Reinforcement yield strength
Reinforcement yield strength
(OP)
what is the effect of having two different yield strengths of bars in the same concrete section , what to do when designing the section ... if the smallest yield strenth to be considered, what about the elastic modulus





RE: Reinforcement yield strength
RE: Reinforcement yield strength
RE: Reinforcement yield strength
RE: Reinforcement yield strength
RE: Reinforcement yield strength
RE: Reinforcement yield strength
RE: Reinforcement yield strength
Aladdin76...your values fall between Grade 40 and Grade 60 rebar (275 MPa to 415 MPa).
Since the values fall with a single grading of rebar (a little above minimum on the low side, to less than the next grade for the high side), I wouldn't worry too much about compensating. The values are not far enough disparate that it is a great concern.
As they say in my area..."You're not building a watch"...it's just a building.
RE: Reinforcement yield strength
I was in a hurry to leave, just typed something pop-up in my head without checking myself. Anyway, I propose to answer aladdin76's question from strength point of view.
If the design was done by using the lower yield strength, then there is no apparent problem. However, if it is another way around, then the member loading capacity will be affected to some extent. The exact effect depends on the quantity of the lower yield bars vs the higher yield ones, and location and pattern of the mix. None of the above can be answered easily since the job is already done, at this juncture, I don't think it is overly conservative to re-compute the strength assuming all bars are to yield at the lowest strength level, thus, reduce the load carrying capacity significantly (almost 28% in this case, fy1/fy2 =380/297=1.279, M=f[Asfyd], assume constant As & d-difference small). Actually, for mixed grade application, after the lower yield strength is reached, the member would still have some additional load carrying capacity until the remaining steel has also yielded. But the total load capacity is still lower than the member reinforced completely with the higher grade bars. Please exam my logic, I think you would agree with me, at least to some extent.
For the case of new design - don't ever contemplate it (mixing two grades of steels).
RE: Reinforcement yield strength
Dik
RE: Reinforcement yield strength
RE: Reinforcement yield strength
RE: Reinforcement yield strength
f = E*s(train). At yield, the corresponding strain of the bars with different yield point would differ though.
RE: Reinforcement yield strength
RE: Reinforcement yield strength
RE: Reinforcement yield strength
More importantly, the percent elongation difference between two different materials could have a significant effect.
RE: Reinforcement yield strength
RE: Reinforcement yield strength
I'm still of the opinion that the yield difference in these two materials is not significant enough to worry about.
RE: Reinforcement yield strength
I could have misunderstood your last statement "the yield difference in these two materials is not significant enough to worry about".
Just for the sake of argument, say you designed a composite floor with concrete on A36 steel beam. At the end of construction, it was found that the beam actually only has a yield strength of 26 ksi, would it be "insignificant"? Or otherwise - the beam might still works but at the load level that corresponding to its true yield.
However, I could be overly "technical" on this issue. And I admit that I have no clue on materials and those associated standard deviations/tolerances. From reading your previous responses in many posts, it seems you possess enormous understanding on many different type of materials, maybe once again you can find the answers and enlighten us on this matter.
RE: Reinforcement yield strength
A 10ksi difference in a rolled steel shape is more significant in several respects. The properties of rolled shapes are controlled to a greater degree than those of rebar. Elongation, yield, and metallurgy are usually kept in reasonably tight constraint. Rebar is an amalgamation of all sorts of scrap steel with less control on the metallurgy and its physical properties, so batch to batch (heat to heat) variations are more broad and common than you would expect in rolled shapes.
RE: Reinforcement yield strength
I would take your words with reserved cautious. Wish I had spent more time and paid interest in metallurgical studies. But I do agree with you without hesitation that the significance of strength deviation in structural steel and reinforcing steel is not directly comparable, because of the stiffening effect from the concrete for the latter case. You are absolutely right on this.
RE: Reinforcement yield strength
What this means, at least to me, is that the controlled metallurgy of the mild steel shapes provides much greater predictability of its properties. It is weldable without significant limitation as to process, filler metals, and pre-weld/post-weld thermal conditions (except for very thick sections). Conversely, rebar has limited weldability, specifically because of its lack of controlled chemistry.
Sorry to ramble on....I just wanted to be clear about the source of my comments.
RE: Reinforcement yield strength
Thanks. It helps to better understand the differences of these metals, and the base of your response on this matter.
RE: Reinforcement yield strength
RE: Reinforcement yield strength
It is a good overview of the properties of rebar. In particular the summary notes the things we have discussed here. Note also the non-linearity of the stress-strain curve of cyclic loading on rebar.
RE: Reinforcement yield strength
Thanks for your tips. Here is the one from Dubai.
RE: Reinforcement yield strength
RE: Reinforcement yield strength
RE: Reinforcement yield strength
The strains will be identical and the % of elongation will be identical for all stresses less than the lower yield stress.
RE: Reinforcement yield strength
RE: Reinforcement yield strength
RE: Reinforcement yield strength
I guess "compatble" means difference is small, but not equal.
My confusion is when E is holding constant, how could the strains be the same for bars with different yield points, albeit the difference COULD be small.
RE: Reinforcement yield strength
If that were not the case, we wouldn't be concerned about mixing Grade 40 and Grade 60 bars in the same section.
RE: Reinforcement yield strength
RE: Reinforcement yield strength
If the OP knew the locations and yield stress of each bar, the ultimate moment could be calculated. Alternatively, he or she could calculate the service load stress and decide if the factor of safety is acceptable for the lower yield bars.
RE: Reinforcement yield strength
E=29000 ksi, Sy=0.003=l/L
fy = E*Sy = E(l/L)
At fy = 40 kai, the elongation l is:
l = fy*L/E = 40 ksi*L/29000 ksi = 0.00138L
At this stage with increase load, the elongation l is:
l = 0.00138L + (60-40)*L/29000 = 0.00138L + 0.00069L
= 0.00207 L = 60*L/29000 = 0.00207L, check.
Note, after yield at 40 ksi, the lower grade bars will continue to lengthen until the higer yield point is reached, but the stress in the lower grade bars would not change. The final elongation of all bars are identical, however, the stresses in bars and the concrete wrapped around the bars are different, they depend on the yield strengths of the bars.
RE: Reinforcement yield strength
RE: Reinforcement yield strength
RE: Reinforcement yield strength
Yes, you are right. I also noted it in my response - no stress changes for grade 40 bars after reaching yield. However, it elongates with the grade 60 bars (at the same rate) after that point (40 kai) until the stress 60 ksi is reached.