steel door, input need

I just got a 3-0 x 7-2 commercial heavy steel door and need to cut it down to 80". What might I find inside as it doesn't really seem hollow, wood core? Both ends are recessed about an inch which I guess was for cutting and fitting? I can weld (TIG, MIG, Stick) so I may be able to add some support back at the cut end. This flat door will be installed on a 20' office trailer and I may have to fab up some kind of frame. TIA

 

Could be a fire door, so the interior could be a cement type material. Doubt if it's wood, except maybe on the edges, if that.

 

Never thought of that, and it makes sense as it came out of a 50 story building in NYC that my Son's are working on. If it is a cement product that will really make the cut harder, no plasma cutter. Thanks and here is mine
http://crayonmedia.com/wayne/1911/

 

Thought about it while I was out, door that big may just be HEAVY. Could still be a foam core, but doubt it.

Depending on what it is, an abrasive metal blade in a circular saw you don't like too much (lots of abrasive dust) may work.

 

I did talk to my son about an hour ago and he said similar doors on that job that were removed had an extra steel slab inside with fiberglass fill. How he knows is an experiment he did shooting 3/8" drop in shields out of EMT with 2200 PSI CO2 at the removed doors.
Another thought is the trailer's framing may not hold a heavy door? I'll have to do lots of checking and measuring next Sunday, and I take photos.

 

The most common cores in order are (1) honey comb (2) foam or glass (3) hollow stiffened.

Honey comb – because that's what it looks like. Actually this door is the basic fire door (12 to 18 gage – with 16 and 18 gage predominating) The honey comb core and thin sheet help dissipate heat better than a solid core, which acts like a heat sink.

Foam or glass – for insulation purposes (exterior & some interior uses). Core may, or may not, include additional stiffeners.

Hollow stiffened - multiple ribs run the entire length (sometimes I shaped, sometimes Z shaped). The basic heavy duty steel door (typically 16, 14, or 12 gage). May be wind load tested.

Others: (1) lead lined. X-Ray rooms, radio active environments (2) Lexan lined – bullet proofing – impact resistance.

Your description: both ends are recessed about an inch fits typical interior construction. My guess is this is a 16 gage door. Usually, but not always, doors opening to the exterior are plugged at the top, and or bottom, then glued or welded closed.

Since you're able to weld. Rip it. See what's inside. Zip it up again.