×
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

Rebar in a suspended slab #60 vs. #40
3

Rebar in a suspended slab #60 vs. #40

Rebar in a suspended slab #60 vs. #40

(OP)
Hello out there - I have an interesting question based on reality.
Suppose a home was built, with a suspended concrete slab. The architect/engineer specified #60 rebar on 12" centers, two layers offset 6" (not technically correct, but you get the picture).
Suppose the guy pouring the slab used #40 rebar (40ksi).
What are the possible failures, etc?

It is the garage floor of a home.

Thanks.

RE: Rebar in a suspended slab #60 vs. #40

Hi
the defects will depend on how the slab was designed and detailed. For example, if the designer has a good factor of safty in his calculation then nothing will happen immediately. If is was designed marginally for the expected loading, then the steel reinforcing will get yields when loading the slab and may develop cracks before it fails. The grade 40 steel means that yield strength of reinforcing is 40000 psi instead of the designed yield strength of 60000psi.

RE: Rebar in a suspended slab #60 vs. #40

(OP)
Thanks - specifically, where would the potentially fatal cracks first appear? On the edge of the slab where it hooks into the supporting wall? Or in the middle?
 

RE: Rebar in a suspended slab #60 vs. #40

Most likely the cracking will occur at mid-span, on the underside of the slab.  Depending on the rigidity of slab connection to the wall, you may also get cracking near the edge of the slab by the wall on the top side of the slab.

Another thing to note, you didn't mention the size of the rebar, just the yield strength.  It is possible that 40ksi steel was used, but a larger size than what was spec'd.  For example, the flexural capacity of a slab with 40 ksi #5 bars at 12" o.c. is approximately the same as 60 ksi #4 bars at 12" o.c.



 

RE: Rebar in a suspended slab #60 vs. #40

Other factors to consider: Span (single garage, 1 1/2 or double? (Likely single, but should ask?!) Also, slab thickness. Concrete strength. Testing?  How did the difference in steel reinforcing strength become known.
How old is the slab? Is it newly built and untried, or
in service with cracks now appearing? Call in a specialist,
to do a full review if you cannot wade through this alone.

RE: Rebar in a suspended slab #60 vs. #40

(OP)
I heard it from the person building the house. It has haunted me ever since I heard it.
The garage is ~25' wide x 30' deep, and will hold some fairly heavy equipment. As far as I know, the rebar used was the same diameter as was spec'd, just lower yield strength.
The slab is between 4-6" thick, new, with very little service (the home is still under construction).
I am not responsible in any way - just want to know exactly what the potential is for problems and what action I need to take as an outsider.

RE: Rebar in a suspended slab #60 vs. #40

The owner needs to have his structural engineer analyze the slab to determine if it has adequate strength.

RE: Rebar in a suspended slab #60 vs. #40

Is this slab on grade or is it over a basement?

It sounds like it's a slab on grade so I doubt the rebar strength is an issue. Heck, if it's new construction it might be grade 60 rebar even if someone "said" it was 40. 40 is not very common any more.  

RE: Rebar in a suspended slab #60 vs. #40

When you say it is a suspended slab, I assume you mean a structural slab with nothing below it but fresh air.  That being the case, if the original design was tight and based on a yield strength of 60 ksi, the structure as built with 40 ksi steel will fall short of what is required by a factor of 40/60 or 0.67.

Failure would be a gradual yielding of the intermediate grade steel.  Deflections would become very evident and there would not be a sudden, catastrophic failure.



  

BA

RE: Rebar in a suspended slab #60 vs. #40

...but....

The actual yield of Grade 40 bars might be higher than 40.  Would require sampling and testing if they did.  I wasn't aware that 40 grade bars were even still produced.

And I'd repeat the questions above - how do you know it was 40?  
 

RE: Rebar in a suspended slab #60 vs. #40

I'm with JAE.

I don't think you could buy grade 40 bars if you tried.  

In grad school we had to order a special heat of grade 40, and the bars still tested out above 50ksi.   

RE: Rebar in a suspended slab #60 vs. #40

You still can buy intermediate grade reinforcement in Canada.  Why you would want to is another matter.  See the following link:

http://pagasasteel.com/products/guide.html

 

BA

RE: Rebar in a suspended slab #60 vs. #40

(OP)
I was told it was 40 - and it was very easy to bend (I was helping him put the rebar into the foundation walls, then bending it over so it could tie into the floor).
I asked who spec'd the rebar, etc. The concrete guy replied that he used #40 instead of #60 'cause the #40 was easier to bend.
 

RE: Rebar in a suspended slab #60 vs. #40

I am not responsible in any way - just want to know exactly what the potential is for problems and what action I need to take as an outsider.

You need to notify the owner and/or the person who originally spec'd grade 60 about the situation so that it can be investigated.

RE: Rebar in a suspended slab #60 vs. #40

(OP)
The owner knows - he was the guy putting the rebar in.
I mentioned my concerns to him - just wonder what else might come of it.

RE: Rebar in a suspended slab #60 vs. #40

Can you see any of the rolling mill marks on the deformation.  This generally indicates the grade.

Dik

RE: Rebar in a suspended slab #60 vs. #40

Hmmm....40 ksi rebar and 60 ksi rebar each have the same modulus of elasticity right?  So effort to bend doesn't depend on fy but rather E.  Unless it goes plastic when bending....but that depends on the radius of bend used.

 

RE: Rebar in a suspended slab #60 vs. #40

JAE,

Are you serious?

BA

RE: Rebar in a suspended slab #60 vs. #40

Uh..ya

 

RE: Rebar in a suspended slab #60 vs. #40

I was afraid of that.  The yield strength of the material is the only factor which determines the effort required to bend the bar.  The moment required to bend a bar (or any other shape) beyond yield is Fy*Z where Z is the plastic modulus of the shape.

BA

RE: Rebar in a suspended slab #60 vs. #40

I said "unless it goes plastic"

which I guess it does

RE: Rebar in a suspended slab #60 vs. #40

It makes a big difference in the effort to bend bars.  In Australia, our standard grade is 500 MPa, but they make 250 MPa bars for the swimming pool builders, who bend 12 mm bars onsite.

RE: Rebar in a suspended slab #60 vs. #40

It does if you want to bend it through an angle theta.  The work done is Mp*theta where Mp is the plastic moment of the bar.

BA

RE: Rebar in a suspended slab #60 vs. #40

hokie,

Many of the housebuilders around here prefer small diameter bars (10M usually) so that they can bend the bars on site.  I have no problem with that, but the fact remains that the effort required to bend a bar is directly proportional to the yield strength of the material.

BA

RE: Rebar in a suspended slab #60 vs. #40

(OP)
No - rebar is already totally enclosed in the concrete.

RE: Rebar in a suspended slab #60 vs. #40

skibum2b,

Quote:

No - rebar is already totally enclosed in the concrete.

No to what?  I don't understand your comment.  Please elucidate.

BA

RE: Rebar in a suspended slab #60 vs. #40

BA,
He was answering dik's query 21/10/09, 1408.

RE: Rebar in a suspended slab #60 vs. #40

"The concrete guy replied that he used #40 instead of #60 'cause the #40 was easier to bend."

Did he also use 6" of concrete instead of 8" because it was easier to pay for?

As already said, contact the original designer. The only way to determine the effect is to have specific knowledge of this particular slab.
If you want generic advice it would be; slab strength is inadequate unless shown otherwise.  

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