THE SAME STEEL AMOUNT IN TENSION AND COMPRESSION OF CONCRETE BEAMS QUESTION
THE SAME STEEL AMOUNT IN TENSION AND COMPRESSION OF CONCRETE BEAMS QUESTION
(OP)
Hello Everyone. I would like to hear your ideas about doubly reinforced beams that has equal amount of reinforcement in compression and in tension. Is that ideal or no?
Thank you. =)
Thank you. =)






RE: THE SAME STEEL AMOUNT IN TENSION AND COMPRESSION OF CONCRETE BEAMS QUESTION
RE: THE SAME STEEL AMOUNT IN TENSION AND COMPRESSION OF CONCRETE BEAMS QUESTION
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: THE SAME STEEL AMOUNT IN TENSION AND COMPRESSION OF CONCRETE BEAMS QUESTION
RE: THE SAME STEEL AMOUNT IN TENSION AND COMPRESSION OF CONCRETE BEAMS QUESTION
RE: THE SAME STEEL AMOUNT IN TENSION AND COMPRESSION OF CONCRETE BEAMS QUESTION
Do you want to rethink your comments regarding conceiving as a steel beam and deflection control
- most of the deflection control in a concrete beam comes from the concrete in compression and the uncracked portion in the tension region, not from the reinforcement.
- Concrete shrinkage warping is not negated by equal top and bottom reinforcement as the effect is relative to the distances of the reinforcement from the neutral axis. Seeing the compression face steel is much closer to the neutral axis than the tension face steel (assuming concrete is cracked) then the tension face steel wins and causes significant increase in deflection.
If you try doing the deflection calculations based purely on the reinforcement, you are wasting your time.
All you are getting from the compression reinforcement is increased ductility which should not be needed in a well dimensioned concrete beam and some reduction in deflection, the amount of which is dependant on how heavily the beam is reinforced in the first place.
RE: THE SAME STEEL AMOUNT IN TENSION AND COMPRESSION OF CONCRETE BEAMS QUESTION
RE: THE SAME STEEL AMOUNT IN TENSION AND COMPRESSION OF CONCRETE BEAMS QUESTION
I'm always happy to rethink my positions when a able colleague suggests that course. So now I've completed that exercise and I stand by my original comments. Consider a case where the symmetrically apportioned top and bottom steel were replaced by a literal wide flange steel beam of an equivalent area encased within the concrete. You're telling me that the stiffness of that composite section would somehow be less than the stiffness of the bare steel section alone? That's a tough pill to swallow.
I'd be wasting my time if my goal were an accurate deflection estimate. If my goal were a lower bound stiffness estimate, I may in fact be saving some of my time as jwilki has intimated.
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: THE SAME STEEL AMOUNT IN TENSION AND COMPRESSION OF CONCRETE BEAMS QUESTION
1. Reduced long term deflections
2. Ductility
3. Compression zone failure is negated
4. Easier to build
and the elephant in the room that jwilki had to blab about, shhhh.
RE: THE SAME STEEL AMOUNT IN TENSION AND COMPRESSION OF CONCRETE BEAMS QUESTION
Your initial comment contained the following
" be conceived of as a steel beam by ignoring the concrete contribution"
If you ignore the concrete contribution then you are not considering it to be composite, purely steel!
JWilki,
What calculation method are you using to justify the reduction in deflections for shallow beams due to the addition of compression reinforcement?
I find in most cases the "compression face" reinforcement in shallow T beams is normally not very effective as it is fairly close to the neutral axis so only has a low compression stress in it so it does not have much effect on creep and it has very little effect on shrinkage warping deflection due to its small lever arm compared to the tension face reinforcement.
