Maximum amount of Longitudinal reinforcement for beams

[mar2805]

My code is EUROCODE 2 for RC structures.
It says that the maximum rinforcement ratio form beams should not exceed 0,04% of cross section area.
Does this mean 0,04% for tension reinforcement and 0,04% for compression compression if the section needs to be doubly reinforced?

 

[mmarlow]

I'm not familiar with that code, but, I'd think its tensile reinforcement. My understanding of Max As, is that it used to limit brittle failures of the compression block before the tensile steel yields and has some ductile performance. I'd be interested to hear other thoughts.

-MMARLOW EIT

 

[BridgeSmith]

Are you sure the limit you're referencing is 0.04%, and not 4% (0.04)?

 

[IDS]

What the code says:

The cross-sectional area of tension or compression reinforcement should not exceed As,max
outside lap locations.
Note: The value of As,max for beams for use in a Country may be found in its National Annex. The
recommended value is 0.04Ac.

Seems pretty clear.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[rapt]

The 4% rule is more for actually fitting the steel in. At 4% tension steel, you will definitely need nearly as much compression steel for ductility as well.

Normal column limit is 8%, so 4% compression and 4% tension if 2 faces reinforced. And I would think you would have problems fitting that in.

If you need that much reinforcement in a beam you need more concrete.

 

[skeletron]

A23.3-14 notes that the "area of longitudinal bars for compression members...shall not exceed 0.08 x gross area of the section." There is an additional note that "the use of more than 4% of reinforcement in a column...can involve serious practical difficulties in placing and compacting the concrete, and in placing reinforcement in beam column joints."

So, while this excerpt addresses compression reinforcement limits, I think you can extrapolate for flexural reinforcement. Anything more than 0.04 x Ag will present some practical issues that preclude the failure mode issues. Practically the reinforcement should be within the 1% to 2.5% range.

 

[rapt]

skeletron

and that recommended 4% limit is in the complete section, so 2% each face.

If a designer is using those levels of tension reinforcement,you really need to look at strain compatibility and not use simplified strength calculations that assume reinforcement yield in tension, unless you are providing a lot of balancing compression reinforcement.

And make sure there are no earthquakes around.

 

[IDS]

and that recommended 4% limit is in the complete section, so 2% each face.

Not how I read it:

The cross-sectional area of tension or compression reinforcement should not exceed As,max
outside lap locations.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[BridgeSmith]

What I surmised from the AASHTO bridge design spec is that the upper limits on reinforcement are primarily to ensure a tension-controlled section, that would have a ductile failure mode.

 

[ukbridge]

From designers guide to EN 1992-2 its limited to 4% to ensure adequate compaction of concreting i.e. what many people have said above.

In practice for beams/slabs I've always taken As as the amount of tensile reinforcement. If theres compressive reinforcement I believe that should not count towards the 0.04Ac.

For piles (admittedly somewhat contradictingly) I take As as the total reinforcement, although any more than 4% you're going to get some moans from the fixers on site because of clashing between the pilecap and piles steel.

I've never really found the limit to be a problem - if you're exceeding it a lot you need a thicker section/different structural solution.

@Hotrod I only partially agree with the AASHOT design spec - it seems like a reasonable number to ensure the steel yields but strictly you should check this from the stress-strain diagram. In the UK/Euroland if you do all the mathematical jiggery pokery you arrive at this formula to check that the steel doesnt yield for a singly reinforced section (theres probably something similar in your code).

 

[BridgeSmith]

For CIP concrete piles or drilled shafts, AASHTO prescribes fairly wide minimum bar spacing that, in practical terms, limits the area of reinforcement to well below 4%, due to the inability to vibrate the concrete in all but the top few feet of the shaft.

@ukbridge, fortunately for us, the AASHTO code has not become that elaborate in that aspect. There are a few things left where it is still simplified (for now).

 

[rapt]

IDS

That rule suggesting maximum8% but recommending 4% was for compression members and applies to the totsl in the section, not on each face.

AS3600 commentary suggests the same maximum for columns 8% allowed but recommend 4% maximum..

 

[mar2805]

At 4% tension steel, you will definitely need nearly as much compression steel for ductility as well.

Correct!

Normal column limit is 8%, so 4% compression and 4% tension if 2 faces reinforced.

Im not talking columns, but beams.
Is it 4% per face, or 4% in total (tension and compression steel together) for doubly reinforced beam section?

 

[IDS]

rapt - the question asked about a specific rule in Eurocode 2, and that rule clearly states that the reinforcement in either face should not exceed Asmax, and the recommended value for Asmax is 4%.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[rapt]

IDS,

I was specifically responding to skeletron's comment. That is why I referenced his name at the start of that post.

I agree with your interpretation of EC2 being 4% in each face in beams. I disagree with EC2 allowing this especially without warnings.

Both ACI and AS3600 recommend 4% maximum in compression members unless you can show that more is buildable. That is 1% on each face for a square column. 4 times that on one face seems an awful lot to get concrete placed around.

 

[IDS]

I was specifically responding to skeletron's comment. That is why I referenced his name at the start of that post.

OK, the discussion has got a bit confused, but referring specifically to the Eurocode limit on % reinforcement in beams, I don't think the quoted clause is intended to indicate that anything up to 4% is OK. It says the absolute upper limit is 4%; you still have to comply with all other requirements.

I agree that close to 4% would rarely, if ever, be a good idea, but 2% would be too low as an upper limit. In many circumstances just over 2% would be no problem for placing concrete, and is well below the balance point.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[rapt]

IDS,

Yes, 2% is just below the balanced point if the concrete strength is 40MPa or more and the section is square and there are 2 layers of reinforcement which you require to fit it in).

At 4% for a 1000mm square section, you would need 32 40mm bars on the tension face so about 4 layers. And probably more than half of that at the compression face. Awfully inefficient!