Cantilevered Slab Deflection

[slickdeals]

Folks,
Assume you have a wrap around 7' cantilevered slab that is 50' wide.

I feel that the deflection will be a combination of a cantilevered deflection at the tip + a deflection due to the slab trying to span in the long direction (transverse to the cantilever span).

Have any your previous experiences shown this type of behavior?

 

[civilperson]

The stiffest dimension will determine the deflection. Given equal load/area, the 7' cantilever deflection will govern.

 

[ron9876]

Depends on the support conditions but probably both will contribute significantly. Always have to be careful if the back span is long enough it may tend to uplift in some loading conditions and could result in drainage problems.

 

[shobroco]

When you say wrap around, do you mean returning back each end of the 50'? If so, you're going to get much different deflections at the corners than in the middle of the long side, depending what length returns you have. You can also get some different effects in the re-entrant corners that will call for careful reinforcing detailing.

 

[TXStructural]

If the load is more or less uniform across the cantilever, the cantilever stiffness is what will determine deflection (as was said previously about relative stiffness.)

The stiffness along the long axis relative to that of the cantilever will determine how wide a section of cantilever is used to resist point loads.

 

[Lion06]

I would say that the cantilever is what you should use to determine deflections. The stiffer portion near the corner will only serve to help deflection, not hurt it. That being said, I wouldn't count on it......... just know it's there.

 

[paddingtongreen]

Without running this, it is difficult to say. At first glance, the load will be distributed to the cantilever and to the slab edge according to their stiffness. So no, the deflection would be that of the cantilever minus a contribution of the beam action of the slab.

The proportion of the load carried by each element varies, near the center, the cantilever supports most of it, near the ends the beam is stiffer.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[asixth]

Are you referring to combined deflections. What I am visualizing that you have is something like I have attached. If it is similar to this then the deflections would be the cantilever portion where this is no contribution from the load bearing elements in the long spanning direction. What are you using for your longterm span-deflection limit.

 

[slickdeals]

Asixth, you got it right with the sketch. Also see attached.

I know that running a Finite element model on this will give me the right results? But I just wanted to hear real life opinions from others.

 

[paddingtongreen]

Slick, please let us know if anything strange turns up when you run this analysis. I have a suspicion that there may be uplift, or less reaction than expected, at the ends of the outermost strip of the slab, or in the corner, similar to what happens to a flat slab supported, or fixed, on all four sides. I think the deflected shape could be quite interesting.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[BAretired]

The slab cannot deflect at point D (see attached). Its deflection at point E will be restrained by the fixity of strip EBG at point B.

Deflection between D and E will be affected by the slab spanning in the long direction between stiff elements but the effect at midpoint is likely to be small.



BA

 

[asixth]

I don't think the stiffness in the long direction will be significant enough to reduce the cantilever deflection. If it were my design, I would design for the deflections considering the cantilever only. I would provide shrinkage steel in the long direction so if it does want to span that direction it will have some capacity to do so but not rely on it in design.

 

[BAretired]

The main reinforcement will be on top of slab. Usually, temperature steel is tied to main steel, so it will be on top and will not resist positive moments.

It might be a good idea in this instance to provide some additional bottom steel to control cracking, but I agree that it would not be prudent to consider any reduction in the cantilever moment as a result of the long orthogonal span.

BA

 

[paddingtongreen]

BAretired I agree with, "The main reinforcement will be on top of slab. Usually, temperature steel is tied to main steel, so it will be on top and will not resist positive moments.", in general, but if the slab is supported at the ends it will require some bottom reinforcing there, at the outermost section, anyway.


Michael.
Timing has a lot to do with the outcome of a rain dance.