RE: THE SAME STEEL AMOUNT IN TENSION AND COMPRESSION OF CONCRETE BEAMS QUESTION
- The Australian bridge code requires all faces to have at least 500 mm2/m reinforcement
- The minimum reinforcement required to exceed the cracking moment often governs
- Continuous beams over closely spaced supports, such as bridge pier footings and headstocks, often have similar positive and negative moments
- Precast elements often have moment reversals from transport and erection to in service conditions
In a lightly reinforced slab, with typical cover for an external structure, top reinforcement often makes very little difference to deflections, because the steel centroid will be close to the neutral axis under working loads. For example, doing some quick calcs on a 350 mm deep slab, with a moment of 120 kNm (not much over the cracking moment), ratios of curvature without compression steel to curvature with equal compression steel are:
For a lightly reinforced section (5 N20 per metre)
Short term loads: 101%
Including creep: 105%
Including creep and shrinkage: 107%
Total long term curvature/ short term: 138% with compression reo, 147% without
For heavier reinforcement (10 N20 per metre)
Short term loads: 104%
Including creep: 114%
Including creep and shrinkage: 121%
Total long term curvature/ short term: 145% with compression reo, 170% without
For a heavily reinforced section (10 N32 per metre)
Short term loads: 112%
Including creep: 142%
Including creep and shrinkage: 174%
Total long term curvature/ short term: 124% with compression reo, 193% without
So for the heavily reinforced section the compression reinforcement gave a significant reduction in long term deflections, but for the other two cases it would be more effective to put the additional steel in the tension zone, if it wasn't required in compression for other reasons.
As for ignoring the concrete if you have symmetrical reinforcement, in these cases it would increase the final calculated curvature, including creep and shrinkage, by a factor of 3.5 to 5. If you want a conservative upper bound estimate of long term deflections it's just as easy, and much less over conservative, to calculate the short term stiffness, ignoring tension stiffening, and multiply by 2.
Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/
RE: THE SAME STEEL AMOUNT IN TENSION AND COMPRESSION OF CONCRETE BEAMS QUESTION
Yes, that was precisely my intent. Shrinkage warping would still work its voodoo on the cross section of course; creep much less so. I contend, as IDS seems to, that the result would still be conservative.
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: THE SAME STEEL AMOUNT IN TENSION AND COMPRESSION OF CONCRETE BEAMS QUESTION
Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/
RE: THE SAME STEEL AMOUNT IN TENSION AND COMPRESSION OF CONCRETE BEAMS QUESTION
I am not disagreeing about it being conservative. It would give a meaningless number that gives no idea of the real deflection and would normally be very conservative.
You would get a much more meaningful estimate of total long term deflection by calculating the elastic short term deflection and multiplying by 6 for an RC member.
Doug,
Agree with your logic. I have been pointing this out in training courses for years.
- For lightly reinforced members, you get a better deflection result by adding tension reinforcement.
- For heavily reinforced members, you get a better deflection result by adding compression reinforcement as long as the depth is sufficient relative to the cover to the compression face reinforcement for the reinforcement compression stress/strain to be relatively high. T beam action makes this very difficult in shallower beams.
RE: THE SAME STEEL AMOUNT IN TENSION AND COMPRESSION OF CONCRETE BEAMS QUESTION
Rapt specifically disputed my assertions and suggested that I rethink my position. So I rethought it and responded.
1) The method that I proposed is a valid, lower bound "idea" of deflection. Obviously.
2) The method that I proposed is very easy to calculate.
3) The method that I proposed has an easy to understand theoretical underpinning.
4) The method that I proposed is employed by many practitioners in my area.
Upper/lower bound estimates are the meat of what structural engineering is. Just because you guys apparently have better ways of doing this doesn't mean that the methods that others are using are meaningless and without benefit.
To my knowledge, neither of the methods that you guys have proposed is used in my area. They sound great and I'd like to know more. Can you elaborate on the methods and explain their theoretical basis? I really would like to start using your methods if they're superior. Unfortunately, I can't just write "6X short term because some interweb guys say so" on my calculation sheets. You know, internal QC and all...
