Support Settlement 1

What is meant by support settlement?Is support settlement due to defective foundation, bad soil conditions.When should a designer account for support settlement?

 

[JAE]

Support settlement is not necessarily a "defective" foundation as most soils will compress to varying degrees when loaded. When the anticipated soil settlements (usually provided by your Geotechnical Report) are fairly moderate (less than 3/4&quot then usually the settlements can be neglected in the design of building frames.

 

[Ron]

To expand slightly on JAE's comment, "support settlement" is not a significant issue unless there is a high magnitude of uniform settlement of all the "supports" of a building, or if there is significant differential settlement between those supports. The amount of tolerable differential settlement depends upon the type of frame, the type of connections, and the distance over which the differential occurs.

 

doubt123,

depending on the soil property, foundations are expected to settle within a given parameter ( a good soil investigation and geotechnical report will give you the maximum allowable ). When foundations are designed based on the parameters given, minor settlements are not of much concern.

Diferential settlements though is a concern since this will add unwanted stresses on certain members which may not have been designed to handle such stress. What we usually do is add "tie beams" or straps to those supports which may settle differentially or if the soil is soft or has low bearing capacity.

good luck.

 

[austim]

doubt123,

A useful thing to remember is one of the basic theorems of plastic design.

If your structure is 100% ductile (with capacity for plastic hinges to develop fully where necessary), then differential settlement will not cause any reduction in the load capacity of your structure, until the rotation at one or more hinges exceeds capacity (which would only occur after gross settlements).

Settlement may, of course, adversely affect some serviceability limits - crack widths in RC beams, excessive movement in brittle finishes, etc.

 

Let us say that the moment at support say A prior to differential settltment was "M" ,
After differential settltment be "M+dM"
Let us say we have a reinforced concrete beam
If the plastic hinge forms at strain 0.0035 in concrete(my country code) which corresponds to bending moment "M" and suppose at this stage enough plastic hinges have formed in the structure(degree of static redundancy+1),
to make a collapse then is'nt it imperative to make correct measure of these differential settlements and the corresponding additional moments?
(Let the last stage correspond to that when not all loads on the structure have been applied (the bending moment due to all loads and total differential settltment is M+dM) then, a slightest increase in loads from the above would cause collapse))Right?

 

[austim]

Memger,

For simplicity, consider what happens to a single span, fixed ended beam carrying a single central point load, and assume perfect elastic-plastic material behaviour.

Without any differential settlement, the beam will collapse when plastic hinges develop at both ends and at the centre.

The load at which this occurs will be 8Mp/L where Mp = plastic moment, and L = span. To get -Mp at both ends and +Mp at the centre, the "free bending moment" due to the applied load must = 2*Mp. (Hence PL/4=2Mp gives P=8Mp/L)

Suppose that one end settles (without rotation). In the worst situation, you could have +Mp (sagging moment) at the lower end, -Mp at the other, even with no applied load.

If you now apply an increasing central point load to this beam, the moment at the upper end will remain constant at -Mp, with a plastic hinge just 'rotating' a bit.

At the other end, the moment will gradually change from +Mp to -Mp. Essentially, until the fixing moment at the lower end reaches -Mp, the beam will behave as a propped cantilever. Collapse can still not occur until there are three plastic hinges forming a mechanism

At collapse condition, the BM diagram will be exactly the same at if there had never been any settlement : -Mp at both ends, +Mp at the centre, and the applied load to cause collapse remains 8Mp/L as before.

Any good reference on plastic theory will confirm this. eg Michael R. Horne "Plastic Theory of Structures" has this to say :
"..provided change of geometry and instability are unimportant, and also provided failure does not occur because of alternating plasticity or incremental collapse... The collapse loads of elastic-plastic structures are unaffected by thermal stresses... Finally, subject to these restrictions, settlement or flexibility of supports and flexibility (as opposed to ultimate strength) of internal connections have no effect on failure loads".

 

Austim,
I am really feeling low for myself for being such a confused and complicated man.Thanks very very much for making me get things right.

memger

 

[austim]

memger,

Please do not feel "low.. for being confused and complicated..." It was not my intention to do that to you.

I would like to think that being confused occasionally is part of the business of being a thinking engineer. Heaven knows that I have had my share of being totally baffled over the years.

 

[jkespd]

i too am confused and complicated at times and i also can't help feeling a little bit low..."support settlement"... but my secret weapon is the forums at eng-tips. all of you guys are just great. keep it up!