Narrow vertical steel columns needed - why is this difficult

[KenExtends]

Hi everybody

My part-one, part-two storey extension has commenced, and I have hit my first stumbling block.

I managed to get walls of 260 mm thickness, however, I cannot get my structural engineer to design columns that will fit in this thickness (100 mm internal blocks, 25mm insulation, 35 mm air, 100 mm brick).

I have to have a box structure to support the top beam which is a 203 x 203 x 60 (and ground beam 203 x 203 x 46). The top beam will support 17.5 Kn/m dead load, and 7.0 Kn/m live load.

The builder has knocked down the columns from 203 x 203 x 46 to 152 x 152 x 30, which shows that he completely overengineered it in ths first place, however, even these smaller columns will protrude into the room.

My builder has proposed three box columns x100 welded together however the engineer has said that this is not possible.

How do others manage to lose these columns into the walls.

Thanks

 

[freddiemercurystwin]

I get my SE to specify sky hooks.

 

[Static]

I get my SE to specify sky hooks.

Yep and im sure they are overengineered sky hooks at that..

Whats the span of the opening?

 

[noseall]

Sounds to me like no one knows what they are doing, it has been badly thought out and that professional advice is being ignored.

260mm wide walls! Why? This thickness of masonry has not been used in donkeys years. No wonder you are having problems.

I bet your S.E. and architect are pulling their hair out.

 

[jeds]

How does that wall comply with thermal regulations? I reckon the u-value would be in the region of 0.60 where the minimum is currently 0.28. i.e. nearly twice as good.

 

[handyman77]

Why are all SE going crazy on box beams. I seem to come against these young fresh out of university SE's and they over complicate a lot of very simple jobs. I am sure there is a time and place but surely not nearly always. Give me the old school engineer any day he's been there and done it and his house are still standing.....

 

[KenExtends]

I have been horrified at the growth in thickness in house walls over the las few years. To me, every inch saved is two inches in internal room width - especially for my key room at the rear of the house with two external walls.

I used Celotex online professional calculator and achieved a u-value of 0.28 by using thermal blocks (250 mm altogether), and slightly more for the brick finish with internal blocks, hence a 35 mm air gap insteac of 25 mm air gap (giving 260 mm) here to achieve the same u-value. Therefore, why have thicker walls...

The horizontal beam is 3.9 m in length.

 

[ABCwarrior]

Maybe all the post 9/11 SE's coming through are being taught that steel will turn to dust if the smoke alarm goes off when you burn the toast, so are just over engineering them a bit, just to be safe like...

 

[RonnyRaygun]

I'm assuming the box frame is being installed where you are knocking through from the existing house to the extension?
If so this is usually done to aid lateral stability where a large section of wall is being removed.
The connections between the beam and the column will be designed in such a way that the stresses are transferred around the corners. This will reduce deflection in the beam, and also the sagging moment, allowing a smaller top beam to be used. It will also mean that lateral loads are resisted so that your house doesn't fall down in a gale!
The thing with 100mm deep sections such as suggested by your builder is that they may be just as strong under axial loads alone, but your SE would never be able to design a connection that would transfer the stresses around the corners. Even if he could, the columns would fail in bending, hence the use of deeper 152 UCs.
It's not necessarily over engineered; it could be more a case of engineering judgement. Smaller columns will definitely mean larger lateral movements and more deflection of the top beam; not desirable as more likely that cracking could occur, but your SE may deem the slightly larger displacements acceptable if still within allowable limits and it will please their customer.

If I'm wrong and it isn't a moment resisting frame, then you could probably bear straight onto the masonry without columns, although the loads will be high and large spreaders may be required; another reason your SE may have chosen columns instead.

 

[KenExtends]

Ronny and others - thanks

Sorry, I didn't make it clear that this is all new build! My builder commented that this would be needed on a knock through.

Apparently, my SE has now updated me that if I use three x100 square tubes welded together, I will need haunches (hence your comments on connections), however, these would not be needed with the UCs

 

[RonnyRaygun]

I think I agree with your builder then...common practice to put in a box frame in a knock through, but can't see why you would do it in a new build.
The base beam is usually used to spread the load back along the existing footing...no need to do that in a new build as you can just bear the beam onto the walls either end and increase the footing size if need be. New masonry means padstones won't need to be particualrly large either...probably 440mm long.
Then you ensure the wall is well restrained with robust strapping details.

We regularly work on new builds where there isn't a single masonry wall internally, let alone box frames, so why you need one in an extension is beyond me.