25 mm settlement origin 1

[lcruzz01]

I remember reading in an article that the 25 mm threshold came from observations of buildings that suffered some kind of settlement. The performance of the buildings that suffered settlements greater than 25 mm was deemed unacceptable. Is this true? Who made this observations? Terzaghi maybe? I can´t recall where I read this the first time. It could be great if you guys could share a reference.

 

[mar2805]

I think this value is referd to differential settlement...I might be wrong

 

[brick2020]

I have an article/paper that I came across on the web, written by Terzaghi, it was for the calculation of Sub Grade Modulus.

If you look at the calculation of sub grade modulus, its SBC/0.04, that is the limit of settlement for 25mm.

So in a way Terzaghi has set the limit for using 25mm. WHY? for that I will have to read the paper to answer that question.

any other views plz..

 

[a2mfk]

25mm/1" is very small for just about any structure, but over what distance is the bigger question?

Differential foundation movements and any limits should be based on the building's ability to accommodate those movements. There are limitless combinations of material types and geometries, so to come up with any set number that would apply to multiple situations would seem illogical.

I deal with foundation movements and causes of damage to homes every day. If someone has a wood-framed house with wood siding, and they have some differential foundation movements that have only caused some minor gyp board damage, I explain to them why they are lucky their house is built that way and can structurally accommodate the movements.

Other Example:
Pre-engineered steel building, all metal panel siding, slab used as a warehouse for storage: very tolerant to differential foundation movements

Three story office building with brick cladding all walls: not tolerant to foundation movements

 

[TXStructural]

Sounds like it came from a one inch convention, and somehow got "metricated" to a very precise "25 mm". It is a pet peeve of mine, that a simple "about an inch" became a precise measurement, instead of "a few centimeters".

Too often we get caught up in finding a computable number, rather than a concept.

 

[mar2805]

This is an interesting topic.
Limits on differential settlement = limits on allowable deflection of members?

My code,EUROCODE is very cheap on giving informations on limiting member deflection.
A base value of span/250 is used but later on it is said that the maximum allowable deflection of any structural memeber should be based on the ductility of the "structure" siting on top of the member for wich the calculation is beeing done...a value of span/500 is mentioned.

I looked at Australian code, AS, since I wanted to see how other codes deal with this matter and they have named few values, and one of them is a limit od L/1000 if member is supporting a masonry partitions.
The last one seems to have the most logic since the masonry wall is brittle and suppose you have a beam or a slab of a span 600cm you should limit you deflection (differntial settlement "beam end" - "midpoint beam") to 600cm/1000=0,60cm.
Compared to European standard600cm/500=1,20cm 600cm/250=2,4cm.
Very large difference in values.

How is this defined in ACI?

This deflection parametar is basicly limiting deflection angle of end point on a member and its mid point.
span/1000 for L=1000cm max.def.=1,00cm for L=750cm max.def.=0,50cm for L=350cm max. def.=0,35cm. all different values but all have the same deflection angle wich is probaly based on a ductility of a member beeing suported.

I found this article that deals with differential settlements of foundations and the damage that is couses in the superstructure.

http://www.concrete.net.au/publications/pdf/TN61ArticulatedWalling.pdf

Based on the material used you can find limiting values for dif.settle or lets call it deflections of bearing members - page 6.
Looking at the values it is clear that your brick partition walls will probably crack if there is a deflection greater then 1cm!

COnfusing thing is that in many foundation books there are values mentioned to allowable diferential settlements wich I think are mentioned for designing your members for their ultimate limiting state (a state before they break, bearing capacity of a member).
Todays design criteria have been updated are more or less all governed by the servicibility demands (cracking, deflections, limiting stresses im members).

 

[BigH]

TXStructural. I know what you mean. I read an article in Heavy Construction News about 20 or so years ago and they reported a contractor placed a 304 mm gravel pad for supporting his pile driving rig. How "a foot" became 12 inches and then 304 mm was quite hilarious! and he used 304 as a round off for 304.8 instead of 305 and wouldn't 300 have been more in line with one significant digit?

 

[TXStructural]

We can calculate inaccurate values quite precisely with our fancy computers, but it does not make the numbers any more correct or useful. And do not even get me started on wind values... a sharper pencil won't make the building any stronger, nor the loads any less.

 

[TomDOT]

Ugh, I already have to deal with this conversion/overprecision issue enough at ASTM.

One pint of (~3/8-1/2") gravel is NOT 473.2 mL

 

[BigH]

@TomDOT - what I liked was when ASTM (and AASHTO) changed from psi to MPa (1000 kPa). . . they told us to report the results to the nearest 79 kPa!! How the 'ell do you do that? I asked them and got a snitty response - they later changed it to 100 kPa (0.1 MPa) like Canadian Standards did it right off the bat. I think that they must have had some interns doing the conversions and didn't check.

 

[TomDOT]

Sadly enough, I frequently have to explain the precision issues to technical committee members. Probably 10% of my negative votes are due to units translation resulting in overprecision - and I'm not that stringent. If they want to translate 1 pint as 470 mL, fine. If they want to translate as 500 mL, fine. I will not abide 1 pint* -> 473.2 mL

Luckily the staff are usually willing to consider it to be an editorial change, so it can get fixed without a reballot.

* And I believe it was "About 1 pint"