not sure if I am getting this right or not??
the timber frame as a 4000mm structural opening that as been supplied with a structural steel beam as part of frame kit??
you mention cladding the external with a weatherboard, which makes me ask why you are blocking up this elevation, it may be you could fix a vertical 2 x 2 treated timber to the face of the osb clad? frame at centres to suit required external cladding. however if there are other reasons that mean you want to block this up and it is a 2 storey with just the block and cladding above as a load then you may get an off the peg HD box lintel, lintel companies can offer loads of technical expertise that they will back up. assuming you are then cladding the blockwork you would still need to fix a vertical batten (2 x 1) to the block work, this could just sail down over the lintel with possibly a squirt of foam/ adhesive to take it up. Obviously you would fit a cavity DPM tray above. closing the cavity to form the head for the door, first consideration structural opening height, if the steel in the timber frame is at the correct opening height your external linel or even steel, may want to be slightly higher to allow you to finish back in to the bi fold door without losing the door frame, that is unless you like the look of the underside of the lintel or steel, on the inside you would finish the underside by sticking plasterboard to underside of steel beam, and fit noggins in the face of this beam for plaster boarding.
the timber frame as a 4000mm structural opening that as been supplied with a structural steel beam as part of frame kit??
you mention cladding the external with a weatherboard, which makes me ask why you are blocking up this elevation, it may be you could fix a vertical 2 x 2 treated timber to the face of the osb clad? frame at centres to suit required external cladding. however if there are other reasons that mean you want to block this up and it is a 2 storey with just the block and cladding above as a load then you may get an off the peg HD box lintel, lintel companies can offer loads of technical expertise that they will back up. assuming you are then cladding the blockwork you would still need to fix a vertical batten (2 x 1) to the block work, this could just sail down over the lintel with possibly a squirt of foam/ adhesive to take it up. Obviously you would fit a cavity DPM tray above. closing the cavity to form the head for the door, first consideration structural opening height, if the steel in the timber frame is at the correct opening height your external linel or even steel, may want to be slightly higher to allow you to finish back in to the bi fold door without losing the door frame, that is unless you like the look of the underside of the lintel or steel, on the inside you would finish the underside by sticking plasterboard to underside of steel beam, and fit noggins in the face of this beam for plaster boarding.
