# Knock through | Steels calculation, one or two required?

#### baker332

Hi all,

I'm in the process of building an extension.

This post is in regard to an 20 year old house, masonry inner and outer leaf, not timber frame, knock-through is ground floor from existing external/outside wall into new extension.

The structural engineer designed goal posts/box section for a 2.7 meter knock-through opening, however, builder suggests it's over-engineered and will cost 3x that of just using an RSJ on pad stones. Builder thinks original structural engineer thought the house was timber frame.

Struc engineer is now MIA, having used a new structural engineer, they're suggesting a single beam on padstones with a 100mm plate welded on top to support the inner and outer leaf. I've gone back to them to ensure this is safe enough, as the inner leaf would support the first-floor floor loads and masonry and assuming roof whilst outer leaf the outer skin brick work.

Any thoughts would single steel be enough resting onto padstones to support that weight?

E.g quick sketch:

Using beam 152 x 152 x 37UC, S275 10mm thick welded plate to support wall At each side 300 x 100 x 140 Concrete padstone

Thanks

Further to this, how is this type of steel beam going to work with a cavity wall? Is there an expectation of building another brick pier? As the steel cannot just be placed within the cavity? Then this defeats the point of the objective.

I've gone back to the struc engineer to ask for two RSJ's over the single, but would this make sense?

For a cavity, two beams would be better design and give loading to the wall.

A 6" column is just stupid.

OK struc eng has come back with the following:

Double 152 x 152 x 30UC, S275, Side by side, bolted or welded together At each side 300 x 100 x 140 Concrete padstone.

Assuming we trust our stuctural engineers, would this be sufficent? 100mm of the beam will sit onto the padstones?

Thanks

Thinking about this is 152 steels even right? Surely if you use two, together they would stick out by 4mm as it's a 300mm cavity? Does it also mean the weight isn't proportioned directly over the 100mm brick?

I don't know why you need two columns for a 2.7m opening. Two 102x203 UBs would normally be adequate.

But a 2mm projection is nothing, and assumes the wall is dead on 302mm. Will a 20 year old house have a 100mm cavity anyway?

Thanks for your feedback Woody, it's appreciated - I'll ask the structural engineer.

It probably won't be dead on, but as I'm no builder maybe I'm asking the wrong questions!

Woody is it a good idea to bolt or weld them together, should this even matter? Is this for lateral forces?

Over that span, two 152 x 152 x 30 would be more than adequate; they would not need bolting together, which is pointless. Bolting together is the SE's mistaken concern that one or other of the beams could fail by lateral buckling, which those sections won't on such a short span. In any case, bolting together is specifically not allowed by the BS Code.

Similarly, two 300mm long padstones at each end are again over the top; a 150 bearing on a 215 long x 100 wide x 140 high padstones would be perfectly OK and less disruptive, even with an inner leaf of Celcon or similar aac block.

Last edited:
If the SE specifies bolting, then bolt them.

If the SE specifies bolting, then bolt them.

....and the client has to pay money for nothing.
In my 115 years of practice, I've never specified beams to be bolted together - whenever I see them, it makes me laugh.

....and the client has to pay money for nothing.
What you or I think does not matter. If someone designs something, then that's how it is built, not doing what the client or anyone else thinks.

Don't do it and there goes any insurance or negligence protection.

What you or I think does not matter. If someone designs something, then that's how it is built, not doing what the client or anyone else thinks.

Don't do it and there goes any insurance or negligence protection.
Agreed.

Wouldn’t it be better to use a single beam with a top plate?
Or a pair of UBs rather than UCs? Can’t imagine a 203 UB would have a buckling issue over such a short span. Maybe even a 178x102...

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