Advice with goal post type structural beam and columns

[Ron-ski]

Hi all, I'm new to the forums but have been renovating and altering houses for well over twenty years.

We've been in our current house for around 14 years, having built a large extension we're now doing the last room in the house that needs doing, once that's done I can put my feet up at the weekend (yeah right ok!)

Anyway, we've decided to take out the back wall of the lounge and fit bi-folds, which will be 3260mm wide. We're planning on having an internal return of 200mm, which means the external is below the limit, so we need goal post type structural steelwork, so the return can be tied into the steel column.

Now the SE has designed the steelwork, which basically has a 152 x 152 x 37 UC across the top, and two 90 x 90 x 5 SHS columns. The columns are bolted to the steel at the top and bolted to the existing foundations at the bottom. The bottom of the column has a 250 wide by 200 10mm plate.

Now this weekend I've done some exploratory work, the foundations are 750mm below internal floor level/DPC (they are the same level). I've dug down outside to find this out, I've also opened up the cavity internally underneath the existing window.

The cavity is about 60mm wide, so the wall in total is about 265mm thick, with the bottom column plate being 250mm. The cavity is also full up with crap pretty much up to DPC level - house was built in the 70's.

Now given the above it looks like we'd have to effectively remove both the inner and out skin of brickwork down to the foundations to install the column and drill the holes for bolting it down, in fact the inner holes will be under the floor slab, which may pose a problem getting the drill in there.

I am planning on contacting the SE on Monday, but is there any other methods we can use, may be a picture frame set up, which means we don't have to go down to the foundations.

I've attached a Sketchup drawing of what we have.

 

[theprinceofdarkness]

I am not sure it of any use to you, but :- I have seen very large satellite dishes (50' diameter) mounted on a large steel tube with an external flange. The flange was bolted to the 8 Cu m foundation block by having a lot of studs sticking upwards as part of the foundation steel work, nuts were put on the studs and run down by hand to the concrete. The dish was then craned in and top nuts were put on. They were all then tightened up to some torque and the gap underneath filled with grouting. 20 years on, its still there
This could mean that you drill and fix your studs into the foundation without the column getting in the way.
Frank

 

[Ron-ski]

@theprinceofdarkness Thanks for the reply, I was hoping not to have dig down 750mm then remove the outer skin down to the foundations. But my SE says it really needs to go down to the foundation, they have modified the baseplate though so it won't be under the floor slab and that means I can actually drill the holes in the foundations now, whereas before there may well not have been a enough room to get the drill in.

Yes you are correct about the nuts, they can be adjusted to level the column, then structural grout is poured underneath.

 

[tomfe]

The only other option I an think of it making your 200mm returns deeper, maybe 3/400mm then forgoing the goal posts. Depends on your foundations though and you'd still have to dig down to them.
Is it single story? Are you sure the se has actually done the calculations to see if 200mm returns would be ok?

 

[tony1851]

I'm with tomfe on this one.
Too often SEs specify goal posts when they are not strictly necessary. Depending on the size of the extension, it can often be proved that relatively small returns can in fact be adequate to maintain lateral stability.
If your internal returns are 200, then allowing - say - 275 thickness for the wall, your outside return could be 2 bricks(?). Depending on the width/length of the extension, that could well be proved OK by doing some wind load/resistance-moment calcs.

 

[Ron-ski]

No, I'm not sure if the SE has done calculations to see if a brick return would suffice. I spoke to BCO and he said a goal post frame would be required.

Its a two storey house, and part of the original house, wall thickness (ignoring plaster) is 265mm, cavity aproximaitly 60mm.

Just been reading this thread.

//www.diynot.com/diy/threads/minimum-internal-wall-return-on-extension.408022/

We're in the south east of Kent, in Broadstairs so it can get pretty windy at times, not like Scotland though!

 

[tony1851]

The BCO has no right to insist on a goal-post - that is a matter for your SE. If the SE provided calcs to show it was not necessary, the council's own checking engineer would go through the figures and confirm that was the case.
Having said that, wind-load and stability calcs are somewhat involved, and few SEs would want to be bothered - far less work for them to design a goal-post in most cases (though more work and expense for the client, unfortunately!).

 

[Ron-ski]

I'd hardly say the BCO insisted, as I never challenged him (I have in the past and won, but that's a different story), he said it would be required and that's what I expected so accepted it. I originally asked the SE to do a design the steel and do structural calcs for the opening and I mentioned that I thought it would need a goal post frame, so that steered them in that direction which was possibly a mistake, but I have asked them if there's any other option and they said 'it does really need to go down to the foundations', which sort of suggests there are other options, but they don't want to go there.

Whether we go for brick pillars or steel columns the steel across the top is going to need to be exactly the same (minus 4 bolt holes), or possibly slightly shorter (but we don't really want the opening any narrower). The same amount of wall or more would need to come out for brick piers as well. So the only additional cost is the columns, which are 90 x 90 x 5 box with 10mm plate welded either end, which will not be that expensive, this would also be offset against the cost of building piers out of engineering brick, also the outer skin can't be in engineering bricks anyway. I work for a company which has accounts with two steel stock holders so the steel will be at trade price, I can weld to a very good standard and I also know a local fabrication shop which specialises in structural work so I should get a good price from them.

 

[theprinceofdarkness]

Correction, In my satellite dish tale , I got the base dimensions wrong, the foundations were 8m X 8m X8m, so 512 Cu m not 8 m.
Frank

 

[mandregomes]

I'm with tomfe on this one.
Too often SEs specify goal posts when they are not strictly necessary. Depending on the size of the extension, it can often be proved that relatively small returns can in fact be adequate to maintain lateral stability.
If your internal returns are 200, then allowing - say - 275 thickness for the wall, your outside return could be 2 bricks(?). Depending on the width/length of the extension, that could well be proved OK by doing some wind load/resistance-moment calcs.

Hi tony1851, could you please recommend some documents/papers/books for me to get more deep in this subject?
Thanks

 

[John D v2.0]

Regarding the necessity of goal posts, i appreciate every situation is different, but our (seemingly very expensive but good) SE designed our removal of the rear wall so the rear beam spans from a 215mm padstone sticking slightly out of the party wall to a brick pier 330x330 and on to the inner leaf of a new wall. However although I don't understand the calculations, I suspect there might be some support assumed from next doors intact rear wall.
I know one of his design options involved no pier, but bigger returns, and he said that wasn't a favoured option for him. I dare say he could have opened the whole thing fully with a box or goal posts but it wouldn't make sense in our budget.
Hope this help, John