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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 building

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:

enter image description here

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.

The problem

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.

enter image description here

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?)
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2 Answers 2

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Disclaimer - No contract exists between us. This post is provided as information and opinion only and does not form "advice" in any way. Although I am a Chartered Civil Engineer I have zero professional experience with masonry. My views here should be treated as those of a keen DIYer (which I also am).

I suspect it'll probably be ok without any lateral support. The vast majority of lateral loading in the UK comes from wind - as long as your builder doesn't cut out the existing wall directly before the highest winds for the past 25 years, I can't see it failing.

What you could do is ask your structural engineer whether they've designed for a positive lateral connection between the UC and the existing brickwork. My guess is not, and if that connection isn't needed for support long term, then it won't be needed during construction either.

Another way of looking at it is to consider patio / bi-fold doors. The vast majority of frames for these are non-structural, and yet there isn't a problem with houses' rear walls falling over from lateral loading. Your hole might be wider than most of these, but I don't think it's higher.

The practical solution for you is of course to just talk to your structural engineer. Construction (Design & Management) regulations (otherwise known as CDM Regs) require the designer to come up with at least one safe method of constructing their design. So your engineer will have already considered a method that is safe. And if that method is "just cut out without support - it's not required" then they'll be able to tell you that.

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  • $\begingroup$ That's an interesting point about doorways/openings! Wind's minor, but it will be taller - and if load scales as the square or cube, then the difference between 1.9m doors and 2.8m is significant (but I don't know if it does). He did specify a moment resisting goalpost/frame specifically, on account of present or future lateral load, although I don't know if that's prudence, actual necessity, or "what if the neighbours also add an extension in future so the rear/party walls have no buttressing". Any other thoughts appreciated - and yes, I will ask, I just want an idea to think over, beforehand $\endgroup$
    – Stilez
    Commented May 18, 2018 at 16:43
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    $\begingroup$ The disclaimer at the top of your answer reminded me that we still don't have a general site disclaimer... See this meta post $\endgroup$
    – hazzey ♦
    Commented May 18, 2018 at 18:16
  • $\begingroup$ @hazzey - Agreed that a general site disclaimer would be useful. I think I'd still have added something here though - to make it clear that although my profile states I am a Chartered Civil Engineer, when it comes to masonry I'm just a keen DIYer. $\endgroup$
    – AndyT
    Commented May 21, 2018 at 8:00
  • $\begingroup$ @Stilez - Wind loading is generally taken as a uniform pressure on a face, so it scales linearly, not as a square or cube. For very tall buildings (skyscrapers) the relationships is probably far more complicated, but for a two storey house we use simplified conservative loads. $\endgroup$
    – AndyT
    Commented May 21, 2018 at 8:02
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I would recommend a temporary shoring of the wall by attaching a vertical 3" x 6" board to the wall near the location of new column.

This board will be attached to the wall by 5x 1/4 inch anchor bolts, and will be laterally braced by a few diagonal 2x4" braces which are secured to the ground. later this board shall be removed with the anchors, and any holes patched.

I would make shore my Engineer has details on positive connection between the new column and adjacent masonry, such as epoxy dowels or other means.

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  • $\begingroup$ By "board" do you mean a length of 3x6 inch timber? Also, is this based on guesswork or do you have experience that you are basing this on? I hadn't thought of doing it that way. Can you update your answer to say a bit more? Thank you! $\endgroup$
    – Stilez
    Commented May 18, 2018 at 16:16
  • $\begingroup$ I am a retired structural engineer in US. No, I mean a 10 feet long, or whatever will be the height of cut for column, nominal 3x6 which is actually 2 1/2 x 5 1/2 inch. In US and Candad such item is sold in the Home Depot for $10-15. $\endgroup$
    – kamran
    Commented May 18, 2018 at 16:29
  • $\begingroup$ We're talking about the same thing then, 3x6 nominal and about 10ft /2.8m long. Gotcha. And the extra info helps! Eating right now, will come back in a bit $\endgroup$
    – Stilez
    Commented May 18, 2018 at 16:47
  • $\begingroup$ I had thought about some kind of face-mounted support, but had not figured a way to do it. This would make sense and it would be easy. It would be even easier if I use some steel angle behind any face-mounted support, to lock around the ends of the cut bricks, and then lock that angle to the rear wall surface. I'll sketch this and add it to the question as an update, see what you think? $\endgroup$
    – Stilez
    Commented May 19, 2018 at 4:56
  • $\begingroup$ I would imagine your engineer will design brakets or other means if connecting and suppting the wall laterally. I would use the opportunity to strengthen existing wall. $\endgroup$
    – kamran
    Commented May 19, 2018 at 21:17

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