Please see attached a mark'up indicating alternative solution for the box frame.
John I am the engineer! I have had extensive education and passed qualification levels to masters level. How can an unregulated, unqualified builder even consider mentioning using a single beam! This is alarming that the builder take decision and even action in some cases by using a single beam in place of a professional design being carried out by a qualified engineer on someone else’s house.
This is not just negligence, it’s deliberate cowboynisem! The builder knows the correct design! But chooses to lie to everybody and uses all the ballshit to try and convince the only person which is the home owner that the box frame design is overkill. “Summery” any builder which challenges a chartered qualified structural engineers should be taken off the project for not complying to UK British building regulations. They then should be reported to trading standards for being unfit to be in the building trade. To achieve similar behaviour of the steel box, we would need to introduce a min. 1000mm long Engineering Brick Pier (215mm wide) with mass concrete footing under - min 650mm wide. This however would need to be installed next to neighbour foundation hence its likely that Party wall award would be needed.
With regards to box frame - please find below few main reasons.
When solid masonry wall (such as external solid wall) is removed - the existing building is loosing part if the lateral stability system which needs to be enhanced by additional solid wall (or long brick piers) or alternatively by steel box frame. It is essential to properly recognize the existing stability system and the implications of the removal of one of its key element (ie solid wall). Steel Box frame is designed not only to resist vertical loads but also designed to resist lateral wind loads. Simple Beam and pier solution may not be sufficient to resist lateral wind pressure.