Under-reinforced beam 1

[BigBakwas77]

A colleague approached me with a structural problem. He issued a drawing for construction where the tension reinforcement for a beam were specified less than required. (As(Reqd)=1783 mm², As(Prov)=1000 mm²). The slab and beam are now cast but formwork is not removed. I have advised him not to remove formwork, or at least keep the beam adequately propped. I also advised him that he can resolve the issue by adding steel plates on either side of the beam or at the bottom. Beam design is OK for shear.
Has anyone come across such an issue? Can someone point me to the material which can help me design the plates and its connection to concrete?

 

[human909]

1. Under reinforced normally has a specific meaning (in most localities), an this isn't it. What you should be saying is it has less that the minimum required tension steel.

2. What is the actual moment capacity of the beam compared to its dead load demand let alone the ultimate limit state. Removing the formwork is highly unlikely to be an issue.

3. If it does meet ULS then many cost allow dispensation for not meeting minimum steel requirements IF failure does not lead to structural collapse. The reason why this is allowed is the beam is 'strong' enough. But the failure mechanism can be brittle, hence the requirement for minimum steel.

4. If the possible failure is catastrophic then yes, you do have an issue and rectification likely can be achieved by plates on the tension face.

 

[Tomfh]

1. Under reinforced normally has a specific meaning (in most localities), a this isn't it.

It’s a rather odd term, isn’t it? One of those counterintuitive phrases—like ‘inflammable’—that seems to imply the opposite of what it actually means to most people, except for those in the know - which as we see here doesn’t even include all engineers. Why they opted for ‘under-reinforced’ instead of something clearer like ‘ductile’ is beyond me

 

[rapt]

I disagree with stripping the formwork if the strength under the construction load is ok.

It is far better to add strengthening before any deflection has occurred.

 

[human909]

True. Rapt. True.

 

[FMJalink]

This is not my area of knowledge, but what I read here amazes me somewhat. Presuming this concerns a new construction where people will be in and/or on. A new beam having only 56% design bearing capacity is to be fixed up to meet the reinforcing cross sectional area by means of adding plate(s) on the outside seems not good construction work.
If a check on real strength of the rebar shows it to be sufficient, then OK, but most likely the rebar will be with more spce between them and with plate(s) how to get an even distribution of the tension to the outside of the beam?

 

[phamENG]

Carbon fiber reinforcing on the beam may be a better option. Adding plates will involve drilling into the beam at the risk of cutting the already insufficient rebar.

 

[BridgeSmith]

What is the criteria for the minimum tension reinforcing area mentioned? Is it needed to carry the loading, or minimum for the cracking moment, or something else?

 

[dik]

How is the reinforcing as far as load capacity? Is is still under reinforced? You may want to loosen the formwork to keep it from adhering to the concrete.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[human909]

what I read here amazes me somewhat. Presuming this concerns a new construction where people will be in and/or on. A new beam having only 56% design bearing capacity is to be fixed up to meet the reinforcing cross sectional area by means of adding plate(s) on the outside seems not good construction work.
If a check on real strength of the rebar shows it to be sufficient, then OK, but most likely the rebar will be with more spce between them and with plate(s) how to get an even distribution of the tension to the outside of the beam?

There was no mention of the beam being understrength in any terms even in ULS. So in my answer I have initially assumed that. By I did seek clarification in my point 2.

The "mininmum steel area" is there to ensure that even if the load exceeds the ULS load then the bending failure will be ductile in nature. Essential if you don't meet it then the concrete tensile capacity could hold the beam together under load, but if this is the case failure can occur in a brittle fashion.

Carbon fiber reinforcing on the beam may be a better option. Adding plates will involve drilling into the beam at the risk of cutting the already insufficient rebar.

This is not my area of knowledge

FWIW, it is barely my area of knowledge either. I'm mostly a steel guy and my concrete hasn't extended much beyond footings at this stage. But it took me a few arguments with engineers several decades more experienced that the minimum steel requirements mentioned in the AS code aren't really applicable for large concrete matt slabs that are mostly designed around mass. (The escape clause here is that failure, even brittle failure of the tension zone of the slab isn't going to result in collapse.) IMO the AS code isn't particularly clear on this aspect especially for matt foundations.

 

[Tomfh]

If a check on real strength of the rebar shows it to be sufficient, then OK, but most likely the rebar will be with more space between them and with plate(s) how to get an even distribution of the tension to the outside of the beam?

The rebar doesn't have to yield simultaneously with the steel plates. It is common to have rebar at different layers in concrete beams. That being said, the rebar and the plate can't be too far out of sync. If you simply bolt on plates, with normal holes with 2mm clearance, the beam will have to deflect too far before the anchors engage.


At the end of the day, how to deal with these "oopsie, our design doesn't work" problems is as much political as it is technical. There are many things to consider apart from how you'd ideally fix it.

 

[dik]

Does this help?

It from CSA A23.3 (14) I don't do much concrete stuff and this is the latest copy I have,

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[JAE]

BigBakwas77, you might clarify the situation a bit.

For concrete flexural design you typically have three limits:

As (minimum) - this is the minimum reinforcement required to ensure that you have a ductile failure. It typically is derived from the beam size, not loads - except for the standard proviso that dik mentions where the codes allow you to simply increase your steel 1.33 times above your required steel based on strength.

As (required) - this is a calculated amount of reinforcement needed to resist design loads. It can end up being higher, or lower, than the As(min.)

As (maximum) - this is the upper limit on reinforcement to ensure that the beam isn't "over-reinforced" such that the concrete will crush before the steel yields should there be an overload of some sort.

The As (provided) is of course just that - the amount actually constructed.

You list in the original question only the provided and the required so we're not sure what we really have here. Can you clarify?

 

[lexpatrie]

Strength issue, carbon fiber on the bottom,

Under the minimum flexural reinforcement (A[sub]s,min[/sub]) then check that provided is what, 50% more than the amount required by analysis and below A[sub]s,max[/sub]?

ACI has a provision for that similar to what was mentioned for CSA (Canada?).

And what is ULS, while we're at it. Ultimate Load Something?

 

[dik]

Thanks lex... didn't know.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[m7rhman]

"Ultimate Limit State".

 

[lexpatrie]

Yeah I don't know the provision but now that more neurons are working, if I recall correctly it's 4/3 of the reinforcing that is required by analysis is the alternate minimum, so it might be justifiable that way. Unless that's only for development length.....

On a related note, this is a really long article.....

Making sense of minimum flexural reinforcement requirements for reinforced concrete members, Seguirant, Brice, Khaleghi. PCI Journal, Summer 2010.

Look at the Graphs!

ETA: I may have invented a provision here, I haven't found it yet.

I did locate the 12.5.3.4 - Excess reinforcement which has a proportional reduction for "excess reinforcement" involving development length, although that can't be used blanket to reduce the development length below a minimum length.... anyway. I'll keep poking about. For whatever reason I'm looking at the ACI 318-95.

ETA: Found it.

Source: ACI 318-95

That's what happens when I text search for 4/3 and the language is "at least one-third greater"..... even looking for "required by analysis" I was getting despondent.