It's not only the weight but you're going to have to figure travel and stopping speeds.
A 5000 lb tractor will weigh a lot more traveling at ( X) MPH and stopping.
Called FRONT PROJECTION limits.
Figuring too close to weight of the tractor is not going to work in an emergency.
While not strictly part of the trailer but worth noting is the transport chains and binders used to secure heavy equipment loads. Chain is not that large an investment; if your chain is the least suspect, better safe than sorry. Chain comes in various grades and thickness; the most commonly used chain in securing loads is Grade 70, 3/8 in., which has a 6,600-lb. maximum pull. For heavier loads, there is a Grade 70, 1/2 in. with 11,300 maximum pull.
Binders, chains, hooks and clevis pins all should be permanently marked with their capacity. If you find you need to upgrade your binders, ratchet binders have twice the strength of mechanical lever binders, and they are also much easier to tighten than the lever ones.
Whether its chains, ropes, wraps or webbing, whatever secures the load must comply with the Federal Motor Carrier Regulations regarding the Aggregate Working Load Limit. The basic requirement is that tiedowns must have the combined strength equal to at least 50 percent of the load being secured.
The way to calculate the aggregate load limit is to add 50 percent of the working load limit of each tiedown used to secure the load; and if the total is greater than 50 percent of the load being carried, it is secured within the limit requirements. For example, assume that a trailer is being loaded with an 11,000-lb. telehandler, which will be held with four Grade 70, 3/8-in. chains. Since each chain is rated at 6,600 maximum pull, the aggregate working load limit is 6,600 lbs. divided by two. The total of the four chains added together is 13,200 lbs. That's more than enough to hold the 11,000-lb. telehandler.
Loading methods
Centering a load in the middle of the trailer is a basic tenet in proper loading, but it sometimes is overlooked when adding attachments or loading a large piece of equipment with limited view of the deck. Using a deck board as a guide to keep the load centered is an effective method of keeping the load centered. Overall, the weight of the load should be slightly forward so that 10 to 15 percent is on the front tongue. Fifth wheel trailers can handle as much as 25 percent of the load on the front.
http:
//auto.howstuffworks.com/auto-parts/towing/towing-capacity/information/overweight-load.htm
http://auto.howstuffworks.com/auto-parts/towing/towing-capacity/information/truck-tow-over-capacity.htm