Canteliver deflection

[Steel Inspector]

i have a cantilever slab that is 1 year old. its span is 2.2m. it has already shown a deflection of 7 cm.
when does deflection become too dangerous? and a sign of failure?

 

[BulbTheBuilder]

7cm after a year is pretty significant. What does the cantilever support and what kind of load come on top? What is the slab thickness? What are the reinforcements? What might be causing this?
We don't know the unknowns.

You can use the basic span-to-depth ratio to have a fair idea of your deflection limit but 7cm in year is significant.

 

[KootK]

Unless something would be prone to sliding off of the slab etc, I would normally look to slab cracking to be more indicative of ULS failure than deflection. Normally, you'd have a hard time convincing me that a cracked / lightly cracked slab is overstressed solely because it has deflected.

Can you tell if the deflection is a product of curvature along the cantilever length versus backspan induced rotation at the support? Those are different problems.

How sure are you that the deflection has taken place in service rather than at or near the time of construction?

We're speaking of downwards deflection of the cantilever tip, correct?

Is there a concentrated load applied to the end of the cantilever? A supported wall etc?

 

[Tomfh]

Significant excess deflection can indicate structural failure. You need to identify the cause of the deflection. As Kootk said - is the cantilever bending? Is the support rotating? Is there a hinge developing? Is the design adequate? Do you know what was actually constructed?

 

[rapt]

That would already be classed as a serviceability failure.

That does not necessarily mean it is about to collapse, but it should be checked. And by someone who knows what he is doing with deflection calculations.

A lot of UAE design is done on computer programs that do not do concrete deflections well and are designed by designers who do not understand deflection calculations (apologies to those who are doing it correctly) and/or base them on L/D ratios.

 

[Steel Inspector]

KootK,
there is no concentrated load
yes i am talking about downward defelection
at the time of construction it was actually lifted up 4cm. and now its 7cm down

 

[Tomfh]

Presumably someone was worried about deflection to begin with if they preset it +40mm. How thick is the slab? What do you know about the design?

 

[JLNJ]

Span/31 ?? THat's hard to do with concrete.

Either there is little stiffness activated or the support has rotated. Most likely the latter.

 

[tomfh2]

It’s actually L/20, as there was 40mm precamber. Someone was presumably expecting a fair amount of deflection.

Maybe they assumed a rigid support and then support rotated? That can give you 2-3x the deflection compared to fixed support.

 

[rapt]

Cantilever deflections are severely affected by the length of the back span. Short back span causes high rotation and the deflection caused by rotation can easily be a lot high than bending deflection.

L/D ratios are based on no rotation. (Eurocode at least mentions this). Rotation has to be assessed separately.

So if someone has designed based on L/D it is easily possible.

At least is is L/20 based on cantilever length, which is really the equivalent to L/40 for normal spans, not that it helps!