Two Layers of Tension Rebar
Two Layers of Tension Rebar
(OP)
How often do you use two layers of tension rebars distance 1 inch apart (the 2 layers). The outer side would yield first before the inner ones. Usually how many percentage yielding of the outside side before the inner ones yield? and why is this allowed in structural books. I only want single layer in my design but I saw others doing 2 layers. What is your experience and say on this? Thank you.






RE: Two Layers of Tension Rebar
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Two Layers of Tension Rebar
In column design.. the contribution of inner rebars are only a lower fraction compared to the outermost rebars (since the separation in column rebars are at least 4 inches). In beam, don't you compute for the contributions of each layers of rebars maybe because the separation is only very close 1 inch? If you had computed it.. what results did you get? Since the distance is only 1 inch, could some be approximate principle that is involved or used?
RE: Two Layers of Tension Rebar
Are you designing a column here? If so, that's important for us to know. I've only used muli-layer cages for lower level columns in pretty tall buildings. And, in those cases, there wasn't really any rebar tension to speak of. All axial.
I don't understand these limits. I've seen column bars closer together than four inches and beam bars typically are spaced as a) 1.4 db or b) Some spacer bar dimension that makes the space greater than 1.4 db. Other spacing minimums associated with aggregates size etc apply too.
As I mentioned previously, all of the bars generally yield for beam designs where I use multiple layers.
The traditional procedure has been this:
1) Assume that all of the rebar yields and treat it as one giant lump of reinforcing located at the centroid of the tension steel including all tension layers.
2) Verify that all layers of reinforcing steel do in fact yield.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Two Layers of Tension Rebar
I'm only referring to beams. For mere 1 inch (or 1.4 db) distance between layers. This centroid thing may work. But if the distance between layers becomes larger like 3" or 5" or 3 db.. the outermost rebars of the beam would surely yield before the inner ones.
For those who have actually computed for the strain of the outermost bars and inner bars layers. What percentage do you usually get? How many percentage of strain of the outermost bars before the inner bars begin to strain (again all for beams only). Thanks.
RE: Two Layers of Tension Rebar
When it's say a 18" deep beam, then a large spacing like that would have a significant effect on the strains in each layer. A 48" deep beam, I feel the centroid method still applies.
Even with a small spacing between bars, the outside layer will always yield first. Why do you feel that is a concern? Are you worried about reaching rupture strain on the outer layer before you get yield of the inside layer?
RE: Two Layers of Tension Rebar
Yes. For let's say a 18" deep beam and 1" between 2 rebars layers.. will it reach rupture strain on the outer layer before you get yield of the inside layer? Or will they still be both elastic. Reviewing 5 structural books. They all use the centroid method. For a 18" deep beam, what spacing of rebars layers before you reach the threshold where centroid no longer applies approximately?
RE: Two Layers of Tension Rebar
I would always calculate the strain at each layer, so an inner layer may not reach yield. You cannot base it on a guess. Normally beams with multiple layers are very heavily reinforced so the neutral axis is fairly deep. There is no general rule about what percentage of yield strain you are going to get in each layer. It depends on steel depths and neutral axis depth.
There is nothing to worry about if you calculate it properly, other than the fact that you are no fully utilizing the steels capacity. So you only do it if you have to.
The other option, if there are a lot of beams is higher strength steel (e.g. PT).
RE: Two Layers of Tension Rebar
Generally people have a spreadsheet that completes this for them for multiple layers of steel.
RE: Two Layers of Tension Rebar
Algebraically, upper bound strain = d_max x (0.002 + 0.0035) / d_min - 0.0035
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Two Layers of Tension Rebar
Definitely an Upper bound as it is the balanced point for the inner layer, bit what use would it be? Except also that .002 does not apply for higher strength steel does it!
RE: Two Layers of Tension Rebar
I developed it with a singular, important use in mind: helping OP out with this thread. With that diagram / equation in hand, OP can easily run a few test cases and convince himself that, while the outer layer strain may indeed exceed 0.002, it's not likely to be anywhere near the rupture strain.
