Question RE high yield rebar in weak concrete 3

[ilyas415]

Hi all!

I am relatively new to structural engineering design and am working on a project in a moderate seismic zone.

To cut to the chase:

My senior engineers want to use 400 MPa yield strength for rebar in the main structural elements.
The concrete compressive strength is 40 MPa (cube strength).

However, the client would rather we used 500 MPa rebar.
The senior guys are fighting this because they're worried about lack of bonding between the concrete and rebar - especially at column-beam joint locations.
They are arguing that if the client wants to use 500 MPa steel, they need to use much higher strength concrete (circa 60 MPa or more).

As the (somewhat) middle man, I have to defend our decision to the client (ノಠ益ಠ)ノ彡┻━┻
...And the client is asking what building code we're using to support our arguments.

Problem is: I can't find any explicit statements in any building code to support our argument. It seems like the issue is related to pull-out or splitting failure of the RC members, and I can only find mentions of these in research papers.

Could anyone help pinpoint a code reference to support the argument?

Thanks in advance

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Ilyas

 

[Settingsun]

Don't look in Australian references. 500MPa in that strength concrete is used routinely. New Zealand (seismic) too, I think, but they need high ductility reinforcement.

 

[Settingsun]

If serviceability is governing your design, 500MPa won't help. Maybe that's something to consider.

 

[ilyas415]

Thanks Steve.

The trouble is, I don't think the clients suggestion is too awful.
But I haven't got so much experience so I sorta trust the senior engineers.

I think the concerns about serviceability are mostly on crack control, and they don't seem TOO fussed about it.

I'm wondering if this is a seismic design-related?

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Ilyas

 

[Settingsun]

With luck, you'll find out tomorrow when NZ wakes up. In the meantime, define what moderate seismic means in case it affects the answer from NZ.

 

[ilyas415]

Thanks. By moderate seismic, I mean the 475-year PGA (before applying a importance factor of 1.25) is 1.0605 m/s2 (i.e. 0.108g ?)

The seismic loads are high enough that they govern the design of the sections, albeit barely in most cases.
The seismic base shear is equivalent to ~12% of the building weight acting in the lateral direction.

Hope that kinda helps.

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Ilyas

 

[hokie66]

Steve is correct in that 500 MPa reinforcement and 40 MPa concrete is a typical combination. But if crack control is the issue, perhaps you just need to use more bars. Use grade 500 bars, but with 400 as the design value. As serviceability often controls, I am one (of the minority) who thinks that 500 is a bridge too far.

 

[Blackstar123]

I think what your seniors are trying to prevent, is the possibility of concrete failing before the steel does because of the increased steel strength.

Using a higher strength rebars could proof beneficial in reducing the member size and reinforcement quantities but members with high strength reinforcement could have serviceability issues due to the higher allowable yield strain/stress. As we know that crack width is a function of steel strain and consequently steel stress, the stress in the steel reinforcement will need to be limited to some extent in order to prevent cracking from affecting serviceability.

If your client’s heart is set on using the 500 MPa steel than you should make him aware of the possible serviceability issues. I think I read somewhere that ACI 318 permits flexure design using steel reinforcement with yield strength not exceeding 550 MPa but the limit for shear and torsion reinforcement remains at 414 MPa. It's possible that some of the code provisions might need adjustment but, in my opinion, if serviceability is not an issue it will not pose any problem if you designed "properly" taking into account the provided yield strain and strength.

Also, don't forget to ask the client for complete tensile test results of the proposed rebars, so that you are sure of the other mechanical properties of the steel as well.

 

[jayrod12]

Higher strength bars would require elongated development and splice lengths as well. So if you have a bunch of typical details on your drawings developed with 400MPa, they would no longer apply correctly.

 

[Agent666]

Firstly in NZ we use grade 500 bars almost exclusively with concrete down to 25MPa (cylinder strength).

Also your seismic pga sounds like it is about slightly less than half of our lowest seismic regions, so fairly low in the big scheme of things.

Now the main issue which I think you are getting at is one of bond strength and bar slip in interior joints. Our code has provisions for limiting the maximum beam and column bar diameters to address this concern. If the bar cannot transfer its force into the joint over the joint width then the bond stresses are too high and its likely under several reversing cycles of an earthquake that the bond breaks down between the bars and the concrete. This can cause the bars to slip/pull through the joint, in effect softening the frame resulting in a loss of stiffness, loss of frame action, generally increased drifts, bad things to occur in your main lateral load resisting system. I'll post the provisions when I fire up my computer in the morning.

 

[r13]

If ductility of the concrete structures can be maintained, I don't see why against the use of higher grade steel as long as code permits. Sure, as mentioned above, it comes with some punishments and head aches, such as requires higher strength concrete, longer splice length, serviceability issues...etc. But the main draw back maybe the potential of needing larger member size to accommodate the bend bars to satisfy development requirement. The owner needs to know all the draw backs before making his decision.

 

[Agent666]

ilyas415, see attached for the maximum bar diameter provisions in NZS3101 for beam/column joints for comparison with the research papers you've read. This has been in our codes since 2006 and is something most designers check first prior to committing to anything else as it will drive the detailing and member sizes in some situations.

Often in a practical sense you might need to use lower grade reinforcement in your beams to achieve the bond requirements, and use higher grade bars in your columns. Especially when operating in the zone where section sizes are being minimised by architects.

 

[ilyas415]

Thanks lot guys! This information is like gold!!

Before I got a chance to do anymore digging this morning, one of the seniors (Japanese structural engineer) in the department decided to weigh in.
He provided the following graph, showing optimal rebar grade versus concrete strength based also on rebar size.

It's apparently based on academic research, not bldg codes, but is widely applied in Japan.

Anyway, I think it is one way of handling and showing the problems that Agent666 and everyone described.

Thank you again for your help!!

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Ilyas

 

[kingnero]

In Europe, I'm not aware of any other grade of structural rebar other than 500 MPa (in both low and high ductility - 2.5 and 5.0% total elongation Agt).
220 MPa is also used, but often for lifting attachments on large prefabricated rebar structures. I didn't know 400 MPa rebar existed.
Does it really matter what you use (400 or 500 MPa)? The profile factor (external geometry) is standardized, so I don't think the adhesion will be problematic.