Cutting out blockwork near a steel support.

[MarkandCath]

I wish to cut out some blockwork for a doorway on the 1st floor. The blockwork has nothing directly above it but is close to a steel that supports the roof. A picture says a thousand words so how close to the edge of the steel can I cut the red line?

Just to be clear, I will be removing the section to the right of the red line.

Thanks.