hi..
 you should consider to include the loads that the columns carry and add it to the proposed truss/beam plus its existing ceiling load...then start the designing...

hope this help..

The proposal is fraught with danger.  The floor was not designed to impose the continuos loads by way of a beam.  I suggest that you design your truss/beam with two point loads at the locations where the columns presently are, that is to maintain the status quo.  Your next problem is to get rid of the loads from the ends of the truss/beam, presumably by columns down to grade on either side of the room, then you will need footings to distibute the loads on the soil.  Check the bearing capacity of the soil.
Dont assume that you know anything about existing construction without having excellant evidence that your assumptions are correct.
Rowland 33

Hi,

Rowland33 is right.
You have to have a general view of the problem beside the details.
For the details i have no problem they are easy to solve.

For the general view,
For this structure the beams could have other behaviors,
 are this beams acting to take lateral forces by now, because when you take out the column is going to take nothing (to long the slenderness have made his work)
Once you replace this two columns the behavior of the building is going to have a drastic change. Have to take an aditional analisys to verify.

Have you think about a safe process of work, do you think that in this process is needed aditional calculations to have no risk?