Well, first place to start is the GCWR, which is the GVWR of truck and trailer combined. If your truck is 11,400 GVW, and Trailer is 18,000 GVW, your total is over 26,000 and you would need a Class A CDL even if the trailer is empty. (most wedges are rated for 18,000 unless you specify lower)
If your GCWR is 26,000 or less, then you do not need a CDL... unless the ACTUAL CURB weight of the rig (with cargo) is 26,001+, then you need a CDL plus you are overweight on your equipment.
You can have a trailer manufacturer down rate your trailer to lets say 14,600 GVW, so your total GCWR is exactly 26,000. In this case, you are legal, as long as your total ACTUAL WEIGHT does not exceed your ratings.
Some people think any trailer 10,000+ requires a CDL, that is NOT TRUE provided your GCWR and actual weight are both below 26,000 lbs. Trailers over 10k do make you a commercial vehicle, so you do need to follow all of the FMCSA rules and regulations, including hours of service.
Does this help?