Jim your on the right track. A single larger cable will do the trick, dont be afraid to go to big. If you have a crane rental company near by, or a large GC, you might be able to get the cable for free, its changed off cranes alot, and really no use after. You have to get rid of all stretching.
Dont be afraid to step back a little with the attachment point, as in right before the platform. This will keep it in line with your steel columns and remove the load that is on an angel to the steel columns and lessen the angle of the cables so your steel columns dont have to hold as much weight.
As bobad said, you want to load them. Keep pulling them till you have raised it above or at least equal to the elevation of the concrete columns. Once you have done this, no cables under will be needed. You have to get the cables holding the same amount of weight that the concrete columns are holding divied by half. Once this is done, it will be like having concrete columns at the end.
With out knowing the weights, your vertical columns are now going to be the weak spot, as in crushing them when the cables are loaded. Quick example, lets say you have 4,000 lbs on the two concrete columns, that means you need 1,000 lbs of vertical force on the cables. If the cables were vertical, it would be easy, but your pulling at an angle, the more you lessen this angle, the less weight you are pulling lateral and more vertical. Example, you have 3,000 lbs of pull on the cable, 2,000 lbs is pulling lateral to get 1,000 lbs vertical, this will change continually all due to the angle of your cable. When your finished, this vertical load now is imposed on your steel columns. trying to crush them, your lateral load is taken up by the back cables, but keep in mind, they are imposing the same vertical force also. In the above example, your going to have 4,000 lbs total of vertical force on your two steel columns.
