Propping for Purlin when steel is installed

[djaychela]

Dear All.

I'm (eventually!) building an extension to my chalet bungalow. GF extension, plus a larger rear dormer. The existing roof has a large purlin (200x70mm) in it, which goes across the entire building, just above FF ceiling level. The new plan calls for steel across the building, which will be supported in the centre by a steel running between the chimney breast and supported on heavy studwork posts down to the GF wall below.

This is straightforward aside from one thing. This steel (152x89UB16) has to be where the purlin's bottom half currently is. Obviously I can't just cut through this, so does anyone have any suggestions as to how to deal with this? Does each side need propping down to the ground? I think one side will be problematic as I'm pretty sure there's a ground floor concrete beam (for FF floor support) directly in the way...

I've attached a drawing to show the situation. Any suggestions will be welcome - I've asked my SE a number of times about this, but he's never really responded. I've mailed him again with just this question alone, but he often takes weeks to reply.

 

[noseall]

Just prop off the floor and use a spreader plank or two if you are worried about the load.

 

[catlad]

Will you be building an ashlar wall? as this can take the place of the purlin.