To retrofit, or not to retrofit? 3

[Blackstar123]

I am checking the design of some RCC beams. The required bottom reinforcement at supports due to seismic load combination is coming out to be less than the minimum reinforcement. Code specifies that in such a case, beam should at least have As = 1.33 times the required reinforcement.
However, provided bottom reinforcement (5-#8) is falling short by 1-#8 bar.

This exercise is part of a big project, in which design of existing structures is being checked for increased loads. We've proposed FRP retrofitting for RCC structures wherever required, because client do not want to disrupt the daily plant operation.

Proposing FRP for these beams, seems a bit overkill. I'd be grateful to know how other engineers will take care of this problem.

 

[r13]

If this is a continuous RC beam, is moment redistribution possible, as hinge has formed at this support?

 

[JAE]

You might look at the beam as a doubly reinforced section instead of a singly reinforced section.
That might change the actual amount of bottom tension reinforcement required.

With compression reinforcement, the req'd tension As is reduced.

Most often, concrete beams are designed with only the single reinforcement condition considered.
There's more work involved but it might bear fruit for you.

 

[Celt83]

tagging on with JAE ....How tall is the beam, any chance there is skin reinforcement that you could take advantage of with a more detailed strain compatibility analysis?

My Personal Open Source Structural Applications:

https://github.com/buddyd16/Structural-Engineering

Open Source Structural GitHub Group:

https://github.com/open-struct-engineer

 

[Agent666]

Minimum is minimum for a reason. That third more capacity than analysis does not apply to seismic cases.

The minimums are there to ensure distributed cracking and hence distributed dissipation of the seismic energy via yielding of the reinforcement under successive cycles of an earthquake over a larger length of the reinforcement. If you don't have sufficient reinforcement to drive this distributed cracking, you increase the potential for a single crack which concentrates the yielding and strain hardening over a very short length of the bar leading to bar fracture due to low cycle fatigue. This behaviour was observed in quite a few structures in our NZ 2011 earthquake series, some bar were observed to fracture in walls predominantly due to the larger strains. Our minimum reinforcement was doubled in wall end regions. Our beam provisions remain unchanged, however designers remain aware that barely reaching minimum reinforcement contents will result in poor performance and risk of bar fracture. Certainly structures that had this single crack behaviour survived the earthquakes, however due to most of the bar capacity being 'used up' due to increased strain hardening, they were ultimately demolished.

Like I said in the beginning, minimums are minimums for a reason.

 

[Blackstar123]

If this is a continuous RC beam, is moment redistribution possible, as hinge has formed at this support?

These beams are continuous tie beams provided to reduce the length of a very long column.

This is at least 20 years old design and was designed to resist only self weight and lateral loads. As far as I know, there's no masonry load on it.

How tall is the beam, any chance there is skin reinforcement that you could take advantage of with a more detailed strain compatibility analysis?

Size of the beam is 750x1200 and yes, it has 4-#6 mid bars on each face. Taking the tensile resistance of the two bars nearest to the tension reinforcement did the trick.

You might look at the beam as a doubly reinforced section instead of a singly reinforced section.

Top reinforcement increased the strength but not by a very big margin as it's not yielding.

The minimums are there to ensure distributed cracking and hence distributed dissipation of the seismic energy via yielding of the reinforcement under successive cycles of an earthquake over a larger length of the reinforcement. If you don't have sufficient reinforcement to drive this distributed cracking, you increase the potential for a single crack which concentrates the yielding and strain hardening over a very short length of the bar leading to bar fracture due to low cycle fatigue.

I chewed on this statement the whole day. And, I've come to the conclusion that you're right about code specified minimum are there for a reason. Even though, I may be able to prove that the provided section has enough theoretical capacity as it is but I may not have code to back me up on this.

On this note, thank you everyone for your responses. It helped a lot.

 

[Celt83]

Blackstar123:

If considering the skin reinforcement make sure you re-check shear with the reduced depth to the tension steel centroid.

My Personal Open Source Structural Applications:

https://github.com/buddyd16/Structural-Engineering

Open Source Structural GitHub Group:

https://github.com/open-struct-engineer

 

[JAE]

Blackstar123,
I was suggesting looking at the double reinforced condition since that can affect the minimum steel (1.33 x As(req'd).
With top steel, the As(req'd) amount may come down a bit, reducing the value of 1.33 x As(req'd)....and possibly getting the beam to work.

The OP stated that they had 83.3% of the required minimum. The double reinf. condition might solve that....probably not but "might".

Having said that - minimum flexural steel in the bottom at the joint may be more than 1.33 x As(req'd) based on the seismic intensity and the type of framing used (Ordinary, Intermediate, etc.) as Agent666 stated above. The OP didn't state the type of moment frame or the seismic design category.

 

[Blackstar123]

JAE,
Oh, now I understand the logic behind your suggestion. Sometimes, my brain catch on to keywords only and make a story of it's own.

And yes, you're probably right, it might solve my problem by reducing the required minimum criteria.

But just saying, finding As for a doubly reinforced section with compression steel not yeilding would be a very cumbersome process. That's why I went for the easiest route and found the capacity instead.

Framing type is IMF for SDC C.

I've check the provided positive moment capacity at support requirement, and it is as required for an IMF frame.

 

[Blackstar123]

Celt83,
Provided shear reinforcement is adequate even for max. shear.

 

[Agent666]

Backstar123

One thing I would add, is why the single crack occurs and its relationship to the minimum reinforcement content:-

While we design to the 28 day 5% characteristic concrete strength typically to work out our design capacities. In real structures the concrete strength increases over time. In real concrete mixes, to ensure the supplier hits the minimum strength specified, you might achieve in the long term a concrete strength of 1.5 to 2.0 times the specified minimum 28 day strength.

In real structures this primarily equates to a much higher cracking moment strength for a given cross section irrespective of reinforcement.

Now the reason for the minimum reinforcement is primarily related to ensuring that the reinforced section also has a higher capacity than this enhanced concrete cracking moment. Especially in seismic design we want to ensure that when the concrete cross section cracks that the reinforcement enhances the cross sections capacity by having a higher capacity than the concrete cracking moment capacity to effectively cause the weaker concrete section to crack at multiple locations. If the reinforcement based moment capacity is lower then when it cracks once, we cannot achieve any further cracks as the reinforcement is the weakest link so continues to yield/strain harden at the location of the single crack (i.e. the scenario I described above).