Reinforcement Congestion in the Precast Columns

JohnRwals · Feb 28, 2023

Hello!

Sometimes a lot of reinforcements can be concentrated at a section like precast columns with corbels taking heavy loads.
And then, can concentrated reinforcements cause brittle failure problem as wide continuous disruption surface can reduce
concrete monolithic property?
Can I ignore this problem as long as minimum reinforcement spacing conditions are observed?

Thanks!

-JRW

HTURKAK · Mar 1, 2023

I'm OK and i would ignore as long as;

- the code specified minimum reinforcement spacing conditions are justified,

- the concrete is poured in horizontal position,( the column formwork with double corbels is at horizontal position and the use of vibration can be done from both sides of crowded area )

My opinion...

Not to know is bad;
not to wish to know is worse.

NIGERIAN PROVERB

MotorCity · Mar 1, 2023

You need to provide the minimum required clear bar spacing so that the concrete can flow between the bars.

dold · Mar 1, 2023

I think we've got another one ^^^

Bob Black said:
Concentrated reinforcements in a section like precast columns with corbels can indeed cause brittle failure problems, especially if the reinforcements are not properly distributed and spaced. The wide continuous disruption surface caused by concentrated reinforcements can reduce the concrete's monolithic property and lead to a brittle failure mode such as shear failure.

Ignoring this problem, even if minimum reinforcement spacing conditions are observed, is not recommended. Proper reinforcement distribution and spacing are essential for ensuring that the concrete can resist loads in a ductile manner and avoid brittle failure. Neglecting this issue can compromise the overall strength and safety of the structure.

Therefore, it is important to carefully design the reinforcement layout and spacing in precast columns with corbels to avoid concentrated reinforcements and ensure that the concrete behaves monolithically and resists loads in a ductile manner. A professional engineer should be consulted for the proper design and detailing of reinforced concrete structures.

Copy/paste the OP into our friendly ChatGPT and one response is:

ChatGPT said:
Concentrated reinforcements, such as those found in precast columns with corbels, can potentially cause brittle failure problems if the reinforcement is not properly designed and placed. When reinforcements are concentrated in a small area, it can create stress concentrations in the concrete, which can lead to cracking and potentially brittle failure.

While following minimum reinforcement spacing requirements can help ensure that the reinforcement is properly distributed, it is not a guarantee against potential brittle failure. It is important to also consider the overall design and placement of the reinforcement to ensure that it is properly distributed and will not create stress concentrations.

To prevent potential brittle failure, it may be necessary to consider additional measures such as using properly graded aggregates, designing adequate reinforcement to distribute loads, and ensuring proper placement and consolidation of the concrete. Additionally, it is important to conduct proper quality control and testing to ensure that the concrete and reinforcement meet the necessary strength and durability requirements.

I wonder if these are bots or people just trying to farm LPS? Either way, what's the point?

Sorry, OP, for the uhhh...congestion...of your thread

KootK · Mar 1, 2023

dold said:
I wonder if these are bots or people just trying to farm LPS? Either way, what's the point?

I'd guess that it's the latter. As for the cause... who knows. Could be:

a) An attempt to generate an undeserved reputation, as you've suggested.

b) Just somebody messing with us. It's a big old world and it only takes one unfortunately. Comsol's revenge perhaps.

Regardless, I'm grateful that a few of your guys seem to be keeping an eye on this.

KootK · Mar 1, 2023

OP said:
Can I ignore this problem as long as minimum reinforcement spacing conditions are observed?

I would say that you can ignore it for the most part. I've executed designs similar to what you seem to be considering successfully. My experience of precasters is that:

1) The miracle of plant fabrication control make it such that they can manage things that would be questionable for EOR/CIP work.

2) Because a precaster's reputation is intimately tied to the quality of their products, they won't hesitate to "do the right thing" if they find themselves questioning their own ability to pull it off. Many precasters will consider concrete cracking of any kind to represent an undesirable serviceability issue. It's mostly impractical do design to that of course.

OP said:
And then, can concentrated reinforcements cause brittle failure problem as wide continuous disruption surface can reduce
concrete monolithic property?

I get what you're saying, and it's healthy of you to question this, but I think that this is fine too. Bottom reinforcing in all beams is anchored at the supports in a very similar way that:

3) Is ubiquitous in both precast and CIP work and;

4) Hasn't been causing problems to my knowledge.

XR250 · Mar 1, 2023

I'm not a concrete guy by any stretch but it always bothers me that in corbels or many other pieces "minimum" cover is always used. A failure would be catastrophic and how much more would it cost to use 2 1/2" cover instead of 1"
Minimum cover does not seem to give much leeway for construction tolerances. I get it, "Factory" but still....

KootK · Mar 1, 2023

XR250 said:
I'm not a concrete guy by any stretch but it always bothers me that in corbels or many other pieces "minimum" cover is always used. A failure would be catastrophic and how much more would it cost to use 2 1/2" cover instead of 1"

In many respects, additional cover simply does little to improve structural performance.

In this situation, the cover is really just about fire protection and rebar passivation for corrosion protection.

In the design of corbels, two things will usually drive the minimization of cover:

1) There are usually architectural constraints on the depth of the corbel. When that is the case, I would way rather have more lever arm than more cover.

2) A corbel will experience significant tensile strain at its topside. More cover = wider cracks = less fire and corrosion protection.

XR250 said:
Minimum cover does not seem to give much leeway for construction tolerances.

I see this as a non issue since there are code mandated tolerances associated with cover, as with most things. If the code says that you need 40 mm cover for a 3HR fire rating, what you really need is 40 mm less whatever that tolerance is. The tolerance issue is baked in.

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