I've got a structural engineer ready to provide calcs on this job, but there is a specific aspect where I'd like a heads-up what the normal approach would be, before I raise it with him, in case he says it's a matter for builders to decide (which it probably isn't!).
The property is a pretty typical London UK suburban 2 bedroom terrace, early 20th century. The wall is 2 x 100mm old London brick, with a 50mm unfilled cavity. The aim is to install a steel 203x203x46kg steel moment-resisting frame on the ground floor (USA:1st floor), as part of sorting out inadequate support for the rear wall as part of an extension. The rear wall is 6m wide and the opening will be about 2800 H x 6000 W resting on the original trench foundations, so there will be a horizontal load spreader, and 3 vertical columns (both sides + middle).
I've sketched one side of the frame and related wall:
I'm pretty confident about installing the horizontal load spreader (I've cut out below a cavity wall and retrofitted lintels/RHS below it, elsewhere). But for this frame, I can't mount the vertical columns against the wall - I need to cut a slot through both leaves of the rear cavity wall, and fit the UC into the slot.
The space for the slot isn't wide (maybe 220mm wide - the width of a single brick), but it is quite tall.
My concern is the lateral bracing shown in the top-right sketch. While most of the load on the removed brickwork is vertical and removing 200mm width isn't an issue, the masonry probably also provides lateral/torsion/racking support to the adjacent rear wall and party wall. I'd be happy if the slot was only a metre or so high, but I'm worried that a 2.8m slot might be too much to trust without appropriate lateral bracing, while the UC is being installed.
This is surely a very common situation for domestic extensions. What should I expect to be told as the usual/proper way to brace the slot, to make absolutely sure there can be no adverse lateral freedom or movement affecting the adjacent wall, during the few days it'll take for the 2.8m slot to be cut and emptied of brickwork, and the UC being installed and mortared in place?
My thoughts so far
I have a couple of ideas, sketched below, but I can't find a definitively good answer.
My own ideas revolve around using short (200mm vertical) steel sections to brace the wall laterally, and hold the slot open properly while the column is installed. Once the column is in, the braces become redundant and can be cut away or left in place.
I'm thinking of either using RHS and slotting the new UC inside it - then it just needs moving sideways to fit - or creating heavy duty "U" shaped sections with 20mm steel bracing at their "open" end, which can be selectively opened and closed one at a time, to allow the UC to be installed. But I've never seen a description of a "proper" or safe way to do this, so these are just guesses.
Questions on my mind:
- What is the realistic risk?
Realistically, how significant is the lateral movement/racking/twisting risk in this situation? How much/how likely is it that there really is a problem?
- How severe and what direction are the non-vertical forces/moments likely to be?
Assuming "normal/ordinary" construction, what should I expect for typical/relevant non-vertical forces/moments and their directions, to help anticipate what the engineer/builder needs to counter?
- What is the usual way this is done safely, and are my ideas any good?
What is the usual way to install UCs without exposing this risk? How heavy duty is this bracing typically, if bracing is needed (Are my ideas adequate, or are there better/cheaper ways?)