×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Reinforcement yield strength

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

First of all, you don't want to do that.  If you use the lower value, you won't achieve compatible strains and thus will "throw" stresses in areas in which you didn't anticipate.  If you use the higher value, the lower strength material would not accept higher stress levels, thus again, distributing stresses or causing failure in unanticipated areas.

RE: Reinforcement yield strength

Is this an engineering/science study, or real world application? What type of solution you are looking for - approximate, or exact?

RE: Reinforcement yield strength

(OP)
it is a real case ,  concrete roof slabs were cast ,, the reinforcement tensile strenth test result came late and they showed different strenthes  

RE: Reinforcement yield strength

How much different are the strengths?  Do you have Grade 40 mixed with Grade 60?

RE: Reinforcement yield strength

(OP)
the yeild strenth varied between 380Mpa and 297Mpa

RE: Reinforcement yield strength

I am tempted to adjust the area of one bar to theother by the relationship As1 = As2*(fy2/fy1), assume the strain and Es are constant for both. I need to think more on that though.  

RE: Reinforcement yield strength

kslee1000...that would work; however, these are already in place...no choice now.

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

Ron:

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

That's the nice thing about limit states design.  Use the respective areas of steel and yield strengths.  If you have different depths to rebar, then strain compatibility should be reviewed.

Dik  

RE: Reinforcement yield strength

The strain of both types of steel are exactly the same in the elastic range if the Modulus of Elasticity is the same, (and they are in this case).  No effect on the reinforced product until the lower yield strength steel yields and then the product will change deflection faster than if all the steel was higher strength.  

RE: Reinforcement yield strength

civilperson....for a given stress, the strains are not the same. If the yield point is different, then so will be the strain.

RE: Reinforcement yield strength

Ron:

f = E*s(train). At yield, the corresponding strain of the bars with different yield point would differ though.

RE: Reinforcement yield strength

kslee1000...thanks.  Yes, that was my point.

RE: Reinforcement yield strength

Civilperson is right that the two types of steel will behave the same in the elastic range.  Ron, I don't understand your reply to his comment.

RE: Reinforcement yield strength

If you have two different yield points, you will have two different strain values at yield. Might or might not be significant, depending upon the difference in yield values.

More importantly, the percent elongation difference between two different materials could have a significant effect.

RE: Reinforcement yield strength

Yes, but I think civilperson meant they behave the same until the yield strength of the lower strength steel is reached.  After that point, the load carried by the low strength bar stays the same, and the high strength bar carries any additional load as it continues up the straight line.

RE: Reinforcement yield strength

hokie66...I agree, and yes, civilperson is correct, but there isn't a problem until yield is reached, at which time strain compatibility becomes an issue.  When yield is reached in the lower strength bar and it progresses into the plastic range, the higher strength bar picks up additional load.  At that point, a beam will react as though it has a torsion component.

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

Ron:

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. bigears

RE: Reinforcement yield strength

kslee1000...no, you're not being overly technical, but those are different situations.  In steel design, the load attenuation is direct, so you mobilize stress and strain directly.  In reinforced concrete, the mobilization is less direct and gets shared differently.

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

Ron:

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

kslee1000...I appreciate your willingness to take my comments at "face value".  If you check the respective ASTM requirements for rebar (ASTM A615) and steel shapes (ASTM A36) you will find that there are essentially no chemical restraints placed on rebar; whereas, A36 has specific chemistry requirements to control the metallurgy.  Further, rebar is allowed to have less ductility, as measured by elongation, than mild steel shapes.  Rebar is required to have a minimum elongation of 12 percent, while A36 steel has to have a minimum elongation of 20 percent.

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

Ron:

Thanks. It helps to better understand the differences of these metals, and the base of your response on this matter.  

RE: Reinforcement yield strength

This weekend seems not a waste. After all, been able to  find two articles discussing properties of reinforcing steels. Please pay attention to Table 5, P.16 of the article from Dubai, Table 5 & Table 7, p.27 & p.28 respecticely, of the article from India. Wish we all gain better understanding after review these articles.

RE: Reinforcement yield strength

kslee1000...thanks for the article. Only the one from India got attached.  You have to do each file separately in separate postings.

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

To calculate the ultimate moment, I think you can just assume strain compatibility with each bar at its own yield strength.  You run into a similar situation when you calculate the ultimate strength of a prestressed beam that also has mild steel.   

RE: Reinforcement yield strength

The trouble he has is the slab has already casted. I don't think there is a record, other than the lab test, to shown where the bars were used - in seperated area, or mixed in the same area; how many were this, or that...

RE: Reinforcement yield strength

Ron,
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

civilperson...I agree, but the strain will be different at the yield points, which is really the only point we're concerned with...the failure.

RE: Reinforcement yield strength

Ron,  Assuming perfect elastic - plastic behavior, after the highest yield strengh bars and/or the bars  closest to the neutral axis have yielded, all bars will be at their individual yield stress.  Strains will still be compatible.

RE: Reinforcement yield strength

graybeach:

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

Agree that the strain difference might be small, but what we could be concerned about, if there were are larger difference in the yield strength (please note that I've already stated a couple of times that I didn't think the yield difference in this case was significant), is that you would develop a strain incompatibility after the first yielding.  That could lead, again if the difference were greater, to a non-ductile failure by quickly overloading the other bar.  

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

No beam in use with service loads achieves yield unless a mistaken design was used or overloaded.  The steel is suppose to yield prior to concrete failure for a ductile failure.  The strains of the steel will be exactly the same as the concrete surrounding them.

RE: Reinforcement yield strength

civilperson has it right.  The strains in the bars will be the same as the surrounding concrete.  Say the strain at a particular level in a beam that has grade 40 and grade 60 bars has reached 0.003.  The grade 40 bars would have yielded at 0.0014, and the grade 60 bars would have yielded at 0.002.  Because both bars are now in the plastic zone, the stress in each bar when the strain in 0.003 would be 40 ksi and 60 ksi respectively.  

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

I don't think the above statement is correct. Let's go back to basics:

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

Note: Sy=0.003=l/L shall be revised as "Sy = l/L". The numerical value 0.003 has no bearing on this relationship.  

RE: Reinforcement yield strength

Kslee1000, Theoretically, after yield, the bar will elongate without any increase in stress.  The 29000 has no effect on the stress after yield.

RE: Reinforcement yield strength

graybeach:

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.

 

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources