Jim, here is some reading on this subject. the first is technical the second is in more laymen terms. You basically don't have enough section in your beam.
1.
http://en.wikipedia.org/wiki/Cantilever 2.
http://www.brantacan.co.uk/beam.htm The following statement is from the second article
"The next two diagrams show the beneficial effect of doubling the thickness of the beam - the sag has been reduced to one eighth of the original value. But twice as much material has to be made, transported, and erected."
This is again your problem your beam doesn't have enough section.
There are many ways to solve this problem suspenion cables, truss beam, etc...
Bottom line is what kinda of look to you want? Golden gate bridge, or Frank Lloyd Wright fallen water cantilevered house. My biggest safety concern is How much concrete did you put into the ancor pad or block? If the beams were to fail they would simple yield and sag even worse but I don't thank anyone would be injured in that event. If the forces were to pull your ancor out of the ground it would be a different story and could happen in a manner of seconds.
Without doing a lot of calulations let me ask this.
First are you going to have hand rails as you stated?
If so, what size and gage tubing are you currently using?
Does the structure sway laterially side to side out on the end given that you have cross braced it?
If it is ridgid enough laterally then I summize that building a truss "hand rail on each side will fix your problem using the same material. As far as added weight concerns voiced by Eddie it appears from scaling the photo that there is about a 20% mechanical advantage in moment arm between the cantilvered end and the fixed end.
If you aren't going to want hand rails and want the walking the plank look then you could simple lift up the structure and slide 2 more of the same size beams underneath. Preload the structure as I've stated to pull the sag out and weld them together and your done. If you would have chosen taller beams to begin with you wouldn't have this problem. I'm still mostly concerned about the ancor I think that is you weakest link. I believe a cubic yard of concrete weighs about 4000 lbs, I hope you have enough mass to overcome the lever arm you have created. I'm not a structural engineer and don't claim tobe but I work with many and could easily get this analysised if I had too. Most of the places that sale I beams like the metal building fab places have a structural engineer that will help with determining and sizing the beam for your application. I would strongly recomend you consult one of them. generally if your buying their product they will work with you at no charge.
Good luck my friend.
