Ld AND Ldh

[h-badawy]

@ THE CONNECTION BETWEEN COLUMN AND BEAM, IF THE WIDTH OF THE COLUMN PARALLEL TO THE BEAM IS LESS THAN (Ldh) OF THE BEAM TOP REINFORCEMENT WHAT CAN I DO, ALSO WHEN CAN I USE (Ld) AND (Ldh)?

 

[TXStructural]

You are correct that you need full development in order to count the moment continuity. It sounds like you are working with an exterior column, since this would not be a concern where the beam reinforcement continues through the column for a full development length. One of the technique used to maintain strong moment continuity is to lap splice column bars and beam bars outside the intersection, this reduces congestion in the intersection while maintaining strength.

You should use Ldh only where the bar is hooked 90 (or 135) or 180 degrees. Ld is for straight bar. A bar is accepted as developed beyond the minimum development length (Ld or Ldh.)

I usually say that one should avoid oddly bent bars since their behavior in concrete is not predicted by the code requirements. Tighter bends than standard hooks can result in non-ductile behavior.

ACI 318 does a fair job of conveying these points without much explanation. The thing to realize about the code is that the provisions work together. One can argue that one provision is not truly representative of how a particular structure will work, but in order to use the simplifications offered by certain provisions they must all be applied as required. Going back to "first principles" only works if you do so with all aspects. For a production engineering situation, getting a safe, constructible design in an economical way is important, and that is the purpose of the code.
As time goes on, refinements to reduce the amount of materials in structures frequently requires increased complexity (cost) in the design process (more equations to get a similar design) and more difficult/expensive construction (more complex formwork or closer tolerances.)

 

[h-badawy]

thank you

but if I will use Ldh instead of Ld the moment will be transferred from the beam to the column as a moment connection ? , also in case of width of support (column or girder) smaller than Ldh length i have to increase the width to achieve this length ?

 

[JAE]

Yes - using Ldh would get you a moment connection at the column (assuming the reinf. can take the moment).
Yes - you have to increase the column width to get a full Ldh inside the column.

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[h-badawy]

thank you JAE

but in this case any connection between beam or slab with any type of supports (girders , columns or shear wall ) I have to take into consideration length Ldh of the top reinf. into the support for example if I used top mesh T20 (200mm) bars diameter the Ldh will be 360 mm so if I have shear wall with width 300mm I have to increase it to be 400mm and the same of the beams .

 

[KootK]

That's correct haithambawdwy. It's a great reason to use small diameter bars. You can also prorate your bar capacity based on available development length but that's pretty inefficient design.

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.

 

[h-badawy]

thank you kootk

 

[WiSEiwish]

Are you close to being able to use the full ldh? ACI 318 allows you to reduce the development length by the ratio of A_s,req'd/A_s,provided. If you are close to getting the development length to work, you could check that ratio and potentially reduce the development length. If the cross section of the column is extremely important you may have to over-design the steel to reduce the development. I'm not sure how efficient that design would be though.

 

[h-badawy]

Thank you PE2012