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: THE SAME STEEL AMOUNT IN TENSION AND COMPRESSION OF CONCRETE BEAMS QUESTION
No-one calculates concrete and steel stresses assuming a cracked section and linear-elastic properties?
Having calculated the stresses it's simple to calculate strains, curvature and flexural stiffness.
Alternatively find the depth of the NA and calculate the transformed second moment of area using standard procedures.
Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/
RE: THE SAME STEEL AMOUNT IN TENSION AND COMPRESSION OF CONCRETE BEAMS QUESTION
I think Doug meant to end his first sentence with !!!!!! instead of ?
We simply use strain compatibility calculations to determine the strain and stress conditions under several different loadings (short term and permanent loads as a minimum) and concrete properties (short term and long term properties to allow for creep and shrinkage) and include a method to allow for tension stiffening to give curvatures and then integrate the curvatures to get deflections. You cannot use rectangular stress block, you need a proper concrete stress strain curve. There is nothing earth shattering about the methodology. It is just going back to a 1st principles calculation method rather than using gross simplifications that are defined in most design codes. Just that computers make it possible where it was too difficult to do by hand in our younger days.
But it allows you to account for the actual amounts of and positions of the reinforcement on short and long term effects. We have been doing the calculations in RAPT since the mid 1980's.
There is a simplified version of the methodology in BS8110 Part 2 and I think Eurocode. Also Bransons and Gilberts books cover it well as do probably many others.
You can still do lower/upper bound checks based on variations in concrete properties, especially elastic modulus, creep and shrinkage if your requirements are that sensitive.
The 6 times elastic basically comes from 2 approximations
- Ieff = .5 Igross and
- Long term component of deflection = 2 times short term
so total long term = 3 times short term.
Combining the 2 you get 3 * 2 = 6 times elastic. There was a British Concrete Society Paper/Technical Report on Long Term Deflections that came to the same conclusion.
RE: THE SAME STEEL AMOUNT IN TENSION AND COMPRESSION OF CONCRETE BEAMS QUESTION
Of course we do. I was asking for some deatail and justification for your 2 x (ST stiffness w/o tension stiffening) shortcut as a reasonable upper bound estimate. Does it show up in print somewhere? Is there a graphical relationship that can be pointed to? Does it have a physical meaning that one might be find intuitive? Is it an extension of old school code recommendations like the 6X algorithm? Or do I just have to be smart enough to infer it from first principles somehow?
Got it, thank you. Certainly, I'm a fan of the the fancier methods when the target is a accurate determination of deflections.
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: THE SAME STEEL AMOUNT IN TENSION AND COMPRESSION OF CONCRETE BEAMS QUESTION
The 6 * elastic was never meant as a suggestion for final calculations. It is a starting point for sizing or a quick check to see if a design is in the ball park, or is that ice rink in Canada!
RE: THE SAME STEEL AMOUNT IN TENSION AND COMPRESSION OF CONCRETE BEAMS QUESTION
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: THE SAME STEEL AMOUNT IN TENSION AND COMPRESSION OF CONCRETE BEAMS QUESTION
How come conservatively ignoring tension stiffening, and conservatively allowing for creep and shrinkage by multiplying by 2, needs detailed justification, but conservatively ignoring the concrete altogether can be accepted with no justification at all?
Anyway, the justification is years of comparing results with more detailed calculations, and measured deflections (both research and on real projects).
That said, having checked the steel only results with my approximation and rapt's, they all give about the same order of conservatism. The steel only approach has more variation than the others, but not by as much as I expected.
