Serviceable limit states, deflection and cracking

[cesaramorim]

I have always had this doubt, there are 3 kinds of service limit states, the one with very short duration, the short duration, and the long duration one.

When checking for deflections or cracking, which one of them should I use?
I've checked the Eurocode 2 (concrete) and for cracking there seems to be the following difference:
For reinforced concrete and prestressed reinforced concrete with no grip on the bars -> long duration limit state
For prestressed reinforced concrete with good grip on the bars -> short duration limit state

Do you agree? What about deflection?

 

[cesaramorim]

bump

 

[IDS]

There are many different definitions of serviceability limit state(s) (although I don't recall any that use the three durations that you mention), but they all come down to the same question:

Will the structure require repair or unscheduled maintenance, or will it fail to satisfy the design requirements, during its service life?

Cracks usually increase in width over time, so should be based on long duration, regardless of the structural system, although with prestressed concrete you will also need to check cracks at the time of load transfer, and for any system you may need to check cracks during construction.

Deflections may be due to sustained loads, which should be checked taking full account of creep and shrinkage, or short term loads, or a combination of both.

So there is no fixed rule. You just have to consider the consequences of the behaviour you are checking for.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[podosci]

I have a question of my own, Creep falls under which limit state?

 

[IDS]

If you are looking at the effect of creep on cracking and deflections, that's the Serviceability Limit State.
If you are looking at the effect of creep on member capacity, that's the Strength Limit State.
If you are looking at the effect of creep on collapse, that's the Collapse Limit State.

The last two are usually combined under Ultimate Limit State, but in my opinion they shouldn't be.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[IDS]

If you are looking at the effect of creep on cracking and deflections, that's the Serviceability Limit State.

Unless the deflections could lead to collapse, in which case it will be the Collapse Limit State.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/