Long-term deflection - consider at all times?

[Malhaneungamzaaa]

Dear community,

Do you have to consider long-term deflection at all times, for any elements of the building - columns/slabs/foundations/beams, etc. when using reinforced concrete?

Thank you always.

 

[oldestguy]

If that would be important (for most sites) we would see consideration of that for all sites. For a building there would be on going need to re-shim or trim doors, etc.

 

[KootK]

I would say it like this: whenever you feel compelled to check the deflection of a concrete element, some consideration should be given to the long term components of deflection. As an example, I don't check concrete columns for axial deflection unless it for a very tall building.

 

[structure567]

For concrete, I've always checked long term deflection for slabs and beams and would provide camber so that it is within deflection limit.

 

[Malhaneungamzaaa]

Thank you all for your responses! They are really helpful.
My concern right now is whether to consider it for a mat slab foundation - as it is also a slab for the ground floor, based on the responses you have provided, I will consider long-term deflection.

Thank you!

 

[MotorCity]

Since this is a mat, it sounds like you are more concerned with long term SETTLEMENT rather than long term DEFLECTION, which is usually associated with an elevated flexural member.

So, yes, I would be concerned with effects of long term settlement.

 

[Malhaneungamzaaa]

Thank you, MotorCity.
This foundation system is a raft slab with helical piles - so would be similar to "deflection".
Please let me know if you think otherwise.

 

[r13]

Unless supporting something sensitive to change in elevation, I usually don't bother to check long term deflection, but specify the slop and sometimes show direction to drain.

 

[rapt]

Disagree completely. There is no logic to checking short term deflection. It is a meaningless number if the total long term deflection is going to be 2.5 - 3 times as much. That is going to be the deflection you will see and feel.

 

[Tomfh]

I generally always check long term
deflection. It’s what people will notice.

As for precambering to avoid long term deflection, I’ve never been comfortable doing this with concrete. Do people do it successfully? How is a Precamber reliably achieved?

 

[dik]

ICYC... for long term deflection calculations I often do them... but, really don't have a lot of confidence in the numbers... sort of a guide...

Dik

 

[KootK]

Do people do it successfully? How is a Precamber reliably achieved?

I don't love it either but it is somewhat common in my area. It's achieved here by jacking up the formwork locally. It's a pretty rough thing as you can imagine. I see it defined as circles or extended circles (like a giant bolt slot) in plan out in the middle / middle strips. Maybe 1/4" increments. If it's not kept pretty simple, it's hopeless. It complicates flatness measurements since:

1) You end up with a slab profile that's more kinked than curved and;

2) If I'm not mistaken, flatness measurements are actually defined on the surface present before the forms are removed, so as not to be impacted by deflections.

I'd love to simply make slabs thick enough that cambering was not required but, since my competitors use this to their advantage, I get stuck with it too.

 

[rapt]

One point regarding precamber, it has no effect on the deflection effect in situations where you are worried about the effect of deflections on something on top of the concrete member.

Precamber reduces the visual and tactile deflection. The amount of movement up or down is still the same, just from a different starting height, and that is what effects something connected to the member.