Here's a summary of my check:
rectangular section, 1000 x 350 thick, 50 cover, equal top and bottom reinforcement:
Light reinforcement: 5 N20, moment = 100 kNm
Medium reinforcement: 10 N20, moment = 200 kNm
Heavy reinforcement: 10 N32, moment = 320 kNm
For each section I calculated the long term curvature, with 5 different methods:
1) Detailed calc, including creep, shrinkage, tension stiffening, loss of tension stiffening to Eurocode 2
2) Sort tem cracked curvature, ignoring tension stiffening x 2 (IDS1)
3) Curvature ignoring tension stiffening and shrinkage, but including creep, x 1.5 (IDS2)
4) Uncracked short term curvature (gross concrete section) x 6 (R
apt)
5) Reinforcement only (KootK)
Here are the results of 2) to 5) as a ratio of 1)
Approx Curve/ Best Estimate
Light Medium Heavy
IDS1 2.6 1.6 1.5
IDS2 2.2 1.4 1.3
Rapt 1.2 1.3 2.4
Kootk 3.0 1.6 1.3
Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/
RE: THE SAME STEEL AMOUNT IN TENSION AND COMPRESSION OF CONCRETE BEAMS QUESTION
Like all things, it does require justification. I assumed that the justification was so abundantly self evident that elaborating upon it would risk insulting folks. See justifying sketch below. Try not to be insulted.
You realize that I can't see these things from here, right? I'm happy to take you at your word but, in practice, it's weird for me to use your experience as my justification.
You and Rapt kinda threw me for loop here. For literally years, we've been having the same, cycling discussion about long term concrete deflections. And the mantra is always the same.
"Span to depth...no good! Simplified north american code deflection checks... no good! Results for lightly reinforced members are not just inaccurate, they're un-conservative! Use our more evolved methods, we have the technology!".
I've been impressed. Truly. I went out and purchased, imported, and digested the texts written by your local heroes. I bought in. When this thread got rolling and you and Rapt said that you knew of better simplified estimating tools, I just assumed that they would be cool new tricks that I was hitherto unaware of.
Surely, you can forgive me for being a little confused to find that 2X this and 6X that are really just the simplified code checks that you normally chastise the commoners for using. I assure you, now that I understand the nature of these algorithms, I require no further justification.
Sometimes I feel like Frodo, chasing you and Rapt around Middle Australia trying to assemble all of the secrets to proper concrete deflection estimation.
Love this. Star for effort. It's interesting to see that the steel only method is as accurate as any of the other approximations for heavily reinforced sections as it is normally heavily, doubly reinforced members where folks will apply the method.
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: THE SAME STEEL AMOUNT IN TENSION AND COMPRESSION OF CONCRETE BEAMS QUESTION
In the past I'm sure we've all been guilty of saying that the reinforcement at the top of the beam contributes, But probably never checked it as a doubly reinforced section to find out if the compression reinforcement is really effective to take up the concrete stress under creep.
Any thoughts while everyone is on the subject?
Codes just treat it as find area of compression reinforcement and apply blanket effect (and no account of actual depth relative to the neutral axis). In many cases I've seen people take all of the top reinforcement even if it below the neutral axis!
RE: THE SAME STEEL AMOUNT IN TENSION AND COMPRESSION OF CONCRETE BEAMS QUESTION
RE: THE SAME STEEL AMOUNT IN TENSION AND COMPRESSION OF CONCRETE BEAMS QUESTION
RE: THE SAME STEEL AMOUNT IN TENSION AND COMPRESSION OF CONCRETE BEAMS QUESTION
My 6 times rule would be used for relatively lightly loaded "normal" structures such as parking/office or retail with logical L/D's and relatively lightly reinforced. Not something heavily reinforced like the extreme of IDS's examples.
My full deflection calculations would take into account the differences that cause the simplification to not work in some cases.
Also, the difference between a rectangular beam and a T beam is very significant in this. I do not run into many rectangular beams in buildings. They are nearly always T beams.
Kootk can rave on all he likes about more accurate methods that we expouse (not the 6* elastic gestimate), but they have shown to be relatively reliable in practice. Cases where they have been compared to deflections in existing buildings that I have been told about have all been within expected variation due to normal variation in load, material properties etc.
