Reinforcement yield strength 1

[aladdin76]

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

 

[Ron]

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.

 

[kslee1000]

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

 

[aladdin76]

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

 

[Ron]

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

 

[aladdin76]

the yeild strenth varied between 380Mpa and 297Mpa

 

[kslee1000]

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.

 

[Ron]

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.

 

[kslee1000]

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).

 

[dik]

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

 

[civilperson]

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.

 

[Ron]

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

 

[kslee1000]

Ron:

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

 

[Ron]

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

 

[hokie66]

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.

 

[Ron]

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.

 

[hokie66]

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.

 

[Ron]

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.

 

[kslee1000]

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.

 

[Ron]

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.