Concrete stress strain curve by ACI 318
Concrete stress strain curve by ACI 318
(OP)
I am study a case which is quite difficult so I will Open this thread to ask my questions, so please help as much as you can
1. I found a sheet calculating moments based on a model of stress strain curve(concrete) by making different layers instead of using Whitney block, and I suppose this method is adopted by ACI318, So any one have a clue,
see the attached document
Also ,if anyone who is structural engineer with concrete experience who is willing to guide me through this design process am going through(Since i feel completely lost),we can chat on private and even speak so I can get the whole picture better
Thank you all
1. I found a sheet calculating moments based on a model of stress strain curve(concrete) by making different layers instead of using Whitney block, and I suppose this method is adopted by ACI318, So any one have a clue,
see the attached document
Also ,if anyone who is structural engineer with concrete experience who is willing to guide me through this design process am going through(Since i feel completely lost),we can chat on private and even speak so I can get the whole picture better
Thank you all






RE: Concrete stress strain curve by ACI 318
As for your comment about wanting help from a structural engineer, this board isn't really for that, unless you are a practicing engineer. Since you say you are "completely lost", ethically, it would be my opinion that you shouldn't take on such a project. If you are just trying to learn about concrete, I suggest getting a book like "reinforced concrete mechanics and design" by Macgregor and wight and learn away.
RE: Concrete stress strain curve by ACI 318
We tend to use the Eurocode model as it gives a good representation of the variation in the concrete stress/strain curve for different concrete strengths.
RE: Concrete stress strain curve by ACI 318
kindly see the attached document to continue discussion
RE: Concrete stress strain curve by ACI 318
RE: Concrete stress strain curve by ACI 318
RE: Concrete stress strain curve by ACI 318
RE: Concrete stress strain curve by ACI 318
A few years ago RISA incorporated this stress block as a more exact option when compared to the whitney stress block. It took a lot of custom programming. A marketing feature, if you will, to demonstrate how much better the RISA solution could be compared to our competitors and how it would give a user a competitive advantage... et cetera.
What, don't remember every seeing that in our marketing literature or when speaking to our engineers? That's because we found out that the member capacities really didn't change very much. So, there wasn't anything for us to promote. It's still nice to have something to compare against. But, I wouldn't expect to get an end result that is significantly different than what the traditional rectangular stress block gives you.
RE: Concrete stress strain curve by ACI 318
Worth a read to compare to ACI, AU and EU codes.
RE: Concrete stress strain curve by ACI 318
In the ACI context I think the important points are:
- The paper references the 2005 ACI code. I don't know if there have been significant changes to the code stress block requirements since then.
- For pure bending or small axial load the stress block makes very little difference.
- For a rectangular section with concrete up to about 50 MPa a rectangular stress block will give very similar results to a more exact formulation.
- For very high strength concrete the ACI stress block becomes increasingly unconservative.
- For a circular cross-section (or similar) any rectangular stress block will be unconservative by about an additional 10%.
- In the most extreme case, with 90 MPa concrete and the axial load that gives the maximum bending moment to ACI provisions, the ACI results are about 30% higher than the most conservative, or 15% higher than results based on actual stress-strain behaviour.
As someone pointed out in a recent thread, we don't have columns failing all over the place, but on the other hand there are not a huge number of structures using very high strength concrete and designed to the limit of loads that will produce the maximum possible moment capacity.Finally I'll reinforce the point made earlier that this is looking purely at ultimate capacity. If you are interested in moment-curvature behaviour under high loads a rectangular stress block is an over-simplification.
Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/
RE: Concrete stress strain curve by ACI 318
Ingenuity and IDS I enjoyed reading the paper
Guys ,I extended the discussion for further clarification kindly see the attachment(I was not able to write here because I need to use word tables),kindly see the attachment
http://files.engineering.com/getfile.aspx?folder=d...
Thank you all
RE: Concrete stress strain curve by ACI 318
Thread UP UP UP
RE: Concrete stress strain curve by ACI 318
rapt,
JoshPlum,
IDS
Ingenuity
Lets us continue guys
Thank you
RE: Concrete stress strain curve by ACI 318
kindly see
page 1
http://postimg.org/image/7l1d1h4yn/
page 2
http://postimg.org/image/j0gj2uylf/
RE: Concrete stress strain curve by ACI 318
RE: Concrete stress strain curve by ACI 318
IDS's paper did jog my memory a little bit:
I now remember seeing some minor differences with columns under mostly laxial load + small amounts of bending. But, the differences between rectangular and parabolic stress blocks were still not enough for us to promote agressively. Though I don't believe we would have tested very high strength concrete. And, we would not have tested versus a Eurocode type of stress block either.
RE: Concrete stress strain curve by ACI 318
Some quick thoughts:
Obviously, he is dividing the beam into 8mm layers.
The assumption is then that the stress in each layer is essentially constant. Therefore, the centroid of the force in that layer is known (i.e. mid-depth of that layer).
If you know the maximum strain and the depth of neutral axis then you can use that parabolic stress block to calculate the stress for each layer.
This in combination with the assumption of constant stress in the layer, allows you to calculate the force in that layer.
Knowing the magnitude of the force and the location of the centroid, you can then know the contribution to total moment contributed by that layer.
Now, there is no way that I can determine how he came up with the depth of neutral axis or maximum strain or such. So, I have no way of knowing how correct or incorrect this method is.
However the general method strikes me as a typical numerical method. Think back to our calculus or pre-calculus days (a long time ago for me!). We did exactly the same thing to sum up an approximate value for the "area under the curve". Then demonstrated how the limit of this method (as delta x approaches zero) will be equal to the exact area. Same concept, the accuracy will get better as the layers get smaller than 8mm and less accurate as the layers get bigger than 8mm.
RE: Concrete stress strain curve by ACI 318
I think we have provided sufficient information for you to investigate further using your engineering education and knowledge. If you want more information, find a good text book.
RE: Concrete stress strain curve by ACI 318