When does RC serviceability limits directly affects strength of an element ?

[MAB70]

I am currently working on a project for structural assessment of an existing 3-underground story car parking building. The system of the slabs are solid slabs of 160 mm thick spanning two way of 8.6 m C/C on shallow RC beams. The slab deflections and vibrations by calculation and what is observed on site (actual) is exceeding the code limits a lot, however the provided reinforcement seems to be sufficient for ultimate limit state checks.

The client is keen to accept the deflections and vibrations of the slabs as long as the slabs wont fail and accordingly will save alot of money for not fixing them;

I am not keen on providing such recommendation but i need backup and demonstrate that excessive deflections and vibrations will eventually end up damaging the slabs through (excessive crack widths, corrosion, deterioration, etc..) but i cant find a way to link directly that the excessive deflection will cause structural failure in the slabs by calculations.

I am just curious if there is any limit in the code that perhaps says maximum effective cracked section should not exceed a certain level or the rotation of the slabs shall be limited to a certain degree before its affects the strength.

Any thoughts??

 

[Agent666]

That's the thing, serviceability generally isn't associated with a strength limit state. The only thing I can think of where this might not be the case is durability and corrosion of the reinforcement getting to a point where it starts affecting the ultimate strength of a structural member.

If the client wants to accept it (presumably more on the basis of the cost than anything else), then its their decision, I wouldn't let that affect your recommendation to them to do something about it.

They can obviously decide either way if you provide the information to make an informed decision. Whether its acceptable or not might depend on the planned occupancy/use for the slab areas in question. Would the proposed loading make things worse than the current state for example, or cause an ongoing deterioration that will just get worse over time? Will the proposed occupancy be sensitive to vibrations, for example people occupied vs being used for storage usage or similar?

If they ultimately go against your preferred recommendations to do something to mitigate the perceived issues, then make sure they provide something in writing accepting any things that might have been highlighted as issues. As long as they understand accepting the serviceability issues becomes their problem essentially and there is no issues with achieving the life safety objectives required of the ultimate limit state. If it is deemed an issue after occupation, you could give them options now to address it at a later date if it is an issue.

 

[GC_Hopi]

The slab deflections and vibrations by calculation and what is observed on site (actual) is exceeding the code limits a lot

I think what you stated is the the most direct answer. Yes deflection is affects strength therefore the code has established a deflection limit.

 

[fp77]

I think what you stated is the most direct answer. Yes deflection is affects strength therefore the code has established a deflection limit.

The deflection and most overall serviceability limits are much more about the perceived strength of the structure (by its users) rather than its actual strength. You can have deflections that are well above the recommended limits and have a rather comfortable security margin in what strength is regarded.

 

[MAB70]

I agree with you all, and I will definitely recommend to do something about them. I was just checking if there is anything out there that is missing, many thanks guys for your help.