Knock through, RSJ and end bearers

[Pru11]

Hi,

I have a 3 bedroom end terrace (ground floor and first floor with truss roof), i have had an extension built to the rear of the house via permitted development (external size 3mx7m with vaulted roof). I now want to knock through the back of the house which is a cavity wall (260mm). The opening will be around 6.5m which i know is a large opening.

I have followed all building regulations regarding the build work and have been advised that i require steel calculations for the opening along with padstone spec and end bearing size/length from a structural engineer, which of course i am happy with.

The SE first stated that due to the opening size i may require steel posts on the sides of the opening, i believe he may of changed his mind now (yes not great). Now he is suggesting that the end bearing walls may need to be 600mm each which is not want i want for a open plan space. i have told him that 600mm is too big for end bearing but he is suggesting that I need to pay his quote so he can conduct the calculation to truly determine how much of the wall i need to leave.

I do not have plans as the build work was done on a building notice, however i have attached some pictures and the SE has modified my estate agent plan.

Sorry for the long post, would just like some guidance. Is the SE advising me correctly.

My building inspector has stated that he doesn't need calcs but as a safety precaution I rather have them and therefore a piece of mind that everything is safe.

Thank you

 

[tony1851]

What your SE suggested at first was a moment-frame (aka 'goal post'). This is a rigid frame intended to provide lateral stability to the house when under wind load.
Frames are expensive and disruptive, and the alternative is to leave reasonable lengths of brickwork in, which also provides lateral stability. This is one of the reasons he had 2nd thoughts and suggested leaving 600mm each side, which of course would look naff. The other reason is that he doesn't want to go to the bother of designing and detailing a frame.
But looking at your house as a whole, and including the brickwork in the extension, I would reckon that you have enough structural walls going across-ways to provide stability, and without leaving big chunks of brickwork in. Small piers - say 200-250 deep would suffice for a bearing.
It would also help lateral stiffness if you diaphragmed the new stud partition with plywood or osb both sides.
But whatever else, you don't need big clunky piers.

 

[Pru11]

Hi Tony

Thank you for your thoughts. That puts my mind at ease as initially I was happy to leave 300mm piers.

Should I approach another SE? If so can you reccomend anyone?

He was charging me around 175 + vat. I don't know if that is good or not.

Thank you

 

[tony1851]

That would be a good price IMO, particularly if you could get him to work with 300 piers, rather than 600.

(Strange that your bco has said he doesn't need calculations; most inspectors would ask for calcs for beams with far
shorter spans than that).

 

[freddiemercurystwin]

Maybe he's 'private!

 

[Pru11]

Thank you.

Yes from reading posts on here even I am cofused as to why the bco has not insisted on calcs.

BCO is part of a reputable company. Regardless I want calcs as safety is the most important thing for me.

Tony, from your experience would I have two steels (for cavity wall) bolted together with spacer or just the one. Just curious.

Thank you

 

[tony1851]

would I have two steels (for cavity wall) bolted together with spacer or just the one. Just curious.

It depends partly on the resources the builders has. On that span, it might be easier to have two steels bolted together, so that they can be installed one at a time. One downside is that the inner skin will probably be considerably more heavily-loaded than the outer skin, due to the house having a trussed roof, so that beam might deflect a little more than the other, even with spacer tubes.
An alternative is to use one of the 203 x 203 beams, with a 250 wide plate welded on top to pick up the cavity wall.

 

[Pru11]

Thank you that makes sense.

My builder prefers to install two steels but the second option also seems viable.

I will be speaking with the SE tomorrow to discuss my options regarding steels and piers.

 

[John D v2.0]

We paid for structural design rather than calculation, or something, but anyway it cost a lot but meant the SE worked with the architect to do the design. In the end he could use next doors wall as we're semi detached, plus a column, and not need any returns on the walls or goal posts. We could have had goal posts and no column but not within our budget.
So if you get an SE that can come up with these trade off you might be able to save more than you pay them.
In our case it ended up a few percent of the build cost. But it does mean next door can't do the exact same extension without adding stability as their wall is supporting our house too.

 

[tony1851]

But it does mean next door can't do the exact same extension without adding stability as their wall is supporting our house too.

Either you were very lucky, or your building control's engineer was asleep.
The convention is that when dealing with overall stability, the house must be capable of standing on its own, regardless of whether its a semi or part of a terrace. Admittedly it seems stupid because the chances of next doors' waking up one morning and deciding to demolish their house is pretty small.
But personally I have been involved in two jobs like this - one where an adjoining semi was demolished to form a vehicle access to adjacent premises, and another involving a narrow three-storey building sandwiched between two existing buildings - one of which was in the clients' ownership. The new building still had to be designed as though it was free-standing; very frustrating.

 

[Pru11]

Would it be possible to sit the steal on the joining wall between me and my neighbour but solely on my brick/block work?

 

[chappers]

Either you were very lucky, or your building control's engineer was asleep.
The convention is that when dealing with overall stability, the house must be capable of standing on its own, regardless of whether its a semi or part of a terrace. Admittedly it seems stupid because the chances of next doors' waking up one morning and deciding to demolish their house is pretty small.
.

Yeah but that's very naughty if your neighbour decides to do an extension and his SE decides to make the same assumption, what then?

 

[John D v2.0]

Oh our SE came and had a look and saw that next door was unmodified rather then making the assumption.
The bco just wanted a sheet of calculations, he didn't question any of them.
I thought buildings could rely on support and protection from next door? Although presumably not best practice. Certainly in London whenever there a big building project they always cover the newly exposed party wall with battens and a membrane. One near kings cross the adjoining building started to get some cracks, then huge temporary steel triangles appeared rapidly. Not sure who paid for them.

 

[John D v2.0]

Would it be possible to sit the steal on the joining wall between me and my neighbour but solely on my brick/block work?

You don't really have your own blocks, the party wall is all owned by both of you.
If you specify in your party wall notice that that's what you plan to do, they'll get a chance to object, if they don't then go ahead. The SE should check what size padstone you need to spread the load through the brickwork.

 

[Pru11]

Thank you, that is something interesting.

Does that mean when I chase the joining wall for new electrical sockets and cable runs that I need a PWA?

Also does that mean the new extension I have built along the boundary is a shared wall?