Steel/concrete ration - limit - why actually?

[BauTom]

Hallo

What is actually the reason for the limit of reinforcement steel in concrete stuctures like for example in columns. I think the most allowed ratio is 10%. Some codes say even 6%.

But what is the theory behind? Is it just to prevent a better workability? To avvoid concrete pockets? fire resistance?

Tom

 

[RHTPE]

The reinforcing limit provides some guarantee of the failure mode of a reinforced concrete member. By insuring that the reinforcing bar(s) yield before the concrete reaches its compressive failure strain, evidence of the condition will become apparent. Flexural members will deform and tension cracks will appear. Too much reinforcing will push the failure mode toward crushing of the concrete (instead of 'stretching' of the steel). Brittle failure of this nature occurs suddenly and with no readily visible warning.


Ralph
Structures Consulting
Northeast USA

 

[PicoStruc]

RHTPE is only refering to the steel in tension in a flexural member (beam) to limit the rupture mode to a ductile one (as Canada code for example limit you to do so). But note that some code accept concrete crushing as a valid rupture mode too in other part of the world (as EURO for example).

What you are refering to, the maximum steel in axial member, serve purpose of workability when placing and compacting the concrete in beam/column joint. See comments in your respective region concrete code (ex : clause 10.9.2 for Canadian A23.3-04 Code)

 

[Lemony]

Keep in mind that at splice regions you are essentially doubling the amount of steel, that gets pretty crowded at 10%. I have always heard the practical limit for efficiency and constructability is 1~5%.

 

[PicoStruc]

Exactly, As Lemony said, these limit usually include the lap splice, that mean the if the limit is prescribed as 8% (canadian code) the real limit is 4% if you splice 100% of rebars !

 

[hokie66]

BauTom,
Maximum column reinforcement limitation is for constructability. Try the search. It works. I found this recent thread. Started by "BauTomTom". Is that you?

thread507-302810

 

[TXStructural]

ACI 318 10.9 gives a maximum reinforcement ratio for columns. Note that all of the reinforcement in conventional columns is around the perimeter. This creates significant congestion as you approach 8%. In a purely compression member, where you are able to assure proper transfer of loads into and out of the member, reinforcement could be also placed inside the column.

R10.9.1 states that the limit is based on practical considerations and the availability of experimental data.

 

[BauTom]

Thank you guys for the interesting answers. It helps a lot

Regards

Tom

 

[Dinosaur]

In my experience column reinforcement around the perimeter gets congested (when it must be spliced) at around 2%. Check the formulas for maximum top size of aggregate and some other bar spacing limits and you will see that it will be quite difficult to get anywhere near 8%.