economy of increasing reinforcement to limit deflection

[ramihabchi]

hi,
Do you think in general that increasing quantity of steel to reduce deflection is economical?

 

[rowingengineer]

Typically not, but it does depend

 

[Jimbio7]

I've often done this for small isolated deflection issues on otherwise appropriately thick slabs. I've been told by contractors that it is cheaper than thickening the entire slab or locally thickening the problem area but this could vary by project.
I've never increased all of the rebar to solve deflection for an entire slab.

 

[KootK]

There are lots nuances (degree of cracking etc) but, in general, deeper is cheaper when it comes to deflection control. In very, very rough terms, I'd expect it to pan out something like this:

1) deflection decreases linearly with added rebar and no depth increase.

2) deflection decreases with the square of the depth increase and no reinforcement increase.

Sometimes the most economical choice will by governed by whether or not the required depth increase would make your member significantly deeper than the stuff around it and, thus, negatively impact formwork costs.

 

[Enhineyero]

Its not economical. However, making a slab "work" for deflection using more reinforcement is normally a last resort when the max. thickness of the slab is already in place, at that point the conversation is about function/design (i.e. headroom, clearances) rather than economy.

 

[Tomfh]

Not generally but sometimes a but extra here and there makes sense.

 

[rapt]

Definitely not the most efficient method.

But if you are going to do it, do not believe the code equations that say to add compression no matter what.

If lightly reinforced (low neutral axis depth) then extra tension reinforcement works best, increasing the neutral axis depth which increases effective inertia.

If very heavily reinforced, tension reinforcement does not work at all (can actually increase deflections due to increased shrinkage warping) but adding compression reinforcement works by reducing creep deflection.

In between a combination of the 2 can work also.