Well yeah, OP would need to apply whatever strain values are appropriate to the materials being used. I do my best to help peers; I'm not a tie-er of shoe laces. Nor is 0.0035 the right number depending on where in the world OP is and what type of concrete is at play.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Two Layers of Tension Rebar
Interestingly most concrete codes do not require that the steel strain does not reach rupture!
Not saying that I agree with the logic!!
RE: Two Layers of Tension Rebar
I agree, that is interesting. Good thing rupture is a remote possibility in most cases.
You know those provisions that we have that ask for the reinforced flexural capacity to exceed the unreinforced flexural capacity by some margin? My understanding is that one of the reasons for that is to promote the development of multiple flexural cracks rather than just one which might lead to excess localized strain and possibly rupture. So, indirecctly there may be some consideration of rupture.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Two Layers of Tension Rebar
That was my assumption, until you look at it and it is completely unrelated to the steel rupture strain!
Also, it is normally based on cracking moment. If you look at a T section,
- for tension on the bottom, the flange is in compression so neutral axis is relatively shallow (as it is very wide) and tension strain is much higher, but Mcr is significantly lower so minimum bottom steel is much lower.
- for tension on the top, the web is in compression so neutral axis is relatively deep and tension strain is lower, but Mcr is significantly higher so minimum top steel is much higher.
So for the face with the higher tension strain we require the lower amount of minimum reinforcement! So the strain in this steel is significantly higher.
I have been complaining about the illogic to the Australian Code Committee for years!
RE: Two Layers of Tension Rebar
I'm computing manually as review taking into consideration the strain-stress curve, the strain diagram, elasticity, and the all others including studying the derivation of each formula to ensure each layer has the right strain. But may I know what is d_max and d_min?
Also note that if the loads is beyond fc'/2, stresses and strain is no longer proportional (concrete has no longer linear stress-strain above fc'/2), so the neutral axis would be deeper or higher from middle point close to the ultimate load.
Anyway. In your sample. I'm assuming you take neutral axis in the middle point just for rough estimate. Ok.. but what is d_max and d_min?
RE: Two Layers of Tension Rebar
d_max is the distance from the compression edge to the most outboard reinforcing layer. d_min is the distance from the compression edge t the most inboard reinforcing layer.
The idea is that strain remains proportional to curvature but the stress does not. Thus the strain gradient that I've proposed remains valid. At least that's our conventional flexural theory. No doubt, strain is not perfectly proportional to curvature either. But then this is practical design, not a PhD dissertation, right?
The neutral axis would not be in the middle. It would be wherever the strain diagram indicates zero strain.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Two Layers of Tension Rebar
I don't agree. At least not yet. Specifically:
1) Once cracking occurs, I think that rebar strain rearranges itself significantly and the rebar strain condition prior to cracking become largely irrelevant with respect to possible encroachment upon rupture level strains.
2) For the same localized curvature, more cracks = more distributed rebar strain = less potential for bar rupture. As having the reinforced flexural capacity exceed the uncracked flexural capacity promotes distributed cracking, I think that logic in this approach is fundamentally sound even if the methods for ensuring Mn > Mcr may be flawed in some cases.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Two Layers of Tension Rebar
I meant that the code formulae for minimum reinforcement/capacity based on an area of reinforcement to resist the cracking moment is unrelated to steel rupture strain.
RE: Two Layers of Tension Rebar
Hmm... would you consider these statements to be accurate:
1) In general, reinforcing sections so that Mn > Mcr will promote more distributed cracking and thus reduce the amount of peak strain in the rebar at any one crack. In this sense, reinforcing quantity bears some relation to peak rebar strain, albeit indirectly.
2) The code formulae do not explicitly relate the quantity of reinforcing provided to the specific amount of rebar strain that can be expected. Thus, designing Mn > Mcr helps to reduce rebar strain but does not reduce it to any particular value.
Thoughts?
