Microsoft version.
I'm having a little trouble, though. Especially with creating the bridge tower pedestals and towers themselves.
The tower pedestals are shaped like a stop sign per se' (shown below in the diagram), but when I try to create that shape, GMAX won't let me. For some reason, it won't allow me to bevel the vertical corners of the box I start with.

Any suggestions?
I'm having a little trouble, though. Especially with creating the bridge tower pedestals and towers themselves.
The tower pedestals are shaped like a stop sign per se' (shown below in the diagram), but when I try to create that shape, GMAX won't let me. For some reason, it won't allow me to bevel the vertical corners of the box I start with.
Any suggestions?