Advice Needed: Extension Steel Work

[Slow_Loris]

So work on our mid terraced town house rear extension is due to start next week. Unfortunately we had to find a new structural engineer (we were getting nowhere fast with the old one). All along we had planned for 400mm masonry nibs for the steel to sit on as specified by the architect and original engineer. However the new engineer is saying we need 800mm nibs to spread the load which is a massive problem for our kitchen design and overall space.

The only workaround I’ve been offered is to have a picture frame steel installed reducing the projection back to around 400mm. The quote for this was £10k.

Does anyone have any advice on other ways to get around this problem? If not is £10k a reasonable cost to expect.

Many thanks.

 

[Notch7]

Seems unecessary to me.... although thats an uninformed guess.

Ive only had to underpin in this situation.....if existing founds arent very big.

 

[Slow_Loris]

Thanks Notch 7. It’s a relative new build (2002 I think). Engineer was quoting 110kN/m2 loading on the existing ground. His concern is smaller nibs will rely on the neighbouring foundations.

In your experience are 400mm piers usually adequate given 3 floors above?

 

[^woody^]

The quote for this was £10k.

Lol. The posts and bases no more than £200 each. The beam, whatever your existing price is. The extra calcs, another £200 at most.

The nibs and nib loading is a nonsense as that's not an issue. Modern foundations are way over designed and capable of point loading. Walls are OK too. The only potential issue is sideways stability, hence the possibility of needing posts.

Find another engineer, one more capable.

 

[Slow_Loris]

Thanks all.

I think I’ll get a second opinion.

 

[Notch7]

His concern is smaller nibs will rely on the neighbouring foundations.

I wondered that.

In your experience are 400mm piers usually adequate given 3 floors above?

Ah 3 floors -that could be the reason, thats quite a bit of weight.

 

[tony1851]

If it's a modern 3-storey town house, there will be the extra loading from the top floor, plus the fact that the rear wall will be supporting half the load from the roof trusses, and there will be the load from the extension roof.
Most bco s and checking engineers would look for a moment frame (aka goal post) to maintain lateral stability. But 10K seems excessive - those I've done turn around 3-4K. It consists of a beam on column each side. If the span is not great, one of the 203 x 203 sections would do for the beam and columns.
With care, the columns can often be partially built into the inner skin of the party walls. The columns usually require separate pad foundations.
I would certainly not put up with 800 returns - that's not engineering - it's incompetence; as above, get another SE.

 

[John D v2.0]

Not sure how directly it relates to you, but in our case the SE was happy to rest the new beam supporting the existing back wall (one floor and the roof above only) directly in the party wall on the corner. Original plans stated to build a column off the existing foundation, but when there were found to be none, he said the existing lime mortar wall would spread the load far enough. I think I did ask what happens if the neighbour does the same, as we are relying on their house now, and he said they would have to work out a way to do it. Our SE was expensive, but I'm sure they saved money overall and got a better result.