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Two Layers of Tension Rebar
Kootk, I understand it's just for rough idea. But would like to confirm if the following computation is correct. See graphics.
I tested the common depth of 19.685". Here d_max = 18.11 and d_min 17.11.. computing for upper bound strain = d_max x (0.002 + 0.0035) / d_min - 0.0035 = 0.00231
Do you know the formula to get the distance to the neutral axis in your sample? I'm poor in trigo.. lol
So the outer strain is 0.00231 for inner bar strain of 0.002. I know this is just for rough idea and illustrating the idea plane remains plane.
(For the actual neutral axis of the beam at ultimate load. It's 6.87 and the neutral axis is higher so the difference is even lesser when the neutral axis gets higher up)
RE: Two Layers of Tension Rebar
Come on, an engineer who cannot do middle school math!
But this calculation you are doing has nothing to do with the real neutral axis depth. You need to do these calculations for the reinforcing pattern you are analysing for, and equate the tension and compression forces to determine the actual neutral axis depth.
RE: Two Layers of Tension Rebar
Neutral axis depth would be: d_min x 0.0035 / (0.0035 + 0.002)
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Two Layers of Tension Rebar
I agree with 2.
1 is the problem. The strain in the steel after cracking is completely unrelated to the cracking moment for a T section.
For tension on the flange side, Mcr is very dependant on the flange width, while steel strain after cracking is dependant on the web width. The flange is no longer an item as it is cracked and has no effect on the strain in the steel after cracking.
For tension on the web side, Mcr is very dependant on the web width, while steel strain after cracking is dependant on the flange width. The flange is the main influence on the strain in the steel after cracking.
So, while adding more steel will reduce the strain, basing the amount you add on Mcr will not give any idea of the strain in the steel and its ability to cause multiple cracking and a ductile section.
RE: Two Layers of Tension Rebar
Kootk, d is the actual depth of the beam which is 19.685" (or 500mm), although I know the d used for calculation is from compression edge to the bars (or the centroid which is what you may be talking about).
Anyway. What is most important is the inner rebars can contribute to strain at elastic service load and not just at yield.. because if you are waiting for inner bars to suddenly yield when the outer ones yield.. this is not good.. What is good is both all layers contribute at elastic strain.. right?
Rapt. Our company mostly used software in design. They never computed each manually. But I want to verify manually. Anyway. I computed for the real neutral axis depth. For the following data:
As = 3.894848
fy = 60000
alpha = 0.72
fc' = 4000
b = 11.8"
d = 19.685"
neutral axis formula at ultimate strength = As*fy/(alpha*fc'*b) = 6.87
(this is the not the neutral axis of Kootk illustration but the actual beam (Kootk's is 10.88818)
Now I can compute for the strain of each layer of rebars and then stress given the real neutral axis at ultimate load.
RE: Two Layers of Tension Rebar
I think that "what is important" with regard to rebar strain is best expressed by a paraphrasing of one of rapt's previous statements, if I have it right:
For ultimate moment resistance, what is important is an accurate estimate of the stress levels in the reinforcing at the ultimate limit state condition. An accurate understanding of rebar strain is required for this. No shortcuts.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Two Layers of Tension Rebar
Of course. But at elastic strain, if the outer and inner bars both contribute, then the beam can functions as designed (in books, they always take the centroid and never argue or give the illustration they both contribute). My initial worry is the outer bar can yield and rupture without the inner bars contributing then the inner bars will engage and rupture too after the main one rupture and failing the beam.. but if the full layers are engaged at elastic load.. then they would function as designed.
Last question. I can't find this in books too. In the following neutral axis for the actual beam given:
As = 3.894848
fy = 60000
alpha = 0.72
fc' = 4000
b = 11.8"
d = 19.685"
neutral axis formula at ultimate strength = As*fy/(alpha*fc'*b) = 6.87
See illustration in the following
How do you compute the strain at the 2 bars given a distance to them? What's the formula (I can't find this in structural textbooks). Sorry I'm really poor in trigo and most designers I know don't do this manual computation style. Thank you!
