Removing rear wall & structural stability

[Mag2.0]

Hello all,

I have a part curiosity question and part second opinion question.

I have a 1950's post war brick house with air cavity.

We are removing the rear wall and installing either 2 Ub's or a single UC section with a plate over. UBs would be be bolted at 600cc through the webs.

The opening created will be fairly large at 5.3m maintaining a 665mm return in compliance with Part A of the BR.

On the rear we will in constructing a single storey extension.

I'm not overly keen on the additional work & cost of installing steel columns which would save us a few hundred mm.

However i did have a think about constructing 440 x 440 solid masonry piers.

As this is less than the 665 return required by the guidance it will need to be justified by calculations.

My question is.... What area of loading should be used when applying the appropriate wind pressures. On the non attached side (semi detached) I don't have any substantial/ continuous internal wall so I assume that the floors provide some diaphragm action taking the load to the central chimney breast.

Therefore if i take the area of half the depth x full height of the house the load and resulting moment on the column will be quite substantial.

The masonry wall at fist floor level will remain so i can consider that looks after itself. Therefore taking half the height of the house x half the depth of the house will result in a much less load and will probably start to make the number work. Does this sound about right? The floor joists and proposed rear roof span parallel to the side wall i'm concerned with so their ability to provide diaphragm action will be limited.

Thoguhts?

Many thanks

 

[tony1851]

For wind-load calcs, I'd take the area hatched on the attached sketch.
If you are sheltered/suburban/low rise, suggest unfactored wind load of 0.6kn/m2.

Assume this acts at top of piers, so bending moment is shared between the two piers.
If you go through the loadings carefully, you generally find that the resistance moment of the 440 x 440 (loaded) piers is greater than the applied moment.

(Note that your Building Control people need to agree the wind load, and the wind-loaded area; if you're more exposed, they might want > 0.6. If they want wind-load calcs for the whole gable wall, you then have to consider the resistance moment of all internal- and external ground floor walls perpendicular to the gable plus contribution from chimney breasts, and add them up, which is very tedious. You can generally assume that the floor and roof/ceiling act as diaphragms and will tend to distribute applied lateral load among all walls).

 

[Mag2.0]

Tony, thanks very much for your response.
I was going to throw 1.0kN/m2 at it (ULS) as a first pass so your figure of 0.6 sounds in the right region.

Thanks for the sketch showing the loaded area. So I can understand better...
You take half the height of the rear extension and ground floor of the existing building. Presumably that is because your taking what would be the reaction force as it is being considered as a beam. If so that is something i would agree with.
However you take 1/4 with depth of the building. Why not 1/2? Does this tie in with your previous comments about considering the building as a whole. What is there were no other internal walls?

Always good to learn

edit: just occurred that your referred to the other internal and external walls. Therefore you consider that the floor and roof structure transmits the load to the opposing walls and therefore only 1/4 has to be directly resisted by the piers. Correct?

 

[tony1851]

The reason I suggest ¼ the width of the main house rather than ½ is that there is usually at least some degree of stiffness across the width of the gable at ground floor level, even if it is only - say - a light studwork partition adjacent to a staircase or a cloak/meter cupboard wall.

The other reason is the purely theoretical one that if you include half the gable for the wind load, you'd considerably increase the applied moment, and it's unlikely the piers would be able to resist that.

At the end of the day, it's a semi; what's the chance of your neighbour waking up one morning and deciding to demolish his house to make yours a stand-alone detached? In the real world, next door's will give you some support, so there's no point in making the theoretical figures more difficult for yourself.

You'll be aware that there's very little published guidance on this, no magic spreadsheets and hardly anything helpful in the 'net, so it's really back to first principles. Curtin; Structural Masonry Designer's Manual gives some help. Also Chanakya Arya: Design of Structural Elements has a masonry section, but focused more on panels.

 

[Mag2.0]

Hi Tony.

The real and interesting point would be "my neighbour has removed most of their rear wall and i'd like to do the same". In that instance your permanent structural support would be subject to a higher degree of loading as a result from their gable side also!

hmmm.... If we have stairs running up the gable side then there is actually a discontinuity of the floor and therefore less ability for the floor to act as a stiff member.

All interesting stuff, although im of the mind that if you maintain 450 x 450 solid piers and all else the same then your general OK.

 

[tony1851]

If you keep to roughly the loaded area on that sketch, and no more than 1.0kn/m2, I'd be fairly sure the 450x450 piers would be ok.

 

[Mag2.0]

Agreed. Particularly as the vertical stress would reduce the tensile stress on the mortar joint on the tension face.

Actually 450 works out to roughly equal to a 665 return when you take into account the external leaf running further to accommodate for the cavity. Although a solid pier would give more benefit where the existing brickwork is poor or there is a high axial load.

Thanks for answer the questions and proving a sound board.

Tony, you seem to know your stuff. are you a SE?