Support Settlement
Support Settlement
(OP)
What is meant by support settlement?Is support settlement due to defective foundation, bad soil conditions.When should a designer account for support settlement?
When was the last time you drove down the highway without seeing a commercial truck hauling goods?
Download nowINTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS Come Join Us!Are you an
Engineering professional? Join Eng-Tips Forums!
*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail. Posting GuidelinesJobs |
|
RE: Support Settlement
RE: Support Settlement
RE: Support Settlement
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.
RE: Support Settlement
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.
RE: Support Settlement
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?
RE: Support Settlement
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".
RE: Support Settlement
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
RE: Support Settlement
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.
RE: Support Settlement