Double Reinforcement in ACI 318

[drasticxx]

Hello,

1. As far as I know the development length should be developed from both side of the maximum moment point,hence for negative moment for the top steel corner ,we use hook to achieve development form the end corner side and use straight length from the other continuous side

2. If what I said is right,then regrading compression steel used in double reinforcement
a) for positive moment the top steel will be under compression,thus the development length with be Ld Compression (both sides)
b)for negative moment at end support, their will be negative moment,thus the bottom steel will be under compression, and hence we have bottom steel already (which we use it for positive moment),we can use it,but the dl compression will be greater the column width,hence I have to use hook,but actually there is no details for hook in compression ,so how can I let the compression steel work,if I cant achieve development

Could anyone please fix the missing part of my understanding here?

 

[KootK]

It sounds as though you understand this just fine drasticxx. For a discontinuous beam span, utilizing the bottom bars as compression steel will be tough. You can:

1) Use very small bars that can be compression developed into the columns or;
2) Prorate the axial force in the rebar based on the amount of compression development length available. This will affect your assumptions regarding bar yielding in your flexural calcs.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[drasticxx]

Thank you

Regarding your second point,fy in the development length equation(according to ACI) ,will be replaced fs developed in compression steel,actually this will be the case for all compression development length calculations,am I right?

but stop ,on the other hand the idea for development length is that steel yields before slip, while actually steel here may not yields???!!! I am little bit confused

 

[KootK]

Regarding your second point,fy in the development length equation(according to ACI) ,will be replaced fs developed in compression steel,actually this will be the case for all compression development length calculations,am I right?

Right.

but stop ,on the other hand the idea for development length is that steel yields before slip, while actually steel here may not yields???!!!

That is the idea for full development. There is such a thing as partial development, however, which is available for some applications (shear friction is not one). In truth, partial development always makes me nervous. If your bar somehow sees more strain than you planned on, you've got a brittle bond failure on your hands.

In practice, I rarely use compression steel. In addition to the issues that we're discussing, some feel that compression steel requires buckling prevention ties like columns which makes things messier still.



I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[drasticxx]

thank you

 

[TXStructural]

In most cases, the confining concrete provides adequate restraint from buckling of compression reinforcement. You will also always have some confining steel in both beams and columns. When in doubt in disturbed regions at ends of beams, strut and tie provides a good way to analyze the section, but it may get difficult in complicated, 3-D situations.

I always encourage design using compression steel, if for no other reason than that construction will require an amount of steel to carry stirrups. You may also find that the added capacity costs almost nothing. However, most designers are content with single reinforcement and it is still the way schools teach design. Many find that there are also added serviceability and performance gains over the life of a structure.

 

[drasticxx]

Just Curious why there is no hook in compression?what is the scientific interpretation?

 

[KootK]

The scientific explanation is that it's been investigated experimentally and found not to help appreciably.

I'm sure that, with infinite concrete cover, a compression hook would help quite a bit. However, we generally use hooks because we're up against a free edge. In that scenario, the significant compression force that would need to develop behind the hook bend in a compression hook would tend to spall the concrete cover. With a properly oriented tension hook, the compression emanating from the hook bend usually points toward, and is anchored by, the member flexural compression zone.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[rapt]

That is correct, you cannot develop compression in a compression bar using a hook or cog.