How to destroy this door?

Ok, so, my group has a person with strength mod 1, and 3 people with strength mad 0. The door has check value 23 (probably. the DM won't actually admit it, but we know that a 20 fails, since the DM doesn't do auto-success, and we are only level one.) He also won't remind us HOW to destroy it, and my players handbook got corrupted (pdf file.) We have tried every check that could do something. We know its a portcullis (please excuse my spelling), Probably wood. I'm pretty sure it has, like, an HP, but I'm not sure how that part works. Please enlighten me on rules in PH about this. We would like to get past the FIRST ROOM in the castle/dungeon/tomb that we are exploring.