ACI Section 10.5.2 for flange in tension 1

[StuSE]

Hi

I'm designing stadia / risers for a stadium. So the flexural shape is a "Z" section with a flange in tension.(See enclosure)

For the design of the "Z" section the limiting factor for my overhanging flange is 1/2 the clear distance, so I proceed to design my section. In my design I don't need much steel, so I have to check for minimum flexural reinforcing.

Section 10.5.1 of the code lists As min = 3f'c^0.5 / fy * bw *d but states (10.5.2) that for members with a flange in tension, bw shall be replaced with 2bw or the width of the flange, whichever is smaller.(in my case 2bw is smaller)

My question is: When I run the formula for As min am I comparing this result to all of my flexural reinforcement used within my L/2 flange design width or is it restricted to only the steel within the 2bw dimension?

Thank you...going to paint a wall now!

 

[rapt]

You should calculate the cracking moment of the section and base the minimum ultimate capacity required on 1.2 * cracking moment. This is the basis of the minimum reinforcement rule.

 

[hetgen]

...1.2 * cracking moment. This is the basis of the minimum reinforcement rule.

.

Wow is that the basis As min? I don't get it!

Cracking moment is a function of tensile strength of concrete, why then do we have to reinforece for 1.2x tensile capacity of the concrete?

 

[hokie66]

To prevent fracture of the steel reinforcement immediately upon cracking of the concrete, leading to sudden collapse. Don't you think that is important?

 

[hetgen]

Thanks Hokie 66
Yes, avoiding brittle failure is important. So we up the minimum reinforcing to be 120% the compressive capacity of the section to avoid failure without warning.
Does this also satisfy the requirement for spread cracking to develop?

 

[rapt]

hetgen,

It gives similar results to the Eurocode rules on minimum reinforcement for crack control to allow multiple cracks to develop.

I have never seen the reasoning fo =r the 1.2, but I ass=ume it is to allow for various factors like variability of tensile strength of concrete, the fact that we use a lower bound concrete strength etc.

The rule is actually in ACI but it is in the section on minimum reinforcement for bonded prestress! Same in BS8110.

But the Rc section rules were originally based on the same logic as they still are in the Australian codes.

 

[hetgen]

Thank you rapt for pointing on the right direction, I will read more about this. I like the simplicity (1.2 x Mcr).

EC2 7.3.2.2 actually states that.... In profiled cross sections like T-beams and box girders, minimum reinforcement should be determined for the individual parts of the section (webs, flanges). And it has this drawn-out formula to calculate min area of steel which allows for the nature of the stress distribution within the section immediately prior to cracking and of the change of the lever arm.