expanding walls?

[Justin R]

Hi everyone, I have a question:
I met a guy on site recently who asserted that a lime built wall which has the roof load removed will expand upwards, sometimes by a significant amount.
He told me of one occasion where they got the engineer/surveyor, who would not believe that this was the case, to measure the wallhead hight of an old two storey agricultural building they were converting. After the roof was removed the wallhead was apparently found to have risen by more than 3 inches!
He said something about frost action on the lime mortar causing microscopic cracks which open up when no longer compressed.
Has anybody else experienced this or was he having a laugh?
Cheers
Justin

 

[MatthewC]

My vote is that he was having a laugh. Just does not sound plausible

Matthew

 

[LadyArowana]

I bet the builder had just dug a really tiny French drain round the outside :wink:

 

[atticman]

Any structure under load will compress, and will therefore "de-compress" when the load is removed, although not to the same "original" dimensions.

3 inches on a standard height wall sounds a bit far fetched, however!

 

[skier-hughes]

I tried it to see................
My wall went from 8' tall to 1' 6" mostly looking like rubble............
Not Pleased :twisted:

 

[Flyfisher]

Let's see . . . .

Two storey wall, say 18 feet, or 216 inches.

Assume 3-inch courses (brick + mortar) = 72 courses.

Assume half-inch mortar per course = total of 36 inches of mortar.

Expansion by 3 inches would be 8%.

Seem a lot to me.

 

[MatthewC]

Any structure under load will compress, and will therefore "de-compress" when the load is removed, although not to the same "original" dimensions.

3 inches on a standard height wall sounds a bit far fetched, however!

The weight of the wall itself is far greater than the weight of the roof, and so at the base of the wall, the load of the roof being removed is only (say) 5% of the total load thus the movement on decompression will be virtually negligible. This ratio of load removed to load remaining will get greater as you get higher until you get to the top mortar where there was only one stone above but lots of roof. However even so I just can't see the sum of the movement at each level being remotely significant, certainly not 3 inches in a two storey building.

 

[Justin R]

Many thanks for your answers everybody.
Overall the conclusion would seem to be that the three inches were rather far fetched to say the least, but that some expansion might be expected.
Maybe just a few percentage points?
The self-weight of the wall might then reduce this further as well.
Would this seem roughly correct?
Cheers Justin.

 

[Justin R]

Many thanks for your responses everybody.
It sounds like you all agree that the three inches is extremely unlikely, but that some expansion might be expected?
8% being too much what do you think might be a possible value?
I guess that if the cumulative effect was very much at all you would see cracks appearing between parts of a building where the roof was removed and parts where it was not?
Cheers
Justin

 

[andrew gayton]

The only time I have seen a wall 'expand' is as a result of fire. unslaked quicklime in mortar, when subject to fire and subsequent soaking by the fire brigade, will slake and expand causing distortion in bed and perp joints.

 

[Andrew Bodenham]

Complete rubbish. The proposed expansion mechanism could not possibly produce such a large distortion.

For sake of argument lets assume that the masonry units are of fixed size and do not contribute to the 'expansion'. Assume bed joints 10mm thick. Assume that the mass of the wall effectively restrains the joints in lower courses, so 'expansion' would only occur in upper courses - for sake of easy maths lets say in the top 10 mortar courses. So we've got approx 70mm 'expansion' occuring in 10 courses, giving 7mm in each course, which is a 70% increase in the volume of the mortar - despite vertical restraint from the self-weight of the masonry. Even if the effect was apparent in all mortar joints the percentage increase in volume would have to be relatively huge to achieve the cumulative total suggested. If the claimed increase was of the order of 1% then I might be less dismissive.

Frost heave can cause significant volume change in unrestrained materials such as surface soils or compacted gravel paths - but not when the material is restrained by significant weight of masonry. It's possible that an exposed wall top may suffer some dislocation of the top course (or top few courses) due to freeze-thaw action over a period of years, but in my experience this tends to exhibit as lateral rather than vertical displacement.

In my opinion the original phenomenon reported is much more likely to have been a result of measurement error - disturbed datum, innaccurate measuring device, failure to measure to the same point on the building - or recording error. Surveys of agricultural buildings are not always carried out to high levels of precision - the accuracy of the 'existing' measurement would need to be known in order to make a valid comparison. Also worth bearing in mind that roof loadings tend to be concentrated by the transmission of load through battens, rafters, purlins etc. to point loads where the principal structure meets the walls. So any 'expansion' related to removal of load would be localized in the vicinity of such point loads, resulting in significant distortion of the surrounding masonry. I have never seen anything like this in a building - has anybody else?

 

[Feltwell]

I agree, it's rubbish. The only movement of that size that I've ever seen in any structure was on a bridge where after the concrete deck had been poured, the weight made the steel bridge beams underneath deflect downwards by 70mm - which is exactly what they were designed to do. This was the weight of a 4-lane bridge deck going all the way over the M62!

 

[stuart45]

Brickwork does expand, but not by that much. Bricks are normally at their smallest when first laid, whereas blockwork is normally at it's largest. This can sometimes cause cracking at the wall plate when the cavity is closed.
If the joints expanded that much the external walls of the cavity on older houses would be much higher than the internal walls which normally take the weight of the roof.
The rusting of wall ties, or sulphate attacks can also lift the brickwork.