From what I understand from Google, Strongboy is a brand name The strongboy is a masonry prop which provides offset support for walls, enabling work to continue beneath the prop without risk of collapse.
To be honest, I've never noticed anything like that.
But while trying to find images I learned a new word (well not quite, as I had heard it before in slang usage): πρέκι= lintel
As for the strongboy, it's a prop, so a στήριγμα or υποστύλωση. But I don't know if there's a specific name for those in Greek.
Thanks, as ever, SBE. When we had our kitchen renovated six years ago, all the talk by the builders was about needing 'acro-props' [from the Greek άκρος be it said] & 'strongboys', which our build took as the technical term, not, it seems, knowing that it was a trade name.