Agent66,
The effect of compression face reinforcement is really too variable to solve by assigning a number. If it is a rectangular beam, it will be more effective than a T beam (the tests for ACI and AS codes on this were based on 600 square rectangular beams initially at least). But then how often do you have a rectangular beam in buildings. They are nearly always T beams.
The effect of shrinkage is normally more significant than creep and it is based on the relative distances of the compression and tension reinforcements from the NA. If they are equal, they cancel and there is no effect. That is possible for an uncracked rectangular section. In any other case, they will not cancel and the more lightly reinforced or the wider the effective flange, the less effective the compression face reinforcement, to the point where the compression face reinforcement might actually be in tension!
For creep, the compression face reinforcement really needs to be in the upper half of the compression zone. Again, very hard to organise in a lightly reinforced T beam or a shallow T beam. Hence our suggestion in earlier posts that compression face reinforcement is much more effective in more heavily reinforced beams and in rectangular beams which have significantly deeper neutral axis because of the lack of a flange.
The only way to allow for all of this is to do the calculations that allow for each of the variables.
RE: THE SAME STEEL AMOUNT IN TENSION AND COMPRESSION OF CONCRETE BEAMS QUESTION
1) I have not once questioned the validity of the accurate methods that you guys espouse. Not. Once.
2) Every last one of these debates has started with you challenging my opinions, not me challenging yours.
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: THE SAME STEEL AMOUNT IN TENSION AND COMPRESSION OF CONCRETE BEAMS QUESTION
In practice, I wouldn't use any of the approximate methods to check deflection. It's just as quick to plug the numbers into a spreadsheet and do a more accurate calculation, but I would do a quick cross-check of neutral axis position and steel strain to make sure the numbers make sense.
On other points raised:
Deflection calculation is a "black art":
I disagree. In my experience, where sections have deflected more than expected, if you do a detailed calculation, including all sources of deflection, the resulting number is close to or greater than the actual deflection. It should be accepted that the "exact" calculation is still an upper bound. In the actual structure the actual maximum moments may be less, the concrete tensile strength may be greater, creep and shrinkage may be less, and differential effects may act to reduce deflections, rather than increase them, so the actual deflection in many members can easily be half or less than the calculated value. That's not important though; it's the maximum deflection that is important, and that can be calculated reasonably accurately, if reasonable upper bound values are used for all the unknowns (including differential temperature, which hardly ever gets mentioned for some reason).
Effectiveness of the compression reinforcement:
Why not just include it in the calculation? Often in slabs it doesn't make much difference, but sometimes it does, so why not include it?
Confinement of compression reinforcement:
I haven't checked the N. American codes, but in the Australian code confinement reinforcement is specifically required for compression reinforcement in beams, if it is required for strength, but there is no requirement if it is for deflection control, or for compression reinforcement in slabs. This makes sense because at service loads spalling will not be a problem, but the compression reinforcement (if it is actually in the compression zone) will have some effect on deflections, albeit usually fairly small.
Finally, for anyone in Sydney on 18th May, please book into the Concrete Institute seminar on Finite Element Analysis, where I will be talking about using computers to understand how structures actually behave (with a focus on deflections), rather than as a tool for design automation.
Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/
RE: THE SAME STEEL AMOUNT IN TENSION AND COMPRESSION OF CONCRETE BEAMS QUESTION
1) You had simplified methods as simple as mine.
2) Your methods could do what mine does.
3) Your methods would be more accurate.
I submit that #2 is false. Precisely because of it's conservatism, I believe that my method could serve as a final deflection check whereas your methods, by your own admission (and my concurrence), would only be suitable for ballpark estimates that would need to be later validated more detailed calculations.
So I do not believe it valid to present your rough estimatation methods as being superior to mine in all respects. There is an apples to oranges dimension to this.
Truth be told, the steel only method is something that I probably only tinker with on average about twice annually. Like you guys, I have developed automated tools for most of this stuff. That said, I teach my crew to start with the simplest possible analysis and only proceed to more detailed methods if necessary. The steel only method is a) one tool in the box and b) a tool that works as advertised.
