max Ratio Reinforcement to Concrete area ?

[BauTomTom]

I was studying different codes and realized that the ration of Reinforcement area to concrete area varies very much from code to code.

What is actually the reason for? Why is it critical when you have to much steel in a column? What if you could still provide enough clearance between the bars, so that the concrete aggregates could still pass?
What if you would even use a concrete with smaller aggregate size?

How critical is it actually if you exceed the ration for example of 10% in the splicing areas?

Bau Tom Tom

 

[ishvaaag]

Quite likely the biggest risk is the rebar spalling the concrete; composite columns have additional steel but is further inwards. One must realize that in concrete some form of conjoint work is in place between the concrete and the steel; in splices for example the bond, interlock etc passes the forces to concrete that then passes the stresse to the corresponding spliced rebar; and the spliced rebars are needed where required to keep the flexural cracks in concrete under control. Upon big amounts of steel at some section it may happen that there's not enough concrete to pass the stresses to the pair in the splice, or to contain some compression or individual pullout of some "distinct" highly stressed bar.

 

[zoparrat]

The maximum reinforcement ratio in reinforced concrete is there to prevent brittle/unexpected failure. When you have excess steel in tension, concrete in compression is going to crush before the tension steel yields/plastically deforms. For a column I don't know why this would be a problem unless the column experiences a lot of bending.

 

[Lion06]

zoparrat-

I don't believe that is true for axially loaded members. Columns, generally speaking are compression controlled by nature, so you expect the concrete to crush. Few, if any, bars are in tension and so you wouldn't get excessive "deflections" or yielding of bars in tension. Your statement is true for beams.

I believe the limit for columns exists for placement of the concrete. Try putting 8% steel into a column and see how easy it is to keep required minimum clear dimensions......... not easy at all.

 

[BauTomTom]

--- to Lion06 ---

I also thing that by columns it should be seen differently and as long I can ensure, that the concrete goes around the bars then is fine. I could even use 12.5mm aggregate concrete.

What do you thing can be the max ratio in SPLICING areas?

BauTomTom

 

[BauTomTom]

The german code says 9%

the british code says 10%

and the south african code 6%

what do your local codes say?

BauTomTom

 

[Lion06]

I think that 8% is a good practical limit. 4% if you're splicing bars in the column. You need to maintain certain clear dimensions between the bars. That being said, I've run into spacing problems using 4% with no splices.

 

[ishvaaag]

Spain's is 8% if I remember well.

 

[hokie66]

The Australian code recommends 4% as a maximum, unless special measures are taken to provide for consolidation of the concrete. So that would be 8% if all bars are spliced at the same height. That much steel is a nightmare. We tend to try to restrict the steel to 1% to 2%, and adjust the concrete strengths to suit. High strength concrete with 1% steel is the most economical solution, at least where I am.

 

[JedClampett]

I don't design a lot of heavily loaded columns. Most of the cross sections I design are controlled by slenderness considerations and I use 1% steel as the minimum. With that, I'm usually over-designed. My question is; Is it common to go up to 8%? I thought I remember hearing in a seminar that if you're up over 4%, you're too high (not for code, but for behavior, constructability, etc.). Same with flexure steel. The allowable maximum is about 2.1%. But if you're over 1.5%, you should rethink your section.
Does this agree with what you others practice, or is it common to use heavily reinforced columns?

 

[hokie66]

Jed,
Heavily reinforced columns are not common, as they are not economical. It is more economical to use higher strength concrete rather than more steel.

 

[ishvaaag]

I have had to go closer to 8% at splices in a good number of columns when working for others that have insisted in having columns of the minimum overall section. No problem so far, surely a virtue of the overall conservatism of codes.

 

[BauTomTom]

---to Lion66---
Spacing problems by 4% already? How this?

---to ishvaaag---
8% in spliced areas you mean, or?

---to hokie66---
Are you from Australia? I also did a rough calculation with the same result (here for South Africa), rather reduce the steel and go higher with the concrete strength. Maybe I must recalculate it one again to get the optimum %.
Which concrete strength are you using over there so in general?
I read a report about a OPERNHAUS in Frankfurt where the rate was around 20% even. They needed to get some special approval for it, which they got also. See attachment

 

[Teguci]

ACI notes 8% as a practical limit and due to the limited testing at higher ratios. For lap splicing, they note 4% should be the max target.

Love to see what the beam-column joints look like in that Frankfurt project.

 

[csd72]

There are also the underlying assumptions to consider. We are relying on the ties and the internal concrete to form a truss to restrain the reinforcement against buckling.

the higher the reinforcement area the higher the stresses on the compression struts. There must be some point at which you exceed the limit.

also at splices, you are relying on the confinement of the surrounding concrete to ensure adequate force transfer. When that surrounding concrete is mostly actually rebar then the splices will not work as designed.

I am sure there are more effects too.

 

[hokie66]

BTT,
Yes, Australia. We typically use concrete strengths up to 65 MPa in columns. Sometimes 80 MPa, and code now extends to 100 MPa as a standard grade.

 

[ishvaaag]

BauTomTom,

yes, 8% in spliced areas, so 4% mechanically speaking. The maximum amount as a section has to be checked per the standing geometry.

 

[BS8110]

Folks,

Max percentage of reinforcement allowed in columns in the UK to BS8110 is 6% of the gross concrete area and 10% at laps. Eurocode recommends 4% and 8% respectively

Hope this helps.

 

[BauTomTom]

---to BS9110--
exactly, i also saw it 6% as normal reinforcement case and 10% by overlapping.

-- to all --
could you also reduce the concrete aggregate size from 19 to 12.5mm to ensure proper surrounding of the rebar’s by the concrete? Or could it have a negative influence on the final strength?

BauTomTom

 

[hokie66]

Reducing aggregate size generally has a negative impact on compressive strength, and that is not what you want in a column.