RE: Two Layers of Tension Rebar
I'm finding your focus on elastic strain confusing. Ultimate moment resistance is your concern, right? At that load level, we are dealing with plastic strain in some or all of our reinforcement. Or, at least, we'd prefer that.
Have you googled the rupture strain of rebar? Do that and compare it the post yield values that we've been discussing. To quote myself:
You must be reading some exceptionally crappy books.
The triangles above and below the neutral axis are similar in the geometric sense (all three angles identical between the two). As such, you can ratio the sides of the triangles to figure out their lengths. 6.87 is to 0.0035 as 10.24 is to ???.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Two Layers of Tension Rebar
My textbook is called Design of Concrete Structures 14th edition by Arthur H. Nilson, David Darwin, Charles Dolan.
I have practically mastered the chapter on flexure.
Elastic strain is strain before reaching yield
Plastic strain is the strain hardening after yield and I'm not mentioning this when I mentioned "elastic strain".
In the book the rupture strain of rebar is many times over after first reaching yield plateau then reaching tensile strength and then go down to rupture strain.
Thanks for the tips on computations. It makes everything clearer.
RE: Two Layers of Tension Rebar
fc' = 2000 (instead of ultimate load 4000 psi)
As = 3.894848
fy = 60000
alpha = 0.72
b = 11.8"
d = 19.685"
neutral axis formula at elastic service load = As*fy/(alpha*fc'*b) = 13.75
now see the following illustration
the strain in the bars seemed to be only 0.000122 and 0.000158.. they are so small.
Can the strain diagram be used at all for small service load or only for ultimate strength? If it can be used for small service load.. the fact the rebar strains are so low means for elastic (service load range) the strains of the two can be taken for granted and you only need it more importantly for plastic (yielding strain hardening) ultimate load? is this why you keep ignoring the elastic service load strain and focusing on plastic yielding strain?
RE: Two Layers of Tension Rebar
Are you even beyond the cracking moment at your service loads? That would be check number one.
I must reiterate Koot's question, why are yu so fixated on service level strain?
RE: Two Layers of Tension Rebar
...not to mention your fixation with INsignificant figures.
RE: Two Layers of Tension Rebar
RE: Two Layers of Tension Rebar
The strain in the concrete is assumed to be .003 or .0035 or whatever depending on your design code and possibly the concrete strength (Eurocode).
The compression force C in your concrete and the steel on the compression side of the neutral axis has to equal the tension force T in all of your steel on the tension side of the neutral axis.
You adjust the neutral axis depth which changes the strain in each layer of reinforcing. From this you calculate the stress in each layer and from that the tension or compression force in the layer.
This gives you C and T. When they are equal you have the right neutral axis depth and the final strains in each layer of reinforcement.
If you cannot calculate the strains in each layer of reinforcement or the associated stresses from an assumed compression face strain and assumed neutral axis depth, you need to go back to school, (and I do not mean university, I mean school).
PS This is all true only if you forget about Strain Localisation, a topic we will leave for the theoreticians.
RE: Two Layers of Tension Rebar
Let's take C=T or alpha*fc'*bc= As*fs and at ultimate load
fc'=4000
fs=60,000
let's use the book example of b=10 and As=2.37
actual reinforcement ratio of the beam is As/bd = 2.37/(10x23) = 0.0103.
Balanced reinforcement ratio is alpha(fc'/fy)(strain(conc)/(strain(conc)+strain(steel))=0.0284.
The book says "Since the amount of steel in the beam is less than that which would cause failure by crushing of the concrete, the beam will fail in tension by yielding of the steel. It's nominal moment, from Eq. (3.206), is
Mn= 0.0103 x 60,000 x 10 x 23^2 (1-0.59 ((0.0103x60,000)/4000) = 2,970,000in-lb= 248 ft-kips
When the beam reaches Mn, the distance to the neutral axis, from Eq, (3.19b) is
c=0.0103 x 60,000 x 23 / 0.72 x 4000 = 4.94"
The c is the neutral axis which is derived from C=T or alpha*fc'*bc= As*fs
At balance point neutral axis corresponding to simultaneous crushing of the concrete and initiation of yielding in the steel, formula is c = strain(conc)/(strain(conc)+strain(steel)*d or c = 0.003/(0.003+0.002069)* 23 = 13.61.
