Is it logical to use slab rebar in calculating negative reinforcement of a T-beam? 5

[NewbieInSE]

Hello all,
Slab requires reinforcement as it supports the loads above it. If I provide reinforcement for slab 10mm@8" at the top to meet the requirement, can I take the full advantage of this slab reinforcement in calculating the T-beam rebar adequacy of an existing RC beam?

I feel like I designed and provided the reinforcement which is required for the slab. Now if I give it as a loan to beam, then where is the reinforcement of the slab to cover its own stress.

Thanks.

 

[NewbieInSE]

How did you assess the slabs required steel area?

That's a good point. I checked it in the FEM considering full flexural stiffness of both beam and slab.

 

[NewbieInSE]

You should consider T-beam if there is a slab attached, as that is how it will behave.

Where I practise, engineers don't do T-beam calculations much . They mostly see how much reinforcement ETABS is showing for the beams, and provide/check the reinforcement.
There is going to remain some fat in the beams which in future might be useful if loads increase.

 

[BridgeSmith]

If you're doing an FEM, don't you just put all concrete and the reinforcing into the model, and let it sort out the the shear flow, effective span, etc.? That would be the whole reason to do the FEM, isn't it?

It's only if you're doing it by hand, that you have to estimate the effective flange, what beam reinforcing is effective for negative bending, etc.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[KootK]

But they usually suggest only bars within 2 bar sizes of the beam bars be included.

I've not heard that before. Do you know what the logic for it is? Do larger bars open up larger cracks that would fracture smaller bars or something? That's a complete WAG, I've no idea.

 

[rapt]

Kootk,

Actually current AS code and Eurocode says to ignore anything less than half the diameter of the main bars.

I think it is because modern crack spacing and width calculations are dependent on the development length of the bars. So you get completely different results from the 2 bar sizes.

 

[JoshPlumSE]

One issue that I've run into with this before, is that if you include too much of the slab bars for negative bending of the tee (i.e. stem in compression, flange in tension) then you an get into an "over reinforced" condition for the beam. Meaning that the ductility of the beam in negative bending is not good.

I think there are ways to use engineering judgement to get around this (i.e. use less of the slab reinforcement when you're looking at negative bending). But, since my experience was with a program that attempted to automate Tee beam design (i.e. RISAFloor) this became something of a problem for the program and its users.

 

[NewbieInSE]

If you're doing an FEM, don't you just put all concrete and the reinforcing into the model, and let it sort out the the shear flow, effective span, etc.? That would be the whole reason to do the FEM, isn't it?

Ummm, I use ETABS, It doesn't have that option to do so as far as I know. Another program from CSI called SAFE can perform what you have mentioned, I think. But I'm not much used to using the SAFE program.

I think there are ways to use engineering judgement to get around this (i.e. use less of the slab reinforcement when you're looking at negative bending)

Doing that only solves the issue in calculation, right? As per what is in place (existing rebar in slab), the beam is over reinforced at top.

 

[JoshPlumSE]

Doing that only solves the issue in calculation, right? As per what is in place (existing rebar in slab), the beam is over reinforced at top.

It's a "code issue" where it get's flagged as an over reinforced tee beam. But, it's not really a Tee beam. It's a slab on top of a beam. So, there is room for engineering judgment here.

Plus, it's doubly reinforced which makes the behavior more ductile. But, only if you do more complex calculations than you normally would want to.

 

[rapt]

JoshPlumSE,

I think you need to explain that one a bit more. The only way it is not a T Beam is if there is no shear connection between the slab and the beam.

Otherwise it is a TBeam and the reinforcement in the effective flange contributes to the beam ductility.