@IDS: any chance that seminar will be recorded or broadcast?
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: THE SAME STEEL AMOUNT IN TENSION AND COMPRESSION OF CONCRETE BEAMS QUESTION
Normally a check on just the steel either proves that a typical conservative design is fine or verifies that an existing structure is okay. It takes three seconds, no reference materials and can be done with one line of hand math if you want.
RE: THE SAME STEEL AMOUNT IN TENSION AND COMPRESSION OF CONCRETE BEAMS QUESTION
I could envisage some applications in heavy industry where strength is the most important design criteria and deflections can be conservatively calculated. With modern computers and spreadsheets i believe it is as quick to run a more sophisticated deflection check as it is to write one line of math.
Ignoring the concrete when calculating the deflection of a concrete element just doesn't logically make sense to me.
RE: THE SAME STEEL AMOUNT IN TENSION AND COMPRESSION OF CONCRETE BEAMS QUESTION
It's only a 12 hour flight from Canada you know, and when you fly home you arrive before you left :)
But yes, it is possible. We are in the process of putting seminars on the web (for a fee), but where we are with state based ones I don't know.
If you send an e-mail to gmail (dougaj4) I'll send a copy of my presentation, when I've done it.
Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/
RE: THE SAME STEEL AMOUNT IN TENSION AND COMPRESSION OF CONCRETE BEAMS QUESTION
RE: THE SAME STEEL AMOUNT IN TENSION AND COMPRESSION OF CONCRETE BEAMS QUESTION
Table 2.2 in the white pages has design tables for instances using compression steel. It uses a method where you superimpose two cases. If you have two bars of compression steel and three bars of tension steel you look at it as:
Capacity of a beam with one bar of tension steel using concrete as the compression block
PLUS
Capacity of the couple between two bars of tension steel and two bars of compression steel.
When you have equal steel top and bottom, the method implies that you just look at steel.
It's clear that they're doing this in the diagram, but the equations kind of hide it a bit. When you work out the equation back through the various steel percentages and things, though, it's straight up just yield of steel times the moment arm between the tension and compression steel.
RE: THE SAME STEEL AMOUNT IN TENSION AND COMPRESSION OF CONCRETE BEAMS QUESTION
This is the method I use to verify beams adequacy against moment. The method for the first case uses the difference between tension and compression bars which is 3-2 = 1. If both top & bottom bars are equal, the difference would become zero. If it is zero, then I'll assume compression bars will not yield, since the first case is always initially used to know whether compression bars will yield or not. Hm.. Is there something wrong with my statement? I would love to hear more from you.
Thanks. =)
RE: THE SAME STEEL AMOUNT IN TENSION AND COMPRESSION OF CONCRETE BEAMS QUESTION
But calculating the ultimate moment capacity including the compression steel is pretty easy anyway.
Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/
RE: THE SAME STEEL AMOUNT IN TENSION AND COMPRESSION OF CONCRETE BEAMS QUESTION
RE: THE SAME STEEL AMOUNT IN TENSION AND COMPRESSION OF CONCRETE BEAMS QUESTION
I use the method mentioned by TLHS if both compression & tension bars yield. If only the tension bars will theoretically yield, I would use the Moment Capacity contributed by the Concrete block and compression steel bars with yield stress lesser than Fy by summing up moments about the tension bar location. This is the way I know and practice. Do u have other way? =)
Thank you. =)
RE: THE SAME STEEL AMOUNT IN TENSION AND COMPRESSION OF CONCRETE BEAMS QUESTION
RE: THE SAME STEEL AMOUNT IN TENSION AND COMPRESSION OF CONCRETE BEAMS QUESTION
RE: THE SAME STEEL AMOUNT IN TENSION AND COMPRESSION OF CONCRETE BEAMS QUESTION