But for ultimate strength and underreinforced for ductile failure,
neutral axis is only 4.94" instead of 13.61". If you will draw the strain diagram.. see following illustration:
Here's the problem. Which of the above is correct? For the one of the left, if the concrete fails at ultimate strain of 0.003, the steel strain is 4 times beyond yield already. For the one of the right. Just as steel yields at strain of fy/Es = 60,000/29,000,000 = 0.002069, the concrete is just tiny strain of 0.000566.. but the right one seems to be correct because remember it is underreinforced, so steel yields first, but how could the concrete strain be only 0.000566 when concrete reaches ultimate strength of 4000 psi corresponding to 0.003 strain. Anyone can give a clue? I've been thinking for it for a day already.
And in the spirit of Rapt message, I know how to solve for the stresses.. but still in the left illustration where the steel is beyond yield.. fs = strain(conc) * Es ((d-c/c)) = 318265 psi instead of 60,000 psi.. but if you set it to 60,000 psi, the concrete strength corresponding to strain of 0.000566 would be less than 1000 psi. Anyone? Many thanks.
RE: Two Layers of Tension Rebar
Depending on the type of steel you are using, the failure strain could be as high as 15 or more times the yield strain (earthquake class reinforcement).
Ultimate strength is determined at the concrete compression limit, so for ACI code a concrete compression strain of .003.
RE: Two Layers of Tension Rebar
But for sake of computation. If you want to get the neutral axis c just when the steel yield at 0.002069 strain (even thought concrete is below 0.003).. how do you compute for it? Most of our reinforcement are way below the reinforcement balance point.. in the case of the book example.. it's only 0.0103 instead of 0.0284 or only 36%. So for b=10, As=2.37
C=T
alpha*fc'*bc= As*fs
what value to be used for fc'..
RE: Two Layers of Tension Rebar
In page 73 of the book Design of Concrete Structures 14th Edition, there is a section called "Stresses Elastic and Section Cracked". It uses transformed section to compute for fs and fc (elastic range below ultimate strength) given the moment. I have 2 questions.
1. Can't it be done without using transformed section.. why doesn't the book mention it? What books have you come across that computes for elastic stresses without using transformed section.
2. How do you solve for the quadratic equation for kd in the above illustration (I drew it so I won't have to scan the book and copyright problem)? I asked the physicsforums site and they thought it was homework and didn't wanna answer. Just give me steps how to solve for "kd" so I can enter it at excel and try different moments and corresponding fc and fs. The book just mentions the kd is 7.6" without giving the steps. Please show the steps. Thank you all! :)
RE: Two Layers of Tension Rebar
But that will not be an ultimate limit condition. And you cannot do rectangular stress block for the concrete compression as the concrete stress block is only for an ultimate condition with concrete strain = .003. You would have to do a curvilinear concrete stress/stain relationship similar to the one defined in Eurocode or elsewhere.
For ultimate condition you set the concrete to .003 and adjust the steel strain to give C = T as discussed earlier.
If you want to have the steel strain as the controlling value at ultimate, that will set the neutral axis depth with compression strain at .003. You then have a value for C and strain in each layer of the steel.
You then have to add more and more steel to the steel layers until T = C.
RE: Two Layers of Tension Rebar
RE: Two Layers of Tension Rebar
You do not have to do it by that formula. Or you can use iteration for kd values to solve that formula if you want to.
You can follow the same procedure I have outlined above and iterate for neutral axis depth.
The straight line concrete stress diagram will only be close to correct for concrete strengths below .4F'c. Higher stress than that and the curve is non-linear.